BENV0003 MSc LL Dissertation
“Discomfort Glare: The Impact of the Spectral Power Distribution and Spatial
Frequencies within Glare Sources on Discomfort Glare Ratings”
by
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Paper submitted in part fulfilment of the
Degree of Master of Light and Lighting
Bartlett School of Energy, Environment and Resources
University College London
Word Count: 14077
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Acknowledgments
I would to thank my supervisor, Peter Raynham, for his support and guidance during the process
of writing this dissertation.
I would also like to thank my family and my girlfriend for their continued support and
motivation over the years.
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Contents
Figures .................................................................................................................................................................... III
Tables .....................................................................................................................................................................VI
Images .................................................................................................................................................................. VIII
Abstract .................................................................................................................................................................... 1
Introduction ............................................................................................................................................................... 2
Disability glare .......................................................................................................................................................... 3
Discomfort Glare ....................................................................................................................................................... 4
Mechanisms and objective measures of Discomfort Glare ............................................................................................... 7
Subjective Methods for Evaluating Discomfort Glare .................................................................................................. 11
The Psychology of Discomfort Glare........................................................................................................................... 15
Research Aim .......................................................................................................................................................... 20
The Experiments ..................................................................................................................................................... 21
Pilot Phase ......................................................................................................................................................................... 21
Main Experiment .............................................................................................................................................................. 31
Discussion ............................................................................................................................................................... 52
Conclusions.............................................................................................................................................................. 57
Bibliography ................................................................................................................................................................i
Appendix 1: Ethics application .................................................................................................................................. iv
Appendix 2: Participant invitation email ................................................................................................................ xxiv
Appendix 3: Experiment information sheet .............................................................................................................. xxv
Appendix 4: Questionnaire .................................................................................................................................. xxviii
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Figures
Figure 1: A representation of the comfort and acceptance scales used by Geerdinck (Geerdinck, 2012). ...................14
Figure 2: The Wilcoxon Signed Ranks Test results for Gerrdinck’s comparison between the acceptance and comfort
scales (Geerdinck, 2012). ...............................................................................................................................................14
Figure 3: The views used in Tuaycharon and Tragenza 2007 study in their ranked order (Tuaycharoen and Tregenza,
2007) ................................................................................................................................................................................16
Figure 4: The main effect of wavelength on discomfort glare ratings predicted for 0.3lx stimuli. (Michael Flannagan,
Michael Sivak, Michael Ensing, 1989)..........................................................................................................................18
Figure 5: The spectrum produced by the white glare images..............................................................................................23
Figure 6: The spectrum produced by the red glare images. ................................................................................................24
Figure 7: The spectrum produced by the blue glare images................................................................................................24
Figure 8: The spectrum produced by the green glare images..............................................................................................25
Figure 9: A normal Q-Q plot showing normality for the pilot study data. .......................................................................28
Figure 10: Comparison of the mean subjective glare ratings between colour for the pilot study ..................................28
Figure 11: Comparison of the mean subjective glare ratings between pattern for the pilot study .................................29
Figure 12: The discomfort glare threshold data from the healthy control group from Main, Vlachonikolis and
Dowson (2000) by the colour of the light source. (Main, Vlachonikolis and Dowson, 2000) ..............................30
Figure 13: The normal Q-Q plot showing normality of the main study data. ..................................................................31
Figure 12: Comparison of the mean subjective glare ratings between colour for the main experiment........................32
Figure 13: Comparison of the mean subjective glare ratings between colours for the main experiment. Only the no
patterns images (images 1, 7, 13, 19) are included in the data. ..................................................................................33
Figure 14: Comparison of the mean subjective glare ratings between colours for the main experiment. Only the
chequered images (images 2, 8, 14, 20) are included in the data. ..............................................................................34
Figure 15: Comparison of the mean subjective glare ratings between colours for the main experiment. Only the
striped right images (images 3, 9, 15, 21) are included in the data. ...........................................................................35
Figure 16: Comparison of the mean subjective glare ratings between colours for the main experiment. Only the
striped left images (images 4, 10, 16, 22) are included in the data. ...........................................................................36
Figure 17: Comparison of the mean subjective glare ratings between colours for the main experiment. Only the
striped vertical images (images 5, 11, 17, 23) are included in the data. ....................................................................37
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Figure 18: Comparison of the mean subjective glare ratings between colours for the main experiment. Only the
striped horizontal images (images 6, 12, 18, 24) are included in the data. ...............................................................38
Figure 19: Comparison of the mean subjective glare ratings between pattern for the main experiment. .....................39
Figure 20: Comparison of the mean subjective glare ratings between pattern for the white data only. ........................40
Figure 21: Comparison of the mean subjective glare ratings between pattern for the red data only. ............................41
Figure 22: Comparison of the mean subjective glare ratings between pattern for the blue data only. ..........................42
Figure 23: Comparison of the mean subjective glare ratings between pattern for the green data only. ........................43
Figure 24-27: The frequency with which participants 5, 6, 7 and 9 used each subjective rating during the experiment.
..........................................................................................................................................................................................44
Figure 28-32: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the white glare
images only......................................................................................................................................................................45
Figure 33-36: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the red glare
images only......................................................................................................................................................................45
Figure 41-44: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the blue glare
images only......................................................................................................................................................................46
Figure 37-40: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the blue glare
images only......................................................................................................................................................................46
Figure 45-48: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the no pattern glare
images only......................................................................................................................................................................47
Figure 49-52: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the chequered glare
images only......................................................................................................................................................................47
Figure 57-60: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the striped left
glare images only. ...........................................................................................................................................................48
Figure 53-56: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the striped right
glare images only. ...........................................................................................................................................................48
Figure 65-68: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the striped
horizontal glare images only. .........................................................................................................................................49
Figure 61-64: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the striped vertical
glare images only. ...........................................................................................................................................................49
Figure 70: The plot shows the negative trend in the data; however, the correlation coefficient shows the trend is
weak. ................................................................................................................................................................................50
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Figure 69: The plot shows the negative trend in the data; however, the correlation coefficient shows the trend. ......50
Figure 71: Comparison of the mean subjective glare ratings between colour for the main experiment of this paper. 52
Figure 72: The transmission curves published in “The wavelength of light causing photophobia in migraine and
tension-type headache between attacks”. High corresponds with blue, medium corresponds with green and
low corresponds with red. (Main, Vlachonikolis and Dowson, 2000) .....................................................................53
Figure 73: The SPD of the glare images characterised as white. ........................................................................................54
Figure 74: The SPD of the glare images characterised as red. ............................................................................................54
Figure 75: The SPD of the glare images characterised as blue. ..........................................................................................54
Figure 76: The SPD of the glare images characterised as green. ........................................................................................55
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Tables
Table 1: A representation of the De Boer 1973 Scale. The original 1967 scale was updated in 1973 so rating 9
became just noticeable instead of unnoticeable. (Fotios, 2015). .......................................................................................12
Table 2: A representation of the scale version of the Hopkinson multi-criterion system. This is the single item scale
version used by many researchers (Tuaycharoen and Tregenza, 2005; Geun, Ju and Jeong, 2011). ....................12
Table 4: The custom scale used in this experiment. .............................................................................................................26
Table 3: The original 1973 De Boer scale .............................................................................................................................26
Table 5: The qualifiers used, and the definitions given to the participants during the pre-experiment briefing. .........26
Table 6: Results from the paired samples tests comparing responses to the different patterns within the glare images.
The only pairs that reach statistic difference (pairs 2, 6, 11, 12, 13) were comparison between the high
frequency patterns and the lower frequency pattern or with the no pattern images, this however was not always
true for this sample. Statistically significant differences are highlighted in green, with the reverse being
highlighted in red............................................................................................................................................................29
Table 7: Paired sample test and correlation data for comparison between glare image colour. Statistically significant
differences are highlighted in green, with the reverse being highlighted in red. .....................................................32
Table 8: Paired sample test and correlation data for comparison between glare image colour with all data from
images containing patterns removed. Statistically significant differences are highlighted in green, with the
reverse being highlighted in red. ...................................................................................................................................33
Table 9: Paired sample comparison between glare image colour for data obtain from images containing a chequered
pattern. Statistically significant differences are highlighted in green, with the reverse being highlighted in red. 34
Table 10: Paired sample comparison between the data for the chequered and striped right conditions. Statistically
significant differences are highlighted in green, with the reverse being highlighted in red. ..................................35
Table 11: Paired sample comparison between the data for the striped right conditions. Statistically significant
differences are highlighted in green, with the reverse being highlighted in red. .....................................................35
Table 12: Paired sample comparison between the data for the striped left conditions. Statistically significant
differences are highlighted in green, with the reverse being highlighted in red. .....................................................36
Table 13: Paired sample comparison between the data for the striped vertical conditions. Statistically significant
differences are highlighted in green, with the reverse being highlighted in red. .....................................................37
Table 14: Paired sample comparison between the data for the striped vertical conditions. Statistically significant
differences are highlighted in green, with the reverse being highlighted in red. .....................................................38
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Table 15: Paired sample test and correlation data for comparison between glare image pattern. Statistically significant
differences are highlighted in green, with the reverse being highlighted in red. .....................................................39
Table 16: Paired sample data for comparison between glare image pattern for the white data. Statistically significant
differences are highlighted in green, with the reverse being highlighted in red. .....................................................40
Table 17: Paired sample data for comparison between glare image pattern for the red data. Statistically significant
differences are highlighted in green, with the reverse being highlighted in red. .....................................................41
Table 18: Paired sample data for comparison between glare image pattern for the blue data. Statistically significant
differences are highlighted in green, with the reverse being highlighted in red. .....................................................42
Table 19: Paired sample data for comparison between glare image pattern for the Green data. Statistically significant
differences are highlighted in green, with the reverse being highlighted in red. .....................................................43
Table 20: The mean glare scores for each participant across all glare images. Scores equal to or lower than 5 are
highlighted in red with scores greater than 5 being highlighting in green indicating the high and low sensitive
groups, red being the high sensitivity group. ..............................................................................................................44
Table 21: The difference between the actual and predicted means for each glare image presentation. The total of
each third and eighth of the data shows a decrease in glare rating as the experiment progressed........................50
Table 22: Shows the order each participant viewed the glare images in. The colour of the cells represents the
perceived colour of each glare image. ..........................................................................................................................51
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Images
Image 3: White, frequency of pattern in degrees: 0.039 ......................................................................................................21
Image 2: White, vertical frequency in degrees: 0.993, Horizontal frequency in degrees: 1.457 ......................................21
Image 1: White, with 0 cycles horizontal and 0 cycles vertical. ..........................................................................................21
Image 6: White, with 0 horizontal cycles, vertical frequency in degrees: 0.086 ................................................................21
Image 4: White, frequency of pattern in degrees: 0.039 ......................................................................................................21
Image 5: White, with 0 vertical cycles, horizontal frequency in degrees: 0.086 ................................................................21
Image 7: Red, with 0 cycles horizontal and 0 cycles vertical. ..............................................................................................21
Image 9: Red, frequency of pattern in degrees: 0.039 ..........................................................................................................21
Image 8: Red, vertical frequency in degrees: 0.993, Horizontal frequency in degrees: 1.457 ..........................................21
Image 12: Red, with 0 horizontal cycles, vertical frequency in degrees: 0.086 ..................................................................22
Image 11: Red, with 0 vertical cycles, horizontal frequency in degrees: 0.086 ..................................................................22
Image 10: Red, frequency of pattern in degrees: 0.039........................................................................................................22
Image 13: Blue, with 0 cycles horizontal and 0 cycles vertical. ...........................................................................................22
Image 14: Blue, vertical frequency in degrees: 0.993, Horizontal frequency in degrees: 1.457 .......................................22
Image 15: Blue, frequency of pattern in degrees: 0.039 .......................................................................................................22
Image 16: Blue, frequency of pattern in degrees: 0.039 .......................................................................................................22
Image 17: Blue, with 0 vertical cycles, horizontal frequency in degrees: 0.086 .................................................................22
Image 18: Blue, with 0 horizontal cycles, vertical frequency in degrees: 0.086 .................................................................22
Image 21: Green, frequency of pattern in degrees: 0.039....................................................................................................22
Image 20: Green, vertical frequency in degrees: 0.993, Horizontal frequency in degrees: 1.457 ....................................22
Image 19: Green, with 0 cycles horizontal and 0 cycles vertical. ........................................................................................22
Image 24: Green, with 0 horizontal cycles, vertical frequency in degrees: 0.086 ..............................................................23
Image 23: Green, with 0 vertical cycles, horizontal frequency in degrees: 0.086 ..............................................................23
Image 22: Green, frequency of pattern in degrees: 0.039....................................................................................................23
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Abstract
Modern discomfort glare research has three key aims: to create an objective measure for
discomfort glare, to find the physiological causes behind the phenomena, and to increase our
understanding of the underlying psychology of discomfort glare perception.
Within the branch of research focused on the psychology relating to discomfort glare perception,
interest in the field of view is one of the proposed factors that could be having an impact. This
paper argues that this concept may be masking the effect of a number of other factors. Two of
these proposed factors are the spectral power distribution of the glare source and the spatial
frequencies within the glare source itself.
Traditionally, this field of research focuses on discomfort glare from windows, with simulated
windows occasionally being used as replacements, allowing for precise control of luminance. The
same thinking has been applied to self-luminous devices in this study.
The results show that spectral power distribution and spatial frequencies do have an impact on
glare perception. Glare sources that contain a higher proportion of high energy wavelengths (white
and blue) are rated as causing increased discomfort glare, in comparison to glare sources that
contain higher proportions of medium or low energy wavelengths (green and red). Also, as shown
in past research, medium spatial frequencies caused increased discomfort. The smaller of those
patterns that contain medium spatial frequencies produced the highest discomfort among the
participants. This indicates that the concept of interest may in fact be masking the impact of a
number of other factors on the perception of discomfort glare.
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Introduction
The Illuminating Engineering Society of North America (IESNA) defines glare as:
“the sensation produced by luminance within the visual field that is sufficiently greater than the luminance to which
the eyes are adapted to cause annoyance, discomfort or loss in visual performance and visibility”
(Rea, 2000)
Voss (1999) proposed eight types of glare, the first four of which are experienced very rarely and
the final four more frequently:
1/ Flash blindness: this is a temporary state of complete bleaching of retinal photopigment
caused by sudden exposure to an extremely bright light source.
2/ Paralysing glare: named after the phenomenon by which a person is paralysed by sudden
illumination.
3/ Exposure to bright enough light to cause retinal damage.
4/ Distracting glare: is produced by bright flashing lights in the peripheral visual field.
5/ Dazzle or saturation glare: this is caused when a part of the visual field is too bright and is
experienced very rarely indoors.
6/ Adaption glare: this is experienced when the visual field is exposed to a dramatic and sudden
increase in luminance of the whole visual field.
The seventh and eighth form of glare are the two most commonly experienced and are essentially
different reactions to the same stimulus: a wide variation in luminance across the visual field
(Boyce, 2014).
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7/ Disability glare
8/ Discomfort glare
The final two types of glare proposed by Voss (1999), will be discussed in greater detail; the
latter of the two is the primary subject of this paper.
Disability glare
Disability glare disables the visual field to some extent and is produced by light scattering in the
eye, which is known as veiling luminance (Vos, 2003; Boyce, 2014; Yingxin Jia, 2014). This has
two effects: it decreases the luminance contrast of the retinal image, reducing visibility, and the
increased retinal illumination reduces threshold contrast which will increase visibility. Of the two
effects of veiling luminance, the first is the most prominent, meaning disability glare is always
accompanied by a reduction in vision (Boyce, 2014).
A comparison between the visibility of an object seen in the presence of a glare source with the
visibility of the same object seen through a uniform luminous veil can be used to measure the
magnitude of disability glare. When the two visibilities are identical, the veiling luminance is a
measure of the disability glare produced by the glare source and is called the equivalent veiling
luminance (EVL) (Boyce, 2014).
The formula for EVL is:
Where:
𝐿𝑣 = 10Σ𝐸𝑛 Θ−2
𝑛
LV = The EVL (cd/m2)
En = The illuminance at the eye from the glare source (lx)
= the angle between the line of sight and the nth glare source (degrees)
n
Since disability glare is a product of illuminance at the eye and the angle between the glare source
and the line of sight, disability glare can occur when the glare source is located outside of the visual
field. Discomfort glare, however, is only perceived when the glare source is located within the
visual field.
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Discomfort Glare
The Commission Internationale de l’Éclairage (CIE) makes a distinction between disability glare,
defined as, “glare that impairs the vision of objects without necessarily causing discomfort‟, and discomfort
glare, defined as, “glare that causes discomfort without necessarily impairing the vision of objects‟ (CIE, 1987)
Although disability glare and discomfort glare are defined as above, the assumption should not be
made that disability glare does not cause discomfort and that discomfort glare does not impair
vision.
Boyce notes:
“As for the disabling effect of what is conventionally called discomfort glare, the failure to find any effect
of visual capabilities is probably more a matter of measurement sensitivity than anything else”
(Boyce, 2014)
While disability glare is well understood, having an effect on visual capabilities which can be
measured using psychophysical procedures and having a plausible mechanism behind the
phenomena, discomfort glare is not as well understood (Boyce, 2014).
Discomfort glare has been the focus of research for many years, starting in the first half of the 20th
century and culminating in more recent research. The focus of this research has been the
physiological causes, the environmental causes, and the prediction of discomfort glare (Einhorn,
1979; Flannagan et al., 1989; Clear, 2012; Boyce, 2014).
Although a number of suggestions have been made for the physiological cause of discomfort glare,
including pupil constriction (Hopkinson, 1956), pupillary hippus (Howarth et al., 1993) and muscle
tension around the eye (Murray, Plainis and Carden, 2002), no cause has been proven beyond
doubt.
However, there is consensus on the environmental factors that cause discomfort glare. The main
factors in the visual field that cause discomfort glare are source luminance levels (Luckish and
Holladay, 1925), background luminance levels (Clear, 2012), angular subtense of the source
(Luckish and Guth, 1946), eccentricity of the source from the line of sight (Holladay, 1926; Clear,
2012), and the arrangement of the glare source and illuminance at the eye (Bullough et al., 2008).
Discomfort glare is a subjective response, as it is essentially a form of pain sensation. However,
researchers and engineers have developed formulae to quantify and predict discomfort glare. These
prediction models include British Glare Index (BGI), Discomfort Glare Index (DGI), Cornell
Glare Index (CGI), Unified Glare Rating (UGR), Visual Comfort Probability (VCP), Discomfort
Glare Probability (DGP), Predicted Glare Sensation Vote (PGSV), Osterhaus’ Subjective Rating
(SR) (Clear, 2012; Yingxin Jia, 2014).
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Of these glare models, specifically related to small sources indoors, only two are in widespread use
today. These are VCP and UGR (Boyce, 2014).
The formula for UGR is as follows:
Where:
𝑈𝐺𝑅 = 8log(
0.25 𝐿2𝑠 𝜔
)Σ( 2 )
𝐿𝑏
𝑃
Lb= The background luminance (cd/m2)
Ls= The luminance of the glare source (cd/m2)
= The solid angle subtending the observer’s eye by the glare source (steradians (Sr))
P= The Guth position index of the glare source
As can be seen from the UGR formula, the main factors in the visual field that cause discomfort
glare are accounted for.
The original UGR formula is restricted in its use to sources within the size range 0.0003-0.1 Sr at
the eye. A new function was added later that makes the formula valid for small sources. By
replacing the function 𝐿2𝑠 𝜔 with 200𝐼2 /𝑅2, increasing for sources below 0.005m2 (Boyce, 2014).
Where:
I = Luminance intensity of the source in the direction of the eye (cd)
R = The distance to the eye from the glare source (m)
For sources with areas greater than 1.5m2, a transitional formula was introduced:
𝐺𝐺𝑅 = 𝑈𝐺𝑅 + (1.18 − (
0.18
)) 8 log 2.55
𝐶𝐶
Where:
(
(1 + (
𝐸𝑑
))
220
(1 + (
𝐸𝑑
))
𝐸𝑖
)
CC = Ceiling coverage equal to 𝐴0 /𝐴1 .
A0 = The projected area of the glare source towards the nadir (m2)
A1 = The area lit by one glare source (m2)
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Ed = The direct illuminance at the eye from the glare source (lx)
Ei = The indirect illuminance at the eye (lx)
GGR and UGR work on the same scale, meaning the same value for GGR and UGR represent
the same level of discomfort glare (Boyce, 2014).
UGR has been shown to correlate with subjective ratings of discomfort glare for a single source,
for multiple sources in a simulated office, and in a real office environment. However, it is worth
noting that the subjective rating was always lower than the predicted value produced using UGR.
This means UGR could be leading to overestimations of discomfort glare, causing some luminaires
to be rejected during the design process when they may not be if another glare model was used.
(Akashi, Y. Muramatsu, R. Kanaya, 1996).
The other most commonly used glare model, VSP, is predominantly utilised in North America.
For a single source, the formula is as follows:
Where:
𝐺𝑙𝑎𝑟𝑒 𝑆𝑒𝑛𝑠𝑎𝑡𝑖𝑜𝑛 = 𝑀 =
(0.50𝐿𝑠 ∙ 𝑄)
(𝑃 ∙ 𝐹 0.44 )
Ls = The luminance of the glare source (cd/m2)
Q = (20.4Ws+1.52Ws0.2-0.075), where Ws is the solid angle of the glare source subtending the eye
(Sr)
P = Guth Position Index
F = the average luminance of the field of view including the glare source (cd/m2)
For multiple sources the value M is summed to give the discomfort glare rating (DGR):
𝐷𝐺𝑅 = (Σ𝑀𝑛 )𝑎
Where a = n-0.0914. n is the number of glare sources.
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As can be seen by comparing the formula for UGR and VCP, they are concerned with the same
environmental variables but process them in different ways.
The modern glare model UGR represents a compromise between a number of older national
models, designed to ensure that there is some international unity on the rating of discomfort glare,
in an attempt to make conversations on the subject easier (Boyce, 2014).
However, there are still flaws in our methods for evaluating discomfort glare. Clear (2012) found
that all glare models, including UGR and VCP, performed poorly when compared to the original
Luckish and Guth data that formed the basis for their creation. Other researchers have also found
inconsistencies while using models including UGR and VCP (Stone and Harker, 1973; Rodriquez
and Pattini, 2014).
In order to improve our methods for evaluating discomfort glare a greater understanding of the
mechanisms and the psychology behind the perception of the phenomena is needed, and this
improved understanding needs to be reduceable to practice.
Mechanisms and Objective Measures of Discomfort Glare
“We cannot measure discomfort glare objectively, since it is essentially a psychological phenomenon”
(Einhorn, 1979)
Discomfort glare is often assessed using subjective methods including rating scales such as the
De Boer and Hopkinson scales. However, this has not stopped researchers from seeking an
objective method to measure and evaluate discomfort glare by researching the physiological
mechanisms behind the sensation.
If, like with disability glare, a plausible physiological cause for discomfort glare was found that
scaled with an increase in the perception of glare, our ability to measure and predict discomfort
glare would be greatly improved. Many attempts have been made to isolate this physiological
cause (Perry, 2002; Yingxin Jia, 2014).
One possible physiological mechanism which contributes to discomfort glare involves the action
of the pupil. Hopkinson (1956) investigated the role that pupil constriction may have on
discomfort glare.
A number of conclusions were drawn from Hopkinson’s work:
1/ The slightest sensation of glare caused the pupil to contract from its maximum.
2/ Pupil diameter is related to the illumination on the eye and to the size of the source
producing the illumination.
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3/ The diameter of the pupil is not directly related to discomfort glare, and instead seems
to be a product of illumination at the eye.
4/ In the presence of intolerable glare, the pupil varies in diameter irregularly, however
this also occurred when participants concentrated on a fixation point when no glare was
present.
Hopkinson concluded, in part, that the sensation of discomfort may be associated with the
actions of the sphincter and dilator muscles surrounding the iris, due to signals received from
exposure to a high luminance area with a low luminance background. However, Hopkinson
states that pupillary hippus is more likely to be caused by illuminance produced at the eye by the
glare source and the background than by a direct effect of discomfort glare. Fry and King (1975),
also support this claim (Fry and King, 1975; Yingxin Jia, 2014).
A correlation between pupil actions and subjective ratings of discomfort glare have also been
found by other researchers. Lin et al. (2015) found that pupil constriction and glare ratings on the
De Boer scale correlate with a coefficient of (R2=-0.61, p<0.001), but they also found that there
was correlation between eye movement (saccades) and ratings on the De Boer scale (R2=0.94,
P<0.001). They noted that severe discomfort glare increased the speed of eye movement and
caused greater pupil constriction. Stringham et al. (2011) also found that greater visual discomfort
is associated with greater iris constriction (R2 =0.429, p<0.037) (Stringham et al., 2011).
However, other studies have failed to replicate this correlation. Howarth et al. (1993) failed to
replicate Hopkinson’s results from his 1956 paper, “Glare Discomfort and Pupil Diameter”.
More recent research has also explored the relationship between pupil responses and discomfort
glare evaluations.
It has been shown that greater source luminance causes greater pupil constriction. 750,000
cd/m2 causes significantly greater pupil constriction compared to 20,000 cd/m2, and that pupil
constriction increases more between 20,000-205,000cd/m2 than between 205,000-750,000
cd/m2. It has also been shown that background luminance has an effect on pupil response, with
a decrease in background luminance being associated with an increase in pupil constriction. This
change in pupil size is significantly correlated with subjective ratings of discomfort glare (r =
0.659 (r2 = 0.434), F = 584.92, p<0.0001) (Tyukhova and Waters, 2018).
Although subjective ratings of discomfort glare and pupil size have been shown to be
significantly correlated, it is reasonable to suggest this is likely caused by increased retinal
illuminance and not the perception of discomfort glare (Tyukhova and Waters, 2018). Retinal
illuminance is a product of source position, solid angle, and source luminance. Therefore, if the
luminance of a glare source increased so would the retinal illuminance, and by consequence pupil
constriction would increase.
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Using pupil size and fluctuations to measure discomfort glare may lead to false readings as they
are influenced by many factors other than light, including the age of the observer, the distance
from the eyes to the object in focus, and emotions such as fear and excitement (Boyce, 2014;
Tyukhova and Waters, 2018).
Another proposed mechanism that could be used to measure discomfort glare is the reaction of
the extra-ocular facial muscles. The sensation of discomfort glare is always accompanied by
flinching of these facial muscles (Berman et al., 1994; Murray, Plainis and Carden, 2002).
Murray et al. (2002), state, that regardless of the physiological causes of discomfort glare, the
predictable accompaniment of the sensation by muscle action may give us an objective method
to measure discomfort glare in the lab and in the field. They propose their Ocular Stress Monitor
(OSM), a device composed of a portable transmitter and a narrow band amplifier allowing
measurements of the electrical activity of the extra-ocular muscles to be taken remotely in a
range of situations.
The OSM uses three electrodes placed below the lower eye lid (active), one lateral to the bottom
eye lid (reference) and one placed on the forehead (ground). The impedance for the experiment
was set to 10K . The electromyographic (EMG) processor consisted of two parts, 1) a radio
transmitter incorporated with a input amplifiers and a narrow band filter, 2) a separate receiver
containing a low and high pass filter, a peak detector and integrator, along with a tone generator,
output buffer amplifiers, and a small loud speaker. (Murray, Plainis and Carden, 2002)
Using the OSM, it was found that the activity of the extra-ocular muscles strongly correlates with
subjective ratings of discomfort glare varying from a maximum coefficient of r2 = 0.817 to a
minimum r2 = 0.659. In all cases the correlation coefficient was significant with p<0.001
(Murray, Plainis and Carden, 2002). Berman et al. (1994) also found agreement between
subjective evaluations of discomfort glare and EMG readings of the extra-ocular muscles.
Although it appears that EMG measurements of the extra-ocular muscles could potentially be
used as an objective method for measuring discomfort glare, there is potential for false readings
if the OSM is used in the field for research. Although Berman et al. (1994) found a strong
correlation between the EMG measurements and subjective ratings, they also reported that
EMG measurements taken around the ear correlated with subjective ratings of discomfort glare.
This indicates that the muscular reaction may be to general discomfort and pain, and if the OSM
is used for field research it may not be possible to limit the experience of discomfort to
discomfort glare alone. More research is needed to fully assess the reliability of using the extraocular muscles as an objective method for measuring discomfort glare, and to assess the
reliability of the OSM.
More recent research collaborating with neuroscience experts has revealed that neural activity
may play a role on discomfort glare perception and severity.
Evidence has emerged that discomfort from the visual scene could be cause by the
hyperexcitability of neurons (Fernandez and Wilkins, 2008; Juricevic et al., 2010).
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In relation to discomfort glare, where an individual is exposed to a high luminance light source,
saturation or hyperexcitability of neurons is likely, and it has been suggested that this is
a homeostatic response to decrease the metabolic load (Wilkins et al., 1984). It follows that under
conditions that cause discomfort glare, we would be likely to find increased activity in the areas
of the brain that process visual information. In a recent study, neural activity was monitored
using fMRI while participants were exposed to stimuli that caused discomfort glare. In this study
participants were organised into low and high sensitivity groups for their perception of
discomfort glare using an adjustment method. The bold responses measured using fMRI were
then compared between the two groups (Bargary et al., 2015).
They found that under conditions that cause discomfort glare bold responses were located in
the bilateral lingual gyrus, bilateral cuneus and superior parietal lobe - all of which have been
shown to play a role in the processing of visual information (Vanni et al., 2002; Koenigs et al.,
2009; Zhu et al., 2018). They also found that bold responses at every light level were significantly
higher among the high sensitivity group (bilateral lingual gyri (left: t(27=6.76, p(c),0.01; right:
t(27)=6.47, p(corr)<0.01), bilateral cunei (left: t(27)=7.01, p(corr)<0.01; right: t(27)=6.31,
p(corr)<0.01), superior parietal lobule (left: t(27)=6.43, p(corr)<0.01; right: t(627)=6.36, p(corr)<0.01).
They found this increased activity even when the light level was below the participants’
discomfort glare threshold, suggesting that these increased neural responses are not necessarily
directly connected to the perception of discomfort glare. However, a positive correlation
between participants’ discomfort glare sensitivity and the magnitude of neural activity was found,
suggesting that there is a relationship between the degree of hyperexcitability and individual
sensitivity to discomfort glare (Bargary et al., 2015).
Although this is strong evidence which supports the idea that neural hyperexcitability is
responsible, at least in part, for the sensation of discomfort glare, it doesn’t necessarily explain
the pain response. It has been suggested that the pain response may be a mechanism to
encourage behaviour to reduce the metabolic load caused by increase neural activity (Wilkins et
al., 1984). However, more research is needed to confirm this conclusion.
There are a number of possible physiological mechanisms that could be the cause of discomfort
glare, and among these, some may be useful as objective measures for the sensation. However,
much more research is needed before firm conclusions are made about the physiological process
that leads to discomfort glare, and as research into objective measurements base their validity on
comparisons with subjective methods, it is unlikely they will completely replace subjective
methods of measuring discomfort glare.
In relation to discomfort glare, Perry is noted as saying:
“There are manifestly no simple and obvious explanations of the phenomenon”
(Perry, 2002)
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Subjective Methods for Evaluating Discomfort Glare
There are many ways to assess participants’ perception of discomfort glare and many of these
have been shown to be reliable and valid, however the use of them comes with the trappings of
all subjective methods.
The perception of discomfort glare is often assessed using introspective methods. These
methods use a number of different procedures, but they all rely on the participants’ ability to
assess the glare they are perceiving and report that accurately to the researcher, either on a
questionnaire or by stating a number or qualifier, or by simply stating that the source is glaring.
Three of these introspective methods are the staircase method (also known as the up-and-down
method), the adjustment method, and rating scales (Stone and Harker, 1973; Tuaycharoen and
Tregenza, 2005; Geerdinck, 2012; Oxford Dictionaries, 2019).
The staircase method was first introduced by Tom Norman Cornsweet in 1962 and is used to
assess absolute thresholds for a stimulus. In the case of discomfort glare, a participant will be
exposed to a glare source which will be adjusted upwards when glare is not perceived and
downwards if glare is perceived. These adjustments happen in smaller and smaller increments
until the threshold for the individual’s perception of glare is reached, meaning that no more
adjustments can be made, or a predefined number of adjustments is reached. Once this has taken
place, the threshold can be calculated using the mean values of a given number of stimuli
(Cornsweet, 1962; Oxford Dictionaries, 2019).
The adjustment method is similar to the staircase method, in that a participant is exposed to a
stimulus, and the intensity of the stimuli is adjusted up or down based on the participants’
responses. The key difference is that, with the adjustment method, the subject is often unaware
of the particular values of the stimulus, meaning the process by which a participant settles on a
value is less clear compared to the computing of a threshold based on the data obtained from the
staircase method (Cornsweet, 1962).
The lack of ambiguity in the staircase method can be a disadvantage. As the participant is fully
aware of the way in which the stimuli are adjusted, they can manipulate the results very easily
while appearing to give meaningful data (Cornsweet, 1962).
The other most popular method for assessing discomfort glare perception are rating scales.
There are a number of rating scales in use, but for glare research, the two most popular are the
De Boer scale and Hopkinson’s multi-criterion scale (De Boer, 1967; Geerdinck, 2012; Fotios,
2015).
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Rating
1
2
3
4
5
6
7
8
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Qualifier
Unbearable
Disturbing
Just acceptable
Satisfactory
Just noticeable
Table 1: A representation of the De Boer 1973 Scale. The original 1967 scale was updated in 1973 so rating 9 became just
noticeable instead of unnoticeable. (Fotios, 2015).
Rating
4.5
4
3.5
3
2.5
2
1.5
1
0.5
Qualifier
Intolerable
Just intolerable
Just uncomfortable
Just acceptable
Just perceivable
Imperceptible
Table 2: A representation of the scale version of the Hopkinson multi-criterion system. This is the single item scale version used
by many researchers (Tuaycharoen and Tregenza, 2005; Geun, Ju and Jeong, 2011).
These scales require participants to give a number rating for their glare perception that is
associated with a qualifier which describes the degree of glare sensation, allowing statistical
analysis to be conducted.
Although scales such as the De Boer and Hopkinson scales have been used for a long time, and
many of the studies that use these scales seem valid and reliable - especially when comparisons
between new physiological measures and these scales strongly correlate - there are issues with
them.
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One major flaw of the De Boer scale, which is not shared by the Hopkinson scale, is the lack of
an option for no glare sensation. Before the 1967 edit, the De Boer scale’s ninth qualifier was
unnoticeable, which is similar to the Hopkinson’s scale’s use of imperceptible. This provides an
option for no discomfort glare sensation. Although it is a useful qualifier since it indicates a level
of discomfort glare that was previously not included in the scale, the change to just noticeable
means that the participants are forced to misreport their no glare sensation as a miniscule glare
sensation, creating bias towards the perception of at least some discomfort glare. Fotios
recommends adding a tenth option to the scale to account for a no discomfort glare response
(Fotios, 2015). This qualifier could simply be no discomfort glare, and as it would have a
corresponding rating higher than just noticeable, statistical analysis would not be compromised.
Another issue with the De Boer scale which is discussed by Tyukhova and Water is the
counterintuitive nature of the scale. Traditionally, on a numbered point scale, people would
assume the higher number equals the highest rating, however on the De Boer scale, this is
inverted with nine becoming the least glaring option. Some researchers state that this may
confuse participants when they make their ratings (Tyukhova and Waters, 2018). However, the
counterintuitive nature of this scale may force participants to play closer attention to the scale
during the experimental briefing, meaning that they spend more time understanding how
qualifiers correspond to each rating.
Researchers have attempted to improve these scales by assessing the impact of specific qualifiers
on the obtained data.
The original 1967 and the edited 1973 De Boer scale contain the qualifiers satisfactory below just
acceptable. Fotios (2015) questions whether feeling any intensity of discomfort glare could be
considered satisfactory. Taking his ideas further, is it reasonable that a satisfactory sensation
could be considered less intense than a just acceptable one? In the Oxford English Dictionary,
satisfaction is defined as “fulfilment of one's wishes, expectations, or needs, or the pleasure derived from this”
(Oxford Dictionary, 2019c), and acceptance is defined as “able to be agreed on; suitable” or
“moderately good; satisfactory” (Oxford Dictionary, 2019a). These qualifiers seem to be inconsistent
with the numerical ratings they are given.
The qualifier disturbing is also placed below unbearable, implying that a sensation “causing anxiety;
worrying” (Oxford Dictionary, 2019b) should be assessed below one that would be “not able to be
endured or tolerated” (Oxford Dictionary, 2019d). Again, this seems inconsistent and has the
potential to cause confusion among the participants using the scale.
Inconsistency within the qualifiers is not exclusive to the De Boer scale. Geerdinck (2012)
criticises the Hopkinson multi-criterion scale for its use of the terms just acceptable and just
comfortable on the same scale. Gerrdinck employed two scales in his experiments, one that was
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exclusively concerned with comfort, and another exclusively concerned with acceptance. Both
scales, like the De Boer and Hopkinson scales, were nine-point scales.
Figure 1: A representation of the comfort and acceptance scales used by Geerdinck (Geerdinck, 2012).
The comfort scale appeared to represent a more critical analysis of the glare setting than the
acceptance scale, and the difference was statistically significant for all settings used other than the
seventh and eighth.
Figure 2: The Wilcoxon Signed Ranks Test results for Gerrdinck’s comparison between the acceptance and comfort
scales (Geerdinck, 2012).
The significant difference between the two scales indicates that the chosen qualifiers have a
significant effect on how participants evaluate the glare they perceive. Taking this into account,
any inconsistency in the chosen qualifiers could potentially confuse the participants’ ratings and
thus decrease the validity and reliability of results.
Fotios (2015) recommends two improvements to rating scales: that the qualifiers are clearly
defined, and that there is a clear option to indicate no glare sensation (Fotios, 2015).
The first recommendation could be improved by specifying qualifiers that describe a consistent
scale of progression, making the scale simpler and therefore easier to understand.
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The Psychology of Discomfort Glare
In previous sections, some potential physiological mechanisms and objective measurement
methods for discomfort glare were evaluated. This section will focus on the potential effect that
psychological factors may have on the perception of discomfort glare.
Considering discomfort glare essentially constitutes a pain response, it is reasonable to assume its
perception is governed by similar mechanisms. It has been shown that emotional states may
either increase or decrease sensitivity to pain, with emotional states traditionally considered
negative causing an increase in pain sensitivity and the reverse being shown for positive
emotional states (Weisenberg, Raz and Hener, 1998; Rainville, Bao and Chrétien, 2005). It
follows that complex sources that cause discomfort glare which have the potential to elicit
emotional responses (for example, self-luminous screens and windows) could potentially cause
increased or decreased subjective ratings of discomfort glare, depending on the information they
contain.
There is some evidence to support this concept, however, it is not direct. Rather than focus on
positive or negative emotional responses, researchers exploring the impact that views, for
example on a screen or through a window, may have on discomfort glare perception instead
focus on the concept of visual interest. This concept implies that subjective interest in the
content of the visual field may have an impact on participants’ tolerance too, or perception of
discomfort glare.
A number of studies support the idea that increased interest in the visual field may reduce
subjective ratings of discomfort glare. In 2007, Tuaycharon and Tragenza conducted two
experiments to explore the impact of visual interest. The first experiment compared subjects’
ratings of discomfort glare of a window covered with a diffusing sheet, with ratings of an
uninteresting view and an interesting view. The second experiment compared subjects’ responses
to natural and urban views, and views with three layers of view (sky/distant view, middle
distance and surfaces close to the window), with views limited to the middle distance. The
subjective ratings were recorded using a Hopkinson-like scale and the DGI equivalents for the
ratings were calculated. The views used in the experiment were assessed and ranked during a
preliminary experiment from one (most interesting) to ten (least interesting) (Figure 3)
(Tuaycharoen and Tregenza, 2007).
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Figure 3: The views used in Tuaycharon and Tragenza 2007 study in their ranked order (Tuaycharoen and Tregenza,
2007)
In experiment one, 72 subjects were assessed (40 men and 32 women), with 20 men and 15
women who wore spectacles, and no colour blindness recorded. They found that both the most
interesting view and the least interesting view were statistically significantly less glaring than the
blank view (with the diffusing sheet) (p<0.01), and that the most interesting view was statistically
significantly less glaring than the least interesting view (p<0.01) (Tuaycharoen and Tregenza,
2007).
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In experiment two, 96 subjects were assessed (44 men and 52 women), with 26 men and 24
women wearing spectacles. They found that natural views were statistically significantly less
glaring than urban views (p<0.01), and that views with 3 layers were statistically less glaring than
views with 1 layer (p<0.01). However, further analysis revealed no interaction between the
factors natural, urban, 3-layer and 1-layer.
These results are supported by a number of other studies (Osterhaus, 2001; Tuaycharoen and
Tregenza, 2005; Geun, Ju and Jeong, 2011; Shin, Yun and Kim, 2012).
However, there are inconsistencies in the findings related to the impact of interest in and the
content of the visual field on discomfort glare. It has been shown that view content may only
influence discomfort glare perception when the view is either very good or very bad, i.e. very
interesting or very uninteresting (Hellinga, 2013), or that there is no effect whatsoever of interest
in view content on discomfort glare ratings (Hirning et al., 2013; Iwata et al., 2017).
Clearly view content does have some effect on discomfort glare ratings, though, the use of the
concept of visual interest may be masking the impact of other factors.
The use of the term interest in research exploring discomfort glare perception may be masking
the impact of other factors, namely colour, defined by its spectral power distribution, and
pattern, defined by its spatial frequency.
It is well established that certain patterns cause discomfort, with the factors that influence
discomfort including shape, spatial frequency, duty cycle, contrast and cortical representation
(Wilkins et al., 1984; Juricevic et al., 2010; Wilkins, 2012). None of these factors have been
assessed in studies which explore the impact view may have on discomfort glare, so they cannot
be discounted as a cause in part of the varying levels of discomfort found in these studies. At a
glance, it is clear that all the views used by Tuaycharon and Tragenza in their 2007 experiments
would clearly be assessed very differently when using these factors, especially when comparing
natural and urban scenes.
The views in Tuaycharon and Tragenza (2007) also clearly differ in the range of colours they
contain, as natural and urban scenes are depicted with differing degrees of sky visible within
them. Since the colour of an object is the product of the intensity of the incident light at each
wavelength and the reflectance of the surface at each wavelength:
Where:
𝐶(𝜆) = 𝐼(𝜆) × 𝑅(𝜆)
C( ) = Colour stimuli
I( ) = The intensity at each wavelength
R( ) = The reflection of the object/surface at each wavelength
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It is reasonable to believe that the resulting spectrum incident at the observer’s eye from these
views may have had an impact on their subjective ratings of perceived discomfort glare.
There is evidence that suggests spectral content has an influence on discomfort glare perception.
Bullough (2009) found that discomfort glare ratings using the De Boer scale for 450nm were
systematically lower than ratings for 510, 590, 650 and 700nm, indicating increased discomfort
glare when exposed to higher energy wavelengths. This was true for both 5 and 10 off-axis to
the left of line of sight (Bullough, 2009).
It has also been shown that discomfort glare thresholds in healthy individuals when exposed to
broadband spectrum stimuli, judged to correspond to blue, green, red and white, were lower for
the blue and red sources compared to the green. A two-way analysis of the variance of
wavelength indicated statistical difference between the wavelengths used (F=91.582; df=2, 116;
P=.0001) (Main, Vlachonikolis and Dowson, 2000). This trend has also been indicated when
using monochromatic sources that contain interference filters with peak wavelengths at 480, 505,
550, 577, 600 and 650nm, with bandwidths at half maximum ranged from 7.1 to 11.4nm. The
main effect of wavelength was found to be significant F(5,70) = 121.23, p<.0001 (Figure 4)
(Flannagan et al., 1989).
Figure 4: The main effect of wavelength on
discomfort glare ratings predicted for 0.3lx
stimuli. (Flannagan et al., 1989)
As can be seen in Figure 4, perceived discomfort glare is increased at higher energy wavelengths
compared to lower energy wavelengths, with the increase in discomfort towards 600nm breaking
the trend.
In direct contradiction to the above, it has been shown that there is a decrease in discomfort
caused by a high luminance light source at 460nm. This decrease in discomfort sensitivity was in
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opposition to a general increase in sensitivity as the wavelengths approached the higher energy
end of the spectrum, which the researchers suggest may be cause by the absorption of light by
macular pigment which has a peak absorption of 460nm (Stringham, Fuld and Wenzel, 2003).
More recently, three experiments were conducted by Sweater-Hickcox, Narendran, Bullough and
Fryssinier, to assess the impact that different coloured luminous surrounds had on discomfort
glare perception from LED sources. The first experiment was designed to assess the impact
different source spectral power distribution (SDP) have on subjective ratings for discomfort
glare, the second experiment was designed to assess if a reduced surround luminance has a
similar effect as in the first experiment, and the third experiment was used to validate the results
from the second experiment (Sweater-Hickcox et al., 2013).
The first experiment showed a significant difference between yellow and blue luminous
surrounds (p<0.05), with the blue luminous surround condition being rated as more glaring. No
significant difference was found between the yellow and white luminous surround. However, in
experiment two and three, no effect of source SPD was found (Sweater-Hickcox et al., 2013).
Similar to previous research, it is suggested that this discrepancy in sensitivity to high energy
wavelengths may be caused by macular pigment (Stringham, Fuld and Wenzel, 2003; SweaterHickcox et al., 2013).
Sweater-Hickcox et al. (2013) varied the viewing angle between their experiments, with
experiment one being 4 and experiment two and three being 2 . It is known that macular
pigment is concentrated within the central 5 of the retina, with the high concentration being
found in the central 2 (Howells, Eperjesi and Bartlett, 2011; Sweater-Hickcox et al., 2013). This
could explain the lack of difference found in subjective ratings for discomfort glare in SweaterHickcox et al. (2013) experiment two and three.
More research is needed to confirm the effect of SPD (colour) on discomfort glare perception,
especially in complex glare sources.
These two factors, SPD (colour) and spatial frequency, may in part be the cause of the decreased
discomfort glare found in research concerned with visual interest, especially considering that
natural scenes were often found to be less glaring than urban scenes. However, biophilia could
also have played a part in causing these decreased ratings.
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Research Aim
Research into discomfort glare caused by complex glare sources including windows and
simulated windows, which are essentially backlit screens, has suggested some complex
interactions between glare source content and the perception of discomfort glare that cannot be
explained by the traditional factors. These include source luminance, luminance of the immediate
surround, background luminance, angular subtense of the source, eccentricity of the source from
the line of sight, the arrangement of the glare source and illuminance at the eye.
As the use of devices that can create complex images (self-luminous displays) is constantly
increasing, and these devices are often being used in environments in which discomfort glare is
likely to be experienced, understanding the interaction between image content (view) and glare
perception is increasingly important.
The research question was:
“Does the SPD of and spatial frequency within a complex glare source have a
measurable effect on subjective evaluations of discomfort glare?”
This question can be split in two:
1/ Is there a measurable effect of the SPD of a glare source on subjective evaluations of
discomfort glare?
2/ Is there a measurable effect of the patterns within a glare source, characterised by their spatial
frequency, on subjective evaluations of discomfort glare?
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The Experiments
Pilot Phase
Methodology
Glare images:
24 different images were created that contained 4 distinct colours and 4 varying patterns. These
images were:
Image 1: White, with 0 cycles
horizontal and 0 cycles vertical.
Image 2: White, vertical frequency in
degrees: 0.993, Horizontal frequency
in degrees: 1.457
Image 3: White, frequency of pattern
in degrees: 0.039
Image 4: White, frequency of pattern
in degrees: 0.039
Image 5: White, with 0 vertical cycles,
horizontal frequency in degrees:
0.086
Image 6: White, with 0 horizontal
cycles, vertical frequency in degrees:
0.086
Image 7: Red, with 0 cycles horizontal
and 0 cycles vertical.
Image 8: Red, vertical frequency in
degrees: 0.993, Horizontal frequency
in degrees: 1.457
Image 9: Red, frequency of pattern in
degrees: 0.039
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Image 10: Red, frequency of pattern
in degrees: 0.039
Image 11: Red, with 0 vertical cycles,
horizontal frequency in degrees:
0.086
Image 12: Red, with 0 horizontal
cycles, vertical frequency in degrees:
0.086
Image 13: Blue, with 0 cycles
horizontal and 0 cycles vertical.
Image 14: Blue, vertical frequency in
degrees: 0.993, Horizontal frequency
in degrees: 1.457
Image 15: Blue, frequency of pattern
in degrees: 0.039
Image 16: Blue, frequency of pattern
in degrees: 0.039
Image 17: Blue, with 0 vertical cycles,
horizontal frequency in degrees:
0.086
Image 18: Blue, with 0 horizontal
cycles, vertical frequency in degrees:
0.086
Image 19: Green, with 0 cycles
horizontal and 0 cycles vertical.
Image 20: Green, vertical frequency
in degrees: 0.993, Horizontal
frequency in degrees: 1.457
Image 21: Green, frequency of
pattern in degrees: 0.039
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Image 22: Green, frequency of
pattern in degrees: 0.039
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Image 23: Green, with 0 vertical
cycles, horizontal frequency in
degrees: 0.086
Image 24: Green, with 0 horizontal
cycles, vertical frequency in degrees:
0.086
Each participant was shown the glare images in a different and random order.
The SPD of the four colours used in the glare image:
SPD of the White Glare Images
2.00E-03
1.80E-03
1.60E-03
W/㎡/nm
1.40E-03
1.20E-03
1.00E-03
8.00E-04
6.00E-04
4.00E-04
2.00E-04
0.00E+00
360nm
410nm
460nm
510nm
560nm
610nm
660nm
710nm
760nm
Wavelength(nm)
Figure 5: The spectrum produced by the white glare images.
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SPD of the Red Glare Images
3.00E-03
W/㎡/nm
2.50E-03
2.00E-03
1.50E-03
1.00E-03
5.00E-04
0.00E+00
360nm
410nm
460nm
510nm
560nm
610nm
660nm
710nm
760nm
660nm
710nm
760nm
Wavelength(nm)
Figure 6: The spectrum produced by the red glare images.
SPD of the Blue Glare Images
1.80E-03
1.60E-03
W/㎡/nm
1.40E-03
1.20E-03
1.00E-03
8.00E-04
6.00E-04
4.00E-04
2.00E-04
0.00E+00
360nm
410nm
460nm
510nm
560nm
610nm
Wavelength(nm)
Figure 7: The spectrum produced by the blue glare images .
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SPD of the Green Glare Images
2.50E-03
W/㎡/nm
2.00E-03
1.50E-03
1.00E-03
5.00E-04
0.00E+00
360nm
410nm
460nm
510nm
560nm
610nm
660nm
710nm
760nm
Wavelength(nm)
Figure 8: The spectrum produced by the green glare images.
Glare source:
The images were displayed on a 13.3” LED retina screen (180mm vertical, 285mm horizontal,
0.053m2), the screen luminance was maintained at 100cd/m2 across all the images, with a black
rest screen with a luminance of 0.3cd/m2 presented between each image. The screen was viewed
from 0.5m away at line-of-sight (0 ), presenting 11.7 on the visual field. Background luminance
was maintained at 0.033cd/m2.
Sample:
5 participants (4 male, 1 female), the lowest age band was 18-25 years and the highest 50-60
years. 2 participants wore correcting spectacles. 4 participants were students on the MSc Light
and Lighting course at UCL.
The sample was an opportunity sample, with a none lighting professional or student included in
the sample.
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Subjective rating scale:
Initially the De Boer scale was chosen as the rating scale for participants to report their glare
perception, however there are two major flaws in the scale that lead to the creation of a custom
scale.
Firstly, the qualifiers are confusing and have conflicting definitions, and secondly there is no
option to report no discomfort glare.
Rating
Qualifier
1
2
3
4
5
6
7
8
Unbearable
9
Just noticeable
Disturbing
Just acceptable
Satisfactory
Table 3: The original 1973 De Boer scale
Rating
1
2
3
4
5
6
7
8
9
10
Qualifier
Unbearable
Just Unbearable
Bearable
Noticeable
Just noticeable
No discomfort glare
Table 4: The custom scale used in this experiment.
The two extremes of the scale (ratings 1 and 9) and the numerical order were kept the same to
give some consistency with the De Boer scale. The qualifiers describing the intermediate
positions on the scale were changed to closer reflect a gradient between the two extremes, with a
10th qualifier added describing no discomfort glare.
The no discomfort glare option was added to the scale as 10 so the option could be incorporated
into the data analysis.
Qualifier
Unbearable
Just Unbearable
Bearable
Noticeable
Just noticeable
No discomfort glare
Definition
Not able to be endured or tolerated
Only just intolerable
Able to be endured or tolerated
Easily seen or noticed; clear or apparent
Not easily seen or noticed; unclear
No discernible sensation of discomfort
Table 5: The qualifiers used, and the definitions given to the participants during the pre-experiment briefing.
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The experiment definition of discomfort glare:
•
Is distracting and is uncomfortable around the eyes
•
You may feel the need to look away from the screen
•
Details on the screen may become blurred
•
The edge of the screen may become blurred
The main indictor of discomfort glare is an uncomfortable feeling around the eye, this feeling may vary in its intensity
but is always present with discomfort glare.
Participants were given this definition to read before the experiment.
Procedure:
Upon entering participants were given a copy of the subjective rating scale to familiarise
themselves with, an experiment information sheet to re-read and keep if they wanted, and the
experimental questionnaire that contained a definition of discomfort glare and some questions
for them to fill out. Once they felt comfortable with the subjective rating scale, they were briefly
tested on their knowledge of the scale by being asked to recall the qualifier that matched a
numerical rating, they were then given the opportunity to spend some more time revising the
scale.
Once the participants felt they were ready to start the adaption phase began. The light levels
were dimmed so the background luminance levels reached 0.033cd/m2. Then a 3 minute
adaption phase was timed before the first glare image was presented.
After the 3 minute mark the participants could activate the timed presentation of the glare
images by pressing the spacebar. This trigged a timed presentation where the glare images were
presented over 5 seconds with 90 seconds rest between images. Participants stated their ratings
of the glare image after the 5 second presentation during the rest period.
The experiment could be paused at any time if the participant required, and when recommencing
the experiment an adaption phase of 3 minutes was implemented as in the beginning of the
experiment.
In order to keep the participants focused on the glare images the questionnaires were filled out
by the researcher when the participant vocalised their rating.
Copies of all the paperwork used in the experiment can be found in the Appendices.
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Test for normality:
Before the data analysis was conducted the data was tested for normality and was found to have
a normal distribution (Figure 9).
Figure 9: A normal Q-Q plot showing normality for the pilot study data.
Results
Mean Subjective Ratings by Colour
The comparison between the
average subjective ratings suggest
a trend similar to previous
research (Flannagan et al., 1989;
Main, Vlachonikolis and
Dowson, 2000). However,
statistical difference was only
reached between the White and
Green conditions (t = -2.892; df
= 29; p<0.007). The White and
Red conditions came close to
statistical difference (t =-1.841; df
= 29; p<0.076).
Subjective Glare Rating
The effect of colour (SPD):
6.0
5.8
5.6
5.4
5.2
5.0
4.8
4.6
4.4
4.2
4.0
5.87
5.50
5.30
4.87
White
Red
Blue
Green
Figure 10: Comparison of the mean subjective glare ratings between
colour for the pilot study
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The effect of pattern:
30/09/2019
The average subjective ratings
compared across pattern suggest
that the patterns with higher
frequencies cause greater
discomfort (images 3, 4, 9, 10, 15,
16, 21, 22 correspond to striped
right and striped left on Figure 6).
This is consistent with past
research (Wilkins et al., 1984;
Juricevic et al., 2010; Wilkins,
2012).
Subjective Glare Rating
Mean Subjective Ratings by Pattern
6.0
5.8
5.6
5.4
5.2
5.0
4.8
4.6
4.4
4.2
4.0
5.65
5.85
5.7
5.8
4.85
4.45
No pattern Chequered Stripped
Right
Stripped
Left
Stripped Stripped
vertical Horizontal
Figure 11: Comparison of the mean subjective glare ratings between
pattern for the pilot study
However, statistical differences
were not always found when comparing the high frequency patterns with the no pattern or other
frequency patterns used (Table 6)
Paired Samples Test for the Pattern Conditions
t
df
Sig. (2-tailed)
Pair 1
No Pattern - Chequered
-0.084
19
0.934
Pair 2
No Pattern – Striped Right
3.269
19
0.004
Pair 3
No Pattern – Striped Left
1.633
19
0.119
Pair 4
No Pattern – Striped Vertical
-0.45
19
0.658
Pair 5
No Pattern – Striped Horizontal
-0.459
19
0.651
Pair 6
Chequered – Striped Right
2.96
19
0.008
Pair 7
Chequered – Striped Left
1.473
19
0.157
Pair 8
Chequered – Striped Vertical
-0.304
19
0.764
Pair 9
Chequered – Striped Horizontal
-0.164
19
0.872
Pair 10
Striped Right – Striped Left
-0.857
19
0.402
Pair 11
Striped Right – Striped Vertical
-3.339
19
0.003
Pair 12
Striped Right – Striped Horizontal
-3.008
19
0.007
Pair 13
Striped Left – Striped Vertical
-2.703
19
0.014
Pair 14
Striped Left – Striped Horizontal
-2.058
19
0.054
Pair 15
Striped Vertical – Striped Horizontal
0.139
19
0.891
Table 6: Results from the paired samples tests comparing responses to the different patterns within the glare images. The
only pairs that reach statistic difference (pairs 2, 6, 11, 12, 13) were comparison between the high frequency patterns and
the lower frequency pattern or with the no pattern images, this however was not always true for this sample. Statistically
significant differences are highlighted in green, with the reverse being highlighted in red.
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Evaluation
Healthy (Control) Data from Main et al (2000)
Discomfort Glare Threshold
The data suggests that there is an
effect of glare source colour
(SPD) on the perception of
discomfort glare.
30/09/2019
7
6.5
6.63
6.31
6.14
6.18
6
5.5
5
Green appears to cause the least
4.5
intense experience of discomfort,
4
which is consistent with previous
Unfiltered
High
Low
Medium
research (Flannagan et al., 1989;
(white)
Wavelength
Wavelength
Wavelength
(red)
(blue)
(green)
Main, Vlachonikolis and
Dowson, 2000). However, the
Figure 12: The discomfort glare threshold data from the healthy
results for the other coloured
control group from Main, Vlachonikolis and Dowson (2000) by the
images are not completely
colour of the light source. (Main, Vlachonikolis and Dowson, 2000)
consistent with other studies.
Figure 7 shows the results of the healthy control group from Main et al. (2000), where the
threshold for the unfiltered (white) condition indicates that a white source would cause a lower
level of discomfort glare compared to a high (red) and low (blue) wavelength sources.
However, the trend in the pilot data for colour seem to follow a similar trend found by
Flannagan et al. (1989). With lower subjective ratings found in the higher energy end of the
spectrum, and the highest ratings being found in the middle of the spectrum and the lower
energy end of the spectrum producing ratings in between (Figure 4). They used an unedited
version of the 1973 De Boer scale. (Flannagan et al., 1989)
The pattern data is consistent with previous research. The patterns that cause the most
discomfort have medium spatial frequencies, which have been shown to cause the most
discomfort when present in the glare images used. (Juricevic et al., 2010; Wilkins, 2012)
However, the sample size in the pilot was very small and in order to draw any meaningful
conclusions a larger sample is needed.
As the data seems to be valid when compared to previous research the methodology was not
altered when moving into the next experimental stage.
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Main Experiment
Methodology
The methodology was identical as the pilot study. This would allow the data from the pilot to be
used in the final analysis.
Sample:
11 participants (4 females, 7 males), the lowest age band was 18-25 years and the highest 50-60
years. 4 participants wore correcting spectacles. 5 participants were students on the MSc Light
and Lighting course at UCL.
The sample was an opportunity sample, with 6 none lighting professionals or students included
in the sample.
The addition of these 11 participants made the final sample for analysis 16, with 5 females, 11
males. 9 participants were students on the MSc Light and Lighting course and 7 were non
lighting professionals.
Test for normality:
The data obtained from the main study was tested for normality and was found to be normal
(Figure 13).
Figure 13: The normal Q-Q plot showing normality of the main study data.
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Results
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Mean Subjective Ratings by Colour
As in the pilot phase the main
experiment’s data suggests that
colour (SPD) has an impact on
the perception of discomfort
glare. However, the trends differ
between the pilot and the main
experiment.
Subjective Glare Rating
The effect of colour (SPD):
6.0
5.8
5.6
5.4
5.2
5.0
4.8
4.6
4.4
4.2
4.0
5.73
5.67
5.21
4.97
White
Red
Blue
Green
In the pilot the green images were
Figure 12: Comparison of the mean subjective glare ratings between
perceived as the least glaring
colour for the main experiment.
(mean subjective rating = 5.87),
whereas after the addition of the
data from the 11 participants from the main experiment, the red images seem to be perceived as
the least glaring. However, the red and green image conditions did not reach significant statistical
difference in either the pilot study (p<0.418) or the main experiment (p<0.793), suggesting the
difference in their subjective ratings is not significant. This was the only change in the overall
trend in the data between the pilot and the main experiment.
As in the pilot the Green and White image conditions reached significant difference (p<0.002),
however unlike in the pilot the Red and White conditions also reached significant difference
(p<0.003). This indicates that images that contain a broader range of the spectrum may cause
increased discomfort glare compared to images that contain a higher percentage of the medium
(green) or low (red) energy wavelengths. This could be explained by the inclusion of the high
(blue) energy wavelengths within the white images. The White and Blue image conditions did not
reach significant difference in either the pilot (p<0.227) or the main (p<0.299) experiment,
whereas the Blue and Red (p<0.045) and the Blue and Green (p<0.031) conditions did reach
significant difference. Indicating that a higher proportion of high energy wavelengths within the
image may be increasing the perception of discomfort.
Paired Sample Test and correlation Results Between Colours
Sig, (2-tailed)
t
df
R2
y=
Pair 1
White - Red
0.003
-3.022
95
0.0442
0.8163x+4.8034
Pair 2
White - Blue
0.299
-1.045
95
0.1871
0.4466x+2.9892
Pair 3
White - Green
0.002
-3.114
95
0.2129
0.4793x+3.285
Pair 4
Red - Blue
0.045
2.027
95
0.0428
0.2412x+3.8264
Pair 5
Red - Green
0.793
0.263
95
0.0174
0.3843x+3.4651
Pair 6
Blue - Green
0.031
-2.194
95
0.2986
0.5499x+2.8026
Table 7: Paired sample test and correlation data for comparison between glare image colour. Statistically significant
differences are highlighted in green, with the reverse being highlighted in red.
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Mean Subjective Ratings by Colours Excluding All
Patterns
6.5
Subjectuve Glare Rating
When looking at the data for
colour while excluding all the
data for the glare images that
contain patterns the overall trend
is different. The data trend more
closely resembles that of the data
obtained from the pilot study,
with statistical difference only
being found between the White
and Green, and the Blue and
Green conditions (Table 8).
30/09/2019
6.00
6.0
5.69
5.5
5.13
5.06
5.0
4.5
4.0
White
Red
Blue
Green
Figure 13: Comparison of the mean subjective glare ratings between
colours for the main experiment. Only the no patterns images (images
1, 7, 13, 19) are included in the data.
Paired Sample Test and Correlation Results Between Colours Excluding all Patterns
Sig, (2-tailed)
t
df
R2
y=
Pair 1
White - Red
0.323
-1.022
15
0.1171
0.3286x+4.0238
Pair 2
White - Blue
0.876
-0.159
15
0.5604
0.7736x+1.2088
Pair 3
White - Green
0.038
-2.27
15
0.5093
0.7189x+2.3604
Pair 4
Red - Blue
0.402
0.863
15
0.0777
0.2999x+3.4193
Pair 5
Red - Green
0.62
-0.506
15
0.1123
0.3515x+4.001
Pair 6
Blue - Green
0.029
-2.406
15
0.6168
0.7657x+2.0759
Table 8: Paired sample test and correlation data for comparison between glare image colour with all data from images
containing patterns removed. Statistically significant differences are highlighted in green, with the reverse being
highlighted in red.
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Mean Subjective Ratings Between Colours for the
Chequered Pattern Data
7
Subjective Rating
The trend in the subjective
ratings between the colours used
in the experiment for the
chequered data is similar to the
data obtain comparing colours
across all the images used.
However, unlike the no pattern
data significant difference was
not reached between any of the
variables tested (Table 9).
30/09/2019
6.4375
6.5
6
5.5
5.625
5.3125
5.1875
5
4.5
4
White
Red
Blue
Green
Figure 14: Comparison of the mean subjective glare ratings between
colours for the main experiment. Only the chequered images (images
2, 8, 14, 20) are included in the data.
Paired Samples Test Between Colour for the Chequered Data
t
df
Sig. (2-tailed)
Pair 1
White Chequered - Red Chequered
-2.029
15
0.061
Pair 2
White Chequered - Blue Chequered
0.198
15
0.846
Pair 3
White Chequered - Green Chequered
-0.536
15
0.6
Pair 4
Red Chequered - Blue Chequered
1.806
15
0.091
Pair 5
Red Chequered - Green Chequered
1.187
15
0.254
Pair 6
Blue Chequered - Green Chequered
-0.788
15
0.443
Table 9: Paired sample comparison between glare image colour for data obtain from images containing a chequered
pattern. Statistically significant differences are highlighted in green, with the reverse being highlighted in red.
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The subjective ratings for each
colour for the no pattern and the
striped right data did not reach
significant difference (Table 10),
this was also true for comparisons
between the variables within the
striped right data (Table 11).
Mean Subjective Ratings Between Colours for the
Striped Right Data
7
6.5
Subjective Ratings
Comparing colours across the
data obtained for the images
containing the striped right
pattern (images 3, 9, 15, 21), the
trend is similar to that of the data
from the no pattern comparison
with the means for the striped
right data being lower than that
of the no pattern data.
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6
5.3125
5.5
4.9375
5
4.5
4.75
4.4375
4
White
Red
Blue
Green
Figure 15: Comparison of the mean subjective glare ratings between
colours for the main experiment. Only the striped right images
(images 3, 9, 15, 21) are included in the data.
Paired Samples Test Comparing the No Pattern Data with the Striped Right Data
t
df
Sig. (2-tailed)
Pair 1
White Striped Right - White No Pattern
-0.824
14
0.424
Pair 2
Red Striped Right - Red No Pattern
-0.645
14
0.529
Pair 3
Blue Striped Right - Blue No Pattern
-0.706
14
0.492
Pair 4
Green Striped Right - Green No Pattern
-0.769
14
0.455
Table 10: Paired sample comparison between the data for the chequered and striped right conditions. Statistically
significant differences are highlighted in green, with the reverse being highlighted in red.
Paired Samples Test Between Colours for the Striped Right Data
t
df
Sig. (2-tailed)
Pair 1
White Striped Right - Red Striped Right
-0.605
15
0.554
Pair 2
White Striped Right - Blue Striped Right
-0.447
15
0.661
Pair 3
White Striped Right - Green Striped Right
-1.066
15
0.303
Pair 4
Red Striped Right - Blue Striped Right
0.296
15
0.771
Pair 5
Red Striped Right - Green Striped Right
-0.878
15
0.394
Pair 6
Blue Striped Right - Green Striped Right
-1.165
15
0.262
Table 11: Paired sample comparison between the data for the striped right conditions. Statistically significant differences
are highlighted in green, with the reverse being highlighted in red.
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Mean Subjective Ratings Between Colours for the
Striped Left Data
7
6.5
Subjectve Rating
The data from the striped left
pattern conditions show that the
white image produced the most
intense sensation of discomfort
glare, with all the other coloured
images producing significantly
less intense discomfort glare
(Figure 16 and Table 12).
Significant difference was not
reach between the other glare
images.
30/09/2019
6
5.4375
5.5
5.125
5.0625
Blue
Green
5
4.5
4.125
4
White
Red
Figure 16: Comparison of the mean subjective glare ratings between
colours for the main experiment. Only the striped left images (images
4, 10, 16, 22) are included in the data.
Paired Samples Test Between Colours for the Striped Left Data
t
df
Sig. (2-tailed)
Pair 1
White Striped Left - Red Striped Left
-2.44
15
0.028
Pair 2
White Striped Left - Blue Striped Left
-2.513
15
0.024
Pair 3
White Striped Left - Green Striped Left
-2.167
15
0.047
Pair 4
Red Striped Left - Blue Striped Left
0.463
15
0.65
Pair 5
Red Striped Left - Green Striped Left
0.706
15
0.491
Pair 6
Blue Striped Left - Green Striped Left
0.105
15
0.918
Table 12: Paired sample comparison between the data for the striped left conditions. Statistically significant differences
are highlighted in green, with the reverse being highlighted in red.
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Mean Subjective Ratings Between Colour for the
Striped Vertical Data
7
6.5
6.5
Subjective Rating
The data from the striped vertical
images shows a similar trend to
the data obtain by comparing the
ratings of the coloured images
while all the pattern data is
included (Figure 17). However, in
this sample no significant
differences were reached (Table
13).
30/09/2019
6
5.5
5.5625
5.5
5.75
5
4.5
4
White
Red
Blue
Green
Figure 17: Comparison of the mean subjective glare ratings between
colours for the main experiment. Only the striped vertical images
(images 5, 11, 17, 23) are included in the data.
Paired Samples Test Between Colours for the Striped Vertical Data
t
df
Sig. (2-tailed)
Pair 1
White Striped Vertical - Red Striped Vertical
-1.426
15
0.174
Pair 2
White Striped Vertical - Blue Striped Vertical
-0.104
15
0.919
Pair 3
White Striped Vertical - Green Striped Vertical
-0.473
15
0.643
Pair 4
Red Striped Vertical - Blue Striped Vertical
1.46
15
0.165
Pair 5
Red Striped Vertical - Green Striped Vertical
1.039
15
0.315
Pair 6
Blue Striped Vertical - Green Striped Vertical
-0.299
15
0.769
Table 13: Paired sample comparison between the data for the striped vertical conditions. Statistically significant
differences are highlighted in green, with the reverse being highlighted in red.
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Mean Subjective Ratings Between Colour for the
Striped Horiontal Data
7
6.3125
6.5
Subjective Ratings
The subjective ratings compared
between colour across the striped
horizontal images show a
different trend to the other data
samples, with the green condition
being considered to cause the
least discomfort glare, and the
other images causing similar
levels of discomfort.
30/09/2019
6
5.5
5.375
5.4375
5.5
White
Red
Blue
5
4.5
4
Green
Figure 18: Comparison of the mean subjective glare ratings between
colours for the main experiment. Only the striped horizontal images
(images 6, 12, 18, 24) are included in the data.
Paired Samples Test Between Colours for the Striped Horizontal Data
t
df
Sig. (2tailed)
Pair 1
White Striped Horizontal - Red Striped Horizontal
-0.136
15
0.894
Pair 2
White Striped Horizontal - Blue Striped Horizontal
-0.202
15
0.843
Pair 3
White Striped Horizontal - Green Striped Horizontal
-1.925
15
0.073
Pair 4
Red Striped Horizontal - Blue Striped Horizontal
-0.128
15
0.9
Pair 5
Red Striped Horizontal - Green Striped Horizontal
-2.267
15
0.039
Pair 6
Blue Striped Horizontal - Green Striped Horizontal
-1.772
15
0.097
Table 14: Paired sample comparison between the data for the striped vertical conditions. Statistically significant
differences are highlighted in green, with the reverse being highlighted in red.
The results of the main experiment indicate there may be an effect of SPD (colour) on the
subjective evaluation of discomfort glare, however statistical difference was only found between
a few of the conditions. It is also reasonable to conclude that the spatial frequency of the
patterns within the glare images would have influenced the glare ratings when looking at the
smaller samples within the main data sample. Further analysis was conducted to assess the
impact of the pattern spatial frequency on glare rating.
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The effect of pattern:
30/09/2019
Mean Subjective Rating by Pattern
Subjective Glare Rating
6.0
5.83
The trend in the data for the
5.66
5.8
5.64
main experiment resembles the
5.47
5.6
trend from the pilot study, with
5.4
5.2
the patterns with a spatial
4.94
4.86
5.0
frequency 0.039 (images 3, 4, 9,
4.8
10, 15, 16, 21, 22) producing a
4.6
4.4
significant difference in subjective
4.2
ratings between these patterns
4.0
and those with spatial frequencies
No pattern Chequered Stripped Stripped Stripped Stripped
Right
Left
vertical Horizontal
of 0.993 vertical and 1.457
horizontal (images 2, 8, 14, 20),
Figure 19: Comparison of the mean subjective glare ratings between
with those with spatial frequencies pattern for the main experiment.
of 0 vertical and 0.086
horizontal (images 5, 11, 17, 23), and those patterns with spatial frequencies of 0.086 vertical
and 0 horizontal (images 6, 12, 18, 24) (Figure 14 and Table 9).
Paired T Test and correlation results between patterns (all colour data)
Sig, (2-tailed)
t value
df
R2
y=
Pair 1
No Pattern -Chequered
0.557
-0.591
63
0.155
0.3769x+3.5795
Pair 2
No Pattern-Striped Right
0.052
1.982
63
0.1157
0.3341x+3.032
Pair 3
No Pattern-Striped Left
0.068
1.857
63
0.1373
0.3262+3.1537
Pair 4
No Pattern-Striped Vertical
0.193
-1.316
63
0.2277
0.466x+3.2763
Pair 5
No Pattern-Striped Horizontal
0.491
-0.692
63
0.2048
0.4127x+3.3992
Pair 6
Chequered-Striped Right
0.01
2.648
63
0.1332
0.375x+2.7443
Pair 7
Chequered-Striped Left
0.013
2.563
63
0.1523
0.3593x+2.9103
Pair 8
Chequered-Striped Vertical
0.49
-0.695
63
0.217
0.4765x+3.1405
Pair 9
Chequered-Striped Horizontal
0.955
-0.056
63
0.1525
0.3725x+3.5553
Pair 10
Striped Right-Striped Left
0.763
-0.303
63
0.2282
0.428x+2.8577
Pair 11
Striped Right-Striped Vertical
0.002
-3.27
63
0.1394
0.3717x+4.0221
Pair 12
Striped Right-Striped Horizontal
0.003
-3.124
63
0.2547
0.4684x+3.38
Pair 13
Striped Left-Striped Vertical
0.001
-3.561
63
0.2572
0.5634x+3.0464
Pair 14
Striped Left-Striped Horizontal
0.002
-3.269
63
0.3452
0.6087x+2.6509
Pair 15
Striped Vertical-Striped Horizontal
0.546
0.607
63
0.1491
0.3601x+3.5577
Table 15: Paired sample test and correlation data for comparison between glare image pattern. Statistically significant
differences are highlighted in green, with the reverse being highlighted in red.
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Subjective Rating
The trend in the data while
The effect of Pattern Accross the White Data
comparing the effect of pattern
7
for only the white glare image
6.5
data is similar to the trend
obtained using the complete
6
5.5
sample (all coloured glare
5.375
5.3125
5.5
5.0625
images). However, the mean
5
subjective rating is lower for all
4.4375
pattern conditions compared to
4.5
4.125
the complete data set, indicating
4
No Chequered Stripped Stripped Stripped Stripped
that colour and pattern both
pattern
Right
Left
vertical Horizontal
impact the subjective rating of
discomfort glare. This effect was
Figure 20: Comparison of the mean subjective glare ratings between
pattern for the white data only.
also found while exploring the
effect of colour, with the white
condition producing the lowest subjective rating (most intense glare perception).
Fewer of the pattern conditions reached significant statistical difference for the white data set,
however those that did were comparisons between the images with spatial frequencies of 0.039
and those with spatial frequencies of 0.993 vertical and 1.457 horizontal, those with spatial
frequencies of 0 horizontal and 0.086 vertical, and those with spatial frequencies of 0.086
horizontal and 0 vertical. (Table 16).
Paired T Test results between patterns for the white data
t
df
Sig. (2-tailed)
-0.42
15
0.68
Pair 1
No Pattern White - Chequered White
Pair 2
No Pattern White - Striped Right White
0.85
15
0.409
Pair 3
No Pattern White - Striped Left White
2.12
15
0.051
Pair 4
No Pattern White - Striped Vertical White
-0.731
15
0.476
Pair 5
No Pattern White - Striped Horizontal White
-0.598
15
0.558
Pair 6
Chequered White - Striped Right White
1.147
15
0.269
Pair 7
Chequered White - Striped Left White
2.643
15
0.018
Pair 8
Chequered White - Striped Vertical White
-0.401
15
0.694
Pair 9
Chequered White - Striped Horizontal White
-0.102
15
0.92
Pair 10
Striped Right White - Striped Left White
0.53
15
0.604
Pair 11
Striped Right White - Striped Vertical White
-1.387
15
0.186
Pair 12
Striped Right White - Striped Horizontal White
-1.431
15
0.173
Pair 13
Striped Left White - Striped Vertical White
-2.756
15
0.015
Pair 14
Striped Left White - Striped Horizontal White
-3.024
15
0.009
Pair 15
Striped Vertical White - Striped Horizontal White
0.19
15
0.852
Table 16: Paired sample data for comparison between glare image pattern for the white data. Statistically significant
differences are highlighted in green, with the reverse being highlighted in red.
40
BENV0003
BXSJ8
The Effect of Pattern Across the Red Data
7
6.5
6.4375
6.5
Subjective Rating
The trend in the data for the
comparison of the effect of
pattern for the red data shows
that the patterns with spatial
frequencies of 0.039 still
produced the lowest subjective
rating, and were therefore
considered to cause the most
discomfort glare. However, in
this sample the Striped
Horizontal condition produced a
similar response on average.
30/09/2019
6
5.6875
5.4375
5.5
5.4375
4.9375
5
4.5
4
No Chequered Stripped Stripped Stripped Stripped
pattern
Right
Left
vertical Horizontal
Figure 21: Comparison of the mean subjective glare ratings between
pattern for the red data only.
As with the white data sample
significant statistical differences
were found between patterns
with spatial frequencies of 0.039 and those with spatial frequencies of 0.993 vertical and 1.457
horizontal, those with spatial frequencies of 0 horizontal and 0.086 vertical, and those with
spatial frequencies of 0.086 horizontal and 0 vertical (Table 17). Statistical difference was also
found between the Chequered and the Striped Horizontal images, however this could be a
product of the sample used, as a number of participants reported an aversion to the red glare
images compared to the other images used.
Paired T Test results between patterns for the red data
t
df
Sig. (2-tailed)
Pair 1
No Pattern Red - Chequered Red
-1.695
15
0.111
Pair 2
No Pattern Red - Striped Right Red
1.399
15
0.182
Pair 3
No Pattern Red - Striped Left Red
0.453
15
0.657
Pair 4
No Pattern Red - Striped Vertical Red
-1.421
15
0.176
Pair 5
No Pattern Red - Striped Horizontal Red
0.62
15
0.544
Pair 6
Chequered Red - Striped Right Red
3
15
0.009
Pair 7
Chequered Red - Striped Left Red
1.777
15
0.096
Pair 8
Chequered Red - Striped Vertical Red
-0.126
15
0.901
Pair 9
Chequered Red - Striped Horizontal Red
2.284
15
0.037
Pair 10
Striped Right Red - Striped Left Red
-0.939
15
0.362
Pair 11
Striped Right Red - Striped Vertical Red
-2.398
15
0.03
Pair 12
Striped Right Red - Striped Horizontal Red
-0.984
15
0.341
Pair 13
Striped Left Red - Striped Vertical Red
-2.512
15
0.024
Pair 14
Striped Left Red - Striped Horizontal Red
0
15
1
Pair 15
Striped Vertical Red - Striped Horizontal Red
1.901
15
0.077
Table 17: Paired sample data for comparison between glare image pattern for the red data. Statistically significant
differences are highlighted in green, with the reverse being highlighted in red.
41
BENV0003
BXSJ8
The Effect of Pattern Across the Blue Data
7
6.5
Subjective Rating
The blue only data seems to be
producing smaller differences in
the subjective glare responses for
the different patterns. The
subjective glare ratings for patterns
with spatial frequencies of 0.039
are closer to the subjective rating
for all the other patterns compared
to the other data samples. The
paired t tests also showed no
statistical difference between the
subjective ratings for the patterns
in this data sample (Table 18).
30/09/2019
6
5.5625
5.5
5.125
5
5.1875
5.5
5.125
4.75
4.5
4
No Chequered Stripped Stripped Stripped Stripped
pattern
Right
Left
vertical Horizontal
Figure 22: Comparison of the mean subjective glare ratings between
pattern for the blue data only.
This could indicate that the colour
of the glare source is interacting with pattern in a way that causes these images to be considered
similarly uncomfortable, even with the differing patterns.
Paired T Test results between patterns for the blue data
t
df
Sig. (2-tailed)
Pair 1
No Pattern Blue - Chequered Blue
-0.085
15
0.933
Pair 2
No Pattern Blue - Striped Right Blue
0.659
15
0.52
Pair 3
No Pattern Blue - Striped Left Blue
0
15
1
Pair 4
No Pattern Blue - Striped Vertical Blue
-0.875
15
0.395
Pair 5
No Pattern Blue - Striped Horizontal Blue
-0.588
15
0.566
Pair 6
Chequered Blue - Striped Right Blue
0.723
15
0.481
Pair 7
Chequered Blue - Striped Left Blue
0.102
15
0.92
Pair 8
Chequered Blue - Striped Vertical Blue
-0.535
15
0.6
Pair 9
Chequered Blue - Striped Horizontal Blue
-0.518
15
0.612
Pair 10
Striped Right Blue - Striped Left Blue
-0.739
15
0.471
Pair 11
Striped Right Blue - Striped Vertical Blue
-1.932
15
0.072
Pair 12
Striped Right Blue - Striped Horizontal Blue
-1.567
15
0.138
Pair 13
Striped Left Blue - Striped Vertical Blue
-0.692
15
0.5
Pair 14
Striped Left Blue - Striped Horizontal Blue
-0.728
15
0.478
Pair 15
Striped Vertical Blue - Striped Horizontal Blue
0.104
15
0.919
Table 18: Paired sample data for comparison between glare image pattern for the blue data. Statistically significant
differences are highlighted in green, with the reverse being highlighted in red.
42
BENV0003
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30/09/2019
Subjective Rating
The green data shows a similar
The Effect of Pattern Across the Green Data
trend as previous data sets, with
7
the patterns with spatial
6.3125
6.5
frequencies of 0.039 producing
6
6
lower mean subjective ratings
5.75
5.625
than the other patterns. Also, the
5.3125
5.5
5.0625
mean subjective ratings for all the
5
patterns are higher compared to
4.5
the white and blue data samples.
This follows from the colour
4
No Chequered Stripped Stripped Stripped Stripped
comparison tests showing that
pattern
Right
Left
vertical Horizontal
the subjective responses to the
green images were higher
Figure 23: Comparison of the mean subjective glare ratings between
compared to the white and blue
pattern for the green data only.
coloured images, indicating that
the green glare images produce lower discomfort glare responses.
As with previous data samples significant statistical difference was only found between the glare
images with spatial frequencies of 0.039 , however in this case statistical difference was only found
between two conditions, the Striped Left and Striped Horizontal images, and the Striped Right
and the Striped Horizontal conditions. The Striped Horizontal images have a spatial frequency of
0 vertical and 0.086 horizontal (Table 19).
Paired T Test results between patterns for the green data
t
df
Sig. (2-tailed)
Pair 1
No Pattern Green - Chequered Green
0.696
15
0.497
Pair 2
No Pattern Green - Striped Right Green
1.047
15
0.312
Pair 3
No Pattern Green - Striped Left Green
1.558
15
0.14
Pair 4
No Pattern Green - Striped Vertical Green
0.473
15
0.643
Pair 5
No Pattern Green - Striped Horizontal Green
-0.512
15
0.616
Pair 6
Chequered Green - Striped Right Green
0.689
15
0.502
Pair 7
Chequered Green - Striped Left Green
1
15
0.333
Pair 8
Chequered Green - Striped Vertical Green
-0.243
15
0.812
Pair 9
Chequered Green - Striped Horizontal Green
-1.337
15
0.201
Pair 10
Striped Right Green - Striped Left Green
0.565
15
0.58
Pair 11
Striped Right Green - Striped Vertical Green
-0.89
15
0.387
Pair 12
Striped Right Green - Striped Horizontal Green
-2.449
15
0.027
Pair 13
Striped Left Green - Striped Vertical Green
-1.58
15
0.135
Pair 14
Striped Left Green - Striped Horizontal Green
-3.596
15
0.003
Pair 15
Striped Vertical Green - Striped Horizontal Green
-1.454
15
0.167
Table 19: Paired sample data for comparison between glare image pattern for the Green data. Statistically significant
differences are highlighted in green, with the reverse being highlighted in red.
43
BENV0003
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30/09/2019
Individual differences in discomfort glare perception:
While collecting the data for the study it became clear that a subset of the participants exhibited
increased sensitivity to discomfort glare. Their mean glare scores taken across every glare image
were noticeably lower compare to the other participants (Table 20).
Participants mean subjective rating
Participant number
Mean Subjective
rating
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
6.25
6.38
5.38
6.08
2.92
7.29
6.67
6.08
3.83
6.50
4.21
6.08
5.58
4.75
3.75
4.63
Table 20: The mean glare scores for each participant across all glare images. Scores equal to or lower than 5 are
highlighted in red with scores greater than 5 being highlighting in green indicating the high and low sensitive groups, red
being the high sensitivity group.
In order to assess whether these differences in participants subjective responses were consistent
across all conditions, the frequency that they used each glare rating was calculated for each
condition and compared to one another. As an example, only participants 5, 9, 6 and 7 will be
included in this section, being the two most and the two least glare sensitive individuals in the
sample.
8
7
6
6
6
4
2
2
1
1
1
0
0
0
9
10
0
1
2
3
4
5
6
7
8
Subjective Rating
Frequency of Each Glare Rating for Participant 9
The occurence of each glare rating
The occurence of each galre rating
Frequency of Each Glare Rating for Participant 5
6
5
4
3
2
1
0
5
5
4
4
2
1
1
1
1
0
1
2
3
4
5
6
7
Subjective Rating
8
9
7
6
6
6
4
2
2
2
1
0
0
0
0
0
1
2
3
4
5
6
7
8
9
10
Subjective Rating
Frequency of Each Glare Rating for Participant 7
10
The occurence of each glare rating
The occurence of each glare rating
Frequency of Each Glare Rating for Participant 6
8
10
8
8
8
6
2
4
3
4
0
0
0
0
1
2
3
4
1
0
0
5
6
7
8
9
10
Subjective Rating
Figure 24-27: The frequency with which participants 5, 6, 7 and 9 used each subjective rating during the experiment.
44
BENV0003
BXSJ8
30/09/2019
4
3
3
2
2
1
1
0
0
0
0
0
0
0
6
7
8
9
10
0
1
2
3
4
5
Subjective Rating
The occurence of each glare rating
Frequency of each glare response to the white
stimuli for Participant 5
Frequency of each glare response to the white
stimuli for Participant 9
4
3
3
2
2
1
1
0
2.5
2
1.5
1
0.5
0
0
1
1
0
2
3
1
0
4
0
5
6
7
1
0
8
9
0
0
0
0
5
6
7
8
0
1
2
3
4
9
10
Subjective Rating
Frequency of each glare response to the white
stimuli for Participant 7
2
1
0
0
Frequency of each glare response to the white
stimuli for Participant 6
10
Subjective Rating
The occurence of each lare rating
The occurence of each glare rating
The occurence of each glare rating
Figure 24-27 indicate that participant 5, 6 ,7 and 9 were reasonably consistent when giving
subjective ratings that correspond with increased or decrease glare sensitivity. However, the
overlap between some of the participants required further investigation. The frequency of each
response was calculated for each condition.
4
3
3
2
1
1
0
0
0
0
1
2
3
4
1
1
0
0
0
5
6
7
8
9
10
Subjective Rating
2.5
2
1.5
1
0.5
0
2
1
2
1
0
1
2
0
3
4
5
0
6
0
7
8
0
9
0
10
Subjectve Rating
Frequency of each glare response to the red
stimuli for Participant 6
2.5
2
1.5
1
0.5
0
2
2
1
0
1
0
2
0
3
0
4
0
5
1
0
6
Subjective Rating
7
8
9
10
The occurence of each glare rating
Frequency of each glare response to the red
stimuli for Participant 5
The occurence of each glare rating
The occurence of each glare rating
The occurence of each glare rating
Figure 28-32: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the white glare
images only.
Frequency of each glare response to the red
stimuli for Participant 9
4
3
3
2
1
1
0
0
0
1
2
3
1
0
1
0
0
0
4
5
6
7
8
9
10
Subjective Rating
Frequency of each glare response to the red
stimuli for Participant 7
4
3
3
3
2
1
0
0
0
0
1
2
3
4
0
0
0
0
8
9
10
0
5
6
7
Subjective Rating
Figure 33-36: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the red glare images
only.
45
Frequency of each glare response to the blue
stimuli for Participant 5
2.5
2
1.5
1
0.5
0
2
2
1
1
0
1
2
3
4
0
0
0
5
6
7
8
0
0
9
10
Subjective Rating
Frequency of each glare response to the blue
stimuli for Participant 6
2.5
2
1.5
1
0.5
0
2
1
0
0
1
2
1
1
1
0
3
0
4
5
6
7
8
9
0
10
Subjective Rating
30/09/2019
The occurence of each glare rating
BXSJ8
The occurence of each glare rating
The occurence of each glare rating
The occurence of each glare rating
BENV0003
Frequency of each glare response to the blue
stimuli for Participant 9
4
3
3
2
2
1
1
0
0
0
0
0
0
0
5
6
7
8
9
10
0
1
2
3
4
Subjective Rating
Frequency of each glare response to the blue stimuli
for Participant 7
4
3
3
2
2
1
1
0
0
0
0
1
2
3
4
0
0
0
8
9
10
0
5
6
7
Subjectuve Rating
Figure 37-40: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the blue glare images
4
3
3
2
2
1
1
0
0
0
0
0
0
0
5
6
7
8
9
10
0
1
2
3
4
Subjective Rating
Frequency of each glare response to the green
stimuli for Participant 6
2.5
2
1.5
1
0.5
0
2
1
0
0
0
1
2
3
4
2
1
0
0
0
5
6
7
Subjective Rating
8
9
10
The occurence of each glare rating
Frequency of each glare response to the green
stimuli for Participant 5
The occurence of each glare rating
The occurence of each glare rating
The occurence of each glare tating
only.
Frequency of each glare response to the green
stimuli for Participant 9
2.5
2
1.5
1
0.5
0
2
2
1
1
0
1
2
3
4
5
0
0
0
0
0
6
7
8
9
10
Subjective Rating
Frequency of each glare response to the green
stimuli for Participant 7
2.5
2
1.5
1
0.5
0
2
2
1
0
0
0
0
1
2
3
4
5
1
6
7
8
0
0
9
10
Subjective Rating
Figure 41-44: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the blue glare images
only.
46
4
3
3
2
1
1
0
0
3
4
0
0
0
0
0
0
5
6
7
8
9
10
0
1
2
Subjective Rating
Frequency of each glare response to the no
pattern stimuli for Participant 6
1.2
1
0.8
0.6
0.4
0.2
0
1
0
1
1
0
1
1
0
2
3
4
0
5
6
0
7
8
0
9
10
Subjective Rating
30/09/2019
The occurence of each glare rating
Frequency of each glare response to the no
pattern stimuli for Participant 5
The occurence of each glare rating
The occurence of each glare rating
BXSJ8
Frequency of each glare response to the no
pattern stimuli for Participant 9
2.5
2
1.5
1
0.5
0
The occurence of each glare rating
BENV0003
2
2
0
0
1
2
3
4
0
0
0
0
0
0
5
6
7
8
9
10
Subjective Rating
Frequency of each glare response to the no
pattern stimuli for Participant 7
4
3
3
2
1
1
0
0
0
0
1
2
3
4
0
0
0
0
7
8
9
10
0
5
6
Subjective Rating
1.2
1
0.8
0.6
0.4
0.2
0
1
0
1
1
0
2
3
0
4
1
1
0
5
6
0
7
8
9
0
10
Subjective Rating
Frequency of each glare response to the chequered
stimuli for Participant 6
4
3
3
2
1
1
0
0
0
0
0
0
0
0
0
1
2
3
4
5
6
Subjective Rating
7
8
9
10
The occurence of each glare rating
Frequency of each glare response to the chequered
stimuli for Participant 5
The occurence of each glare rating
The occurence of each glare rating
The occurence of each glare rating
Figure 45-48: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the no pattern glare
images only.
Frequency of each glare response to the chequered
stimuli for Participant 9
4
3
3
2
1
1
0
0
0
3
4
0
0
0
0
0
6
7
8
9
10
0
1
2
5
Subjective Rating
Frequency of each glare response to the chequered
stimuli for Participant 7
2.5
2
2
1.5
1
1
1
0.5
0
0
0
0
1
2
3
4
0
0
0
9
10
0
5
6
7
8
Subjective Rating
Figure 49-52: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the chequered glare
images only.
47
Frequency of each glare response to the striped
right stimuli for Participant 5
5
4
3
2
1
0
4
0
0
1
2
0
3
0
4
5
0
6
0
7
0
8
0
9
0
10
Subjective Rating
Frequency of each glare response to the striped
right stimuli for Participant 6
2.5
2
1.5
1
0.5
0
2
1
1
0
1
2
0
0
3
4
0
0
0
5
6
7
0
8
9
10
Subjective Rating
30/09/2019
The ocurence of each glare rating
BXSJ8
The occurence of each glare rating
The occurence of each glare rating
The occurence of each glare rating
BENV0003
Frequency of each glare response to the striped
right stimuli for Participant 9
4
3
3
2
1
1
0
0
1
2
0
0
0
0
0
5
6
7
8
0
0
3
4
9
10
Subjective Rating
Frequency of each glare response to the striped
right stimuli for Participant 7
4
3
3
2
1
1
0
0
0
0
1
2
3
4
0
0
0
0
9
10
0
5
6
7
8
Subjective Rating
1.2
1
0.8
0.6
0.4
0.2
0
1
1
1
1
2
1
3
4
0
0
0
0
0
0
5
6
7
8
9
10
Subjective Rating
Frequency of eash glare response to the striped
left stimuli for Participant 6
4
3
3
2
1
1
0
0
0
0
0
0
0
0
0
1
2
3
4
5
6
Subjective Rating
7
8
9
10
The occurence of each glare rating
Frequency of eash glare response to the striped
left stimuli for Participant 5
The occurence of each glare rating
The occurence of each glare rating
The occurence of each glare rating
Figure 53-56: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the striped right
glare images only.
Frequency of eash glare response to the striped
left stimuli for Participant 9
1.2
1
0.8
0.6
0.4
0.2
0
1
0
0
1
2
1
1
1
0
3
4
5
6
7
0
0
0
8
9
10
Subjective Rating
Frequency of eash glare response to the striped
left stimuli for Participant 7
4
3
3
2
1
1
0
0
0
0
1
2
3
4
0
0
0
0
8
9
10
0
5
6
7
Subjective Rating
Figure 57-60: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the striped left glare
images only.
48
Frequency of eash glare response to the striped
vertical stimuli for Participant 5
2.5
2
1.5
1
0.5
0
2
1
1
0
1
2
0
3
4
0
5
0
6
0
7
8
0
9
0
10
Subjective Rating
Frequency of eash glare response to the striped
vertical stimuli for Participant 6
1.2
1
0.8
0.6
0.4
0.2
0
1
0
0
0
0
1
2
3
4
5
1
6
1
0
0
7
8
9
1
10
Subjective Rating
30/09/2019
The occurence of each glare rating
BXSJ8
The occurence of each glare rating
The occurence of each glare rating
The occurence of each glare rating
BENV0003
Frequency of eash glare response to the striped
vertical stimuli for Participant 9
4
3
3
2
1
1
0
0
1
2
0
0
0
0
0
5
6
7
8
0
0
3
4
9
10
Subjective Rating
Frequency of eash glare response to the striped
vertical stimuli for Participant 7
2.5
2
1.5
1
0.5
0
2
1
0
0
0
0
1
2
3
4
1
0
5
6
7
8
0
0
9
10
Subjective Rating
Figure 61-64: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the striped vertical
glare images only.
Frequency of eash glare response to the striped
horizontal stimuli for Participant 5
3
3
2
Axis Title
Axis Title
4
Frequency of eash glare response to the striped
horizontal stimuli for Participant 9
1
1
0
0
0
0
0
0
0
0
5
6
7
8
9
10
0
1
2
3
4
2.5
2
1.5
1
0.5
0
2
1
1
0
1
2
3
4
Axis Title
0
0
1
2
3
4
5
1
0
0
0
6
7
8
Axis Title
9
10
Axis Title
Axis Title
1
0
0
0
0
0
5
6
7
8
9
10
Frequency of eash glare response to the striped
horizontal stimuli for Participant 7
2
0
0
Axis Title
Frequency of eash glare response to the striped
horizontal stimuli for Participant 6
2.5
2
1.5
1
0.5
0
0
2.5
2
1.5
1
0.5
0
2
1
0
0
0
0
1
2
3
4
0
5
1
0
6
7
8
0
9
10
Axis Title
Figure 65-68: The frequency with which participants 5, 6, 7 and 9 used each subjective rating for the striped horizontal
glare images only.
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The deeper analysis of the frequency of each glare response given by participants 5, 6, 7 and 9
show they consistently rated at either end of the scale with few ratings diverging from this
pattern. This indicates that these participants were in fact either more sensitive (participants 5
and 9) or less sensitive (participants 6 and 7) to discomfort glare. This is consistent with a
number of previous studies exploring discomfort glare. (Stone and Harker, 1973; Bargary et al.,
2015)
The effect of order:
An analysis on the impact order may have had on the participants ratings of each glare image was
conducted.
Difference between the actual and predicted means for each glare source
1st Third
0.6
0.7
0.6
0.3
-0.2
2nd Third
-0.5
-0.4
0.2
-0.4
-0.3
0.0
1.3462
Totals
1.84
0.3
0.1
3rd Third
-0.3
-0.1
0.3
0.0
-0.3
0.6
-0.4
-0.4460
-0.32
-0.60
-0.04
0.3
-0.8
-0.1
-0.3
-0.9001
-0.24
-0.02
0.51
-1.14
Table 21: The difference between the actual and predicted means for each glare image presentation. The total of each third and
eighth of the data shows a decrease in glare rating as the experiment progressed.
The totals of each eighth of the data from Table 21
The differences between the predicted and actual means for
each glare presentation
0.8
2.00
y = -0.0206x + 0.2573
R² = 0.1402
0.6
y = -0.0602x + 0.6927
R² = 0.2502
1.50
0.4
1.00
0.2
0.50
0.0
-0.2
0
5
10
15
20
25
-0.4
30
0.00
0
5
10
15
20
25
-0.50
-0.6
-0.8
-1.0
Figure 69: The plot shows the negative trend in the data;
however, the correlation coefficient shows the trend.
-1.00
-1.50
Figure 70: The plot shows the negative trend in the data;
however, the correlation coefficient shows the trend is
weak.
50
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There is a small effect of the order of presentation causing a decrease in the subjective rating of
the glare images. This could indicate a number of effects. The decrease in the subjective rating
could indicate that the rest period of 90 seconds was not enough to return to the original state of
adaption, and the accumulative effect of this caused a decrease in the subjective rating over the
course of the experiment. This decrease in subjective rating could also indicate increased
accuracy in the use of the scale, however a longer experiment and repeated tests with the same
sample and the same scale would be needed to confirm this.
The order each participant viewed the glare images were randomised, therefore reducing the
impact order effects had on the final data set.
The order each participant viewed the glare images in can be found in Table 22.
Order of stimuli presentation for each participant
participant
number
Glare source number
1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
2
3
2
12
1
5
11
7
4
9
6
8
17
13
10
15
20
18
21
19
16
24
22
14
23
3
19
24
10
13
12
6
7
3
11
9
8
20
1
21
15
17
5
2
23
16
18
22
14
4
4
21
2
12
3
10
11
7
6
9
4
24
17
13
1
15
18
20
16
5
23
8
22
14
19
5
11
13
2
12
23
3
18
9
15
19
7
17
24
10
2
20
5
21
4
16
1
22
14
8
6
13
7
3
11
24
6
17
8
10
1
9
20
21
19
5
12
15
2
23
22
16
4
14
18
7
11
6
3
13
24
7
10
8
17
1
9
18
21
15
5
14
20
2
23
19
16
4
22
12
8
7
11
13
24
3
6
10
17
8
1
21
15
18
9
14
5
2
20
23
19
12
4
22
16
9
3
11
7
10
24
1
13
6
21
14
17
8
16
9
23
5
18
22
15
20
12
4
19
2
10
10
13
7
21
24
3
11
17
1
14
23
8
12
9
20
5
18
22
2
6
16
4
19
15
11
3
21
7
13
17
10
11
24
8
14
9
20
12
23
1
5
18
22
2
6
16
4
19
15
12
11
9
10
14
1
13
3
24
17
22
21
5
15
19
7
20
18
8
4
23
16
2
6
12
13
14
3
13
19
22
4
15
24
10
1
21
5
23
18
7
20
11
12
6
9
17
8
16
2
14
22
15
1
21
10
14
3
24
13
4
18
23
20
5
7
19
9
12
8
2
6
17
16
11
15
11
16
17
6
2
8
12
9
19
13
7
5
4
23
18
20
24
3
14
10
21
1
15
22
16
8
23
17
1
19
11
20
16
2
9
22
5
4
13
3
12
24
15
7
18
21
6
14
10
Table 22: Shows the order each participant viewed the glare images in. The colour of the cells represents the
perceived colour of each glare image.
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Discussion
The effect of colour (SPD):
The psychology surrounding the perception of discomfort glare is an extensive field with a
number of areas of specialty, one of which is the impact interest in the content of a glare source
(view) may have on the perception of discomfort glare. However, this concept may be masking
the impact of a number of separate variables on the perception of discomfort glare. This paper
poses the question if SPD, when it has an impact on colour appearance, and the patterns within
images, characterised by their spatial frequencies, could be among the variables masked by the
concept of visual interest.
The results from this paper suggest there is an effect of SPD on the perception of discomfort
glare, with SPDs containing a majority of low or medium energy wavelengths producing high
subjective ratings (lower sensations of discomfort glare), and the reverse being true for SPDs
containing a majority of high energy wavelengths. The SPDs measured in this experiment
correspond to colour appearances assessed as white, red, blue and green. Previous research
assessing the impact of spectral content may have on discomfort glare has produced similar
results to my own (Flannagan et al., 1989; Main, Vlachonikolis and Dowson, 2000; Stringham,
Fuld and Wenzel, 2003; Bullough, 2009; Sweater-Hickcox et al., 2013).
Subjective Glare Rating
A number of studies has shown that sources producing SPDs containing a high proportion of
higher energy wavelengths cause
participants to increase their subjective
Mean Subjective Ratings by Colour
ratings of discomfort glare (Flannagan
6.0
5.73
et al., 1989; Main, Vlachonikolis and
5.67
5.8
Dowson, 2000; Bullough, 2009). This is
5.6
5.4
also found when comparing the
5.21
5.2
subjective ratings by colour from the
4.97
5.0
main experiment (Figure 71). However,
4.8
some researchers have not found this
4.6
to be the case, with participant
4.4
4.2
responses indicating a decrease in
4.0
discomfort glare perception at 460nm
White
Red
Blue
Green
(Stringham, Fuld and Wenzel, 2003).
This decrease has been speculated as
Figure 71: Comparison of the mean subjective glare ratings between
colour for the main experiment of this paper.
being caused by the action of macular
pigment absorbing the high energy
wavelengths with its peak absorption at 460nm, and is supported by research in the relevant
medical areas (Sharpe et al., 1998; Howells, Eperjesi and Bartlett, 2011; Stringham et al., 2011;
Lima, Rosen and Farah, 2016).
As macular pigment density is most concentrated at the fovea, with its concentrations decreasing
outwards towards 5.5 of angular subtense (Snodderly, Auran and Delori, 1984), with a source
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subtending 5.75 of visual angle, it is reasonable to conclude that macular pigment is likely
responsible for the decrease in discomfort glare ratings at 460nm. With the source for this
experiment subtending 11.76 , it is likely that there was a reduced effect of macular pigment,
supported by the fact the SPDs corresponding to blue and white were rated as causing increased
discomfort glare.
Although there are examples of previous research that support the findings that SPDs with
higher proportions of medium and low energy wavelengths cause decrease discomfort glare
compared to those SPDs with high proportions of high energy wavelengths, comparisons
between experiments should be made with care. One such example is the study by Main et al.
(2000); although their results show similar findings to my own, their SPDs will have differed
significantly from those in this study. They have not published the SPD of their halogen emitter
and instead have published transmission curves (Figure 72-76).
Figure
72:
The
transmission
curves
published
in
“The
wavelength
of
light
causing photophobia in
migraine and tension-type
headache
between
attacks”.
High
corresponds with blue,
medium
corresponds
with green and low
corresponds with red
(Main, Vlachonikolis and
Dowson, 2000).
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SPD of the White Glare Images
2.00E-03
1.80E-03
1.60E-03
W/㎡/nm
1.40E-03
1.20E-03
1.00E-03
8.00E-04
6.00E-04
4.00E-04
2.00E-04
0.00E+00
360nm
410nm
460nm
510nm
560nm
610nm
660nm
710nm
760nm
710nm
760nm
Wavelength(nm)
Figure 73: The SPD of the glare images characterised as white.
SPD of the Red Glare Images
3.00E-03
2.50E-03
W/㎡/nm
2.00E-03
1.50E-03
1.00E-03
5.00E-04
0.00E+00
360nm
410nm
460nm
510nm
560nm
610nm
660nm
Wavelength(nm)
Figure 74: The SPD of the glare images characterised as red.
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SPD of the Blue Glare Images
1.80E-03
1.60E-03
W/㎡/nm
1.40E-03
1.20E-03
1.00E-03
8.00E-04
6.00E-04
4.00E-04
2.00E-04
0.00E+00
360nm
410nm
460nm
510nm
560nm
610nm
660nm
710nm
760nm
710nm
760nm
Wavelength(nm)
Figure 75: The SPD of the glare images characterised as blue.
SPD of the Green Glare Images
2.50E-03
W/㎡/nm
2.00E-03
1.50E-03
1.00E-03
5.00E-04
0.00E+00
360nm
410nm
460nm
510nm
560nm
610nm
660nm
Wavelength(nm)
Figure 76: The SPD of the glare images characterised as green.
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Although a direct comparison cannot be made because of the missing SPD for the halogen
source used in Main et al. (2000), it is clear that it would have differed greatly from those in this
experiment. The use of a halogen source edited using subtractive colour mixing (Lee Filters
lighting gel) would produce a much smoother curve than a LED retina screen using additive
colour mixing to create the required colour appearance.
Despite the lack of a direct comparison, there is evidence suggesting that SPD, when it is
sufficiently different to cause a change in the colour appearance of a glare source, does influence
the perception of discomfort glare. However, as the analysis has shown, the differences in
subjective ratings are not always significant, therefore no firm conclusions can be drawn, and
further research is needed.
The effect of pattern:
The results for the pattern research are consistent with previous studies, with patterns containing
medium spatial frequencies producing increased discomfort (Wilkins et al., 1984; Fernandez and
Wilkins, 2008; Juricevic et al., 2010; Wilkins, 2012). The smaller of those patterns that contain
medium spatial frequencies produced the highest discomfort among the participants, although
these responses were not always statistically different to the responses for the other patterns.
This indicates that the spatial frequencies within a complex glare source can have an impact on
the observer’s perception of discomfort glare.
General discussion:
As this study was conducted using a glare source with a low luminance (100cd/m2), it is not
known if the findings of this study could be replicated at high luminance levels. Other studies
have used threshold methods, in which once the participants reported the sensation of
discomfort glare, a lux level was recorded at the subjects’ eye (Flannagan et al., 1989; Main,
Vlachonikolis and Dowson, 2000; Sweater-Hickcox et al., 2013). However, none of these
exceeded 17lux at the eye. In terms of this experiment, repeating the procedure at luminance
intervals of 50cd/m2 until 500cd/m2 was reached would give an indication of whether the same
effect is present at higher luminance levels. This could be repeated for SPD and spatial
frequency.
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Conclusions
Researchers exploring the impact that interest may have on discomfort glare perception should
attempt to record the spatial frequencies and SPD of the views they use, as there is evidence that
this may be a contributing factor leading to different ratings for discomfort glare when assessing
complex glare sources.
Colour preference was not controlled in this experiment and may have influenced the subjective
ratings given. However, it must be noted that some participants anecdotally stated an intense
dislike for the red glare images used in this experiment, and yet these images were rated as the
least glaring, suggesting this may not have a significant impact.
SPD, when it is different enough to cause a change in colour appearance, may have an impact on
discomfort glare perception. However, as the experiment was only conducted at line of sight (0 )
and at low luminance levels (100cd/m2) with a small opportunity sample, the findings should be
considered with caution. Further research is needed to confirm the impact of SPD on
discomfort glare perception.
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Yingxin Jia (2014) A Study of Mechanisms for Discomfort Glare, CIRP Annals - Manufacturing Technology. City University
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Zhu, P. et al. (2018) ‘Altered intrinsic functional connectivity of the primary visual cortex in youth patients with
comitant exotropia: a resting state fMRI study’, International Journal of Ophthalmology. doi: 10.18240/ijo.2018.04.22.
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Appendix 1: Ethics application
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Appendix 2: Participant invitation email
Participant invitation email
Dear IEDE Staff and Students,
I am currently looking for participants to take part in a study exploring the impact colour may have on the
perception of discomfort glare. The study involves exposure to 24 different glare sources and rating your
experience of glare on the de Boer rating scale. This study is being carried out by a student from MSc Light
and Lighting.
Not including any travel time your participation in the experiment will take a maximum of one hour on the
17st or the 24th of May 2019 depending on your availability.
If you are interested in taking part in this research project a full information sheet is attached along with a
consent form for you to sign. If you have any questions, please email Daniel Spreadborough at
ucbqdjs@ucl.ac.uk.
If you would like to take part specific location information will be provided at a later date.
Thank you for considering being part of the project.
Kind regards,
Daniel Spreadborough.
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Appendix 3: Experiment information sheet
Experiment Information Sheet
Research title: “Discomfort Glare: Colour and Subjective Perception”
You are being invited to take part in a research project. Before you decided it is important for you to
understand why the research is being done and what participation will involve. Please take the time
to read the following information carefully and discuss it with others if you wish. Ask us if there is
anything that is not clear or if you would like more information. Take time to decide whether or not
you wish to take part. Thank you for reading this.
What is the project’s purpose?
This project aims to build on research into discomfort glare with the hope of clarifying the impact
colour may have on our perception of glare.
Why have I been chosen?
You have been chosen for this study because you self-reported normal eye sight and no visual related
diseases or disorders that could affect the study, or mean you are harmed during your exposure to
bright light sources.
Including yourself a number of other participants that fulfil the above criteria have been selected for
participation. They were approach by email or responded to the experiment advert under the same
circumstances as yourself.
Do I have to take part?
Taking part in the study is entirely voluntary and that refusal to agree to participate will involve no
penalty. You may discontinue participation at any time without penalty.
It is up to you to decide whether or not to take part. If you do decide to take part you will be given
this information sheet to keep and be asked to sign a consent form, which you will also be given a
copy of. You can withdraw at any time without giving a reason.
What will happen to me if I take part?
The experiment takes place during a one-hour time slot which will include a briefing by the
experimenter, the experiment and a de-briefing.
During the briefing the experiment will be reexplained and any questions you have will be answered.
The experiment will consist of exposure to 24 glare stimuli, exposure will last 5 seconds, and you will
have 1:30 to rest and report your glare assessment before exposure to the next stimuli. You will be
able to stop the experiment at any time if you need a break or you wish to completely discontinue the
experiment.
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If uninterrupted the experimental stage will last 10 minutes, an hour is set aside to give you time for
any potential breaks you may need, the briefing and debriefing.
During the debriefing you will be given the opportunity to have any remaining questions you have
answered; these include any questions you have about the processing of your data. You will also be
given the opportunity to be contacted with the results of the study once the final report has been
completed.
No expenses that occur on your way too, during, or on your way from the experiment will be covered
by the researcher or the institution responsible for awarding the degree.
What do I have to do?
There are no restrictions placed on you while taking part in this experiment. All you need to do is arrive
at the experiment location in time for your chosen time slot.
What are the possible disadvantages and risks of taking part?
As the study is researching the impact colour may have on the perception of discomfort glare, you, as
a participant, may experience brief intervals of discomfort while viewing a glare source. The level of
discomfort will be no more than can be experienced while viewing a retail standard computer screen,
and discomfort may not be experienced at all.
There will be no lasting effects of participating in the experiment.
What are the possible benefits of taking part?
Whilst there are no immediate benefits for those people participating in the project, it is hoped that
this work will contribute to the understanding of the perception of discomfort glare, and therefore
allow designers to create more comfortable environments.
What if something goes wrong?
If you feel that you have been treated unfairly or feel you have been put in a situation you have not
consented to as stated in the consent form, you may contact the research supervisor with your
complaint.
Should you feel your complaint has not been handled to your satisfaction by the researcher or the
research supervisor you can contact the Chair of the UCL Research Ethics Committee to take further
action.
Will my taking part in this project be kept confidential?
All the information that we collect about you during the course of the research will be kept strictly
confidential. You will not be able to be identified in any ensuing reports or publications.
If any identifying information is written on your experimental questionnaire it will be destroyed and
you will be provided with a new blank one.
Any contact information you provide will be stored separately from any consent forms you have
signed and from the experimental questionnaire. Contact information will only be kept with your
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permission and only for the purpose of contacting you with the results of the experiment upon the
completion of the written report.
What will happen to the results of the research project?
The data you provide as part of the experiment will only be used in the dissertation project
“Discomfort Glare: Colour and Subjective Perception” in part fulfilment of the Degree of Master of
Light and Lighting.
Your raw data will be deleted on the 31st of August 2019, and the only surviving data will be
included in the written dissertation.
Who is organising and funding the research?
The organiser of this research is the researching student Daniel Spreadborough. There is no outside
funding and all equipment was either provided by UCL as part of the researcher’s degree program or
was provided by the researcher himself.
Contact for further information
Principle contact: Daniel Spreadborough
Contacts email address: ucbqdjs@ucl.ac.uk
A copy of this information sheet will be provided by the researcher before the experiment begins for
you to review. Please also keep a copy of this document for future reference, and please contact the
researcher on the above email address if you are interested in taking part.
Thank you for your interest in taking part in this study.
Yours sincerely Daniel Spreadborough
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Appendix 4: Questionnaire
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