ASSESSMENT OF DAYLIT GLARE PARAMETERS WITH
IMAGING LUMINANCE MEASURING DEVICES (ILMD) AND
IMAGE PROCESSING
Porsch, Tobias; Schmidt, Franz
Outline
• What is the aim of this study?
• Prior studies & applications
– Veiling glare-spread function (Lv)
– Threshold Increment (TI)
– UGR (Unified Glare Rating)
• Theoretical & practical realisation
Aim
Prior studies & apps
Realisation
Tests
Conclusion
–
–
–
–
Vertical illuminance (Ev)
Positioning index (p)
Threshold luminance
Calculating the Daylight Glare Probability (DGP)
• Tests and verification
• Conclusion
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What is the aim of this study?
•
Developing an easy to use tool for the assessment of discomfort glare
parameters based on imaging luminance measuring devices (ILMD)
Used hardware:
LMK mobile advanced
(Canon EOS 450D qualified for measuring luminances)
circular Fisheye lens
(Sigma F2.8/4.5mm EX DC)
Used software:
LMK LabSoft
(luminance image analysis software)
MS Excel
Aim
Prior studies & apps
Realisation
Tests
Conclusion
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Prior studies & applications
Veiling glare-spread function (Lv)
LV 
c  Evert
n
Threshold Increment (TI)
 = observing angle
c = age depending constant
TI 
n = 2 (common use)
65
( Lave,road )
0 ,8
UGR (Unified Glare Rating)
Aim
Prior studies & apps
Realisation
Tests
Conclusion
L2Si  i
0,25
UGR  8  log(

)
2
LA
pi
i
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 LV
Theoretical & practical realisation
•
Most glare indexes bases on classified ‘glare’ luminances LS(ϑ,ϕ) related
to its spatial positions (ϑ,ϕ), to its solid angles ΩS and to the state of
adaptation of the subject’s eye – i.e. the ambient luminance LA or there
caused vertical illuminance EV.
glare_ index_ X  f glare Lsource (,);source ;;; Lambient 
Aim
Prior studies & apps
Realisation
Tests
Lsource
Lambient
Ωsource
= luminance of the glare source
= luminance of the ambient (adaptation luminance)
= solid angle of the glare source to the area it illuminates
,
= spatial position of the glare source
( = 0° viewing direction)
Conclusion
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Theoretical & practical realisation
•
Aim
The first task in using ILMDs is to evaluate luminances “as seen” in the
image – using high dynamic range algorithms
OECF calibration
Prior studies & apps
optical calibration
(i.e. vignetting)
absolute calibration
Realisation
Tests
Conclusion
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Theoretical & practical realisation
•
The second approach is to combine the luminance information with the
position information of the measured luminance values for extracting
additional relevant information – geometrical adjustment.
•
Coordinate transformation rules x, y   , 
  arctan
y
x
  f (r)  f ( x²  y²)
      sin 
Aim
Prior studies & apps
Realisation
Tests
Conclusion
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Theoretical & practical realisation
•
The result of the performed geometrical adjustment and calculation is
an index image for the solid angle increment of each pixel: i, j
•
The information is used to
summarize the solid angle of a
detected luminance source and
hence to calculate the vertical
illuminance out of this data:
Evert   L(i, j )  (i, j ) cos (i, j )
  arctan
y
x
  f (r)  f ( x²  y²)
i, j
for (i, j )  lightsource
Aim
Prior studies & apps
Realisation
Tests
Conclusion
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Theoretical & practical realisation
•
The main focus at the current state of our studies was on the
measurement of the Daylight Glare Probability (DGP) by Wienold.
DGP  5.87 105  EV  9.18102 
log(1  
i
•
Aim
L2s ,i   s ,i
Ev1.87  Pi 2
)  0.16
The Position Index P is calculated
according to the proposal of the
IES for the visual field above and
the equations by Iwata and Tokata
for the visual field below the line of
sight.
Prior studies & apps
Realisation
Tests
Conclusion
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Theoretical & practical realisation
•
The DGP requires to indicate a luminance threshold level which is
needed to define light sources in the field of view as a glare sources or
rather as a part of the background.
•
Three principle methods were tested:
Prior studies & apps
Realisation
luminance histogram
1000000
Pixel (count N)
Aim
• using the average luminance of
the entire image multiplied by
factor x (x = 4)
• using a task zone area for
averaging a threshold luminance
• analysing the luminance histogram
of the entire image to adjust the
threshold level as proposed by
Wolf for the UGR assessment
100000
10000
1000
100
10
1
0
2000
4000
6000
8000
10000
L (cd/m²)
Tests
Conclusion
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12000
Theoretical & practical realisation
•
To perform all steps before and inside the calculation formula for the
DGP a MS Excel based user interface is used. This front-end has direct
access to LMK LabSoft image analysis software, via ActiveX.
Aim
Prior studies & apps
Realisation
Tests
Conclusion
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Theoretical & practical realisation
Aim
Prior studies & apps
Realisation
Tests
Conclusion
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Tests and verification
•
The major possibility to establish a measurement series for comparisons
of the calculated DGP was given with the use of the ‘evalglare’ tool.
•
In a first step a software tool was created
to convert the 4 byte luminance image
format *.pf into 4 byte RGB *.pic format
for the Radiance image viewer tool.
In the next step the ‘evalglare’ tool was
used to perform the glare analysis:
>> evalglare -d -c %str%_small.pic -y
%str%_small.pic
•
Aim
Prior studies & apps
Realisation
Tests
Conclusion
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Tests and verification
•
Simultaneously to the High-Dynamic luminance capturing a comparison
measurement for the vertical illuminance with a Luxmeter was made.
•
The comparison of the results shows a high correlation of the vertical
illuminance for both:
Aim
– Luxmeter and luminance image calculation
 the used solid angle correction is reliable
– Luxmeter and the ‘evalglare’ calculation
 the image conversion is correct
– additionally the correlation of the average
luminances is high
 the image conversion is correct
Prior studies & apps
Realisation
Tests
Conclusion
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Tests and verification
Camera/Luxmeter
Evalglare/Luxmeter
1800
1400
1200
1000
800
Camera/Evalglare
600
600
400
200
0
Aim
0
200
400
600
Prior studies & apps
Realisation
Tests
vertical illuminance [lx]
800
average luminance [ccd/m²]
vertical illuminance [lx]
1600
500
400
300
200
1000
1200
1400
1600
100
0
0
50
100
150
200
250
300
350
400
450
average luminance [ccd/m²]
Conclusion
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500
Tests and verification
•
The comparison of the calculated DGP results shows also a high
correlation.
29,00%
27,00%
25,00%
23,00%
21,00%
19,00%
17,00%
Aim
15,00%
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Prior studies & apps
Realisation
DGP (ave_evalglare)
DGP_ave
DGP_hist
DGP_task
Tests
Conclusion
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Tests and verification
•
Case study: situation number 6 shows a higher mismatch of the DGP
result can be found.
E vert
(evalglare)
E vert (LMK)
48,8 lx
45,8 lx
DGP (ave_evalglare)
16,56%
DGP_hist
19,94%
Lave 90°
(LMK)
18,96 cd/m²
DGP_ave
19,80%
Lave
(evalglare)
18,1 cd/m²
DGP_task
19,88%
luminance histogram
Aim
Prior studies & apps
Realisation
Pixel (count N)
1000000
100000
10000
1000
100
10
1
0
1000
2000
3000
4000
5000
6000
7000
L (cd/m²)
Tests
Conclusion
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Conclusion
ILMDs are mainly used for the assessment of glare values. This is
currently state of the art and commonly used in a wide field of scientific
researches, concerning this topic.
Simplifying the workaround by implementing heuristic identified formulas
into image processing algorithms can be a solution for non-experts to
identify glare values under real environment conditions.
Problems:
Aim
Prior studies & apps
Realisation
Tests
Conclusion
• High contrasts between ambient and glare source do generate
scattered light effects in the imaging system of the ILMD
• software scattered light correction (available)
• use of optimized imaging systems (available)
• Geometrical and photometrical imaging resolution
• mostly very small glare sources  dependance of the glare
value from the geometrical resolution
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Thank you for your attention!
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