Applied Ergonomics 32 (2001) 485–499
Polarized task lighting to reduce reflective glare
in open-plan office cubicles
D.A. Japuntich*
Office Supplies Division, Office Ergonomics and Cleaning Systems, 3M Company, 3M Center, Building 230-2S-13, St. Paul, MN 55144-1000, USA
Received 25 February 2000; accepted 13 March 2001
Abstract
This ergonomic study deals with the common situation where a glossy document is placed between a viewer and an under-shelf
task light source in a common open-plan office cubicle workstation. With a task lamp in front, when looking at a document a viewer
sees two images, the document itself and specular glare, which is the reflected image of the light source. Specular glare or veiling
reflection causes eye discomfort, makes it difficult to read a document and has been thought to contribute to eyestrain. This paper
analyzes the application of polarized lighting for this specific situation. The use of a linear polarized light source helps to minimize
specular glare by darkening the reflected image of the light source on the document. The performance and predictive optimization of
the use of polarized lighting in this situation is investigated according to female and male viewer height demographics. Theoretical
predictions and light measurement analysis of specular glare reduction are compared with empirical results from testing on a panel
of humans on semi-gloss finish and matte finish papers. This study shows that with the right alteration of a polarized light source
position, specular glare may be significantly reduced, and correlations exist between the theory, empirical measurements and the
human response to specular glare reduction. r 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Task light; Polarized light; Glare; Eyestrain; Optometry; Architecture
1. Introduction
This study deals with the common situation where a
glossy document is placed between a viewer and an
under-shelf light source in a constrained open-plan office
cubicle workstation environment. Under-shelf and desktop task lights are commonly used in open-office plan
workstations to augment or even replace inadequate
over-head lighting. With a task lamp in front, a problem
exists when looking at the document in that a viewer
sees two images, the printed image and specular glare,
the reflected image of the light source. Having a close,
bright source of luminance like a reflection in the field of
view negatively affects eye comfort through elevated
luminance ratios. The use of a linearly polarized light
source may help to minimize specular glare by darkening
the reflected image of the light source on the document.
The ability of a polarized light source to alter the
luminance or brightness of a reflected lamp image on a
*Tel.: +1-651-736-9041; fax: +1-651-736-7924.
E-mail addresses: dajapuntich@mmm.com (D.A. Japuntich).
document is greatly affected by the viewing angle of the
observer. Studies by Allphin (1963), Crouch and Kaufman (1963), Lumsden(1976), Rea et al. (1985), and Rea
et al. (1990) on task reading or viewing angles have
shown a range between 01 and 401 with an average of
around 251, elevating above 351 depending on the height
of the viewer. The measured location of lamp images on
office tasks has been studied by Siminovitch (1993), and
worker glare comfort surveys were taken by Bernecker
et al. (1993a, b) and by North (1991). Discomfort from
reflective glare at a desk is one of the most common
complaints in offices, and North (1991) found a gender
difference in complaints: 32.7% for females, as compared to 21.3% for males. The degree of glossiness of a
surface affects reflective glare, as does the material that
the surface is made of. Glossy surfaces are dielectric
materials, which include plastics, glass, waxes, transparencies, glossy paper coatings and even the toner image
on photocopies.
Many researchers have focused on the amount of
visual performance improvement using a polarized light
source to alter contrast when the specular reflection of
0003-6870/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 0 3 - 6 8 7 0 ( 0 1 ) 0 0 0 2 0 - 5
486
D.A. Japuntich / Applied Ergonomics 32 (2001) 485–499
the lamp image is superimposed over a reading task.
Edwin Land (Grabau, 1940) invented the first
polarized lamp to reduce veiling glare, and Marks
(1959) presented an excellent explanation of contrast
and polarization. Blackwell (1963, 1984), Blackwell
and DiLauria, 1973 and Bernecker et al. (1993a, b)
extensively studied glare and polarized light, pioneering
quantitative methodologies for optimizing reading
contrast resulting in higher visual performance at
lower illumination, giving possible energy use reductions. Rea (1981) investigated combinations of visual
task characteristics, orientation and polarization.
Further polarized lighting investigators, Jones (1992)
and Karpen (1995), have focused on energy savings
and the use of full-spectrum fluorescent lamps with
radial polarization. Boyce and Rea (1994) found little
improvement in visual performance and a slight
improvement in color sorting tasks using full-spectrum
polarized lighting from overhead luminaires with
incident light percent degree of polarization of less
than 15%, measured at angles from the vertical of 401 or
less.
Clear and Mistrick (1996) wrote a general review
of the state of the research on polarized lighting up to
that time. An objective of the use of polarized lighting to
reduce visual discomfort was lost in the debate of the
difficulty of proving visual performance. In the
discussion section of Clear and Mistrick (1996), P. Boyce
asked the question, ‘‘if multi-layer polarizers are to be
treated as special purpose devices, under what conditions
are they useful?’’ This is an ‘‘ergonomic question’’, and it
is hoped that this paper will provide some answers to that
question for an application of polarized lighting for
under-shelf lighting in open-plan office cubicle workstations. Certainly, at the minimum the reduction of
specular glare will result in a better, more comfortable
workplace.
The IES RP-24 (1989) Recommended Practice for
Lighting Offices Containing Computer Visual Display
Terminals states, ‘‘The logical place for fixed local task
lighting is under a cabinet or shelf and directly over the
work station. These locations are often in the offending
zone, thus producing veiling reflections at some locations on the work surface.’’ The great increase in
open-plan office cubicles has created a familiar
situation where uncomfortable specular glare from
under-shelf lighting is always present on a document
or desk surface. This is a two-dimensional situation
where linear polarized light may be used to reduce glare
if the proper optimized design specifications may be
shown.
Previous work has been done with radial polarizers
(Karpen, 1995; Boyce and Rea, 1994). These not only
give a wide-spread and multi-directional polarization,
but also give a lower degree of polarization. The
application shown in this paper uses a multi-layer linear
polarizer, which in this constrained, two-dimensional
use gives a very high degree of polarization of the light
in the direction of the viewer.
One simple, inexpensive method to see if linear
polarized light is indicated to reduce specular glare is to
sit at a workstation viewing the reflective glare while
rotating a lens from polarized sunglasses. If the
luminance from glare is altered in intensity by rotating
the lens, a polarized light source may help to reduce it. If
there is no change then polarized lighting is not indicated.
This study shows a method of determining the
location of the offending lamp reflection zone on an
office desk according to ergonomic gender demographics using viewer height, lamp height and viewerlamp distance. Theoretical predictions and empirical
measurements of the level of specular glare reduction
using linear polarized lighting are presented, including
optimization based on the location of the reflection.
Further testing using human subjects was done to show
their agreement with the theory and empirical measurements of specular glare reduction optimization.
2. Theoretical predictions of the location of a light source
image on the surface in front of a viewer
The location of the ‘‘offending zone’’ or the reflective
image of the lamp on a document needs to be
determined for each of a population of office workers.
The problem solved in this section is the location of the
lamp reflected image in the plane of vision of seated
male and female workers for their wide range of height
demographics.
As shown in Fig. 1, lamp reflected image distance (X)
is a function of the height of the viewer (h), the height of
the light (Y) and the horizontal distance (L) of the
viewer from the light. Viewing angle (i, equal to the
incident angle) is the angle from the vertical to the line
of view and is a function of the horizontal distance of
the image from the lamp and the height of the lamp. If
the height of the viewer increases, the lamp image moves
towards the lamp, and if the height of the lamp
increases, the lamp image moves away from the lamp.
Solving simultaneous equations gives the Eqs. (1) and
(2) for the lamp image distance from the light, X, and
subsequently the viewing angle, i. In the very special
case where the light and viewer heights are identical, the
lamp image distance is one-half the viewer distance. The
image distance from the viewer is L@X. The image
distance (D) from the desk edge is the difference of W,
the light distance from the desk edge, and X.
X¼
YL
hþY
X
L
i ¼ arctan
¼ arctan
:
Y
hþY
ð1Þ
ð2Þ
D.A. Japuntich / Applied Ergonomics 32 (2001) 485–499
487
Fig. 1. Dimensions relating to forward illumination of a desk surface.
Table 1
The effect of gender height demographics on image viewing angle and
specular glare image distance from the front edge of a desk
Seated
eye height, h,
above 737.6 mm
desk surface (mm)
Females 5th percentile
Females 50th percentile
Females 95th percentile
Males 5th percentile
Males 50th percentile
Males 95th percentile
315.4
391.4
471.4
409.4
490.4
569.4
Image
viewing
angle, i
(Deg)
36.3
33.6
31.2
33.1
30.6
28.6
Image
distance
from front
desk
edge,
D (mm)
232
260
285
266
290
310
It is important to determine where the image of a
fluorescent lamp falls in the range of viewing for a
common reading task in an office cubicle. As an average
viewing position, it is assumed that the eye of the viewer
is directly above the front desk edge, as in Siminovitch
(1993). A common cubicle arrangement is a lamp height
(Y) of 410 mm (16 in), a lamp horizontal distance (L) of
the viewer from the lamp of 533 mm (21 in), with the
viewer’s eye directly above the front desk edge. The
seated eye height of the viewer above a 736.6 mm (29 in)
high desk surface (h) is population based for females and
males as derived from the anthropometric data in
Gordon et al. (1988). Eye height was calculated as the
combination of popliteal height and eye height sitting to
obtain the distance above the floor, and then subtracting
a desk height, as shown in Table 1. It should be noted
that the 50th percentile female height of 391.4 mm is
below the common lamp height of 410 mm, while the
male 5th percentile height of 409.4 mm is equal to this
lamp height. For non-recessed luminaires, 95% of the
males are not looking directly into the light source, while
more than 50% of the females are exposed to direct
glare from the light source.
Fig. 2. The location of fluorescent lamp images on reading material
for female height demographics.
Another way of looking at the lamp image distance
from the front edge of the desk for the population data
in Table 1 is to show it the way a viewer would see it
superimposed on the reading material. This is shown in
Figs. 2 and 3. The location of the image of the lamp is
shown for each percentile. Also added to Figs. 2 and 3 is
the location of the 401 viewing angle for each of the
female and male 50th percentile of the populations,
respectively. A viewing angle of 401 is judged by most
researchers as the maximum viewing angle for reading,
although reflections above 401 can be uncomfortable,
even when located on the desk surface. In all cases the
specular glare images of the lamp fall on the reading
material within the 401 viewing angle. The equations
may also be used to calculate image location for subjects
leaning back or forward and for other circumstances.
They also work for other configurations, such as
forward overhead lighting.
From observing reading behavior, it was found that in
order to keep their bodies from blocking their view a
majority of readers pushed the reading material away
from the edge of the desk by about 50 mm (2 in).
It is interesting to note in Figs. 2 and 3 that the 401
upper limit for 50th percentile females falls exactly
on the length of US letter size paper pushed up 50 mm
from the desk front edge (279+50 mm=329 mm),
and the same limit for men falls exactly on the
length of US legal size paper pushed up 50 mm from
488
D.A. Japuntich / Applied Ergonomics 32 (2001) 485–499
Fig. 4. Reflection components from a dielectric surface for an
unpolarized light source.
Fig. 3. The location of fluorescent lamp images on reading material
for male height demographics.
the desk front edge (355.6+50 mm=406 mm). Perhaps
to some extent, American paper sizes have evolved
according to ergonomic attributes. A4 paper size
(297+50 mm=347 mm) falls in between these two
distances.
3. Light polarization and reflected images
It is important to understand how light reflects and
the value of the polarization of that light on reducing the
intensity of a specular reflection. As seen in Fig. 4,
unpolarized incident light from a lamp interacts with a
reflective surface and produces un-refracted, reflected
light (reflected specular glare) and refracted scattered
light (image). The reflected light as specular glare is the
unfocused image of the light source(s). The refracted,
emitted light contains the information of color and
pattern from the surface and is what we ‘‘read’’.
Reflected specular glare maintains the same color as
the incident light, carries no information from the
surface, and has two transverse components: the
vertically vibrating p-waves and the horizontally vibrating s-waves. The linear polarization characteristics of
the reflected light from the surface is a function of the
degree of polarization of the incident light, the viewing
angle from the vertical (i), and the angle of refraction (r),
which is a function of the refractive index of the surface
(Z) and the viewing angle.
An unpolarized ray of light consists of light waves
having transverse vibrations of equal magnitude that
oscillate about the line representing the direction of the
light ray. For simplicity, it is common to resolve the
amplitude of the light ray vibrations into components
vibrating in two planes at right angles to each other
along this line, explained in depth in IES (1993). For this
paper with respect to light striking a horizontal surface,
the two principal components are the vertically vibrating
p-waves and the horizontally vibrating s-waves. When
these two components are not equal, the light is partially
or totally polarized. Depending on the viewing angle,
the veiling glare of the reflection of an unpolarized light
source on a horizontal surface in front of a viewer may
consist of a majority of s-waves. If the incident light at
certain viewing angles is polarized to increase the
amplitude of p-waves and decrease the amplitude of swaves, the brightness or luminance of the veiling
reflection may be reduced, resulting in a darker reflected
light source image. Simply stated, the brightness of the
specular, veiling glare is reduced with incident light
made of p-waves. The effects on reflected images from
light with and without polarization are well described in
Shurcliff (1962) and Collett (1993).
Polarized sunglasses reduce reflected glare from swaves by adsorbing them. These sunglasses have lenses
analogous to vertical microscopic gratings that absorb swaves and let through p-waves. Instead of having a
viewer wear sunglasses, a polarizing filter may be placed
over a light source to give the same effect on the reflected
image. However, although s-waves make up a proportion of reflected glare, p-waves may also be part of the
glare. Contrary to common knowledge, at small angles
and large angles the specular glare is composed of both
s-waves and p-waves. When a vertical polarizing lens is
used to eliminate the s-wave glare, the p-wave glare will
still be present, superimposed on printed material on a
document or on a highway while driving. This is why the
glare from the image of the sun superimposed on the
road in the early morning with ‘‘large’’ viewing angles
approaching 80–901 is not totally eliminated by the use
of polarized sunglasses while driving. Viewing angle is
D.A. Japuntich / Applied Ergonomics 32 (2001) 485–499
very important, and the viewing angles most encountered in offices range between 201 and 451.
4. Theoretical predictions of glare reduction using a
polarized light source
In the real world of office workstations, reflective
glare appears on messy, non-uniform surfaces like
paper, magazines and desktops. In order to determine
if polarized light theory may be applied to this problem,
one needs to start with a standard surface, which may be
mathematically described by the theory and later find
out if the theory predicts actual measurements. In this
case, that standard surface is glass.
Polarized incident light may be used to reduce the
glare from the reflected image of a light source on a
document, depending on the viewing angle. Linear
polarized light is produced by placing a polarization
lens in front of a light source. No polarizing lens is
perfect, and how well the light is polarized is shown by
proportions of the two wave components, as described
by the degree of polarization of the incident light, %Pi .
The greater the degree of polarization of the incident
light, the darker the reflected image of the lamp will
appear to the viewer, depending on the viewing angle.
When the light is not polarized, the degree of polarization is zero (%Pi ¼ 0).
The theoretical intensity (IT )of the lamp image
reflection is dictated by percent degree of polarization
of the incident light (%Pi ), the intensity of the incident
light (I0 ) and the reflection intensities for the s-waves (Is )
and the p-waves (Ip ) as given by the Augustin Fresnel
(1821a, b, 1847) equations for the viewing angle (i) and
the angle of refraction (r). These may be added to give the
intensity of the glare of the reflected image, IT ¼ Is þ Ip
at a viewing angle, in a method similar to that used in
Clear and Berman (1992), as shown in Eqs. (3) and (4).
2
sin ði@rÞ
IT ¼ Is þ Ip ¼ I0 ð0:50@%Pi =200Þ
sin2 ði þ rÞ
2
tan ði@rÞ
þ I0 ð0:50 þ %Pi =200Þ
; ð3Þ
tan2 ði þ rÞ
where
r ¼ arcsin
sin i
:
Z
ð4Þ
A percent specular glare reduction, %R, may be
calculated using Eqs. (5) and (6) as the percent change
of the reflected image intensity from Eq. (3) for incident
light with a given percent degree of polarization, ITð%Pi Þ;
to the intensity of incident light with no polarization,
ITð%Pi ¼0Þ , for a given polarization film transmittance, t.
All polarization films have light transmission losses. For
instance, the absorbing polarizer medium for polarized
489
sunglasses typically has a transmittance of less than
0.40, while a multiplayer reflective polarizing film may
have a transmittance greater than 0.70. The true glare
reduction of the polarization medium alone is calculated
in Eq. (5) by correcting for the transmittance in Eq. (6),
the ratio of the intensities of the light transmitting
through the film to the intensity without the film.
tðITð%PI ¼0Þ Þ@ðITð%PI Þ Þ
;
ð5Þ
%R ¼ 100
tðITð%PI ¼0Þ Þ
where
I0ð%Pi Þ
:
t¼
I0ð%Pi ¼0Þ
ð6Þ
In Fig. 5, the theoretical performance for glass,
Z ¼ 1:5, using Eq. (5) shows the percent glare reduction
from incident light through polarizing lenses of different
effectiveness, giving different degrees of polarization at
different viewing angles, assuming the same incident
light intensity. At low angles, less than 251, eliminating
the s-waves and increasing the p-waves to get the same
light intensity reduces the glare by less than 20%. In
other words, the p-waves are part of the specular glare.
As the viewing angle increases, the p-wave absorption
and, therefore, the glare reduction increases as the angle
increases to Brewster’s angle, where the reflected
intensity of the reflected image of the lamp is darkest.
Note that at Brewster’s angle of about 571 the percent
glare reduction is equal to the percent degree of
polarization of the incident light, showing that the
condition of the polarization of the incident light is very
important.
5. Empirical measurements of specular glare reduction on
a standard glass surface
Percent specular glare reduction of a lamp’s reflection
on a standard glass surface may be calculated from light
meter measurements with and without the polarizing
filter covering the lamp. If the theory predicts glare
reduction from measurements, the theory may be used
to predict situations of optimized glare reduction in the
workplace.
As demonstrated in Fig. 6, empirical measurements of
image luminance and lamp illuminance were taken on a
desk in a typical cubicle arrangement with an undershelf T-8 fluorescent luminaire. The luminaire had a 901
metallic reflector and no diffuser lens. The polarizing
filter was a polycarbonate tube, lined on the inside with
an efficient multi-layer reflective polarizer film (Weber
et al., 2000), mounted on and completely covering the
T-8 fluorescent lamp.
Familiar standard polarizing films as used in sunglasses produce polarized light by adsorbing the
horizontal component of the incident light. These films
490
D.A. Japuntich / Applied Ergonomics 32 (2001) 485–499
Fig. 5. Theoretical and empirical specular glare reduction for glass at different viewing angles from polarized incident light of different degrees of
polarization.
have the inherent energy disadvantage of a light
transmittance of less than 0.40. Multi-layer polarizing
films utilize alternating layers of transparent films,
which transform incident light transmitted through
them into polarized light by internal reflectivity of the
many layers. Past multiplayer polarizers, as in Marks
(1959), were shown to be much more light transmission
efficient than adsorbing polarizers, giving a light
transmittance of up to 0.65. Weber et al. (2000)
describes new, unique, thin, multi-layer reflective films,
which utilize hundreds of alternating layers of highly
birefringent polymers, giving a typical transmittance of
0.94.
As shown in Fig. 6 for transmittance measurements,
the Minolta Illuminance Meter was positioned at a
distance of 600 mm from the lamp, perpendicular to the
incident light at an angle from the vertical of 351. The
351 angle is well within the viewing field of a reader
sitting at the desk. The transmittance, t, of the
polarizing filter (combination of the tube and the
multil-layer film) was calculated to be 0.89 from
Eq. (6) from the output of the illuminance meter with
and without the polarizing filter tube over the lamp. The
illuminance meter was checked for polarization sensitivity for the lamp with no polarization (degree of
polarization equals zero) by covering the illuminance
meter lens with a polarizing lens cap cover and then
rotating the lens cap cover full circle, which showed no
changes in the illuminance reading. As another check,
the transmittance of 0.89 for the polarizing tube was
found to be identical to the transmittance of a flat
laminate of polycarbonate film (same thickness as the
mounting tube) and the multi-layer reflective polarizer
film mounted on a flat light-box.
Fig. 6 also shows the configuration of how the
incident light degree of polarization, %Pi , was calculated
from luminance measurements with the reflective desk
surface removed so that the luminance meter could be
focused directly at the surface of the lamp polarizing
filter. A linear polarizing lens cap cover was placed on
the luminance meter lens aperture, and the meter was
focused on the lamp polarizing filter tube surface at a
distance of 600 mm at an incident light angle from the
vertical of 351. The meter polarizing lens cap cover was
rotated to give the s-wave minimum luminance (I0s , lens
cap polarization horizontal) and p-wave maximum
luminance (I0p , lens cap polarization vertical). The
degree of polarization was then calculated using
Eq. (7). Since the polarized incident light should be
Fig. 6. Experimental configuration for specular glare reduction
measurements.
491
D.A. Japuntich / Applied Ergonomics 32 (2001) 485–499
dominated by the vertical p-waves, the incident light
percent degree of polarization is positive according to
the convention in the IES (1993) Lighting Handbook.
The measured percent degree of polarization of this
system was 87.5%.
%Pi ¼ 100ðI0p @I0s Þ=ðI0p þ I0s Þ:
ð7Þ
The reflective luminance measurement methodology for
glare reduction calculated by Eq. (5) follows the ASTM
(1994) Standard Test Method for Specular Gloss: D52389, modified to measure specular reflection (gloss) at
different angles with and without the polarization
medium in place on the lamp. This method is also
similar to the Blackwell (1963) methodology for
reflected image luminance reduction. To measure IT as
shown in Fig. 6, the Minolta LS-100 Luminance Meter
was mounted on a tripod in place of the ‘‘viewer’’ and
was focused on the reflected image of the lamp on the
surface. The luminance meter was checked and found
not to have polarization sensitivity in any orientation.
Smooth glass was chosen not only for a standard,
traceable reflectivity, but also because its performance
may be theoretically calculated. The standard glass
reflective surface (Z ¼ 1:5), traceable to ASTM D523-89,
was Hunterlab D48 Glossmeter Standard: 98.8 gloss
value at 751-TAPPI, manufactured by Hunter Associates Laboratory, Reston, VA. Other non-standard
surfaces may give different results due to surface texture,
index of refraction, light absorbency and multi-surface
reflectivity.
In the configuration of Fig. 6, the corrected percent
glare reduction of the reflected lamp image, %R, was
calculated by Eq. (5) from the luminance meter measurements (cd/m2) of the reflected lamp image at a
chosen viewing angle with the polarization filter in place,
giving ITð%Pi Þ , and with no polarization filter, giving
ITð%Pi ¼0Þ .
In Table 2 the theoretical percent glare reduction
values were calculated at the measured incident light
degree of polarization of 87.5%, using Eqs. (3) and (4)
for the polarizing filter percent degree of polarization
and transmittance, and were compared to the empirical
corrected percent glare reduction. Data were gathered at
two lamp heights with different viewer heights and
viewer distances from the lamp. Viewer distance and
height were also varied by changing the tilt angle from
Table 2
Comparisons of theoretical and empirical percent specular glare reduction values at many different lamp and viewer configurations for incident light
with an 87.5% degree of polarization
Gender/
percentile
Chair tilt
(Deg)
Lamp height
(y) (mm)
Viewer height
(h) (mm)
Viewer distance
(L) (mm)
Incident angle
(Deg)
Theoretical
% glare reduction
Measured % glare
reduction
Female 5th
Female 50th
Female 95th
Male 5th
Male 50th
Male 95th
Female 5th
Female 50th
Female 95th
Male 5th
Male 50th
Male 95th
Female 5th
Female 50th
Female 95th
Male 5th
Male 50th
Male 95th
Female 5th
Female 50th
Female 95th
Male 5th
Male 50th
Male 95th
Female 5th
Female 50th
Female 95th
Male 5th
Male 50th
Male 95th
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
15
15
15
15
15
15
20
20
20
20
20
20
410
410
410
410
410
410
381
381
381
381
381
381
381
381
381
381
381
381
381
381
381
381
381
381
381
381
381
381
381
381
315
391
471
409
490
569
315
391
471
409
490
569
315
391
471
409
490
569
304
378
455
395
473
550
296
368
443
384
461
535
533
533
533
533
533
533
533
533
533
533
533
533
655
655
655
655
655
655
615
634
655
639
660
680
641
667
694
673
701
728
36
34
31
33
31
29
37
35
32
34
31
29
43
40
38
40
37
35
42
40
38
39
38
36
43
42
40
41
40
38
50
43
37
42
36
31
53
46
39
44
38
33
69
61
54
59
52
46
65
60
55
59
54
50
69
65
60
64
60
56
50
41
35
39
34
29
53
45
38
44
36
29
68
60
53
59
51
48
64
57
53
57
52
48
67
63
56
62
58
54
492
D.A. Japuntich / Applied Ergonomics 32 (2001) 485–499
the desk edge, as if the viewer were leaning back in a
chair. The viewer’s eyes were directly over the table edge
at 01 tilt angle.
As seen in Fig. 5, when plotting the Table 2 measured
percent specular glare reduction data points for a filter
with 87.5% degree of polarization, the theory agreed
very well with this data, predicting performance over a
wide range of angles. Also, because of smaller height,
females had larger incident angles and larger resultant
glare reduction at the same configurations.
6. Optimization of glare reduction and the location of the
light source image on reading material
Because the theory successfully predicts the actual
glare reduction, it may be used to predict optimal lamp
conditions for the best glare reduction for the viewer. At
the same time, the position of the reflected image may be
optimized to move it further up and perhaps off the top
of the page of the reading material as in Siminovitch
(1993). Viewing angle is difficult to measure directly, so
the easily measured and understood lamp height and
lamp to desk edge distance were used in this optimization, just as they would be by an ergonomist. As people
of smaller height are most affected, the height of the
female 50th percentile was used.
A dimensional analysis of the equations for the lamp
image location on reading material and the percent glare
reduction showed that the lamp height and the distance
of the lamp to the desk edge may be simultaneously
varied to give improved results. For the female 50th
percentile of 391 mm, a grid was created within the
bounds of a standard American office cubicle by varying
the lamp height from 350 to 500 mm and the lamp to
table edge distance from 400 to 850 mm. Location of the
reflection, D, was calculated for W by Eq. (1) for X, and
the percent glare reduction was calculated by Eq. (5). It
was assumed that the viewer’s eyes were directly above
the desk edge, and the predictive results were sorted to
give output ranges as shown in Figs. 7 and 8, for the
reflected image location and the percent glare reduction,
respectively.
Figs. 7 and 8 show that simultaneously decreasing the
lamp height and increasing the lamp to table edge
distance moves the image up the page and increases
percent glare reduction. In some cases the image may be
moved off the page completely, where glare reduction of
the nuisance reflected image on the desk may be
achieved. However, in actual use for a certain luminaire
design, there are limits that may need to be placed on the
changes to the luminaire position and installation, based
upon the illumination field: the intensity of the
illumination and its uniformity in the reading area (see
Siminovitch, 1993, p. 239).
Fig. 7. Predictive grid of fluorescent lamp specular image location optimization at different lamp heights and lamp to viewer distances for 50th
percentile female height.
D.A. Japuntich / Applied Ergonomics 32 (2001) 485–499
493
Fig. 8. Predictive grid of fluorescent lamp specular image glare reduction optimization at different lamp heights and lamp to viewer distances for 50th
percentile female height.
7. Glare reduction testing on humans and correlation to
empirical measurements
This section deals with glare reduction ratings on
printed-paper by a panel of people and correlation of
these ratings to light meter measurements of glare
reduction and, as a reference, the theoretical percent
glare reduction on glass. The easily measured variables
of fluorescent lamp height and lamp to desk edge
distance were used to vary the viewing angle and,
subsequently, the percent glare reduction. If the results
for glare reduction ratings from human testing give the
same optimizational trend as in Fig. 8, the use of the
theory for workstation lighting design is justified.
Rather than a glass surface, this study includes testing
on two different papers with and without bright, diffuse
ambient lighting illuminance superimposed on the
illuminance of the fluorescent task light.
The selection of 30 experimental subjects (17 females
and 13 males) provided for near equal weighting of age
and gender within the four decades of age groups (20,
30, 40, 50 years) normally encountered in the workplace.
Test subjects with eye correction totaled 17 with
eyeglasses and 7 with contact lenses.
The Fig. 8 plot of theoretical percent glare reduction
optimization for different lamp heights and distances
may be regarded as a contour plot (response surface) of
the change of the independent variable (% glare
reduction) to the interaction of two independent
variables (lamp height and lamp distance). This is a
typical example of the output of an analysis of variance
for a two-level factorial designed experiment. Consequently, in order to give a clear picture or contour plot
of how to optimize human glare reduction response for
the two paper sample types, a two-level factorial design
format was chosen for the analysis of the glare reduction
rating response of the test subjects to different lamp
positions.
For these two-level factorial designs (as shown in
Table 4) there were six experimental conditions. The
lamp height and the lamp distance from the viewer were
varied high and low, and for the statistical analysis two
center points were added at half the differences between
each of these two levels. An analysis of variance or
ANOVA for each response was performed to create a
statistical model and equation describing the system
mathematically. The experimental set up was similar to
that shown in Fig. 1 with each person positioned at a
50th percentile female eye height of 391 mm (15.3 in)
with their eyes directly above the desk edge. The lamp
could be easily adjusted horizontally and vertically in
the same space as a typical under-shelf lighting fixture.
The experimental set up of the fluorescent lamp
fixture was in a typical open-plan office cubicle workstation. The test lamp was a 1.2 m (4 feet) long T-12,
40 W fluorescent tube. The front of the luminaire facing
494
D.A. Japuntich / Applied Ergonomics 32 (2001) 485–499
the test subject was shaded to eliminate any direct glare
from the lamp, and a 901 metallic reflector was used in
the luminaire behind the fluorescent lamp. The lamp was
covered except for a 0.38 m length open area facing
downward in the center of the lamp, positioned directly
in front of the test subject. A polycarbonate tube of
0.8 m length fitted over the lamp was shuttled back and
forth across this open area. One-half of the interior of
the polycarbonate tube length was lined with a multilayer reflective polarization film with an 87.5% degree of
polarization (Weber et al., 2000) and the other half was
lined with a polyester film of the same transmittance. In
this way, the light from the lamp could be easily changed
from a polarized to an unpolarized light source of the
same illuminance by shuttling the tube left or right. With
the polarization, Tables 3 and 4 shows the distribution
of luminance parallel to the lamp at a 200 mm distance
from the desk edge, measured using the luminance meter
at a lamp height of 410 mm, lamp distance of 505 mm
and a viewing height of 391 mm, giving a viewing angle
of 271. Table 3 shows that the luminance varies less than
6% across the 20 cm width of the reading material target
and varies less than 25% across the entire exposed width
of the lamp from end to end.
The overhead, ambient lighting was provided by two
luminaires perpendicular to the test lamp, each containing two 1.2 m (4 feet) T-8 fluorescent lamps, covered
with plastic diffusers. They were positioned to provide
diffuse, ambient non-polarized lighting 2 m above the
reading surface, each 1 m on either side of the reading
surface. The illuminance provided by this lighting at the
center of the reading surface, 200 mm toward the lamp
from the test subject was 480 lux. This illuminance can
be considered the high end of the recommendations
given in IES Lighting Handbook (1981), which recommends 200–500 lux for reading tasks at office desks.
The two paper samples represented typical A4 size
(295 mm long) reading materials of semi-gloss and matte
finish papers, the lower edge positioned 50 mm toward
the lamp from the desk edge. For the convenience of the
readers of this journal, the semi-gloss paper was chosen
as the inside of the back cover page of the June 1998,
vol. 29(3) issue of Applied Ergonomics (Notes for
Authors), and the matte finish paper sample was chosen
from P. 163 of the interior of the same journal volume.
Both samples were two columns of text on white paper.
Gloss measurements using the ANSI/ASTM (1994)
Standard Test Method for Specular Gloss: D523-89
were 45.6 at 601/86.3 at 851 for an un-inked area on the
semi-gloss paper, 4.5 at 601/17.2 at 851 for an un-inked
area on the matte finish paper, 59.8 at 601/79.5 at 851
for an inked area on the semi-gloss paper and 10.8 at
601 /25.2 at 851 for an inked area on the matte finish
paper. Papers and surfaces with this degree of specular
gloss are common in any office workplace.
The two-level factorial design is shown in Table 4.
The experiment run order selection was random. The
lamp height and lamp distance from the viewer levels are
typical ranges that could be achieved in under-shelf task
lights for open-plan office cubicles. The illuminance of
the lamp output changed with lamp design position (see
Table 4), and the illuminance with the polarizing film
was the same as for the polyester film. The glare
reduction rating difference at each design position was
judged as the change in glare with and without
polarization. In Table 4, the viewing angle of the main
reflection of the lamp on the page was calculated using
Eq. (2). In all positions, the glare from the lamp could be
seen as a reflection on the papers.
The change in glare on the paper was judged by each
of the test subjects as a glare reduction rating from 0 to
10, with 10 the highest change or difference in glare. A
rating of 0 was judged as no change or difference in glare
at all. No explanation or definition of glare was given to
the test subjects, leaving the assessment of what they
were judging up to him or her.
The experiments and their average (n ¼ 30) results are
shown in Table 4. Design points 5 and 6 are replicates
and center points, and show good reproducibility of the
average glare reduction ratings from each experiment.
The size of the standard deviations may be the result of
the differences in sensitivity of each test subject and their
picking of a different starting level for glare reduction
ratings. For instance, one may start with a glare
reduction of 5 and judge around that level while another
started at 7. However, it is the differences between the
glare reduction averages in the design points that are
most important in the two-level factorial analysis.
Table 5 shows the results of the analysis of variance
(ANOVA) for each of the experiments as analyzed using
the Design-Expert t software made by Stat-Ease t, Inc.
of Minneapolis, MN, USA. Each analysis gives a
predictive equation for each model. The excellent degree
of linear model fit is shown by the high R2 values (R2 in
Table 3
The luminance distribution with the polarizing tube from the lamp center: bond paper at a 200 mm distance from the desk edge at a lamp height of
410 mm, lamp distance of 505 mm, viewing height of 391 mm and a viewing angle of 271
Lamp edge
Position (cm) in front of lamp
Luminance (lux) at 200 mm from desk edge
@30
49.9
Lamp center
@20
67.9
@10
84.8
0
90.3
Lamp edge
10
84.3
20
69.7
30
50.9
495
D.A. Japuntich / Applied Ergonomics 32 (2001) 485–499
Table 4
Two-level factorial design of glare reduction rating for differences in lamp height and lamp to viewer distance: experimental test conditions and
average results on 30 human subjects
Design Run Lamp Lamp
Lamp
Viewing
Semi-gloss
point
order height distance illuminance angle (Deg) paper/lights off
(mm) (mm)
(lux)
Glare reduction
rating
1
2
3
4
5
6
4
2
3
5
6
1
330
490
330
490
410
410
410
410
600
600
505
505
582
398
206
239
351
351
29.6
25.0
39.8
34.3
32.2
32.2
Matte paper/
lights off
Semi-gloss paper/
lights on
Matte paper/
lights on
Glare reduction
rating
Glare reduction
rating
Glare reduction
rating
Ave n ¼ 30
SD
Ave n ¼ 30
SD
Ave n ¼ 30
SD
Ave n ¼ 30
SD
6.3
3.8
7.9
5.6
6.5
6.4
1.8
2.0
1.6
2.0
1.7
1.3
2.8
2.0
5.2
3.5
2.9
3.3
2.5
1.6
2.3
1.9
2.1
2.4
4.4
3.1
6.3
4.9
4.9
4.4
1.8
1.5
1.7
1.6
1.8
1.7
1.5
0.9
2.4
1.0
1.2
1.5
1.1
1.1
1.7
1.1
1.3
0.9
Table 5
Two-level factorial design ANOVA models of glare reduction rating for two different papers with and without bright, non-polarized ambient lighting
Test condition
Model fit R2
Intercept
Lamp height coefficient
Lamp distance coefficient
0.998
0.976
0.951
0.859
7.532
3.217
1.502
2.684
@0.01500
@0.00844
@0.00797
@0.00625
0.00895
0.00974
0.01022
0.00263
a
Predictive factorial model
Semi-gloss paper/lights off
Semi-gloss paper/lights on
Matte paper/lights off
Matte paper/lights on
a
Rating=intercept+Coeff (Lamp height)+Coeff (Lamp distance).
this case being a measure of the amount of variation
around each mean as explained by the model). These
models for each of the two papers may be presented as
contour plots as shown in Fig. 9 for no ambient lighting
and Fig. 10 for bright ambient lighting.
Figs. 9 and 10 contour plots of the results in Table 5
show the same optimization trends as the theoretical
results in Fig. 8. Even though the levels of the responses
are different, it is possible that both the theoretical
percent glare reduction and the test subject glare
reduction rating may be used to optimize an office
workstation for lighting to give minimal glare.
The glare reduction rating changes with the glossiness
of the paper are seen by comparing the two plots in
Fig. 9 for no ambient lighting. If an under-shelf task
light is the sole source of lighting in front of a worker,
polarized lighting is a good tool for glare reduction for
both semi-gloss finish and matte finish papers.
The glare reduction rating results with the addition of
strong, diffuse ambient non-polarized lighting may be
seen in Fig. 10. In most cases the ambient lighting was a
greater source of illuminance than that of the undershelf task light. In strong ambient lighting, polarized
task lighting still successfully reduces specular glare on
semi-gloss finish, but to a lesser extent than without
ambient lighting. However, for the matte finish paper
with bright ambient lights, so little reflection from the
test lamp may be perceived that glare reduction afforded
by polarized light is minimal.
Fig. 11 shows all of the design points of the four
experiments, plotting glare reduction rating against the
calculated reference of theoretical percent glare reduction on glass (task lighting degree of polarization of
87.5%) from Table 6. The data of glare reduction rating
for the two papers at the two test conditions follow the
same general patterns. This strongly suggests simple
linear correlations for this range of viewing angles. For
the two common papers studied, the glare reduction
rating for most actual work conditions will fall between
the plots of ambient lights off and bright, ambient lights
on, with the amplitude of the rating dependent on lamp
position (viewing angle).
Simple linear regressions were run for each of these sets
of data in Fig. 11 and are shown in Table 7. The degree of
fit for each of paper and lighting condition shows that for
these materials the specular glare on a chosen surface as
sensed by a human may be directly related to theoretical
glare calculation on a reference surface. The data in
Table 7 could also be used to generate the same type of
contour plots as in Figs. 9 and 10.
The empirical percent specular glare reduction was
measured using the luminance meter at all the same
Table 4 design points that had been used in the human
testing for each of the two papers at the two lighting
496
D.A. Japuntich / Applied Ergonomics 32 (2001) 485–499
Fig. 9. No ambient lighting: ANOVA model contour plots of human glare reduction rating at different lamp heights and lamp to viewer distances for
50th percentile female height for semi-gloss finish and matte finish papers.
Fig. 10. Bright ambient lighting: ANOVA model contour plots of human glare reduction rating at different lamp heights and lamp to viewer
distances for 50th percentile female height for semi-gloss finish and matte finish papers.
conditions. When this data was compared to the human
testing glare reduction ratings, Fig. 12 resulted, showing
a reasonably good correlation (R2=0.91) for these very
different papers and conditions. It may be concluded
that the luminance (brightness) meter and the eyes of the
human subjects respond in the same manner to changes
in glare reduction from the use of polarized lighting.
Glare reduction ratings of over 5 on papers may be
achieved at measured specular glare reductions of 20%
or greater. In this case, the correlation is non-specific to
materials over a wide range of conditions. So, in place of
using a luminance meter, one simple, inexpensive
method to see if linear polarized light is indicated to
reduce specular glare is to sit at a workstation viewing
497
D.A. Japuntich / Applied Ergonomics 32 (2001) 485–499
Fig. 11. Human glare reduction rating versus theoretical percent glare reduction on glass for semi-gloss and matte finish papers.
Table 6
Two-level factorial design for differences in lamp height and lamp to viewer distance: theoretical percent glare reduction on glass, glare reduction
rating averages and measured percent glare reduction for each design point
Design
point
1
2
3
4
5
6
Viewing
angle (Deg)
29.6
25.0
39.8
34.3
32.2
32.2
Theoretical
% glare
reduction
on glass
33.4
23.4
59.5
44.8
39.7
39.7
Semi-gloss paper/
lights off
Matte paper/
lights off
Glare
reduction
rating
Measured
% glare
reduction
Glare
reduction
rating
6.3
3.8
7.9
5.6
6.5
6.4
20.7
14.1
35.8
26.7
23.2
26.6
2.8
2.0
5.2
3.5
2.9
3.3
Table 7
Linear regressions of glare reduction rating versus theoretical percent
glare reduction on glass for semi-gloss and matte finish papers at two
lighting conditions
Semi-gloss paper/lights off
Semi-gloss paper/lights on
Matte paper/lights off
Matte paper/lights on
Intercept
Coeff
R2
2.273
1.276
@0.204
0.079
0.0945
0.0844
0.0875
0.0333
0.733
0.957
0.956
0.588
the reflective glare while rotating a lens from polarized
sunglasses over a 901 arc to see the magnitude of
luminance difference.
Semi-gloss paper/
lights on
Matte paper/
lights on
Measured
% glare
reduction
Glare
reduction
rating
Measured
% glare
reduction
Glare
reduction
rating
Measured
% glare
reduction
10.1
6.2
14.4
10.9
9.3
10.3
4.4
3.1
6.3
4.9
4.9
4.4
11.6
6.5
22.0
14.8
12.1
15.4
1.5
0.9
2.4
1.0
1.2
1.5
2.3
1.3
4.8
2.2
2.6
2.4
8. Conclusions
This is a study of the specular reflection on a task
placed between a viewer and an under-shelf light source
in a constrained open-plan office cubicle workstation
environment. Specular glare reduction and the optimization of the location of the reflected image of a lamp
may be measured and accurately predicted by the
methods presented in this paper. These may be achieved
by proper and optimal installation of light sources
producing light of a high degree of polarization in openplan office cubicles according to the height of the viewer
to help in minimizing both direct glare and reflected
specular glare. A diverse panel of human subjects using
498
D.A. Japuntich / Applied Ergonomics 32 (2001) 485–499
Fig. 12. Correlation of measured percent glare reduction and human glare reduction rating.
a glare reduction rating at the 50th percentile female eye
height has confirmed these methods. It is surmised that
human subjects of progressively higher eye heights will
see similar changes, but less specular glare reduction
because of smaller viewing angles.
This study successfully uses simple linear polarization
optics for lighting optimization. However, luminaires
have a definite width and length, may be covered by
different light diffuser lenses and also may have different
reflector designs. The effects of these dimensions on glare
reduction for different viewing positions needs to be
considered in future studies and in specific applications.
Other lighting configurations such as side-lit lamps,
different reflective systems and lenses will not follow the
same optimization procedures as shown in this study.
From human testing of the glare reduction rating, this
study clearly shows that polarized lighting affords predictable specular glare reduction if the task light is the sole
source of illuminance, even for matte finish paper. If the
ambient, diffuse lighting is increased, then the benefits of the
use of polarized task lighting are less, but still significant,
depending on the gloss value of the viewed material and the
ratio of the illuminance of ambient light to the task light.
Human glare reduction ratings correlate well with
empirical specular glare reduction as calculated from
luminance meter measurements. For a specific surface
and lighting condition, the glare reduction ratings may
also be simply correlated to the theoretical percent glare
reduction on a reference surface, such as glass.
The ergonomics of lighting clearly needs to take
worker stature into consideration for worker comfort
and productivity. Because of their heights, 95% of
the males are not looking directly into an undershelf cubicle light source of the common height of
410 mm, while more than 50% of the females are
exposed to direct glare from the light source. Female
viewers who are of smaller heights with larger viewing angles will see an increase of 6–10% reflective,
specular glare over taller male viewers when using nonpolarized light, as seen in Table 2. This may help to
explain the results of the research of North (1991) that
shows a 50% increase of glare discomfort in women
over men.
These methods may be applied to other areas of
study, such as in the field of education, where children
are even shorter than adult females, and their heights
should also be taken into consideration for proper
reading and work illumination. The analysis of the
geometry of lamp placement versus the viewer may be
used for other illuminating studies, such as for forward
over-head lighting.
References
Allphin, W., 1963. Sight lines to desk tasks in schools and offices.
Illum. Eng. 58 (4), 244–249.
ASTM Standard Test Method for Specular Gloss: D523-89, 1994.
American Society for Testing and Materials, West Conshohocken,
PA.
Bernecker, C.A., Davis, R.G., Webster, M.P., Webster, J.P., 1993a.
Task lighting in the open office: a visual comfort perspective. J.
Illum. Eng. Soc. 22 (1), 18–25.
D.A. Japuntich / Applied Ergonomics 32 (2001) 485–499
Bernecker, C.A., Beebe, L.A., Shi, J., 1993b. Blackwell: a look back.
LD+A. 23 (8), 62–68.
Blackwell, H.R., 1963. A general quantitative method for evaluating
the visual significance of reflected glare, utilizing visual performance
data. Illum. Eng. 58 (4), 161–243.
Blackwell, H.R., 1984. Studies of visual effectiveness of polarized light,
Report to the Ohio State University Institute in Vision to the
Lighting Engineering Research Institute, November 30.
Blackwell, H.R., DiLauria, D.L., 1973. Application procedures for
evaluation of veiling reflections in terms of ESI. J. Illum. Eng. Soc.
2, 254–283.
Boyce, P.R., Rea, M.S., 1994. A field evaluation of full-spectrum,
polarized lighting. J. Illum. Eng. Soc. 23 (2), 85–107.
Clear, R., Berman, S., 1992. Polarization and glare on VDU’s and
other tilted specular surfaces. J. Illum. Eng. Soc. 21 (1), 29–40.
Clear, R., Mistrick, R.G., 1996. Multilayer polarizers: a review of the
claims. J. Illum. Eng. Soc. 25, 70–88.
Collett, E., 1993. Mueller Matrices for Reflection and Transmission,
Polarized Light: Fundamentals and Application. Marcel Dekker
Inc., New York, NY (Chapter 8).
Crouch, C.L., Kaufman, J.E., 1963. Practical application of polarization and light control for reduction for reduction of reflected glare.
Illum. Eng. 58 (4), 277–291.
Fresnel, A.J., 1821a. M!emoire sur la double r!efraction. M!em. de
l’Acad. des sc. vii, 45.
Fresnel, A.J., 1821b. Ann. de Chim.(2) xxviii, 263.
Fresnel, A.J., 1847. Pogg. Ann. xxiii, 372, 424.
Gordon, C.C., et al., 1988. Anthropometric survey of US army
personnel: methods and summary statistics. Technical Report
Natick/TR 89/044.
Grabau, M., 1940.The Major Applications of Polarized Light.
Introduction to Polarized Light. Polaroid Corporation, Cambridge,
MA, 29–30 (Chapter V).
499
IES, 1993. Light and optics. In: Rea, M.S. (Ed.), Lighting Handbook,
8th Edition, Illuminating Society of North America (IESNA), New
York, NY, p. 22 (Chapter 1).
Jones, B.F., 1992. Polarized light: effects on vision, energy use and cost
reduction. LD+A. 22, 34–36.
Karpen, D., 1995. Progress in full-spectrum polarized lighting systems.
Energy Eng. 92 (6), 18–48.
Lumsden, W.K., 1976. Effective task lighting using polarized light.
Light Light. Environ. Des. 69 (4), 160–162.
Marks, A.M., 1959. Multi-layer polarizers and their application to
general lighting. Illum. Eng. 54 (2), 123–135.
North, V.A., 1991. Factors influencing worker’s impressions of glare in
open offices. Ph.D. Thesis, University of Michigan, Ann Arbor, MI.
Rea, M.S., 1981. Visual performance with realistic methods of
changing contrast. J. Illum. Eng. Soc. 19 (3), 164–177.
Rea, M.S., Ouellette, M.J., Kennedy, EM.E., 1985. Lighting and task
parameters affecting posture, performance and subjective ratings. J.
Illum. Eng. Soc. 15, 231–238.
Rea, M.S., Ouellette, M.J., Tiller, D.K., 1990. The effects of luminous
surroundings on visual performance, pupil size, and human
preference. J. Illum. Eng. Soc. 19 (2), 45–58.
RP-24. 1989. IES Recommended Practice for Lighting Offices
Containing Computer Visual Display Terminals. Illuminating
Engineering Society of North America, New York, NY.
Shurcliff, W.A., 1962. Reflection polarizers. In: Polarized Light:
Production and Use. Harvard University Press, Cambridge, MA
(Chapter 6).
Siminovitch, M.J., 1993. Experimental development of efficacious task
source relationships for interior lighting applications. Ph.D. Thesis,
University of Michigan, Ann Arbor, MI.
Weber, M.F., Stover, C.A., Gilbert, L.R., Nevitt, T.J., Ouderkirk,
A.J., 2000. Giant birefringent optics in multilayer polymer mirrors.
Science 287, 2451–2456.