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OCR for page 111
5
Lighting and Reflections
Experience reveals that lighting and reflections may cause
problems for video display terminal operators. The reflection of a
lamp or a bright window on any viewing screen makes it difficult
or impossible to see the picture. Even if the picture is visible
through the glare, the reflection may be distracting and annoy-
ing. Almost everyone who has watched television or worked at a
VDT, microfilm viewer, or similar display device has probably had
such experiences. Many complaints reported by VDT operators are
specifically related to workplace lighting and reflections. Several
studies involving such complaints are reviewed and analyzed later
in this chapter.
The basic principles and most of the details of how lighting and
reflections affect visual performance and comfort are known.
Lighting specification systems based on principles of geometry,
the physics of light and reflectance, and characteristics of the
human visual system are routinely used by illuminating engineers
and other lighting specialists to design appropriate lighting for
workplaces. Good lighting design also includes esthetic and other
considerations intended to promote appropriate psychological and
social reactions. Although lighting specification systems differ in
their specific assumptions, criteria, and spatial resolution, they
are fundamentally the same. An experienced, well-trained light-
ing specialist could use these systems to design appropriate
lighting for VDT workplaces and to predict or explain problems
that may result from inappropriate lighting design.
Workplaces in which VDTs are used require lighting designs
that differ in simple ways from those required in no - VDT
workplaces. The differences primarily involve geometrical
relationships--the presence of VDTs may complicate lighting
design by adding (or substituting) work surfaces in different
positions and planes.
OCR for page 112
112
Just as some workplaces are poorly designed in terms of
lighting and workstation arrangements for non-VDT work, so some
are poorly designed for VDT work. It is likely that many offices
designed for desk-top paperwork are now being used for VDT work
without appropriate modifications of the lighting. Higher rates of
complaints for work involving VDTs compared to other types of
work may largely be due to inappropriate lighting for the VDT
situations. As lighting is improved for VDT installations, many of
the problems attributed to VDTs may vanish.
ILLUMINATION
Illumination in offices and other workplaces comes from light
sources, windows, and reflections from a variety of objects and
surfaces. VDTs differ from most other task objects or surfaces in
that they emit light, and they usually have a highly specular
curved glass surface in a more vertical plane. These differences
have important consequences with respect to illumination and
reflections. For example, VDT operators in some situations see
reflections of their faces and clothing on the display screen. Such
reflections can be annoying and distracting, and in some cases may
reduce task visibility enough to affect performance. An analysis
of the basic characteristics of illumination should be helpful in
determining the possible role each may play in causing problems
for VDT operators.
The four major characteristics of illumination are spectral
composition, temporal changes, intensity, and spatial or
directional aspects. There is little indication or reason to believe
that spectral composition or temporal changes of illumination are
responsible for complaints or problems peculiar to VDT use.
Illumination having unusual or extreme color or flicker character-
istics may interact with display characteristics in special ways
that may cause problems. (Display characteristics, including color
and flicker, are discussed in Chapter 4.) It is unlikely that such
conditions were involved in any of the studies reviewed in this
report. Although some people have negative attitudes and
reactions to commonly used illumination (certain spectral
compositions and flicker frequencies), there is no indication that
these conditions have special importance in VDT situations.
For the other two major characteristics of illumination,
intensity and spatial aspects, there are important differences
between VDT and non-VDT work situations. Our review and
analysis of these characteristics of illumination is divided into
three parts: problems caused by successive viewing of different
luminances (which can lead to transient adaptation), reflections,
and glare.
OCR for page 113
113
Transient Adaptation
Transient adaptation refers to the temporary loss in visibility that
occurs when a person changes his or her point of regard to sur-
faces having different luminances or when illumination changes
occur naturally in the visual environment. In general, the greater
the ratio of change or difference in luminance levels, the greater
the loss in visibility (Boynton et al., 1969; Rinalducci and Beare,
1974, 1975~.
The results of research on transient adaptation have typically
been interpreted in terms of the ratio of the contrast threshold of
the target in the transient state of adaptation to the contrast
threshold after complete adaptation to the new prevailing lumi-
nance level. This ratio (symbolized by t) represents the increased
amount of light needed to see the target in the transient versus
the steady state and thus is indicative of loss in visibility as a
function of luminance change. Figure 5.1 shows ~ as a function of
the ratio of background field change. Figure 5.2 shows log ~ and
plotted as a function of the log ratio of background field change
and compares data obtained by Boynton and coworkers (1969) and
Rinalducci and Beare (1974~. Here, as with most research of this
nature, the transient state threshold is measured 300 msec (~)
after the change from the prevailing luminance level, B1, to the
new luminance level, B2.
Visibility losses due to transient adaptation in VDT operations
occur when an operator looks toward a glare source (e.g., a
window or a luminaire) and then back to the display screen. The
decrement in visibility should be particularly large for a positive-
contrast display (light characters on a dark background). A second
situation that could involve losses in visibility due to transient
adaptation is when a positive-contrast display is combined with a
negative-contrast source document, such as a typewritten page.
Visibility losses may also occur when secondary task lighting is
used on the source document. In this situation, a VDT operator
using a positive-contrast display may be particularly prone to the
effects of transient adaptation, especially when a negative-
contrast source document is used. Rupp (1981) has reviewed a
number of European and Canadian documents that recommend
standards for VDT design and use. Two of the documents
reviewed- - report of the Technical University of Berlin (Cakir et
al., 1978) and the German DIN draft Standard 66234- - xpress some
concern with transient adaptation effects that may occur when an
operator continually looks back and forth between a positive-
contrast display and a negative-contrast source document. Rupp
appears unconvinced that such effects are significant, citing the
review of MacLeod (1978) and the research on scotopic adaptation
OCR for page 114
114
8
7
6
l
Maximum for /
Al I Conditions /
Investigated /
5 _
4
3 _
2
1
Overage
/
/
~ /
//
_' / ,o'~—Minimum
_ ~
3 1 0 30 1 00 300 1 ,000
FACTOR BY WHICH PREVAILING ADAPTING LEVEL IS CHANGED
FIGURE 5.1 Phi as a function of the factor of change from one
luminance to another. SOURCE: Boynton and Miller (1963~.
by Barlow and Andrews (1973), and suggests that the visual
system's level of adaptation is determined by the luminance of the
light symbols and not by an integrated luminance level or
background luminance level. Thus, if this hypothesis is correct,
one might be more concerned with matching the luminance of the
light symbols with the source document background. However, the
evidence presented in support of Rupp's hypothesis is either
unconvincing or inappropriate, and we believe the situation needs
to be examined further (also see the discussion in Chapter 4~.
OCR for page 115
115
Tau = +300 ms
0.7
0.6
0.5
0.4
o
0.3
1
0.2 _ ~~
%~%
0.1 _ In_
Q
1 1
-
Log (B2/B1 ): -2
(B2/B1 ): 0.01
FIGURE 5.2 Log phi as a function of log ratio of backgrounds.
Data from an experiment employing the same ratios by Boynton et
al. (1969) are included to allow comparison with effects of similar
changes from higher initial luminances. NOTE: The right-hand
ordinate and lower abscissa scales make the figure direct-reading
for phi as a function of the ratio of the backgrounds. SOURCE:
Rinalducci and Beare (1974~.
o
._ B1 = 0.02 fL Size = 10.6'
0~0 B1 =4Q.OfL ~
B 40 f J Size = 12' (from Boynton etal., 1969),
5.01
_ 3.98
p
,~ / _ 2.51
"'/ ~
/ ,'
/ ,'
~ /
,' /,
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+1
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3.16
1.99
1.58
1.26
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100
_ 1 .00
v,
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o
-
-
Reflections
CRT screens are usually convex, spherical shells of glass with a
radius of curvature of approximately 63.5 cm. Reflections from
the mirrorlike front surface of the screen form images. A very
distant object seen by reflection in the screen will appear to be
located at one-half the distance from the screen to its center of
curvature, i.e., at 31.75 cm behind the screen. For an operator at
70 cm from the screen, dioptric accommodation levels for the
screen and the reflected image are 1.43 diopters and 0.98 diopter,
respectively. The reflected image of an object located close to
the screen will also be closer to the plane of the CRT face. For
example, if an operator at 70 cm from the screen sees his or her
face reflected from the screen, the image will appear to be
located about 22 cm behind the screen. Reflected images of
luminaires or windows can produce a veil of light (reflected glare)
over a portion of the screen. They can also serve as distracting or
annoying stimuli that may cause discomfort or affect perfor-
mance indirectly by distracting or changing the motivation of the
OCR for page 116
116
operator (see Petherbridge and Hopkinson, 1955, for evidence from
a non-VDT study).
Since reflected images form at distances other than that of the
screen surface, accommodation and convergence may fluctuate or
otherwise be inappropriate for viewing the screen. This effect
may be annoying, induce discomfort, or affect performance.
Reflected images may also affect performance if an operator
looks directly at them (possibly due to phototropism), which could
cause transient adaptation problems (DeBoer, 1977~. Reflected
images could also induce binocular rivalry, which might cause
-
discomfort or affect task performance (Reitmaier, 1979)e
The inside phosphor surface of a CRT screen reflects light in a
diffuse manner rather than imaging it. Light also excites the
phosphor, increasing its illuminance. Both effects can reduce
contrast. Many VDTs have adjustments for screen brightness
(illuminance) and contrast that can compensate for this type of
effect except in extreme cases.
An analysis and discussion of relationships between problems
reported by VDT operators, such as ocular discomfort and fatigue,
and physiological optics variables, such as accommodation, fixa-
tion, convergence, and binocular rivalry, is included in Chapter 7.
Glare
Glare is the sensation produced by luminances within the visual
field that are sufficiently greater than the luminance to which the
eyes are adapted to cause annoyance, discomfort, or loss in visual
performance and visibility (Kaufman and Christensen, 1972~. The
magnitude of the sensation of glare depends on factors such as the
size, position, and luminance of the light source or reflecting
surface, the number of light sources, and the luminance to which
the eyes are adapted. Reflected glare is the result of specular
reflections from polished or glossy surfaces or diffuse reflections
that produce a veil of light that reduces contrast. Disability
gore, which may be caused by light scattered within the eye
(reducing contrast at the retina), or by reflected glare, reduces
visual performance and visibility. Discomfort glare produces
discomfort, and it may, but does not necessarily, interfere with
· . ~ . .. ...
visual performance or VlSlDlllty, Just as disability glare may or
may not be accompanied by discomfort.
The large reported differences among individuals in sensitiv-
ity to glare, as well as the great variability among studies, may be
due to problems in methodology, especially in studies of discom-
fort glare (see, e.g., Lulla and Bennett, 1981~. The lack of a clear
understanding of how glare induces discomfort (see Chapter 7j
OCR for page 117
117
also makes the analysis and interpretation of problems attributed
by VDT operators to glare difficult and uncertain. Nevertheless,
several models and mathematical expressions for describing the
effects of glare on visual comfort probability (VCP) have been
formulated (see, e.g., Kaufman and Christensen, 1972~. Extensive
research dating from the 1 920s (Holladay, 1926; Nowakowski,
1926) has provided the basis for such models. References to much
of this research and other important issues related to discomfort
glare are included in a report of the Commission Internationale de
l'Eclairage (1 980~.
The report also includes a proposed CIE glare formula by H. D.
Einhorn with a discussion of its rationale, quantitative aspects,
and the significance and choice of scaling factors. The formula
identifies the important variables and indicates how they are
related:
CGI = 10 log 0.1 (L w) X Ed/500
(Ee)
where
CGI = CIE glare index (provisional name)
L = Luminance of a glare source, in cd/m2
w = Solid angle of source, in steradian
P = Guth position index
Ed = Direct vertical illuminance at eye due to all sources,
in lux
Ee = Vertical illuminance at eye, in lux. Ee includes the
indirect illuminance: Ee = Ed + Ei
The position index P is based on Luckiesh-Guth's research. For
computer work it is best expressed as:
1 d2E
+ 0.12 (1-E)
P d2 + 1.5d + 4.6
with E = exp (- 0.18 s2/d ~ 0.011 s3/d)
where d = forward distance of source/height
s = sideways distance of source-height (forward)
means in the direction of the line of sight,
sideways means perpendicular to it, height means
height above eye level.)
OCR for page 118
118
In general, the higher the luminance of a glare source, the larger
the source, the lower the background luminance, and the closer
the source is to the line of sight, the greater is the capacity of the
source to produce discomfort. The position index, P. is direc-
tional; a glare source located horizontally to the line of sight has
greater potential for producing discomfort than an equivalent
source located the same angular distance directly above the line
of sight.
The use of comprehensive formulas of this type has not been
reported in studies of VDTs. However, the potential for VDTs to
induce discomfort glare can be estimated from reported lumi-
nance measurements and data from basic research on discomfort
glare. Table 5.1 shows data (Gush, 1951) relating background
luminance and glare-source size to glare-source luminance at the
borderline between comfort and discomfort (BCD). Assuming that
these data accurately represent discomfort glare thresholds for at
least some VDT operators, it can be seen from Table 5.1 that some
situations would induce discomfort. Fellmann and coworkers,
using an illumination of 150 lux on eight different brands of
VDTs, reported !uminances ranging from 2-7 cd/m2 for screen
backgrounds, 8-110 cd/m2 for consoles, and 8-45 cd/m2 for
keyboards (Fellmann et al., 1981~. These values represent the
background luminances (the approximate levels to which th
operators would be adapted), that are best represented in the table
by the 3.4 and 34 cd/m values. Luminances in excess of 1,000
cd/m2 from potential glare sources (e.g., windows and luminaires)
have been measured in actual VDT workstations (Cakir et al.,
1978; National Institute for Occupational Safety and Health,
1981~. These values are glare source luminances. Values above
the BCD values in the table would induce discomfort. (Note,
however, that Guth used a flashing glare source. It is not clear
how much the BCD values from steady sources in natural settings
would differ. Eye movements and blinks would interrupt the
retinal images of steady sources.) Several combinations of the
measured or assumed conditions would produce discomfort glare.
A lighted environment that is properly designed and therefore
comfortable for workers performing traditional desk-top tasks
may not be comfortable for workers performing tasks involving
VDTs for two reasons. First, the design of general office lighting
assumes a depressed line of sight; however, when a VDT screen is
viewed, the line of sight is at or near horizontal. The higher line
of sight needed to view the screen brings ceiling luminaires closer
to the line of sight, resulting in a higher glare index and a greater
likelihood of discomfort glare. Second, the temporally and
spatially averaged luminance is lower for positive~ontrast VDTs,
which results in a higher glare value.
OCR for page 119
119
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OCR for page 120
120
Glare problems have been reported in several studies involving
VDTs. Although there are problems in the methods used and the
interpretation of results in some of these studies (see Chapters 2
and 7 and Appendix A), there is little doubt that lighting and
reflections were responsible for some of the reported operator
complaints and problems.
A general analytic model for predicting task visibility under
different lighting conditions may be useful in analyzing VDT task
situations. This model is
~ n - S
VL = Cref X - of-- X CRF X DGF X TAF
0.0923
where (as defined in Commission Internationale de l'Eclairage,
1981 ):
VL = Visibility level, a measure of the extent to which the
equivalent contrast of a task visual display exceeds
the visibility threshold of an observer for the same
display at the same level of task background
luminance, measured in units of the observer's
threshold contrast.
-
Cref = Reference equivalent contrast, the value of
equivalent contrast of given task details under
reference lighting.
RCS = Relative contrast sensitivity, proportional values of
contrast sensitivity expressed relative to the value
, · ~
obtained with a luminance of 100 cd/m~.
CRF = Contrast rendering factor, the measure of the
visibility of a task in a real lighting installation in
comparison with its visibility under reference
lighting conditions, account being taken of lighting
geometry and polarization of illuminance.
DGF = Disability glare factors a measure of a task in a
_
given lighting installation in comparison with its
visibility under reference lighting. The measure
takes account of the two effects of the ocular stray
light produced by the pattern of luminance in the
surround of the task: (a) the reduction in image
contrast, and ~) the increase in RCS due to visual
adaptation to the sum of the focused light and stray
light.
OCR for page 121
121
TAF = Transient adaptation factor, a measure of task detail
visibility in a given lighting installation in
comparison with its visibility under reference
lighting, account being taken of the transient
adaptive effect that occurs when the eyes of the
observer view luminances in the environment
different from the task background luminance.
0.0923 = The value of the visibility threshold for the 4-minute
disc task obtained by the reference observer at a
level of task background luminance equal to 100
cd/m2.
This model is based on extensive laboratory data and has been used
in analyzing complex realistic visual performance, for example,
proofreading, visual search, and numerical verification. While it
has not yet been applied to VDT tasks, there is no obvious reason
why it should not be applied to such tasks. It incorporates several
factors, including transitional adaptation and disability glare
effects. Although this model has been criticized (see, e.g.,
Yonemura, 1977; Padmos and Vos, 1980) and may have important
limitations, it seems worthy of further validity tests, some of
which could include VDT applications.
Some aspects of an earlier version of the model incorporated a
visual discomfort formula (Commission Internationale de
l'Eclairage, 1975), and some recommended standards have been
used by player and Barlier (1981) to evaluate 73 VDT work-
stations. Luminance measurements were made of screens,
keyboards, and documents. Contrasts of screen characters and
their backgrounds for 40 of the VDTs are plotted with CIE
visibility curves in Figure 5.3. Of the 40, 2 VDTs were above the
comfort limit; 3 were approximately at threshold (VL = 1), which
should make task performance very difficult or impossible; and 15
were in the range that would be expected to have a significant
effect on the performance of some tasks (VL < 8~.
In summary, VDTs differ from objects and surfaces in non-VDT
workplaces because they usually have a highly specular, curved
glass surface in a more vertical plane. Consequently, workplaces
in which VDTs are used require lighting designs that differ from
those required in non-VDT workplaces. Lighting and equipment
arrangements that are appropriately designed for a particular task
and working situation should prevent glare and most other prob-
lems arising from lighting and reflections. Application of
established knowledge and principles of design in the field of
illuminating engineering can be expected to alleviate most of the
difficulties related to lighting and reflections in VDT-related work.
OCR for page 122
122
100
at
o
J
A
UJ
A:
UJ
IL
11
~ 0.1
10
1
\\
\
\\
Comfort Limit
\
No Visibility
r
Satisfactory Visibility
- ~ ~ VL8
VL 4
l
Poor Visibility
_ VL 1
o.olL I I I I I I I I I
0.001 0.01 0.1 1 10 100 1,000
J
Lum inance Characters Lum inancegackground
Luminance
Background
BACKGROUND LUMINANCE (cd/m2)
FIGURE 5.3 Illuminating Engineering Society (IES) visual
performance curve: visibility of screens. SOURCE: Mayer and
Barlier (1981~.
REVIEW OF VDT STUDIES
Field Surveys of VDT Workers
Several field surveys have attempted to determine the opinions of
VDT operators regarding problems caused by lighting and reflec-
tions in the workplace. In some of these surveys, measurements of
various aspects of the lighting conditions have also been made, and
some investigators have attempted to relate those measurements
to visual symptoms and complaints reported by VDT operators.
Most of the surveys suffer from the kinds of limitations in method
discussed in Chapter 2. Consequently, while the results of these
surveys reveal that many VDT operators have complaints and
symptoms related to workplace lighting conditions, they do not
establish whether such complaints and symptoms are more
frequent, more severe, or of a different nature than those that
may be associated with non-YDT near-visual work. Our purpose in
briefly reviewing several of these surveys is simply to provide an
OCR for page 123
123
overview of the kinds of studies that have been conducted and the
kinds of problems that have been reported.
There have been several published studies that used various
types of questionnaires and interviews to determine the opinions
of VDT operators about their work. In a study conducted by
Ghiringelli (1980), 63 percent of operators reported that badly
working equipment was a problem, 43 percent reported problems
with reflections, and 43 percent also reported problems with
"luminance." Using an unstructured interview technique, Grieco
and coworkers (1980) interviewed an unspecified number of
selected newspaper photocomposition VDT workers and reported
that operators had problems with lighting, particularly with screen
reflections. Dainoff (1980) and Dainoff and coworkers (1981) also
used an unstructured interview technique in a study that covered
90 clerical workers, who reported that they worked with VTDs
from 0-100 percent of the time (median 47 percent), and 31 data
entry workers, who reported that they worked with VDTs 75
percent of the time. Complaints about lighting related
specifically to the VDT were significantly higher in the data entry
group; complaints about general workplace lighting, however, were
made by the same percentage (37 percent) of workers in both
groups. No attempt was made to relate complaints to specific
aspects of the physical environment of the workplace. The designs
of these studies do not permit an analysis of possible causal
factors in reported complaints.
Several published studies present data on the types and
frequency of complaints, accompanied by measurements of certain
aspects of the lighting and reflection conditions. Hultgren and
Knave (1974) studied an insurance office that had 17 VDTs at
various locations in a room. A questionnaire was used to deter-
mine operator feelings about discomfort glare, eyestrain, and
specific discomfort associated with reading the text on the
screen. The luminance of the screen, illuminance of the work
area, and angles of incident light and reflection were measured for
6 representative terminals. The operators reported problems with
13 of the 17 terminals in at least one of the three areas covered
by the questionnaire. The luminance ratio of the screen to the
brightest region in the immediate vicinity of the screen was a
maximum of 1:500. In the worst case, reflected images had double
or triple the luminance of an area of the screen containing a
character. The character luminance was not measured directly.
This study involved such a small sample that firm conclusions
about the relationship between complaints and the VDT
environment cannot be drawn. It did indicate, however, that
operators in this one location had problems with glare.
OCR for page 124
124
Cakir and coworkers (1978) conducted a field study of 30
companies in which more than 1,000 VDT operators participated.
Statements from operators about lighting problems were obtained
by means of a questionnaire, and various aspects of the lighted
environment were measured. The authors reported that the
luminaires providing the general room lighting were a source of
operator complaints. Luminaires of different construction elicited
significant response differences to a question concerning direct
glare. Bare fluorescent lamps were complained about more
frequently as a source of direct glare than were luminaires
constructed with some type of diffusing cover. Operators also
rated bare fluorescent lamps as poorest with regard to the
visibility of screen characters. More than 50 percent of the
workers reported seeing reflections in their screens and, again,
bare fluorescent lamps were singled out by the operators as
producing more problems with screen reflections than luminaires
with diffusing covers.
At selected sites and on selected VDTs, Cakir and coworkers
measured the luminances of ceilings, luminaires, display docu-
ments, keyboards, and screen backgrounds. Ceiling luminances
ranged between 15 and 35 cd/m2, while luminaires in the ceilings
had luminances over 1,000 cd/m . Luminance ratios between
screen background and display documents were found to be 1:6 if
no antireflection filter was used over the VDT screen and 1:100 if
a filter was used. The luminance ratio between the screen and the
keyboard ranged from 2:1 to 1:70. Problems with beat frequencies,
which might arise from the flicker of fluorescent lights and the
refresh rate of the VDT screen, were not found.
On the basis of the responses to a questionnaire, Elias and
coworkers (1979, 1980) determined that 70 percent of operators
who worked in an office that had windows almost all the way
around had complaints about general lighting, while 45 percent of
those who worked in an office with fewer windows had such com-
plaints. The tasks differed in the two offices--in the windowed
office the task was predominantly data entry; in the other office
the task was interactive—and so did complaints about reflections
on the screens: 45 percent of the data entry group and 65 percent
of the interactive group had such complaints. The authors ascribe
the higher frequency of complaints about screen reflections in the
interactive group to the greater period of time these operators
viewed the screen. The two groups also differed in complaints
1Elias and coworkers (1980) define the task as Data acquisitions;
however, the description of the task (Elias et al.' 1979) corre-
sponds more closely to what is referred to in this report as data
entry (see Chapter 1).
OCR for page 125
125
about glare: 80 percent of the data entry operators and 52
percent of the interactive operators reported complaints related
to glare that occurred, "sometimes" or "often" (four responses
were available: "often," "sometimes," "rarely," "never"~. The
authors ascribe the greater number of glare complaints in the data
entry group to various factors, including range of contrast.
Luminance levels were measured at a typical worksite in the
data entry (windowed) office. The luminance ratio between the
screen and the document was 1:8.5; between the screen and its
periphery, the maximum ratio was 1:500, with daylight from a
window providing general illumination on a sunny day; when
illumination was provided only by a luminaire, the luminance ratio
was 1:108. A higher percentage of data entry than of interactive
operators reported discomfort glare. The authors ascribed this
finding to the higher frequency of measured saccadic eye
movements between objects of disparate luminances made by the
data entry operators.
Stewart (1980a) assessed environmental problems at 80 VDT
workplaces. The most frequently occuring environmental problem
was thermal (100 percent of the workplaces), but the next most
frequently occurring problems were glare (71 percent of the
workplaces) and reflections from windows and luminaires (83
percent of the workplaces). Measured illumination levels at the
workplaces ranged from 100 to 2,500 lux.
A study conducted under the auspices of the New Zealand
Department of Health at selected VDT workplaces showed that
screen reflections occurred on 42 percent of the VDTs surveyed
(Coe et al., 1980)e No statistical relationship between room
lighting intensity and presence of screen reflections was found.
Of the workplaces sampled, approximately one-third exceeded a
3:1 ratio of luminances between hard copy and screen, and more
than half exceeded that ratio between the immediate background
of the screen and the screen itself. For those VDT tasks requiring
hard copy, 54 percent of the operators viewing hard copy illumi-
nated with less than 250 lux reported asthenopia (sore eyes and
visual discomfort), 55 percent of the operators viewing hard copy
illuminated between 250 and 500 lux reported asthenopia, and 34
percent of the operators whose hard copy was illuminated with
more than 500 lux reported asthenopia. A comparison group was
used in this study, but no data are reported on lighting complaints
of the non-VDT comparison group.
Field Surveys Comparing VDT and Non-VDT Work
The studies reviewed above indicate that lighting and reflections
do cause problems for VDT workers; however, these problems may
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not be unique to VDT use, but may be present in general office
work. Without evaluating non-VDT work, no comparisons can be
made regarding such complaints in VDT and non-VDT tasks. Two
studies have attempted such comparisons.
Laubli and coworkers (1980) conducted a field study in which
illumination and luminance levels were measured at workstations
used in four different types of office tasks: (1) data entry VDT
work; (2) interactive VDT work; (3) traditional clerical work; and
(43 typing. At 90 percent of the VDT workstations, the illumi-
nation levels of source documents were between 100 and 1,900 lux;
at workstations used for traditional clerical work or typing, the
levels were between 100 and 3,200 lux. The luminance ratios
between source document and screen background ranged from 7:1
to 87:1 at VDT workstations. The frequency of ocular and visual
symptoms pains, burning, fatigue, shooting pain, red eyes, head-
aches, blurring of near and far vision, flicker vision, and double
images (all referred to as eye impairments in the study)--
increased as luminance ratios increased. The incidence of
symptoms was highest in the interactive VDT operators; it was
lower in data entry VDT operators and in typists (and approxi-
mately the same for these two groups), and considerably lower for
workers performing traditional clerical work. Although images
reflected by the VDT screen had lower luminances than screen
characters, the measured intensity of the reflections was
correlated with reported annoyance; it was not, however,
correlated with frequency of ocular or visual symptoms.
Stammerjohn and coworkers (1981) conducted a study at four
newspapers and one insurance company. At selected workstations,
they measured illuminances and luminances in operators' general
visual field, including the luminance of the screen background, but
not that of the display characters. Subjective operator ratings on
a five-point scale from "no bother or problem" to "constantly
bothersome" were obtained on screen brightness, character
brightness, readability, screen angle, keyboard angle, screen
height, keyboard height, distance to the screen, distance to the
keyboard, screen glare, keyboard glare, noise from the VDT, and
screen flicker.
The majority of workstations had illuminances between 500 and
700 lux, with a low of 300 and a high of 1,200 lux. The range of
luminance ratios in the immediate visual field of the operators
was from 1:2 to 1:60. Potential glare sources (windows, lumi-
naires) were reported at 46 of the 5) workstations. Luminances of
these sources were near 2,100 cd/m . Reflected glare from
windows or overhead lights was present on most of the screens
surveyed at one site. [uminances of the reflected images had
maxima of 3~60 cd/m . Although character luminance was not
measured, the investigators had difficulty in reading the screen
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text from VDTs that had particularly high reflected glare leve
(17 percent of those examined).
Is
Several aspects of the VDTs were reported as bothersome by
operators: screen glare (85 percent), character brightness (70
percent), readability (69 percent), flicker (68 percent), and screen
brightness (62 percent). A slight majority were satisfied with
workstation and background illumination. However, 80 percent of
the VDT operators reported trouble with glare from the work-
station lighting. More than 60 percent of the non-VDT operators
also reported problems with glare from workstation lighting.
Because of employee anonymity involving the use of the
questionnaires, specific complaints cannot be related to specific
VDTs and therefore to specific design features. This study did,
however, determine that a significant relationship exists between
complaints regarding visual function and employee rating of
workplace design parameters, including glare, screen angle, noise
from the VDT, and screen flicker.
Laboratory Studies
In a laboratory investigation, Radl (1980) demonstrated the
importance to operator comfort and to one measure of perfor-
mance of graded luminance from the screen to its surround. The
subjects had to transcribe letters from the screen to a paper
sheet. They viewed a VDT screen with a back2ground luminance of
18 cd/m2 and symbol luminance of 120 cd/m with a surround of
4,200 cd/m2 (i.e., a glare source) encompassing about 75 degrees
of the subjects' visual field. The initial experiments used a black
frame of varying dimensions around the VDT screen. The results
are summarized in Table 5.2. Radl also compared the effects of
positive and negative contrast on performance of the same task
and on rating of visual comfort, using 24 subjects. The subjects
rated negative contrast (dark characters on a light background) as
more comfortable (general illumination of 500 lux; screen surround
not described), and performance was also greater with this pre-
sentation. Both findings were reported as statistically significant.
Bauer and Cavonius (1980) examined the differential effects of
positive and negative contrast on performance of a letter identi-
fication task. The display~resentations were (1) low-luminance,
positive contrast (10 cd/m background), (2) high-luminance
positive contrast (80 cd/m2 background), and (3) negative contrast
(80 cd/m2 background). The error rate was lowest for the
negative-contrast display and highest for the hig~luminance
positive-contrast display.
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TABLE 5.2 Graded Luminance and Operator Performance and
Comfort
Width of Frame
Rated Visual Comfort
Performance(%)a (Worst, 0; Best, 7)
.
No frame 42 0.32
7° 67 1.45
1 1° 69 1.80
16° 84 2.05
21° 63 3.07
11°b 85 3.15
a Relative to performance tested in the absence of glare.
b Frame continuously shaded from black at the screen to white at
the outer edge.
In a second experiment, Bauer and Cavonius (1980) compared
the speed and accuracy of subjects using positive- or negative-
contrast displays in detecting discrepancies between a VDT
presentation and a typewritten presentation. The two conditions
compared were positive Contrast display (symbol luminance 40~5
cd/m2, screen background luminance < 10 cd/m2, ambient
illumination 270 lux) and negative-contrast display (symbol
luminance set as low as possible, screen background luminance
50-70 cd/m2, ambient illumination 550 lux). The negative-
contrast display produced the fewest errors and fastest
performance time, and 18 of the 19 subjects preferred the
negative contrast; the one subject who preferred positive contrast
actually performed better on the negative-contrast display. As
noted in Chapter 4, both the Bauer and Cavonius (1980) study and
the Radl (1980) study should be interpreted cautiously because
changes in contrast polarity were combined with changes in
ambient illumination and absolute contrast magnitude.
In summary, the field surveys and a few limited laboratory
studies of VDT-related work indicate that workers who use VDTs
have problems caused by lighting and reflections. However, we
found no studies that compared equivalent, appropriately
illuminated and arranged VDT- and non-VDT tasks and working
situations, and so we cannot draw conclusions about the relative
number, types, and severity of complaints and problems related to
lighting and reflections in VDT and non-VDT work.
Representative terms from entire chapter:
data entry
