Seeing is believing: HPS lighting
How LightIng Affects Us
When we think about lighting in the workplace, the first thing that comes to mind is the
obvious physical effect it has on us. Inappropriate lighting can lead to a host of
problems, ranging from eyestrain to serious musculoskeletal injuries. In fact, more
than two-thirds of those responding to a workplace survey (April 1999) indicated that
they experienced serious physical problems associated with a poorly lit workplace.
This isn’t new. These responses are consistent with what people have been saying in
studies and surveys for years.
The physical effects
*Workers surveyed by the Kensington Technology Group (1998) listed eyestrain as a
leading cause of physical stress in their workplace.
*According to a 1997 study sponsored by the American Society of Interior Designers
(ASID), 68 percent of all office workers were concerned about their lighting. Office
workers consistently rated poor lighting as the first or second concern that needed to
be addressed. In addition, they rated the physical workplace third, after compensation
and benefits, in the list of factors that influenced whether they accepted or left a
position.
*The Steelcase Worldwide Office Environment Index conducted by Louis Harris and
Associates (1991) found that 64 percent of computer users listed eyestrain as the
number one health hazard in the workplace.
These aren’t just isolated complaints. The experiences of these people reflect genuine
patterns of user discomfort and dissatisfaction that translate in to the potential for
substantially reduced productivity. Historical studies reinforce this strong relationship
between light quality and productivity.
Measurable differences
The Cornell University Study (1989-1990) of a Xerox facility in upstate New York
found that 24 percent of the workers in a poorly lit environment reported a loss of
work time due to vision problems and discomfort. In most cases, the time lost was
well over fifteen minutes per day – a two percent productivity loss per individual, per
year. To help measure this, it would be equivalent to giving everyone in an
organization an extra week of paid vacation per year.
The Reno Post Office Study (1986) suggests that quality lighting is more than a
luxury. When the Reno Post Office set out to trim their energy costs, they saved on
electricity and also realized an unexpected benefit of a sustained six percent increase in
worker productivity. This increase was enough to recover the cost of the new lighting
system in less than a year.
The physiological effects
While the physical impact of lighting is obvious, its physiological and psychological
impact can be just as strong. Light sends a visual message which can affect mood and
motivational levels. Light also affects our biological clocks in the following manner.
It is well known that circadian rhythms, such as sleeping or waking cycles, are
influenced by light. Many business travelers use melatonin in tablet form to help them
maintain their work efficiency and performance when they travel to locations in
different time zones. What many people don’t realize is that simply increasing their
exposure to light could also help them naturally alter their melatonin levels.
In addition, researches suspect that Seasonal Affective Disorder (SAD) is associated with
a disruption in circadian rhythms. Though researchers don’t know exactly why, light
therapy appears to relieve the depression and lack of energy associated with SAD.
HPS and the Factory & Warehouse Environment
How does high-pressure sodium lighting stand up in today’s factory and warehouse
environment? Are there any drawbacks to this lighting system that we are unaware of?
The purpose of this “knowledge paper” is to address these questions in regards to HPS.
There is no doubt that high-pressure sodium lighting is one of the most energy efficient
systems in warehouses today. However, are we sacrificing effectiveness for the cost of
efficiency?
In order to address these issues, we need to first understand the environment in which
work is being done. Today’s modern factories and warehouses can be considered a
scotopic environment. To put it simply, it’s dark with very minimal natural “daylight”
present. In this type of environment, it is the “rods” of the eye that play a prevalent
role in our ability to see. Rods are responsible for what is commonly termed, night
vision. The drawback, however is that these “rods” are very sensitive to the blue region
of the spectrum and for the most part are responsible for our peripheral vision.
In the factory and warehouse environment where HPS is used, the light spectrum
becomes shifted to the yellow region. This has a negative effect on the “rods” of the
human eye. Since the “rods” pick up primarily the blue spectrum, a “tunnel vision”
effect (seeing primarily straight ahead with very little peripheral imaging) results.
Also, HPS’s blue deficiency appears to cause problems with workers task visibility; they
can’t detect detail nor differentiate color under HPS light. This has tremendous
repercussions for the warehouse environment where productivity is based on the ability
to locate product, which is often stacked and tagged by color code; it impacts the
pick/pack process where detail is needed to ascertain that the correct product is sent to
the correct customer, and labels are clearly marked and read. The safety of the
environment comes into question as it pertains to hi-lo drivers and their ability to
safely circumnavigate the factory and warehouse with the loss of peripheral vision.
The presentation of the facility is also impacted when the strongest proponent for
selling a client on the capabilities of today’s third party warehouse is the facility itself.
Giving a tour of a “cave-like” environment could have severe results on attracting and
retaining both customers and employees alike.
Clearly, HPS is a cost efficient light source. However, based on the aforementioned
issues, cost is merely being measured by one parameter. Productivity, facility
presentation, and safety issues also need to be addressed. In a typical business
expenditure breakdown, labor accounts for 85 percent of the operating costs while
lighting accounts for only 1 percent. A productivity increase of even 1 percent can
offer savings in excess of an entire electricity bill. There is certainly a need in today’s
factory and warehousing environment to balance efficiencies with effectiveness. The
question that needs to be answered – Does HPS achieve this balance?
We believe the answer to be NO. With today’s technological advances in lighting,
there are better solutions available for the warehouse and factory worker. At Light
Corporation we believe in enlightening workspaces; not just in the front office, but
everywhere people work.
Reassessing the Value
Of the High–Pressure Sodium Lamp
High-pressure sodium (HPS) lamps, the ubiquitous golden-white light source, are the
darlings of public works engineers, who have installed them in more than 90 percent
of all roadway lighting fixtures in the United States. You can find them in the
decorative acorn fixtures in almost every redeveloped downtown are as well as in
warehouses, industrial work spaces, building and monument floodlighting, and airport
terminals. They are everywhere.
From the moment they were introduced in the 1960’s, HPS lamps became the light
source of choice for applications requiring high intensity discharge lighting. Heavily
advertised as the replacement for mercury vapor in applications where color quality
was apparently of no concern, HPS lamps quickly became recognized as the most
energy efficient, long-life light source available.
During the energy crisis of the 1970’s, HPS’s popularity soared. The technology was
rapidly applied – and misapplied – across the nation. In the mid-1970’s, newspaper
reports began to appear linking the HPS lamps installed in grade-school classrooms to
save energy with instances of headache and nausea in children. While there was no
explanation for the phenomenon, the specialty fluorescent lamp industry took
advantage of it to promote “full-spectrum” lighting. Many HPS installations were
changed back to fluorescent systems, and the children’s symptoms disappeared.
In the 1980’s similar problems showed up in industrial applications as mercury vapor
or fluorescent systems were changed to HPS. In the one case, a major automobile
manufacturer’s workers complained that under HPS light they had trouble reading fine
print and doing detail work; in spaces with mercury vapor light, they said, even at
lower levels they could see well enough to perform the same tasks easily. In another
case, a manufacturer had to remove an HPS retrofit and replace it with a high-output
fluorescent system to prevent a union walkout after workers complained of nausea and
disorientation.
What’s Wrong with HPS?
Much of the explanation rests in the human eye’s response to the visible spectrum. In
photopic vision, commonly called day vision, where there is plenty of light, a great
deal of information is provided to the cones of the eye. Cones are concentrated around
the fovea, or focal point of the retina, and in normally sighted individuals they are
sensitive to a full range of colors and send visual signals to the brain at high
resolution. At high illumination levels such as those during the day, the eyes depend
primarily on the cones to perform detail-oriented tasks like reading. The cones’ peak
response comes at 555nanometers (nm) where the light is yellow. This is perfect for
sodium sources, whose average peak output is at approximately 580 nm.
In the dark, the cones are almost useless, and most visual information is sensed by the
rods. The rods, which are located everywhere around the retina except for the fovea,
are responsible mostly for peripheral vision. Rods provide low-resolution black-andwhite
images but are much more sensitive to light than cones and are especially good
at detecting motion. The rods product scotopic vision, or night vision. But rods are
sensitive to different wavelengths than cones. Their peak sensitivity occurs at 507 nm,
well into the blue region of the spectrum. The change in peak sensitivity form the
cones’ yellow to the rods’ blue is called the Purkinje shift. Rods do not respond very
well to yellow light. Whereas the HPS peak yellow light excites cones to around 90
percent of their peak output, it excites the rods to only about 10 percent of their peak
output. It has long been known that lower lighting levels cause an increasing reliance
on rods and, therefore, change the effective spectral response of the vision system.
People always sensed that there was a problem with HPS light. And it was easy to
blame the color, which HPS aficionados call golden or gaslight, although HPS is really
a very poor color-rendering source. But the problems weren’t just with bad color.
The first clues were revealed in the work of Dr. Sam Berman and his colleagues at
Lawrence Berkeley National Laboratory (LBNL) in Berkeley, California. Berman
reported improved visibility at low lighting levels when sources rich in blue light
were used. Among the findings Berman reported throughout the 1990's was an
exaggerated dilation of the pupil for a given light level when a blue-poor source was
used. In “Bluer Light, Better Sight” (RECORD, February 1991, page 83), it was reported
that Berman had proposed a correction scale to help account for the improved visibility
caused by these bluer light sources.
On this scale, HPS compared poorly to other sources. Later, a demonstration booth
built by LBNL allowed people to experience the phenomenon of scotopically enhanced
light. It showed that human visions could actually be improved by adding bluer light
to the spectrum even at indoor task-light levels. In other words, the rods could be
made to respond to light and contribute to visual acuity at much higher levels than
anyone had previously imagined.
More evidence to support these findings has been found in the years since Berman’s
experiment. In 1995, Dr. Mark Rea and his colleagues at the Lighting Research Center
(LRC) conducted experiments to determine whether the differences in visual response
between metal halide outdoor lighting and HPS could be measured. In a paper with
dramatic implications, the LRC reported that peripheral vision was 50 percent better
with metal halide than with HPS, when both were at equal, ordinary, parking-lotlighting
levels. Since visibility tends to change only a small amount for relatively
large changes in illumination, the finding that such a high increase in visibility could
be achieved simply by changing light sources was extraordinary.
There is an unmistakable correlation between LBNL’s work and the LRC’s findings.
Spectrum does make a difference in human vision. The blue portion of the spectrum,
which is abundant in sun-, moon, and starlight, is needed for the proper function of the
human eye, and it appears that its importance to a person’s visions increases as light
levels decrease. Blue-deficient light sources like HPS do not provide the same amount
of visual stimulation as sources that produce spectra rich in blue.
The impact of this revelation is profound. The lumen, the basic measurement of light
quantity used in all lighting calculations, is based on the photopic, or day-vision,
curve. But at lower light levels, the color of the light has a greater influence on
vision than the quantity of light. Therefore, everything based on lumens becomes
questionable or incomplete, starting with typical indoor light levels and decreasing to
almost total darkness. Footcandles and lux are suddenly no longer complete
measures of light as it relates to human vision. Neither are other common
measurements, such as luminance or brightness. And if a metal-halide source
stimulates the eyes better at low light levels than HPS, even source efficacy, the
energy efficiency of a light source as measured in lumens per watt, becomes invalid.
This suggests that there is a need to revisit the entire scientific foundation upon which
we base lighting standards, design, and measurement in the mesopic (twilight vision)
and scotopic regions. Berman, Dr. Alan Lewis of the Michigan College of Optometry, in
Big Rapids, Michigan, and others have developed proposed scotopically corrected
factors for various light sources. Not surprisingly, sodium-based sources are inferior to
most other sources.
Because of the eye’s poor rod and peripheral-vision response to HPS, this type of
lighting may be unacceptable in many of the places it is traditionally used. It certainly
explains what has prompted those who operate parking lots and garages in the retail
and gaming industries to replace their HPS with metal halide. When people can see
better, they feel safer. One can only wonder what the implications of vastly improving
the peripheral vision of people who drive at night might be.
At higher levels, HPS’s blue deficiency appears to cause problems with task visibility.
Workers can’t see detail because they have trouble focusing on the task. This can be
explained by using a simple analogy comparing the way the pupil works to the
mechanics of a camera lens. Berman showed that the human eye’s pupil dilates much
more when it is viewing an object under HPS light than it does when viewing surfaces
under bluer sources. This is true even though the scene illumination is identical. The
pupil is acting like the aperture of a camera lens – the wider the lens aperture, the
shallower the zone of sharp focus. It can be assumed, then, that using HPS to light fine
work is a questionable practice.
But neither the night-vision deficiency not the focusing difficulties of HPS explain the
instances of illness in schools and industrial facilities. Those were caused by the
flickering of the light source. In each of these cases, HPS luminaires were used in
relatively small spaces – small enough for all the lighting to be powered from one
branch circuit. High-pressure sodium lamps exhibit the greatest flicker of any normal
light source, and all of these lunimaires were on the same phase, flickering in sync.
The result was that the environment took on a stroboscopic effect, which can cause
people to become ill.
Based on the evidence to date, there are literally millions of locations lit with HPS that
would be better served by other light sources, or at least by the addition of task
lighting. In many instances where HPS has been used, designers should consider other
options: metal halide, fluorescent, compact fluorescent, electrodeless fluorescent,
and even sulfur lamp are all energy-efficient alternatives that offer long life.
Low-pressure sodium (LPS) lamps, which produce bright yellow light like the
incandescent A-lamps that are used to repel insects, have their own problems. They
are considered to be a monochromatic source because they produce no color except
yellow, and compared to HPS, they are far worse for night vision. But LPS is
aggressively supported by astronomers and has become mandatory in some American
cities and counties because its rays don’t interfere with viewing the sky through
telescopes. Limits on unshielded light sources, hours of operation, and even lighting
levels all make sense in meeting astronomers’ needs to minimize light pollution, but
requiring the use of a light source that limits visibility and peripheral vision in a way
that significantly affects safety and security is no longer justified.
How did HPS gain such wide acceptance?
Many will wonder why lighting designers and manufacturers didn’t perceive that there
were problems with HPS long ago. At the time it was introduced, HPS was a real
improvement over mercury vapor in lamp life, efficiency, and cost, and there were not
practical alternatives. HPS entered the mainstream very quickly. It is also important
to consider that one cannot easily “see” the problems discussed here. On the other
hand, if lighting research had been better funded when HPS was gaining in popularity,
perhaps knowledge of its fatal flaws would have spurred manufacturers to develop
alternative sources.
There are still many challenges ahead for those involved in lighting research – it is a
serious problem that basic lighting science doesn’t address how the eye sees light at
low levels. And there is a great need for establishing a quantity that could replace the
lumen, that could be used to make calculations for applications where mesopic and
scotopic vision are in use. And on a practical level, perhaps there is a need for a
popular movement that would send HPS the way of mercury vapor.
The Effect of HPS Light
On Performance of a Multiple Refocus Task
Hypotheses are presented which describe some possible effects of narrow bandwidth
light on the oculomotor functions. The hypotheses are tested by using a simulated
clerical task in an office environment. When compared to CW fluorescent light (or
“White Light”), HPS light is found to reduce many subjects’ abilities to perform the task.
Background
Narrow bandwidth light has long been known to allow better visual acuity than does
white light. Based on this fact, it might seem that high-pressure sodium (HPS) would be
a good choice of light source in cases where visual performance is critical and where
color rendering is not important. Consider, however, that while the perceptual
function of acuity may be improved as the bandwidth of the illumination is reduced,
certain oculomotor functions, also important to visual performance, may be impaired.
This increase in acuity is related to chromatic aberration of the eye. The refractive
media of the eye bend short wavelength light more than they do long wavelength
light. Thus, when illuminated by white light, a point object results in a relatively
blurred retinal image and it seems reasonable that narrow bandwidth light would
reduce this blur, thereby improving acuity.
Consider, however, a coincident effect. The eye has a given range of refractive power
over which it can be adjusted to bring objects into focus (accommodation). With
monochromatic light, yellow, for example, this range of refractive power sets a limit
to the range of viewing distance. With white light, however, added refractive power
for the blue component and reduced refractive power for the red component might
allow objects to be focused for closer and farther distances respectively, provided that
the observer focuses for different colors of light at different viewing distances.
Millodot has measured the wavelength of light for which young observers, wearing
their normal corrective lenses, focus as a function of viewing distance. At about one
meter, most observers focused for yellow light. At further distances, most observers
focused for red; and at near distances, such as reading distances, most observers
focused for blue. Thus, it seems possible that reduced bandwidth light may reduce the
range of viewing distance for some observers, especially for older people who have
less flexible lenses (presbyopia).
Convergence of the eyes upon an object to produce singular binocular vision is closely
linked to accommodation. Fincham and Walton have shown that accommodation and
convergence are coupled such that a change in one of these oculomotor movements
results in a proportional change in the other (normal A/C relationship). The
convergence required for a given viewing distance is independent of wavelength light.
It is a simple geometric relationship. From this convergence angle, a particular and
reproducible amount of accommodation will automatically result due to the A/C
relationship. Consider, however, the possible results of narrow bandwidth light. For
example, if a normal observer viewed an object at reading distance, his eyes would be
convergent for that distance and would probably be accommodated to blue light. If
the blue component of the light were missing, more accommodation would be required
of the observer. This added amount of accommodation might cause further
convergence thereby tending to de-fuse the singular binocular image (abnormal A/C
relationship). A possible conflict between accommodation and convergence may result
and this must be absorbed by the A/C coupling tolerance (fusional reserves) of the
observer. Wyszecki has shown that changes in the color of light account for the
equivalent of about one diopter change in accommodation. It has yet to be seen
whether or not narrow bandwidth light can cause a sufficiently abnormal A/C
relationship to be disturbing to observers.
One of the most interesting properties of accommodation is the means by which it is
stimulated. It seems possible that for a change in viewing distance, the change in
convergence required to form a single image would induce the necessary
accommodation change through the normal A/C relationship. It also seems possible
that because an out-of-focus image is blurred, a trial-and-error process of
accommodation could result in a clear image. Apparently, other processes are also
employed because, as Fincham has shown, many people cannot accommodate properly
under monochromatic light. Campbell and Westheimer have verified the phenomenon.
The hypothesis of Fincham is based on chromatic aberration. If an object were in front
of the accommodation distance, the retinal image would be surrounded by a red fringe.
If the object were located beyond the accommodation distance, the retinal image would
be surrounded by a blue fringe. Fincham concluded that the stimulus for
accommodation arose from this colored fringe. Of the observers who could
accommodate properly in monochromatic light, Fincham found a certain degree of
spherical aberrations (astigmatism, a condition in which the refractive media are
slightly cylindrical rather than spherical). He also found that when observers who
could not accommodate under monochromatic light were provided with slightly
cylindrical lenses, many learned to accommodate properly. This suggests that
spherical aberrations may also result in useful information for accommodation. Other
stimuli for accommodation are known, but discussion of these will not be necessary in
the development of this thesis. It is sufficient to recognize the possible role of white
light and chromatic aberration in the normal accommodation process.
Preliminary studies
Several brief preliminary studies were used to evolve a testing procedure that would
facilitate detailed study of the hypothesized effects of HPS light on visual performance.
The first of these studies was designed to test the hypothesized abnormal A/C
relationship. A finely spaced pattern of black printed dots on white paper (Zip-a-Tone)
was presented to observers who were asked to judge subjectively their “ability to count
the dots” and to note their impression of “clarity” of the pattern. This pattern was such
that when seen at distances beyond normal reading distance it appeared to be a
homogeneous grey tone rather than a pattern of dots. Typical viewing distances were
less than ten inches. The use of this distance was thought to insure the need for a blue
component in the illumination. Observers examined the dots under 40fc of HPS light
and under the same light with 1 fc of supplemental blue light added. Thus, one
illuminant appeared yellowish and the other appeared whiter in comparison.
Although some subjects did report that under HPS light the pattern seemed to “fuse”
and that the dots seemed to “jump around” more often, it was decided that there was
not sufficient preliminary evidence of an effect to pursue this kind of experiment.
At this point, to better address real-world situation, it was decided to use visual tasks
that approximate those found in office building, such as typical printed reading
material.
The second preliminary study involved subjective impressions of “ease of reading” and
was designed to test the hypothesized effect of an increased requirement for refractive
power while reading at close distances under red light. Pages from a telephone book
were located in equal luminance light booths, one illuminated with red light and one
illuminated with blue light from typical colored PAR lamps. Observers were asked to
compare the booths in terms of “ease of reading.” Many subjects, wearing their usual
corrective lenses, reported more difficulty when reading by the red light. For most of
these people, positive 0.8 diopter lenses were found to reverse the choice of light that
allowed “easiest reading.” This was taken as confirmation of the Millodot results and
of the Wyszecki data on chromatic aberration.
The same reading task was then set up in two different light booths, one 70-fc HPS
booth and one 70-fc cool-white fluorescent (CW) booth. Care was taken to avoid veiling
reflections. Because HPS light is relatively weak in blue components, it was thought
that this comparison would yield similar but less obvious results than the red-blue
comparison. The results were in fact contradictory. Most people preferred to read
under HPS light, and lenses did not reverse the preference. The paradox seemed
satisfactorily explained by the comments of many observers who implied that the
yellowish color of the paper under HPS light was more “soothing to look at” than the
bluish-white color of the paper under CW light. It was concluded from this test that the
hypothesis of increased refractive power could not be easily studied by comparing HPS
To explore the hypothesis suggesting reduced ability to quickly change accommodation
with bandwidth spectrum light, the light booths described above were slightly
modified. Two telephone book samples were located in the HPS booth and two samples
in the CW booth. In each booth, one sample was located at normal reading distance
and the other sample was located about 20 inches further from the observer. In each
booth, observers were required to read numbers sequentially from the near task and
from the far task. Results showed that the ability of many observers to change
accommodation quickly and repeatedly was the most affected visual function of those
tested in the preliminary studies. Subjects required a measurably longer time to read a
given set of numbers in the HPS booth than in the CW booth.
Continued preliminary experiments with this effect resulted in the final study procedure
which was intended to simulate a typical clerical task in an office environment.
Apparatus
The task adopted was based on the visually similar “a” and “s” in pica type. The “a”s
and “s”s were systematically grouped into 36 five-letter nonsense words. From these
36 words, six test sheets were prepared, each sheet having the same words but
arranged in a different random order. The words for each sheet were typed with a
clean mylar ribbon onto non-glossy rag bond typing paper. Thus, the experimenter
had three sets of two sheets (three near tasks and three far tasks), each sheet having 36
words arranged in a single column.
In order to avoid warm-up difficulties, the HPS luminaires were provided with
mechanical dimmers in a way that allowed the lamps to be left on for long periods
with essentially no light output and without overheating. In the fully dimmed setting,
light leaks accounted for less than 1fc of HPS illumination on the work surface. This
was considered negligible. The CW lighting system was electrically switched on and
off as needed.
The light distribution and the work station locations were adjusted such that 50 fc of
diffused in direct illumination was provided on both test sheets from each lighting
system in a way which was not affected by body shadows. Illumination measurements
were made using a cosine-and color-corrected light meter as defined by the CIE system
of photometry.
During the experiment, subjects sat at the desk directly in front of the left-hand sheet
(near task). The right-hand sheet (far task) was vertically aligned with the left by
using a T-square. For each subject the far task was located at the maximum horizontal
distance which would allow the subject to distinguish “a”s and “s”s without errors.
Thus, a unique separation distance was found for each subject and was used
throughout the testing of that subject. Subjects did not lean over toward the far task
in order to see it more clearly, but were required to turn their heads and refocus for
farther distance.
Each subject was instructed to count to himself and read aloud the number of “a”s in
the top left-hand word, then turn his head and read aloud the number of “a”s in the top
right-hand word, then turn his head back to the left-hand word, then turn his head
back to the left-hand sheet and read the number of “a”s in the second row from the top,
and so on until the number of “a”s in each of the 72 words had been said aloud. Thus,
in this sequential refocusing task, the subject changed accommodation and
convergence 72 times during each test.
Because subjects in the preliminary tests found it difficult to keep their places in the
test sheets, the words were arranged in groups of three. The larger spaces between
the groups allowed a T-square to be used as an orientation aid. Although the T-square
introduced an additional motor function, subjects quickly learned to use it.
The same three sets of test sheets were used in the same order for every subject. One
set was for the learning procedure in which each subject performed a complete
“rehearsal” test. The other two sets were used for collecting performance data.
Further preliminary studies using this method showed that subjects were repeatedly
able to rate certain kinds of subjective impressions concerning the way in which the
two lighting conditions affected their ability to work the test. These subjective
variables were used to develop a follow-up test, in the form of semantic differential
scaling, to be taken after the performance tests under both lighting conditions were
completed. These procedures were based on the methods of Flynn, et al.
After working two tests under each lighting condition, the subject was immediately
given the subjective rating from. Regardless of the order in which the lighting sources
were used (that is, HPS first then CW, or CW then HPS), the subject was instructed to
consider the first lighting condition as “neutral” on the rating form and then to compare
the second lighting condition to the first lighting condition; that is, “rate the second
lighting condition in terms of the first lighting condition.”
Analysis
Raw data for each subject consisted of:
- The time required and the number of errors made on each of two tests under
the CW lighting condition.
- The time required and the number of errors made on each of two tests under
the HPS lighting condition.
- Comparative subjective ratings of the second lighting condition in terms of
the first lighting condition.
From this data a measure of performance for a test was defined as follows:
Performance
Number of words perceived correctly
=
Time required to perceive all words
= Correct words perceived per second
This simple metric seemed appropriate because of its limited scope. It was only
required to confirm or nullify the cumulative hypothesis that HPS light might reduce a
person’s ability to perform a complex oculomotor task.
The only meaning attributed to the performance scores was derived from comparisons
of a subject’s performance under HPS light to the same subject’s performance under CW
light. Because of the design of the experiment, the only variable which was thought to
influence a subject’s performance was the SPD of the light.
The following method was used to evaluate the change in performance of a subject
due to the lighting. The scores of the subject for the two tests under each light were
first averaged:
P(CW test 2) + P(CW test3)
2 = P(CW average)
P(HPS test 2) + P(HPS test3)
2 = P(HPS average)
P(CW average) was then used as a base performance condition so that a relative change
in the subject’s performance under HPS light could be expressed in terms of the
subject’s performance under CW light.
percentage of change
in performance due to =
HPS light
% DeltaP = P(HPS average) – P (CW average)
P(CW average)
Thus, a subject who worked better under HPS light than he did under CW light would
show a positive % DeltaP, indicating the percentage of improvement in his performance
due to the change from CW light to HPS light. Conversely, a subject who did not work
as well under HPS light as he did under CW light, would show a negative % DeltaP.
It seems possible that subjects could improve their performance scores on successive
tests by learning perceptual “short-cuts” or by having become more practiced at the
oculomotor movements. In order to cancel the effect of learning, half of the subjects
worked first with CW and half of the subjects worked first with HPS. Regardless of the
order of the lighting conditions, % DeltaP was computed in the same manner.
The subjective rating data were also restructured by the author so that impressions of
HPS light could be compared to impressions of CW light as a base condition. This was
trivial for the half of the subjects who began the test sequence with CW light because
these subjects were instructed to consider the first lighting condition as “neutral” and to
rate the second lighting condition (HPS) in terms of the first. For the other half of the
subjects who began with the HPS lighting condition and who were also instructed to
consider the first lighting condition as “neutral”, the ratings were inverted by the
author so that the CW impressions became the base condition for purposes of analysis.
For example, if a subject reported CW to be more “clear” than HPS, that report was
taken as being equivalent to a report of HPS being more “hazy” than CW. The
magnitudes of the differential ratings were not changed. Because CW impressions were
defined as neutral, CW impressions were, by definition, center on the scales. HPS
impressions were tabulated to the right or to the left of the CW impressions.
A group of 24 subjects who were known not to have any previous knowledge of the test
procedure or of the test hypotheses were selected without regard to age, sex, or visual
defects. The average age was 30; 12 subjects were under 25; 8 were over 40. All
subjects wore their normal corrective lenses. No clues were given to the subjects
concerning the test hypotheses. Mean performance under HPS light was 4.1 percent
less than the mean performance under CW light (% DeltaP = 4.1). This result
correlates with impressions of HPS as being relatively “hazy” and causing relatively
more “trouble with focus” than CW light. It appears that HPS light does affect visual
performance in this kind of test and the cumulative hypothesis that HPS light affects the
oculomotor functions may be accepted at the 99 percent level of confidence (using the
F-test).
It is interesting to note that the HPS condition was rated as slightly dimmer than the CW
condition. This phenomenon of varying subjective brightness as a function of SPD, at
constant illuminance, is relatively well-known and has been discussed recently by
Corth.
A learning effect can be identified in the data. The twelve subjects who began the test
sequence with CW showed a % DeltaP of only –0.3 percent whereas the twelve subjects
who began the test sequence with HPS showed a % DeltaP of –7.9 percent. If it is
assumed that the average subject improved 3.8 percent due to learning while being
tested under the first lighting condition, then certain reasonable conclusions follow.
First, when corrected for learning, the % DeltaP of both twelve-subject groups becomes
–4.1percent which agrees with the average for all subjects together. Second, the
standard deviation of scores is decreased when 3.8 percent learning is accounted for.
An adjustment for learning is not necessary to confirm the test hypothesis. It does,
however, alter the distribution of % DeltaP scores in an interesting way. It appears
that subjects fall rather neatly into three groups: 1) those who showed a moderate
improvement due to HPS light; 2) those who were only marginally affected (possibly
not affected); and 3) those who showed an extreme reduction in performance due to HPS
light. Further studies were made in order to understand this grouping.
Further Studies
The 24 subjects were recalled and their resting points of accommodation found by
using a laser optometer. Readers interested in the principles of laser optometry are
referred to the work of Leibowitz and Owens. The resting point of accommodation is
the distance for which an observer accommodates when the lense and related muscles
are in their natural, or tonus, condition (resting focus). It seems probable that the
resting focus is the distance for which an observer will focus with yellow light. This
assumption follows for two reasons. First, because if yellow were used at the resting
focus, then chromatic aberration could best be utilized to allow a maximum range of
viewing distance. Second, because measurements by Leibowitz and Owens, on a
similar subject population as that used by Millodot, showed that most of the subjects,
while wearing their normal corrective lenses, had a resting focus of about one meter.
Millodot’s subjects accommodated for yellow light at this distance.
If follows that a subject with a resting focus beyond the distance of his far task in the
experiment, is farsighted (hyperopic) for the test and this subject is hypothesized to need
the blue component in order to maintain a normal A/C relationship. It is also
hypothesized that an extreme hyperope, especially when presbyopic, may not have a
sufficient range of viewing distance to focus for the near task under HPS light.
No correlation between hyperopes and subjects who showed a reduction in performance
or indicated “trouble with focus” due to HPS light was found. No correlation between
task separation distance or age (indicators of presbyopia) and reduced performance or
“trouble with focus” was found. These findings agree with the conclusions of the
preliminary studies.
In order to further test the remaining hypothesis, that visual difficulty may be
encountered due to reduced stimulus for accommodation, another experiment was
conducted based on the following reasoning. If the narrow bandwidth of HPS light is
the cause of insufficient chromatic blur fringe to stimulate fast, accurate
accommodation, then if a third light having a bandwidth narrower than HPS light is
compared to HPS light, subjects should indicate this third light causes more difficulties
than does HPS light. That is, as bandwidth becomes narrower, the difficulties should
become more pronounced.
For this experiment the third lighting condition was generated by filtering HPS light.
This third light source, hereafter called FIL, differs from HPS only in the strength of the
far blue components. Mechanical dimmers allowed the setting of equivalent
illuminance levels (50 fc) for both lighting conditions. The test procedure was
identical to the first experiment. For analysis, HPS was considered the neutral or base
condition to which FIL light was compared. Every subject reported subjective
difficulties with FIL light and all but one subject showed reduced relative performance
due to FIL light as compared to HPS light. This trend was taken as confirmation of the
hypothesis and testing was stopped. As previously found, there was no correlation
between rest foci data and reduced relative performance data or “trouble with focus”
data.
Summary and conclusions
Three hypotheses were explored for the purpose of providing information that would be
useful to illuminating engineers when evaluating the use of HPS light in office
situations. It was found that many people did not perform as well in a multiple
refocus task under HPS light as they did under CW light. This finding was independent
of age and refractive state.
An abnormal A/C relationship or reduced range of accommodation due to HPS light
was not found. Thus, it does not seem likely that building users, who complain of eye
troubles due HPS light, will be aided by the use of simple corrective lenses.
A reduced stimulus for accommodation was hypothesized based on previous research by
Fincham and was confirmed in preliminary studies, the main experiment, and in a
follow-up experiment. Thus, it appears that a weak blue spectral component in HPS
light is causing reduced relative performance and impressions of “trouble with focus”.
In the author’s opinion, a very small amount of blue light, when added to the HPS light
would rectify the difficulty. The HPS spectrum seems to be only marginally inadequate
in its ability to provide sufficient stimulus for accommodation. The possibility that
subjects would learn an alternative method of accommodating over a period of time
under HPS light cannot be ignored. It also seems possible that slightly cylindrical
lenses could be used to provide an alternate stimulus of accommodation in some
situations. Investigation of these possible corrective effects was beyond the scope of
this study.
Three distinct degrees of effect were identified in the performance data. It was not
possible, however, to correlate the degree of effect with other characteristics of the
subjects. Further research is called for in order to devise standards of SPD and in
order to make recommendations for individuals who are unusually affected by HPS
light.
References
Bard, Donna, “Lighting Design for Low Light Levels,” Electrical Contractor, May 2000
Benya, James Robert, “Reassessing the Value of the High-Pressure Sodium Lamp,”
Architectual Record, May 1998.
Piper, H.A., “The Effect of HPS Light on Performance of a Multiple Refocus Task,”
Lighting Design and Application, February 1981.
“Seeing the Difference: The Importance of Quality Lighting in the Workplace.”
Steelcase Inc, 1999.
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