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.