Effects of Lighting on Work Function
Subjects find parabolic more bothersome than lensed
June 1990 – One Year Later
The researchers returned to verify their findings after the groups had each had a further year of acclimatization to the two systems. They wanted to earn whether satisfaction levels or the frequency of visual health problems had changed, and to see what actual effects the visual health problems created. The second survey brought answers from 121 workers (82% return).
Work-related Health Complaints
Problems under parabolic cut into worker productivity
Almost half the parabolics in the whole office area now
modified or disconnected
Indirect group still strongly prefers lensed indirect,
parabolic group now prefers indirect by equal margin
Effects of Lighting on Work Function
Lighting problems now tied to lost work
Some Questions Raised by the Cornell Study
Any study which produces such important and potentially influential findings will immediately raise questions. The more concerned, interested and informed the questioner, the more specific the questions become. This is a sampling of some of the most commonly asked questions about the Cornell Study.
Were there any differences between the two groups that might have influenced the results?
The respondents in both the 1989 and the 1990 survey showed no significant differences in demographics or work profile between the parabolic and the lensed indirect group. All subjects spent a large portion of their working hours at the computer.
Generally, the subjects were highly-skilled workers whose tasks would better be defined as “creative” rather than “routine.” They were involved with graphics, language translation, technical documentation and software development. Many were working with two or three computers at the same station.
Their offices were similar and they were divided up into approximately the same balance between open-plan cubicles and enclosed offices. Almost everything other than the lighting was consistent between the two groups: office temperature, ventilation, noise level, chair comfort, desk space, storage space, privacy, furniture, wall and partition color and so forth.
Was the test structure biased in favor of either system?
The researchers began with certain hypotheses based on earlier laboratory studies. The study report states on page 11, “It was hypothesized a priori that the lensed indirect system would create a more favorable luminous environment, i.e., higher satisfaction, fewer glare problems, better visual health, than that produced by the parabolic system, and consequently one-tailed tests were used for all analyses of questions concerning the effects of the different lighting systems. Two-tailed tests were used wherever analyses were performed in the absence of a priori directional hypotheses.”
The survey questions were specifically structured to be neutral. In order to provide the most useful comparative data, the Cornell research team maintained contact with members of other research groups conducting work on office lighting. A number of the questions were identical to those used in survey questionnaires from other recent lighting studies.
One factor made the test something less than a strictly fair comparison. Since this was a real-world study, the workers were free to behave towards their lighting as they would have if no study had been conducted. Almost from the beginning, the workers under the parabolics requested that a high percentage of the fixtures be modified. BY 1990, 6% of the 164 parabolic office fixtures were disconnected entirely and an additional 42% had had one or two fluorescent lamps disconnected.
In the small offices, 76% of the parabolics were modified.
Does the cost differential between the two systems modify the impact of the findings?
From the beginning of the test, the researchers were well aware that the lensed indirect system cost more than the parabolic. Therefore, study results which favored the lensed indirect system over the parabolic wouldn’t be meaningful unless they also proved a superiority sufficient to offset the added cost. In this particular installation (a building with no unusual installation problems for either type of fixture), the cost differential for installed fixtures was between $100 and $200 per workstation, depending on the methods used to calculate the cost. The 1990 study revealed that the parabolic workers were losing substantial amounts of productive work time each day due to lighting-related health problems. The following three examples give an indication of the financial impact of that lost time, assuming a conservative installed cost differential of $200 per workstation and an average work loss differential of ten minutes per day.
Economic Impact: Three Scenarios
(Based on 25% salary burden and 8-hour work day)
Did the “Hawthorne Effect” influence the results?
Since the findings of the study were based on subjective criteria, the researcher went to considerable length to avoid having the results jeopardized by this much-discussed phenomenon. Earlier studies suggest that workers respond positively to any change in their work environment and the involvement of a researcher.
In this study, the office lighting was replaced as part of a scheduled renovation, and the evaluation questionnaire covered many aspects of the office environment, not just lighting. The researchers were careful to disguise the fact that the study was primarily about lighting, and the 28-question, 169-item survey was presented as an “evaluative study of the three important factors in office design.”
Both halves of the building received lighting systems which claim to support computer use considerably better than the previous lighting. Therefore, any influence from the Hawthorne Effect should have been minimal and should have been equal for both study groups.
Why weren’t workers shifted during the test so they could experience working under both systems?
This was a real-world test, not a controlled laboratory test. A large-scale movement of employees from one system to the other would have added an unrealistic element to the test. It also would have made it apparent to the subjects that a comparison between the two lighting systems was the true focus of the test.
In fact, because this was a phased renovation, some parabolic workers moved in the indirect area before the parabolic area was completed, which may have had some influence on overall preference figures. Within each group, some changed offices and some didn’t. Whether workers moved offices within the group or not was examined statistically and shown to have no significant effect on their opinions about lighting.
Are the results specific to the particular luminaires used in the study?
In the case of the parabolic system, the answer is “no” with minor qualifications. Regardless of the sophistication of its design, direct parabolic downlighting provides a cone of light from an otherwise dark ceiling. Direct lighting creates direct shadows, contrast between the fixture and the ceiling and an environment that is more brightly lit within the cone of light than outside it. The answer is less clear-cut in the case of the indirect system. There is great variation in the ability of indirect lighting systems to distribute light across a wide area. An indirect system like the systems used in the study will provide a superior luminous environment if it can produce even luminance ratios across the entire ceiling. However, one with poor distribution will produce hot spots on the ceiling bright enough to cause contrast and shadow problems similar to those created by downlights. Substituting a lesser system for the one used in the study could have created considerably different results.
Why did the study use indirect luminaires with visible lenses rather than totally indirect fixtures?
The researchers felt it was important to compare the best indirect lighting then available with the best parabolic lighting then commonly available in order to provide as fair a comparison as possible. In the open office areas, the low-brightness visible lenses are claimed by the manufacturer to provide a higher satisfaction level because they provide a higher level of perceived illumination, a contention supported by earlier studies (Bernecker, Penn State University, 1984 and 1986). In the enclosed offices, the lensed indirect fixture provides a superior luminous environment to other commercially available indirect luminaires because the lens provides an extremely widespread distribution designed especially for enclosed areas.
Why were self-reported evaluations of productivity used instead of objective measurements?
At the beginning of the study, representatives from Xerox made it clear that, in their opinion, self-reported evaluation would be the most useful information these employees could provide. The creative nature of the high-salaried job functions created broad variety in the amount and quality of job output. Quality of product meant far more than the number of data bits entered. An attempt to quantify individual output would have been somewhat meaning less in any event, because much of the output in this installation was based on team effort rather than individual production.
Why are measures of preference included, since they seem to have little to do with productivity?
Preference is more important than might first appear. In the positive sense, the very fact that a group of workers prefers a specific type of lighting makes it superior lighting, regardless of whether an outside observer might insist that some other alternative is a “better” system. Preferred lighting contributes to the sense of “a nice place to work” and helps workers feel better about their environment and their job. It also aids in recruitment, since recruiters can point the lighting out as a job benefit. In the negative sense, any irritant in the workplace will contribute to employee dissatisfaction, and at a certain level, dissatisfaction turns to departure. Each personnel expert may place a different dollar value on any phase of the attrition-recruitment-rehiring-retaining cycle, but all agree that any reasonable technique to reduce employee dissatisfaction is a useful and practical tool.
Did differences in light level influence the test results?
Commonly recommended light levels for parabolic systems in computer installations are somewhat higher than those recommended for indirect systems. Normally, parabolic installations have an average illuminance of 50 to 70 fc and indirect installations have an illuminance of 30 to 50 fc. Both systems were installed according to standard practice and to provide average illuminances within the standard boundaries.
© Cornell University, 1991