Advancing the effective use of light for society and the environment.

Thursday, July 15, 2004

LRC study explores the effects of lamp spectrum and pupil size on task performance
Photo: Close-up of human eye

Lighting scientists are continually looking for ways to help us to see better, perform visual tasks with more accuracy, and save energy at the same time. This quest for better quality lighting inevitably leads to studies to understand how light helps or hinders, and how our visual systems perform under various lighting conditions. LRC researchers recently were awarded an international prize for their published study evaluating whether light sources that produce smaller pupil sizes result in better visual task performance.

Study seeks to evaluate task performance with common office lighting conditions

Some researchers have argued that light levels could be reduced without affecting performance if the light source had a spectrum that could more effectively stimulate the visual system. Professor Peter Boyce of the Lighting Research Center says this argument is based on the finding that smaller pupil sizes improve visual acuity. “The prevailing thought is that despite the reduction in illuminance, performance will improve because smaller pupils provide a greater depth of field and reduced distortions, which all lead to a better quality retinal image,” says Dr. Boyce. However, the literature on pupil size and its effect on task performance has been inconsistent, he says, noting that some studies have shown positive effects of a small pupil size while others have not.

Photo: Office lighting

Illuminance level and lamp spectral power distribution (SPD) are the two major drivers of pupil size. Research has shown that a lamp with an SPD containing a greater scotopic component will reduce pupil size. Therefore, selecting a lamp with the optimum spectrum to reduce pupil size means, in theory, that illuminance levels could be reduced, resulting in energy savings. Several past studies have looked at ways to use lamp spectra to change pupil size and improve task performance. Boyce and LRC researchers Dr. Yukio Akashi, Dr. John Bullough and former LRC research specialist Claudia Hunter last year published their own study to determine the effects of pupil size and lamp spectra for everyday, practical conditions, such as lighting systems and reading materials commonly found in offices. This study, sponsored by the U.S. Department of Energy and GE Lighting, provided sound evidence relevant to a long-standing argument in the international lighting field.

“We chose to test the hypothesis that light sources that produce smaller pupil sizes ensure better achromatic visual task performance, but to do our tests under realistic conditions that had not been explored previously,” says Dr. Akashi. The realistic conditions included typical office light levels and normal contrast, black-on-white printed materials. The materials presented included a wide range of print sizes to vary the difficulty level of the tasks. The researchers also wanted to know whether people noticed and liked the enhanced clarity predicted to occur with smaller pupil sizes.

Researchers assess pupil sizes and their effect

To test the hypothesis, the team studied the speed and accuracy with which participants from two age groups examined charts of Landolt rings, a broken circle commonly used as a test object to evaluate visual acuity. The researchers tested subjects under four lighting conditions using T8 fluorescent lamps at two illuminances (500 lux and 344 lux) and two correlated color temperatures (3000 K CCT and 6500 K CCT). Each combination of illuminance and CCT produced a different pupil size. Subjects’ pupils were recorded on video and measured for each condition.

The findings showed the researchers’ hypothesis to be false. “For normal office tasks, we found the change in pupil size produced by a change in the spectral power distribution did not affect the level of task performance,” says Boyce. Overall, the lamp type had little effect on the work speed and accuracy of both young and old participants. The researchers noted that even with the most difficult visual task using a very small Landolt ring—conditions where light can enhance or hinder performance—a smaller pupil size did not help subjects under the tested conditions. “Instead, the factors determining task performance were the size of the gap in the broken ring and, to a smaller extent, the illuminance level,” says Dr. Bullough.

Participant surveys show minimal and unexpected results

Survey ratings from the younger participants showed that illuminance and lamp type did not influence their opinions of task difficulty, but older subjects did find some significant interaction between lamp type and ring gap size. “The older people in our study tended to perceive the 6500 K light source as making the task less difficult, but not in the way we expected,” says Akashi. The older participants, he says, found it easier to see the larger ring gap sizes under the 6500 K light source than the 3000 K light source, but there was no difference for the smaller gap sizes. The results also showed some indication that the 6500 K lighting was perceived by all participants as brighter than the 3000 K lighting at the same light level.

In May, the authors were given the Walsh-Weston Award by the UK Society of Light and Lighting for the best lighting research paper published in Lighting Research and Technology in 2003. According to Boyce, this award is recognition of the quality of work done at the LRC.

Details of the study by Boyce, Akashi, Hunter, and Bullough can be found in the journal article, “The impact of spectral power distribution on the performance of an achromatic visual task,” published in Lighting Research and Technology, volume 35, issue 2. If you would like a copy of the paper from the LRC library, please email our Resources Manager at

About the LRC

The Lighting Research Center (LRC) is part of Rensselaer Polytechnic Institute and is the leading university-based research center devoted to lighting. Founded in 1988, the Lighting Research Center has built an international reputation as a trusted and reliable source for objective information about lighting technologies, applications, and products. Its mission is to advance the effective use of light and create a positive legacy of change for society and the environment.

2004 Rensselaer Polytechnic Institute, Troy, NY 12180 USA.

Rennselear Polytechnic Institute