Pupil Lumens

How people see and are psychologically impacted by lighting has been a subject of much study and discussion for years. Describing light as “lumen output” and measuring it as “foot candles” on a work surface have been the traditional ways of describing and defining how much light is required to perform a variety of tasks. However, that is being re-examined based on the results of studies on visual performance and the psychological impacts of lighting. Additionally, the “Color Rendering Index” (CRI), “Correlated Color Temperature” (CCT) and Kelvin color temperature describe the quality of the light (relating to how true colors appear compared to under a noon north sky on a clear day).

As lighting technology evolves into various types and colors, simply measuring the lumens proves not to be fully adequate in predicting how well people can see. An excellent example is the low sodium lamp which produces many lumens, but only two colors (yellow  and gray); the ability to make out details, beyond shape of objects, is lost under this light source. Different light sources produce light in different spectral ranges and there is a wide variety of spectral output in LED, LVD and fluorescent lamps.

Vision itself is affected by many factors, from light intensity, distribution, color and contrast to reflections, glare, air quality, motion of subjects and viewers. Our eyes use different parts to see in bright light and low light conditions. The eye contains cones and rods which were thought to work in opposite conditions. Cones provide color vision and fine detail (photopic) in bright light and rods take over in dim light (scotopic). In bright light our pupils contract allowing more detail to be perceived, while depth of field and perceived brightness also increase. In low light our eyes dilate to allow more light in.

Light meters and recommended light levels for tasks have traditionally been calibrated for daytime viewing and general interior lighting based on the photopic response. However, studies are indicating that the scotopic vision is more involved in interior lighting than thought and affects pupil size. At recent conferences, some presenters encourage designers to specify the photopic/scotopic (P/S) ratio of lamps when selecting them in order to get better design, efficiency and better vision for occupants.

Sam Berman – formerly with Lighting Systems Research Group at Lawrence Berkeley Laboratory and a major supporter of the importance of the P/S ratio lighting selection – developed a conversion factor that applies that P/S ratio to lumen output to various light sources an then expresses the effective lumens the eye will perceive for vision based on the size of the pupil and the effect on vision. Some lamps like Low Pressure Sodium lose most of there output using this methods. Others like high quality fluorescent lamps, LED lamps and LVD lamps gain substantially.

The correction factors applied to conventional values of lumens per watt yield a value for pupil lumens per watt, which is a measure of how effectively the eye sees the light that is emitted. This is due to the pupil being more receptive to light at the blue end of spectrum in low light conditions.

Recent studies seem to favor white light for viewing moving objects in low lighting conditions, such as spotting a pedestrian or animal on the side of the road at night. Some cities opt to use white light rather than the yellowish light or high pressure sodium in hopes of reducing accidents.

White light is proving to have advantages for visual performance. Current codes and standards are based on measurement that do not address the impact of pupil lumens, which can be vastly different from traditionally measured lumen output of lamps. Studies on the relevance of light spectrum and the mechanics of vision are on going, codes and standards may reflect this in the future.