What is Visually Efficient Lighting?
by: Dr. Samuel M. Berman, Ph. D.

For more than 100 years lighting users have often found that their sense of brightness does not tally with light meter values. Areas lit by lighting with bluish tint appear brighter than the same areas lit by lighting with more of an orange or reddish tint even though a light meter may indicate the opposite. Lighting professionals have puzzled over this observation for just as long but have failed to provide a satisfactory explanation. However, recently an eminent research team of scientists has finally discovered what is behind this perception and have demonstrated how this new understanding can lead to lighting that provides better vision and energy savings.

When we view a scene, light arrives at the eye from all portions of the illuminated space. Even as we look straight ahead we are aware of light coming all the way from the right side tot he left side, and from near top to near bottom. This is the full field of view of which we normally partake in everyday common vision. Instead of this normal viewing, envision an exercise where all viewing is accomplished by looking through long, thin, matte black mailing tubes that restrict vision so that one can see just what is directly ahead. Although this very restricted view is hardly normal, it is the condition used in photometry to evaluate the sensitivity of our eyes to different colors (different wavelengths of visible light). This restricted visual sensitivity as generated and measured by vision scientists is the basis by which all present light meters are calibrated. Furthermore, such calibrated light meters have become the accepted practice by lighting practitioners as the sole valid means of determining the quantity of light. The tacit assumption that it can be applied without alteration to the normal scenes of full field of view appears not to have been questioned.

However, the new research findings demonstrate that this assumption is not valid and is the origin of the brightness discrepancy between sensation and meter values. To understand how this comes about let us take a brief glimpse of the eye. At the back of the eye there is a light sensitive membrane named the retina, which contains millions of very tiny light receptors. These receptors convert received light into electrified signals that are sent to the vision centers of the brain. The retina contains two major categories of light receptors, called cones and rods because of their geometric shapes. Remarkably, in the very central portion of the retina, which is the portion receiving light when the black mailing tubes are employed, there are only cones and no rods. In the rest of the retina there are both rods and cones with the number of rods dominating the cones by about 10 to one. Because of the small field of view used in generating the eye sensitivity calibration function, only the cone sensitivity is included and rod sensitivity is excluded. Lighting practice accepted this single sensitivity function because it was (erroneously) assumed that the more light sensitive rods only functioned at very dim light levels as occur under starlight on a moonless night. The new research findings, however, reveal that at typical interior light levels the rod receptors are not disabled and are functioning quite well. Light that is received by these rod receptors also contributes to the perception of brightness, and because rods are more sensitive than cones to bluish light, the sensation of brightness is enhanced with the bluish light. But this only happens when the scene is the full field of view and it would not occur with the restricted field of view provided by the mailing tubes, which allow only cones to sense the viewed light.

Vision scientists have given names to the cone and rod sensitivity functions, which are referred to as photopic and scotopic responses. The new research findings explain how to put these together to yield a valid measure of brightness for the full scene viewing of lighting practice. One might think that it would take two light meters to achieve the evaluation, but in many lighting applications the traditional light meter can be used if an additional property of the lighting is known. For any light source or any lighting that has a stable and approximately constant color temperature, the ratio of scotopic to photopic output is a fixed constant independent of intensity, that can be measured with instruments and can be supplied by the lamp manufacturer (this is the case for fluorescent lamps). Once this ratio is known, a scotopic value can be determined simply by multiplying the known ratio by the measured or given photopic value. The specific ratio is noted as the quantity S/P where S is the scotopic measure and P is the analogous photopic measure. The new research explains that brightness perception is simply correlated to the value P "S/P rather than just P alone.

When we read, view a computer screen or carry out the multitude of near vision tasks of the workplace, our eyes automatically accommodate in order to bring the viewed objects into focus. This act of accommodation requires the eye to do work by changing the shape of the lens in order to bring the desired light rays into focus. It is a known fact of vision that, as the pupil of the eye gets smaller, the net amount of accommodative work is also automatically reduced. Thus, if we can arrange to have small pupils, the eye would work less and there would be less visual fatigue. Of course, pupil size can be made smaller by simply ramping up light levels. However the new research findings demonstrate that ramping up light levels is not the efficient way to achieve smaller pupils. This is because pupil size is predominantly controlled by signals from the rod receptors. Since rod receptors are tuned to the scotopic content of the light they are most efficiently activated by a light spectrum with the highest possible S/P ratio consistent with any other spectral requirement.

Both vision and brightness are enhanced by scotopically rich lighting. Adoption of such lighting can also lead to substantial energy savings if vision and brightness are maintained at the same levels as achieved with standard lighting. Scotopically rich lighting is the preferred lighting for interior spaces because it is the most visually efficacious.