Implications of Rod Sensitivity to Interior Lighting Practice

Conventional photometry is based on the psychophysical response of observers with the field of view confined to 2 deg. That region of the human retina contains only cone photoreceptors and does not contain rod photoreceptors. However, the majority of the retina is dominated by rods. Perhaps because the measurement of rod spectral response requires extremely low light levels in order to remain below cone threshold, it has been erroneously assumed that rods are inactive at normal interior light levels. Thus, any effects of rod receptors have not been included in the measurement of light quantity that is applied to lighting practice. Our studies demonstrate that rods are indeed active at typical interior light levels and that under conditions of full field of view they dominate the spectral determinants of pupil size, as well as, contributing significantly to brightness perception. Rod intrusion is well known to vision scientists, but nevertheless many useful past studies of lighting effects have fallen by the wayside because the obvious invoking of rod contributions have not been part of interior lighting science. We present here evidence that indicates that the time has come for the C.I.E. to recognize that lighting practice requires an enhanced photometry which incorporates realistic viewing conditions and the resultant scotopic sensitivity.