Measuring Light for Vision, Health, and Everything Else

Everybody in the lighting world knows (or should know) that the basis of photometry is the spectral luminous efficiency function V(λ), but did you know that its basis is research conducted in the USA under the auspices of the US National Committee of the CIE?  2024 marks the centenary of V(λ) being adopted internationally by the CIE and the beginnings of physical measurement of optical radiation.

The 1924 definition of V(ʎ) is based on psychophysical experiments measuring brightness perception. It has long been recognized that it underestimates visual sensitivity at short wavelengths and is appropriate only for small (2o) visual fields. Recent research has made it possible to propose a new standard based on the physiological properties of the visual system—but is changing the foundation of photometry the right decision? The presentation will describe the pros and cons. 

V(λ) best describes light for vision, but in recent years, we have learned about new photoreceptors, the intrinsically photoreceptive retinal ganglion cells (ipRGCs), and have learned to measure light for its effects on these and the classically known rods and cones. This presentation will celebrate the history of V(λ) and show how we use it together with the CIE system of metrology for ipRGC-influenced effects of light to characterize light exposures, with pointers to how these measurements are coming to be reflected in lighting recommendations, including ANSI/IES RP-46-23 Recommended Practice: Supporting the Physiological and Behavioral Effects of Lighting in Interior Daytime Environments.