Circadian and visual photometry.

Photometry is the metrology of light-optical radiation seen by the human eye due to its action on retinal photoreceptors. Its origins are closely tied to the International Commission on Illumination (CIE), which remains responsible for photometry standards and the language of light used in science and technology. When in 1931 it had become possible to model the response to light of the human eye based on reliable spectroradiometry data, the CIE published standard formulae for predicting the luminance of a stimulus. These and related colorimetry formulae are still in use, having been internationally agreed and adopted. Both fields continue to be the subject of active research and increasing accuracy. CIE S 026:2018 represents another milestone for the metrology of light (CIE, 2018a). It is the first standard where light is considered for its ability to evoke circadian and neurophysiological responses, and includes the spectral sensitivity of melanopsin-a retinal photopigment discovered, and shown to be contributing to and influencing responses from human intrinsically-photosensitive retinal ganglion cells (ipRGCs), only 20 years ago (Berson et al., 2002; Hattar et al., 2002; Provencio et al., 1998). These accessory visual functions also depend to some extent on inputs from the rods and three types of cones; until very recently, rods and cones (or "classical photoreceptors") were the only photoreceptors in visual models. If photometry standards are replaced with modern physiological data, consistent changes should be expected in the photometry of these accessory functions. This chapter outlines the current standards, their definitions and calculations, and how the main elements are related.