ISO/CIE 23539:2023 Photometry — The CIE system of physical photometry

The purpose of photometry is to measure light as perceived by human eyes. The brightness of a luminous surface depends not only on the amount of radiation it emits, transmits or reflects, but also on its spectral composition and on the visual response function of the observer viewing it. Because human visual response varies at different light levels and from person to person, precise photometry requires the definition of representative standard observers. The CIE system of physical photometry specifies procedures for the quantitative evaluation of optical radiation in terms of internationally agreed spectral luminous efficiency functions for human vision. V(lambda) represents photopic vision, V prime (lambda) represents scotopic vision and Vmes;m(lamda) represents mesopic vision, the latter being intermediate between photopic and scotopic vision. Furthermore, V10(lambda) represents 10 degree photopic vision. These luminous efficiency functions adopted from CIE 018:2019 and BIPM-2019/05, together with the SI base unit, the candela, constitute a system that enables the calculation of values of photometric quantities for optical radiation as well as light-emitting, light-transmitting or light-reflecting surfaces, to be precisely determined based on the International System of Units (SI), regardless of the spectral composition of the radiation emitted, transmitted or reflected. The CIE system of physical photometry has some limitations in respect to the brightness of coloured surfaces: two light sources of different colour but with the same measured luminance value will not necessarily be perceived as equally bright. CIE has therefore published a more complex model (CIE 200:2011) for specific situations. For eye-mediated non-image-forming effects of light induced partially or completely by the intrinsically photosensitive retinal ganglion cells (ipRGCs), CIE S 026/E:2018 is used.