scope:

Introduction

Photometric procedures are used to measure light, i.e. measuring the universal and essential attribute of all perceptions and sensations which are peculiar to the human visual system and which are produced through the agency of that system. Thus, photometric quantities must take into account both the purely physical characteristics of the radiant power stimulating the visual system and the spectral responsivity of the latter. The psychophysical nature of the second of these two factors sets photometric quantities apart from purely physical quantities.

The purpose of this document is to describe and explain the usage of the photometric quantities as well as their units, and to explain briefly the conventions relating them with the purely physical quantity, radiant power, and its unit, the watt. These definitions and conventions aim at enabling photometric measurements to be made by purely physical methods, but to yield results that correlate adequately with visual experience. The relationships between these photometric quantities and photobiological, photochemical, and colorimetric quantities are also given. The practical realization of the units for these photometric quantities is outside the scope of this document and is available in the mise en pratique for the definition of the candela and associated derived units for photometric and radiometric quantities (Zwinkels et al., 2016).

If electromagnetic radiation is of sufficient power and of wavelengths within the range 360 nm to 830 nm (approximately), it stimulates the visual organ, i.e. the eye. The retinal receptor elements involved may be principally the cones (photopic vision, eye adapted to higher levels of luminance), the rods (scotopic vision, eye adapted to lower levels of luminance), or a combination of the cones and rods (mesopic vision, intermediate between photopic and scotopic vision).

This 3rd edition of CIE 18 has been updated to include, in the definitions of photometric quantities and units, the spectral luminous efficiency functions for mesopic vision based on CIE 191:2010 (CIE, 2010), as well as the 10° spectral luminous efficiency functions based on CIE 165:2005 (CIE, 2005), and to introduce the defining constant for photometry, denoted K_cd, adopted in the International System of Units (SI) 2019 (BIPM, 2019).