ABSTRACT
Photometry is a system of language, mathematical formulations, and instrumental methodologies used to describe and measure the propagation of visible (VIS) light through space and materials. Photometry is considered a subset of radiometry, and the study of electromagnetic radiation both within and outside the VIS range, over a large range of wavelengths. This entry begins with a brief introduction to fundamental radiometric quantities and units and then provides a single defining equation for the corresponding photometric quantities.
