Photometry is the branch of optics that deals with the measurement of light, specifically the visible portion of the electromagnetic spectrum, as perceived by the human eye. It focuses on quantifying light in terms of its intensity, distribution, and effects on the human visual system, rather than solely considering physical aspects like power or energy.
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Photometry weights visible electromagnetic radiation by standardized visual response functions; radiometry measures its energy and power without visual weighting.[ref]
The question is how much light a source supplies under a stated visual weighting. A beam containing at the defining frequency has luminous flux . Moving the same radiant flux to a region of smaller spectral luminous efficacy reduces its luminous flux. Changing the optical spectrum changes this conversion even when its total power stays fixed. This makes photometry useful for comparing lighting, while radiometry remains necessary for an energy budget. The lumen counts luminous flux; the candela additionally specifies its distribution over solid angle.
Two measurement routes implement the weighting: measure an optical spectrum and evaluate a weighted integral, or use a photometer whose spectral responsivity approximates the prescribed weighting. Both require a specified collection geometry. Neither is a complete prediction of an individual's perception of a scene.[ref]