= Photometry
{disambiguate=optics}
{wiki}
= Photometry
{synonym}
<Photometry> weights visible <electromagnetic radiation> by standardized visual response functions; <radiometry> measures its <energy> and <power> without visual weighting.<OSA Handbook of Optics, Volume III Visual Optics and Vision/Radiometry and Photometry: Review for Vision Optics>{ref}
The question is how much <light> a source supplies under a stated visual weighting. A beam containing $1\,\mathrm{mW}$ at the defining <frequency> $540\,\mathrm{THz}$ has <luminous flux> $0.683\,\mathrm{lm}$. 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.<OSA Handbook of Optics, Volume III Visual Optics and Vision/Radiometry and Photometry: Review for Vision Optics>{ref}
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