= Photometer
{wiki}
A <photometer> measures a visually weighted radiation quantity. A filtered <photodiode> approximates the <photopic luminous efficiency function> with its <spectral responsivity>; calibration establishes how its <electric current> maps to <illuminance> or collected <luminous flux>. NIST's detector-based realization traces the <spectral responsivity> to a <cryogenic radiometer> and the aperture <area> to length measurements. <Realization of the candela>{ref}
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