A photometer is an instrument used to measure the intensity of light. It quantifies the amount of light that passes through or is emitted from a source, and it can be employed in various scientific and practical applications. Photometers can measure light in different contexts, such as luminous intensity (in terms of lumens), irradiance (in watts per square meter), or illuminance (in lux), depending on the specific type of photometer.
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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. [ref]
