The word photometry is derived from Greek: photos = light and metron = measure. It is the measurement of the visual aspect of radiant energy (visible light). As such, it is distinguished from radiometry in that photometry takes into account the varying sensitivity of the eye to different wavelengths of light. The units of photometry are luminous quantities that include luminous intensity, luminous flux, luminance and illuminance.
The photometer is a light detector with a spectral response similar to that of the human eye, by using a measurement head representing what the eye sees in photopic vision.
