Luminous efficiency, luminous coefficient Ratio of luminous flux to power consumption The candela per square metre is sometimes called the nit. Luminous flux per unit solid angle per unit projected source area. The lumen second is sometimes called the talbot. Luminous intensity of artificial light sources is typically measured using and a goniophotometer outfitted with a photometer or a spectroradiometer. The curve which represents the response of the human eye to light is a defined standard function y(λ) or V(λ) established by the Commission Internationale de l'Éclairage (CIE) and standardized in collaboration with the ISO.

Light with the same radiant intensity at other wavelengths has a lower luminous intensity. When adapted for bright conditions ( photopic vision), the eye is most sensitive to yellow-green light at 555 nm. The human eye can only see light in the visible spectrum and has different sensitivities to light of different wavelengths within the spectrum. Photometry deals with the measurement of visible light as perceived by human eyes.

The SI unit of luminous intensity is the candela (cd), an SI base unit. In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye.

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