Luminous intensity

Luminous intensity measures visible-light output in a particular direction, weighted by the standard human eye’s sensitivity to wavelength. Its SI unit is the candela.

Candela: The SI base unit of luminous intensity, defined using monochromatic radiation at a specified frequency. It is the SI unit used to express luminous intensity.
Luminous intensity distribution: The variation of a source’s luminous intensity with direction. It maps how the source’s light is concentrated across angles.
Goniophotometer: An instrument that measures a light source’s photometric output over a range of angles. It measures the angular intensity data used to characterize luminaires.
Illuminance: Luminous flux incident per unit area on a surface, measured in lux. Unlike luminous intensity, it describes light arriving at a surface.
Solid angle: A measure of the apparent angular size of a region, expressed in steradians. Intensity is luminous flux emitted per unit solid angle.
Luminous efficiency function: A standard function describing the eye’s relative sensitivity to different wavelengths of visible light. Its wavelength weights convert radiant power into photometric intensity.
Light-emitting diode: A semiconductor device that emits light when electric current passes through it. LED intensity and beam direction shape its suitability for lighting applications.
Luminance: Luminous intensity emitted or reflected per unit projected area in a given direction. It adds source area to the directional measure of intensity.
Luminous flux: The total rate of visible-light emission, weighted by the standard eye’s wavelength sensitivity. Its distribution across directions determines luminous intensity.
Radiant intensity: Radiant power emitted per unit solid angle, without weighting for human visual sensitivity. Comparing it with luminous intensity reveals the effect of visual weighting.

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