Candela

The candela is the SI base unit of luminous intensity, measuring light emitted in a specified direction according to the sensitivity of human vision.

Luminous intensity: Luminous intensity is the luminous flux emitted per unit solid angle in a specified direction. The candela is the SI base unit that measures this quantity.
Candlepower: Candlepower is an obsolete measure of luminous intensity based on a standard candle. The candela replaced candle-based standards with a reproducible photometric unit.
Radiant intensity: Radiant intensity is radiant power emitted per unit solid angle in a specified direction. It measures physical energy rather than light weighted for human visual sensitivity.
Photometry: Photometry measures light according to its perceived brightness to the human eye. It uses the candela and related units to quantify visible-light sources.
Lux: The lux is the SI unit of illuminance, measuring luminous flux received per unit area. Lux describes light arriving on a surface, unlike candela’s directional source intensity.
Luminous efficacy: Luminous efficacy expresses how effectively a source converts radiant power into visible-light output. The candela’s definition uses a fixed efficacy for monochromatic radiation at 540 terahertz.
International Candle: The International Candle was an early international unit of luminous intensity defined through reference lamps. Its adoption preceded the candela in the effort to standardize light measurement.
Luminous flux: Luminous flux is the total visible-light output of a source, weighted by human visual sensitivity. It is the photometric quantity from which luminous intensity is derived by direction.
Goniophotometer: A goniophotometer measures a light source’s luminous intensity at different angles. Its angular measurements are reported in candelas.
Nit: The nit is a non-SI name for one candela per square metre of luminance. It measures apparent surface brightness, not the source’s luminous intensity alone.

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