Candelas

Candela is the SI base unit of luminous intensity, symbol cd. It expresses how bright a light source appears in a given direction. The candela is defined by a fixed amount of luminous intensity corresponding to a monochromatic light of frequency 540 terahertz (about 555 nanometers) with radiant intensity of 1/683 watt per steradian in the specified direction. This ties the unit to the peak sensitivity of the human eye, yielding a luminous efficacy of 683 lumens per watt at that wavelength.

Luminous intensity is the luminous flux per unit solid angle. If a source has constant luminous intensity I in a given direction, the luminous flux emitted into a solid angle dΩ is dΦv = I dΩ. The total luminous flux from a source is Φv = ∮ I dΩ. Consequently, 1 lumen equals 1 candela × 1 steradian. In photometric practice, lumen and candela relate perceived brightness to radiometric power.

The candela is one of the seven base units of the SI. In 2019, SI definitions were re-baselined by fixing fundamental constants: the candela is defined by fixing the luminous efficacy of monochromatic radiation at 555 nm to be exactly 683 lumens per watt. The name candela derives from candle, reflecting historical brightness standards.

In practice, devices are specified by luminous intensity in candelas (cd) or by luminous flux in lumens (lm). Lighting design, displays, and photography rely on these photometric quantities to quantify brightness in a direction.