- mcd - Milli-candela symbol
- Mcd - Mega-candela symbol
The candela (symbol cd) is the unit of measurement of light intensity, defined as follows:
A candela is the luminous intensity in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and radiant intensity in that direction of 1/683 watt per steradian.
Although this definition is based on another SI (International System) derived as the watt, the candela is one of seven basic units of the International System of Units.
Before 1982, the year in which was introduced the present definition, the candela was defined as the luminous intensity, in the direction perpendicular to a surface of 1/600 000 m2 of a blackbody at the melting temperature of the platinum at a pressure of 101 325 N/m2 (the standard atmospheric pressure).
The arbitrary term 1/683 that appears in the current definition was chosen to make it to coincide with the old. The frequency chosen is that of the visible spectrum to the next green. The human eye is more sensitive to this frequency.
The ANSI Standard has a different light intensity measurement unit, the lumen.