Like other SI base units, the candela has a definition in terms of a physical process that will produce one candela of luminous intensity. Since the 16 th General Conference on Weights and Measures (CGPM) in 1979, the candela has been defined as:
< >The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency
and that has a radiant intensity in that direction of 1⁄683 watts per steradian (unit of sold angle. A sphere has a solid angle of
).The definition describes how to produce a light source that (by definition) emits one candela. Such a source could then be used to calibrate instruments designed to measure luminous intensity.
A common candle emits light with roughly 1 cd luminous intensity. A 25W compact fluorescent light bulb puts out around 1700 lumens; if that light is radiated equally in all directions, it will have an intensity of around 135 cd. Focused into a 20° beam, it will have an intensity of around 18 000 cd.
The luminous intensity of light-emitting diodes is measured in millicandela (mcd), or thousandths of a candela. Indicator LEDs are typically in the 50 mcd range; "ultra-bright" LEDs can reach 15,000 mcd, or higher.
The luminosity of various sources in cd is shown in the table
Sources | Luminosity (cd) |
Fission Bomb |
|
Lightning Flash |
|
Sun at the Earth's Surface |
|
Camera Flash |
|
60 W Incandescent Bulb |
|
Clear Blue Sky |
|
Moon Observed From Earth |
|
Starlit Sky |
|
