The Candela

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 frequencyand 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

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