Candlepower Lumen Candela Definition

The lumen and the candela are two units used to measure light.

The lumen and the candela measure slightly different things. The candela is a unit of luminous intensity that measures the power emitted by a light source in a particular direction. The lumen measures luminous flux, which is the power of a light as perceived by the human eye.

Definition of a Candela

The typical candle has a luminosity of one candela, but the actual definition of a candela is much more specific. According to the General Conference on Weights and Measures in 1979, 1 candela is the luminous intensity of a light source that emits radiation with a wavelength of 555 nanometers with a radiant intensity of 1/683 watt per steradian. A steradian is roughly 1/12 of a sphere, in terms of degrees.

Definition of a Lumen

One lumen is defined as the luminous flux produced by a light source that emits 1 candela over an angle of 1 steradian. In other words, if a source of light emits 1 candela equally in all directions, it is emitting 12.57 lumens, because the sphere is broken up into 12.57 steradians.

Adjustment to the Human Eye

Both the candela and the lumen are adjusted depending on the wavelength of light being measured. The base definition for a candela uses 555 nanometers, in the green part of the spectrum, because the human eye is most sensitive at that frequency. At other frequencies, a greater radiant intensity is required to create the same perception of brightness.

References

• The NIST Reference on Constants, Units, and Uncertainty: Base unit definitions - Candela
• he NIST Reference on Constants, Units, and Uncertainty: Essentials of the SI - Base & derived units