The candela (IPAEng: kanˈdɛlə, /-ˈdiːlə/, symbol: cd) is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function (a standardized model of the sensitivity of the human eye to different wavelengths, also known as the luminous efficiency functionSharpe, Stockman, Jagla & Jägle (2005) 2-deg V*(l) luminous efficiency function). A common candle emits light with a luminous intensity of roughly one candela. If emission in some directions is blocked by an opaque barrier, the emission would still be approximately one candela in the directions that are not obscured.

Definition

Like other SI base units, the candela has an operational definition—it is defined by a description of a physical process that will produce one candela of luminous intensity. Since the 16th 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 540e: 12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

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, for example.

The candela is sometimes still called by the old name candle 1, such as in foot-candle and the modern definition of candlepower.

Explanation

The frequency chosen is in the visible spectrum near green, corresponding to a wavelength of about 555 nanometers. The human eye is most sensitive to this frequency, when adapted for bright conditions. At other frequencies, more radiant intensity is required to achieve the same luminous intensity, according to the frequency response of the human eye. The luminous intensity for light of a particular wavelength \lambda is given by

I_v(\lambda)= 683.002\,\overline{y}(\lambda) I(\lambda)

where I_v(\lambda) is the luminous intensity in candelas, I(\lambda) is the radiant intensity in W/sr and \overline{y}(\lambda) is the standard luminosity function. If more than one wavelength is present (as is usually the case), one must sum or integrate over the spectrum of wavelengths present to get the total luminous intensity.