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The candela (cd) - an S.I. Base Unit of luminosity

 

How well we (and optical instruments designed to mimic human vision) see things depends on the level of light reaching the eye (or light detecting system). It is therefore important to develop a scientific system to measure light levels.

The candela is the base unit within the International System of Units (SI) that is used for such measurements of light. The power of optical radiation is measured in watts. However the eye cannot 'see' all colours, or wavelengths of light, equally well and thus another unit is needed to assess the visual effect of optical radiation - the candela.

The eye is most sensitive to light in the yellow-green region of the spectrum, close to the wavelength which corresponds to the peak output of sunlight reaching the earth's surface. It is less sensitive to red and blue light. A special function, known as the V(λ) function, describes the way in which the eye responds to different wavelengths of light. The shape of the V(λ) function affects how we perceive different types of light source.

For example, a 60 watt tungsten lamp, consumes four times the electrical power of a 15 watt compact fluorescent lamp but they are both perceived as producing approximately equal amounts of light, i.e. producing roughly the same number of candelas.

The realisation of the candela at the National Physical Laboratory is based on the use of a cryogenic radiometer. This equates the heating effect of optical radiation with that of electric power, and provides measurements of optical radiant power at specific wavelengths with an uncertainty of better than 0.01%.

A solid-state photometer has been developed to evaluate light of other wavelengths according to the V(λ) function, enabling the candela to be realised with an uncertainty of 0.2%.

The current definition of the candela was made in 1979, in terms of the watt at only one wavelength of light. It is defined as: The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 Hz and that has a radiant intensity in that direction of 1/683 watts per steradian (a unit of solid angle).

Extracted from: http://www.npl.co.uk/reference/measurement-units/si-base-units/the-candela