The ease with which we see things depends on the level of light present. 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 frequencies 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 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 parts of the visible spectrum.
The shape of the V(λ) function affects how we perceive different types of light source. For example, a 60 watt tungsten lamp, of the type found in many households, 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 NPL is based on the use of a cryogenic radiometer which, by equating the heating effect of optical radiation with that of electric power, can provide measurements of optical radiant power at specific frequenices with an uncertainty of better than 0.01%. A solid-state photometer has been developed to evaluate light of other parts of the visible spectrum 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 frequency 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 hertz and that has a radiant intensity in that direction of 1/683 watts per steradian (a unit of solid angle).