candela (cd)

The candela is used to measure the visual intensity of light sources, like light bulbs or the bulbs in torches. It is the only SI base unit based on human perception.

In the past, the unit used to measure visual intensity was called the candle, based initially on a ‘standard’ wax candle and then on specially designed oil or gas flame standards. However, these standards varied from country to country, and even from one device to another, meaning they lacked sufficient accuracy and reproducibility. The ‘new candle’, or ‘candela’, was introduced in 1948 and replaced these earlier standards with a black furnace operating at a specified temperature, with a further refinement in 1979 to link visual intensity directly with the radiant intensity of light at a specified wavelength.

The human eye has different sensitivities to different frequencies of light. The peak sensitivity is at approximately 540 THz, which is in the greeny-yellow region of the spectrum. So we see this light more intensely than other colours of the same physical power.

The ‘lumen’ is derived from the candela and measures total light in all directions from a source. It can be seen on light bulb packaging, allowing you to compare the 'light output' or 'performance' of different lighting technologies.

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 luminous intensity.

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%.

Find out about NPL's research in this area

More information on the SI unit definitions from BIPM