National Physical Laboratory

SI units > Candela (cd)

Candela (cd)

CandelaThe candela is the SI base unit for luminous intensity. It is the brightness of a light source in a particular direction.

The candela is used to measure the brightness of light sources, like light bulbs or the bulbs in torches. It is the only SI unit based on human perception, as 'brightness' is something experienced in different ways by different people.

In the past, a 'standard' candle defined the candela. However, this standard varied from country to country, and from candle to candle, meaning it lacked sufficient accuracy. The candela, or 'the new candle', replaced these definitions in 1948 and it measures luminous intensity.

The human eye has different sensitivities to different wavelengths of light. The peak sensitivity is at approximately 555 nm, which is in the greeny-yellow region of the spectrum. So we see green light more brightly than other colours.

Redefinition

The candela is not expected to be redefined in 2019, as it is already defined using the Planck constant. It is defined as the luminous power emitted by a point light source in a particular direction.

Did you know?

  • The candela is the only SI unit based on human perception, it takes into account the sensitivity of our eyes to different colours
  • A 'lumen', which you might find on light bulb packaging, is a measure of total light in all directions from a light source

The science behind the unit

The efficiency and 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 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, 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 National Physical Laboratory 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 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 hertz and that has a radiant intensity in that direction of 1/683 watts per steradian (a unit of solid angle).

Kilogram Metre Second Ampere Kelvin Mole Candela

 

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