Candela (cd)
Definition, realization and luminous intensity measurement in the International System of Units
Luminous intensity is a fundamental quantity in photometry and lighting technology and describes how strong a light source appears to the human eye in a given direction. Unlike several other physical quantities, the candela is not tied solely to objectively measurable energy flows, but also takes human vision into account.
The SI unit of luminous intensity is the candela. The modern definition of the candela is tied to fundamental physical quantities, but also contains an explicitly perceptual element. To understand this it is, as with the other SI base units, necessary to distinguish between definition, realization and practical use.
The definition of the candela in the SI system
The candela is defined within the International System of Units (SI) as follows:
The candela is the SI unit of luminous intensity in a given direction. It is defined such that the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10¹² hertz and has a radiant intensity in that direction of 1/683 watt per steradian, is 1 candela.
The chosen frequency, 540 THz, corresponds to green light, close to the wavelength range where the human eye is most sensitive in daylight.
The definition thus connects:
- radiant intensity (watt per steradian),
- frequency,
- a fixed numerical constant (1/683),
- and the visual response of the human eye.
Historical background
Early definitions of the candela were based on actual light sources, for example candles or glowing metal filaments. These definitions were practically useful but difficult to reproduce exactly.
During the twentieth century, definitions gradually moved to physically well-defined radiation. The current definition is completely decoupled from specific light sources and instead rests on radiation physics and standardized visual physiology.
Definition, realization and use
For luminous intensity too, it is important to distinguish between three levels:
The definition states what the candela is in principle and specifies how luminous intensity relates to radiant intensity.
The realization refers to the methods by which the candela is made real in practical measurements.
The use refers to how luminous intensity is used in lighting, imaging technology, optics and everyday applications.
Photometry and the relationship to radiometry
The candela belongs to photometry, which differs from radiometry in that it weights electromagnetic radiation according to the sensitivity of the human eye.
Radiometric quantities, such as radiant intensity in watts per steradian, describe the physical energy in the radiation. Photometric quantities, such as the candela, are obtained by multiplying radiometric quantities by the spectral luminous efficiency function V(λ), which describes the response of the eye at different wavelengths.
The candela is thus a physical unit with an explicitly biological element.
Realizing the candela
The candela is realized through carefully controlled light sources and optical measuring systems. In practice, the following are used:
- stable monochromatic light sources,
- broad-spectrum sources combined with spectral measurement,
- calibrated photodetectors whose response is traceable to radiometric standards.
By measuring the radiant intensity emitted in a given direction and weighting it with the value of the visual function, the luminous intensity in candela can be determined.
The candela and other photometric units
The candela is the base unit for several derived photometric units, including:
- the lumen (luminous flux)
- the lux (illuminance)
The lumen is defined as the candela multiplied by solid angle, while the lux is defined as lumens per square meter. These units are used in practical lighting engineering and workplace assessment.
Practical use of the candela
In practice, the candela is used to:
- describe the directed luminous intensity of light sources
- specify the performance of lamps and luminaires
- assess visual comfort and glare
Unlike the lumen, which describes total luminous flux, the candela states how strong the light appears in a particular direction.
Summary
The candela is the only SI base unit that explicitly accounts for human vision. It defines luminous intensity from radiant intensity at a specific frequency, weighted by the sensitivity of the eye, so it combines a physical measurement with how the eye actually responds.
This definition makes the candela a practical tool in lighting technology, optics and visual ergonomics, and it fits fully within the constant-based SI system.