Lighting Technology Laboratory
UEEN FEEC BUT in Brno
Core quantities and concepts of lighting technology with a concise definition, symbol, unit and relation. The glossary is growing; terms will be linked to the places where they appear on the site.
A measure of how faithfully a source renders colours against a reference source.
Expresses how faithfully object colours appear under a given source compared with a reference source of the same colour temperature. The general index Ra is the average over eight test colours; 100 means perfect rendering, lower values indicate colour distortion. Quality indoor lighting usually requires Ra ≥ 80. It is a dimensionless quantity.
Temperature of a black-body radiator whose light colour matches the source.
Temperature of a Planckian (black-body) radiator whose chromaticity exactly matches that of the source. It applies only to sources lying directly on the Planckian locus in the chromaticity diagram. Lower values (~2700 K) correspond to warm white light, higher values (~6500 K) to cool white. The unit is the kelvin (K).
Temperature of the black-body radiator closest in colour to a source off the Planckian locus.
Temperature of the Planckian radiator whose chromaticity is closest to that of the source (the shortest distance in the u'v' diagram). It is used for real sources — fluorescent lamps and LEDs — that do not lie exactly on the Planckian locus and therefore have no true colour temperature. It is often supplemented by the Duv deviation describing the offset from the Planckian locus.
Luminous flux incident on a unit area of a surface.
Areal density of luminous flux falling on a surface — how much light reaches a given area. 1 lux = 1 lumen per square metre. It is a quantity measured at the lit surface (e.g. a desktop), not a property of the source. Illuminance is the value most often specified by indoor lighting standards.
Luminous intensity of a surface in a direction per unit projected area.
Areal density of luminous intensity — the intensity of a surface element in a given direction divided by the projection of that surface onto a plane perpendicular to the viewing direction. Luminance is the only photometric quantity the eye perceives directly as the 'brightness' of a luminous or lit surface. The unit cd·m⁻² is called the nit.
Total light power of a source weighted by the eye's spectral sensitivity.
Radiant power weighted by the spectral sensitivity of the human eye, V(λ). It expresses the total amount of light emitted by a source in all directions per unit time and is the basis for luminous efficacy (lm/W). The unit is the lumen (lm).
Luminous flux emitted into a unit solid angle in a given direction.
Spatial density of luminous flux — flux per unit solid angle in a given direction. The candela (cd) is an SI base unit. Luminous intensity describes how 'strongly' a source emits into a particular direction and characterises the directional distribution of luminaires.
The angle under which an area is seen from a point — the spatial analogue of a plane angle.
Ratio of the area cut out on the surface of a sphere to the square of its radius. The full space around a point equals 4π steradians (sr). The solid angle links luminous flux and intensity through Φ = I·Ω and is essential for describing a source's emission into space.