|
Quantity/concept |
Designation |
Unit |
Formula |
Explanation |
|
Luminous intensity |
l |
Candela (cd) |
I=Φ/ω |
Luminous power emitted by a light source in a particular direction. |
|
Illuminance |
E |
Lux (lx) |
|
The total luminous flux on a surface. |
|
Luminance |
L |
cd/m2 |
L=l/A |
A value also called luminous intensity. The greater the luminance of a surface, the brighter it appears. |
|
Luminous flux |
Φ |
Lumen (lm) |
Φ=l/ω |
A measurement describing the total luminous energy emitted by a light source. This indicates the perceived power of the light. |
|
Light output ratio |
ηA |
|
|
The proportion of the total luminous energy emitted from a light source that lands on a surface either directly or via reflection. |
|
Colour temperature |
|
Kelvin (K) |
|
A measurement that describes the colour impression of a light source. This is normally perceived as hot at <4,000 K and cold at >4,000 K. |
|
Colour rendering index |
Ra |
|
|
An index describing the ability of a light source to render test colours in comparison with a reference light source at a particular colour temperature. |
|
Luminous efficacy |
- |
lm/W |
|
The ratio of the emitted luminous flux to the power consumed by the light source, including the ballast. |
|
Glare |
- |
|
|
Conditions wherein reduced visual ability occurs, caused by inappropriate distribution or levels of luminance, or by extreme contrasts. |
|
Uniformity |
- |
|
|
The ratio between the lowest value and the average value over a specific surface. |
|
Useful life |
- |
hours (h) |
|
The amount of use after which the light output of a lighting installation has declined to 70% of initial lumens, on account of luminaire failure and lumen depreciation. |