Glossary - FAQ
Frequently Asked Questions
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Color temperature enables one to determine the (actual or “virtual”) temperature of a light source according to its color. It is measured in Kelvins (K).
The higher the color temperature, the closer the light emitted by a light fixture is to cool white (it is bluer). Inversely, a low color temperature corresponds to what is called “warm white” (it is yellower).
The values that one comes across most frequently are:
- Cool white: 6500 K
- Neutral white: 4000 K
- Warm white: 2700 K
The CRI (Color Rendering Index) or “Ra” is the ability of a light source to render the true colors of the visible spectrum. This index is used to rate the quality of light compared with sunlight.
The maximum index, Ra = 100, corresponds to white light that has the same spectrum as sunlight. A CRI that is greater than 90 is considered to be very good.
A lamp with a very low CRI, such as the low-pressure sodium lamps (Ra < 15) that are used on Belgian motorways, alter our color perception greatly: Everything has an orangey tinge.
Illuminance corresponds to the mean luminous flux reaching a unit of surface area and, by extension, the luminous flux hitting a point on this surface (luminous flux/surface area). Its symbol is “E”. The notation is lux (lx), which is lumens per square metre.
Luminance is the ratio of the luminous intensity at a point on the surface of a source in a given direction per unit of projected area on a plane perpendicular to the given direction (intensity/surface area of the source). Its symbol is ‘L’; its standard unit is candelas per square metre (cd/m²).
The luminous flux is the visual quantity that corresponds to the luminous intensity (or light output) emitted in space by a source (total of the luminous intensity emitted in all directions). Its symbol is “F” or “Φ”. Its standard unit is the lumen (lm).
The luminous intensity (or light intensity) is the luminous flux incident on a small surface that lies in a specified direction from a light source and is normal to this direction, divided by the solid angle (in steradians) that the surface subtends at the light source (F/Ω). It is used to establish the photometric distribution curves of all lighting devices. Its symbol is “I”; its standard unit is the candela (cd).
The UGR (UNIFIED GLARE RATING) method relates to glare from ceiling fixtures that are placed in a regular pattern.
European standard EN 12461, Lighting of Indoor Work Places, contains a table in which the maximum allowed UGR value is prescribed per type of room and per type of activity. The UGR method produces a scale figure: below 13 no glare, 13 – 16 suited for accurate eye tasks, 16 –19 suited for average eye tasks, 19 –22 suited for moderate eye tasks, 22 –28 suited for simple eye tasks, above 28 not suited for work lighting.