View source for Lumen

The total brightness of a lamp is measured in the [https://en.wikipedia.org/wiki/International_System_of_Units '''SI base unit'''] of [https://en.wikipedia.org/wiki/Candela '''Candelas'''] but for practical design purposes, it is more important to measure the amount of visible light available in a particular direction and this is measured in [https://en.wikipedia.org/wiki/Lumen_%28unit%29 '''Lumens'''].


The amount of light actually reaching a particular surface can vary according to the type of lamps used, their total wattage, their distance from the surface, and whether or not they are modified by a shade or diffuser for example. This is known as '''Illuminance''' and is measured in '''lumens per square metre''' or [https://en.wikipedia.org/wiki/Lux '''Lux'''].


For example:
* a 100 watt filament bulb or its equivalent may generate 1200 lumens ''(from tables or manufacturer's information)''
* it is possible that, because this lamp is shaded and positioned on a high ceiling, only 15% of its light reaches a work-surface below;
* the actual light reaching the work-surface is therefore only 15% of 1200 which is 180 '''Lumens''';
* assuming a standard length of kitchen worktop 3 metres long x 600mm wide ''(i.e. area = 1.8 sq.m.)'';
* '''Illuminance''' ''(i.e. light arriving at the work-surface divided by its area)'' is 180 ÷ 1.8 = 100 '''Lumens per square metre''' or [https://en.wikipedia.org/wiki/Lux '''Lux'''].


[[Category:Terminology]]