Last edited 24 May 2019

Lux

In physics, lux (symbol lx) is a standard unit used as a measure light intensity (illuminance, or illumination). It is the intensity with which a surface is illuminated and is part of the SI system of measurement.

One lux is equal to one lumen per square metre (1 lm/m2) where a lumen (lm) is the ‘...SI unit of luminous flux, describing the quantity of light emitted by a lamp or received at a surface.’

Typically:

  • A bicycle lamp may produce 10 lm
  • A 100W incandescent light bulb 1,700 lm
  • A 150W tungsten spot lamp 2,000 lm
  • A 140W sodium street lamp 13,000 lm

The illumination of a surface S that is 1m x 1m will be 1 lux (or 1 metre-candle) when a lamp of 1 candela (see below for definition) is placed one metre away from it. In other words, 1 lux of light intensity is spread over the 1m2 surface of S which will be equal to 1 lm/m2).

A light source producing 1,000 lm that is 1m away from a surface of area 1m2 produces an illuminance on the surface of 1,000 lux. When the same light source is applied to an area of 10m2, the illuminance level is one tenth of the original, or 100 lm. Lighting larger areas requires greater intensity light sources…or more bulbs.

A 100W incandescent light bulb produces 1,700 lumens and is regarded as sufficient for general domestic and task lighting applications.

The candela (cd) is a unit of source intensity used in the specification of lighting and can be thought of as being equal to one large domestic candle. Examples are:

  • A torch bulb (not LED) = 1 candela (emits one lumen of light flux)
  • A 60W tungsten bulb = 50 candela
  • A single fluorescent tube = 400 candela

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