lux

lux

SI coherent derived unit with special name and symbol
Name Symbol Derived quantity Expressed in terms of SI base units
lux lx illuminance m-2 cd

Definition

The lux, symbol lx, is the SI coherent derived unit of illuminance and luminous emittance, measuring luminous flux per unit area.

One lux is equal to one lumen per square metre.

The lux, symbol lx, is the special name for the lumen per square metre, symbol lm/m2.

Photometry

In photometry, the lux is used as a measure of the intensity, as perceived by the human eye, of light that hits, or passes through, a surface.

The lux, or lumen per square metre, symbol lm/m2, is analogous to the radiometric unit watt per square metre, symbol W/m2, but with the power at each wavelength weighted according to the luminosity function, a standardised model of human visual brightness perception.

Illuminance

Illuminance is a measure of luminous flux per unit area of illuminated surface. It is a measure of the intensity, as perceived by the human eye, of light illuminating a surface.

Illuminance, EV, is directly proportional to luminous flux, ΦV, and inversely proportional to the area, A, of surface illuminated.

E_V \propto \dfrac {\phi_V}{A}

Using SI coherent units, the proportionality constant is 1.

E_V = \dfrac{\phi_V}{A}

where:

  • EV is illuminance, measured in lux, symbol lx,
  • ΦV is luminous flux, measured in lumen, symbol lm,
  • A is area of surface illuminated, measured in square metres, symbol m2.

1\ \text{lx} = 1\ \dfrac{\text{lm}} {\text{m}^2} = 1\ \dfrac{\text{cd} \ \text{sr}} {\text{m}^2}

One lux is equal to one lumen per square metre.

A luminous flux of one lumen, concentrated onto an area of one square metre, lights that square metre with an illuminance of one lux. However, the same one lumen, spread out over ten square metres, produces a dimmer illuminance of only 0.1 lux.

Examples of luminous flux and Illuminance for different areas of illumination
luminous flux illuminated area illuminance
100 lm 1 m2 100 lx
100 lm 10 m2 10 lx
1000 lm 10 m2 100 lx

Luminous emittance

The lux is also the SI coherent derived unit of luminous emittance, the number of lumens emitted by a surface per square metre, regardless of the angular distribution of the emitted light.

Photometry units

Name Symbol Quantity Expressed in terms of SI base units Expressed in terms of other SI units
candela cd luminous intensity cd lm/sr
lumen lm luminous flux cd cd sr
lux lx illuminance m-2 cd lm/m2
candela per square metre m-2 cd luminance m-2 cd cd/m2
lumen second lm s luminous energy s cd lm s
lumen per watt lm W‑1 luminous efficacy kg‑1 m‑2 s3 cd lm/W