lux

SI coherent derived unit with special name and symbol

Name	Symbol	Quantity	Base units
lux	lx	illuminance	m^-2 cd
The lux, symbol lx, is the SI coherent derived unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is the special name for the candela steradian per square metre, symbol m^-2 cd sr. One lux is equal to one lumen per square metre.
Definition			h c^-2 Δν_Cs⁴ K_cd
$1 \mspace{4mu} \text{lx} \mspace{6mu} = \dfrac{(299 \mspace{4mu} 792 \mspace{4mu} 458)^2}{(6.626 \mspace{4mu} 070 \mspace{4mu} 15 \times 10^{-34})(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)^4(683)} \mspace{6mu} \dfrac{h \mspace{4mu} {\Delta \nu _{Cs}}^4 \mspace{4mu} K_{cd}}{c^2}\\ \\ \\ 1 \mspace{4mu} \text{lx} \mspace{6mu} \approx 2.781 \mspace{4mu} 027 \mspace{4mu} 075 \mspace{4mu} 972 \mspace{4mu} 537 \mspace{4mu} 548 \times10^{7} \mspace{6mu} h \mspace{4mu} c^{-2} \mspace{4mu} {\Delta \nu _{Cs}}^4 \mspace{4mu} K_{cd}$

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, E_V, is directly proportional to luminous flux, Φ_V, 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:

E_V is illuminance, measured in lux, symbol lx,
Φ_V 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

candela	cd	luminous intensity
lumen	lm	luminous flux
lux	lx	illuminance
candela per square metre	cd/m²	luminance
lumen second	lm s	luminous energy
lumen per watt	lm/W	luminous efficacy