Illuminance Illuminance Gosford City Council Category="Photometry"Category="Physical quantity"In photometry, illuminance is the total Luminous flux incident on a surface, per unit Area. It is a measure of the perceived intensity of the incident light.

In SI derived units, it is measured in lux (lx) or lumens per Square metre (cd·sr·m^-2).

Illuminance was formerly often called Brightness, but this leads to confusion with other uses of the word. "Brightness" should never be used for quantitative description, but only for nonquantitative references to physiological sensations and perceptions of light.

The human eye is capable of seeing somewhat over more than a 2 trillion fold range: The presence of white objects is somewhat discernable under starlight, at 5 logfile.inc.0.1.14 10^-5 lux, while at the bright end, is it possible to read large text at 10⁸ lux, or about 1000 times that of direct Sunlight, although this can be very uncomfortable and cause long-lasting afterimages.

SI photometry units

SI photometry units
Quantity SI unit Symbol Notes
Luminous energy lumen seconds lm · s lumen seconds are sometimes called Talbots
Luminous flux lumen or (candela · Steradian) lm also called luminous power
Luminous intensity Candela or (lumen / steradian) cd
Luminance candela / Square metre cd/m² also called luminosity
Illuminance Lux or (lumen / square metre) lx
Luminous efficacy lumens per Watt lm/W maximum possible (@ 555 nm) is 683

SI photometry units
Quantity	SI unit	Symbol	Notes
Luminous energy	lumen seconds	lm · s	lumen seconds are sometimes called Talbots
Luminous flux	lumen or (candela · Steradian)	lm	also called luminous power
Luminous intensity	Candela or (lumen / steradian)	cd
Luminance	candela / Square metre	cd/m²	also called luminosity
Illuminance	Lux or (lumen / square metre)	lx
Luminous efficacy	lumens per Watt	lm/W	maximum possible (@ 555 nm) is 683