# Illuminance

In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance.[1]

Illuminance
Common symbols
Ev
SI unitlux
Other units
phot, foot-candle
In SI base unitscd·sr·m−2
DimensionL−2J

In SI derived units these are measured in lux (lx), or equivalently in lumens per square metre (cd·sr·m−2). In the CGS system, the unit of illuminance is the phot, which is equal to 10000 lux. The foot-candle is a non-metric unit of illuminance that is used in photography.[2]

Illuminance was formerly often called brightness, but this leads to confusion with other uses of the word, such as to mean luminance. "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 more than a 2 trillion-fold range: The presence of white objects is somewhat discernible under starlight, at 5×10−5 lux, while at the bright end, it is 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.

## Common illuminance levels

Lighting conditionFoot-candlesLux
Full daylight1,000 [3]10,000
Overcast day1001,000
Very dark day10100
Twilight110
Deep twilight0.11
Full moon0.010.1
Quarter moon0.0010.01
Starlight0.00010.001

## Astronomy

In astronomy, the illuminance stars cast on the Earth's atmosphere is used as a measure of their brightness. The usual units are apparent magnitudes in the visible band.[4] V-magnitudes can be converted to lux using the formula[5]

${\displaystyle E_{\mathrm {v} }=10^{(-14.18-M_{\mathrm {v} })/2.5}}$,

where Ev is the illuminance in lux, and Mv is the apparent magnitude. The reverse conversion is

${\displaystyle M_{\mathrm {v} }=-14.18-2.5\log(E_{\mathrm {v} })}$.