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.

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.

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

 
A lux meter for measuring illuminances in work environments

 

Lighting condition
Foot-candles
Lux
Full daylight
1,000
10,000
Overcast day
100
1,000
Very dark day
10
100
Twilight
1
10
Deep twilight
0.1
1
Full moon
0.01
0.1
Quarter moon
0.001
0.01
Starlight
0.0001
0.001

 

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. V-magnitudes can be converted to lux using the formula

E v = 10 ( − 14.18 − M v ) / 2.5 {\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

M v = − 14.18 − 2.5 log ⁡ ( E v ) {\displaystyle M_{\mathrm {v} }=-14.18-2.5\log(E_{\mathrm {v} })} .

 

  • SI photometry quantities

     
    QuantityUnitDimensionNotes
    NameSymbolNameSymbolSymbol
    Luminous energyQvlumen secondlm⋅sTJThe lumen second is sometimes called the talbot.
    Luminous flux, luminous powerΦvlumen (= candela steradians)lm (= cd⋅sr)JLuminous energy per unit time
    Luminous intensityIvcandela (= lumen per steradian)cd (= lm/sr)JLuminous flux per unit solid angle
    LuminanceLvcandela per square metrecd/m2L−2JLuminous flux per unit solid angle per unit projected source area. The candela per square metre is sometimes called the nit.
    IlluminanceEvlux (= lumen per square metre)lx (= lm/m2)L−2JLuminous flux incident on a surface
    Luminous exitance, luminous emittanceMvluxlxL−2JLuminous flux emitted from a surface
    Luminous exposureHvlux secondlx⋅sL−2TJTime-integrated illuminance
    Luminous energy densityωvlumen second per cubic metrelm⋅s/m3L−3TJ 
    Luminous efficacyηlumen per wattlm/WM−1L−2T3JRatio of luminous flux to radiant flux or power consumption, depending on context
    Luminous efficiency, luminous coefficientV  1Luminous efficacy normalized by the maximum possible efficacy

     

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