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The science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology.

Measuring Light

The "brightness" of light can mean different things: for example, the amount of light coming from a light source is luminous flux (lumens), the amount of light falling on a surface is illuminance (lux), and the amount of light reflected off a surface is luminance (cd/m2).


Illuminance is a measure of how much luminous flux is spread over a given area. One can think of luminous flux (measured in lumens) as a measure of the total "amount" of visible light present, and the illuminance as a measure of the intensity of illumination on a surface. A given amount of light will illuminate a surface more dimly if it is spread over a larger area, so illuminance (lux) is inversely proportional to area when the luminous flux (lumens) is held constant.

One lux is equal to one lumen per square metre: 1 lx = 1 lm/m2 = 1 cd * sr/m2

A flux of 1,000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1,000 lux. However, the same 1,000 lumens, spread out over ten square metres, produces a dimmer illuminance of only 100 lux.

Definitions and units

  • Radiant flux (W (watt)) is a measure of radiometric power. Flux, expressed in watts, is a measure of the rate of energy flow, in joules per second. Since photon energy is inversely proportional to wavelength, ultraviolet photons are more powerful than visible or infrared.
  • Luminous flux (lm (lumen)) is a measure of the power of visible light.
    • Photopic flux, expressed in lumens, is weighted to match the responsivity of the human eye, which is most sensitive to yellow-green.
    • Scotopic flux is weighted to the sensitivity of the human eye in the dark adapted state.
  • Irradiance (W/cm2 (watts per square centimeter)) is a measure of radiometric flux per unit area, or flux density.
  • Illuminance (lx = lm/m2 (lumens per square meter)) is a measure of photometric flux per unit area, or visible flux density. Illuminance is typically expressed in lux (lumens per square meter) or foot-candles (lumens per square foot).
  • Radiance (W/cm2/sr (watts per sq. cm per steradian)) is a measure of the flux density per unit solid viewing angle, expressed in W/cm2/sr. Radiance is independent of distance for an extended area source, because the sampled area increases with distance, cancelling inverse square losses.
  • Luminance (lm/m2/sr (lumens per sq. cm per steradian)) is a measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted or reflected from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square metre (cd/m2).
  • Radiant Intensity (W/sr (watts per steradian)) is a measure of radiometric power per unit solid angle, expressed in watts per steradian.
  • Luminous intensity (cd = lm/sr (lumens per steradian)) is a measure of visible power per solid angle, expressed in candela (lumens per steradian). Intensity is related to irradiance by the inverse square law.

Light levels under various conditions

Illumination          Condition
 ftcd        lx
9,290   100,000       Direct Sunlight
1,000    10,752       Full Daylight
  100     1,075       Overcast Day
   10       107       Very Dark Day
    1        10.8     Twilight
    0.1       1.08    Deep Twilight
    0.01      0.108   Full Moon
    0.001     0.0108  Quarter Moon
    0.0001    0.0011  Starlight
Luminance   Condition
7,000       Average Clear Sky
2,000       Average Cloudy Sky
  250       Peak Luminance Of An LCD Monitor
   80       Monitor White In SRGB
   30       Green Electroluminescent Source
    2       Floodlit Buildings, Monuments, And Fountains
    1       State-Of-The-Art (2015) GITD Pigment After 10 min
    0.04    Phosphorescent Markings On A Watch Dial After 1 h
    0.001   Night Sky


See also: