Light - Units and Measures

Units and Measures

Light is measured with two main alternative sets of units: radiometry consists of measurements of light power at all wavelengths, while photometry measures light with wavelength weighted with respect to a standardised model of human brightness perception. Photometry is useful, for example, to quantify Illumination (lighting) intended for human use. The SI units for both systems are summarised in the following tables.

Table 1. SI radiometry units
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Quantity Symbol SI unit Symbol Dimension Notes
Radiant energy Qe joule J M⋅L2⋅T−2 energy
Radiant flux Φe watt W M⋅L2⋅T−3 radiant energy per unit time, also called radiant power.
Spectral power Φeλ watt per metre W⋅m−1 M⋅L⋅T−3 radiant power per wavelength.
Radiant intensity Ie watt per steradian W⋅sr−1 M⋅L2⋅T−3 power per unit solid angle.
Spectral intensity Ieλ watt per steradian per metre W⋅sr−1⋅m−1 M⋅L⋅T−3 radiant intensity per wavelength.
Radiance Le watt per steradian per square metre W⋅sr−1⋅m−2 M⋅T−3 power per unit solid angle per unit projected source area.

confusingly called "intensity" in some other fields of study.
Spectral radiance Leλ
or
Leν		 watt per steradian per metre3
or

watt per steradian per square
metre per hertz

 W⋅sr−1⋅m−3
or
W⋅sr−1⋅m−2⋅Hz−1		 M⋅L−1⋅T−3
or
M⋅T−2		 commonly measured in W⋅sr−1⋅m−2⋅nm−1 with surface area and either wavelength or frequency.
Irradiance Ee watt per square metre W⋅m−2 M⋅T−3 power incident on a surface, also called radiant flux density.

sometimes confusingly called "intensity" as well.
Spectral irradiance Eeλ
or
Eeν		 watt per metre3
or
watt per square metre per hertz		 W⋅m−3
or
W⋅m−2⋅Hz−1		 M⋅L−1⋅T−3
or
M⋅T−2		 commonly measured in W⋅m−2⋅nm−1
or 10−22W⋅m−2⋅Hz−1, known as solar flux unit.
Radiant exitance /
Radiant emittance		 Me watt per square metre W⋅m−2 M⋅T−3 power emitted from a surface.
Spectral radiant exitance /
Spectral radiant emittance		 Meλ
or
Meν		 watt per metre3
or

watt per square
metre per hertz

 W⋅m−3
or
W⋅m−2⋅Hz−1		 M⋅L−1⋅T−3
or
M⋅T−2		 power emitted from a surface per wavelength or frequency.
Radiosity Je or Jeλ watt per square metre W⋅m−2 M⋅T−3 emitted plus reflected power leaving a surface.
Radiant exposure He joule per square metre J⋅m−2 M⋅T−2
Radiant energy density ωe joule per metre3 J⋅m−3 M⋅L−1⋅T−2
See also: SI · Radiometry · Photometry · (Compare)

Table 2. SI photometry units
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Quantity Symbol SI unit Symbol Dimension Notes
Luminous energy Qv lumen second lm⋅s T⋅J units are sometimes called talbots
Luminous flux Φv lumen (= cd⋅sr) lm J also called luminous power
Luminous intensity Iv candela (= lm/sr) cd J an SI base unit, luminous flux per unit solid angle
Luminance Lv candela per square metre cd/m2 L−2⋅J units are sometimes called nits
Illuminance Ev lux (= lm/m2) lx L−2⋅J used for light incident on a surface
Luminous emittance Mv lux (= lm/m2) lx L−2⋅J used for light emitted from a surface
Luminous exposure Hv lux second lx⋅s L−2⋅T⋅J
Luminous energy density ωv lumen second per metre3 lm⋅s⋅m−3 L−3⋅T⋅J
Luminous efficacy η lumen per watt lm/W M−1⋅L−2⋅T3⋅J ratio of luminous flux to radiant flux
Luminous efficiency V 1 also called luminous coefficient
See also: SI · Photometry · Radiometry · (Compare)

The photometry units are different from most systems of physical units in that they take into account how the human eye responds to light. The cone cells in the human eye are of three types which respond differently across the visible spectrum, and the cumulative response peaks at a wavelength of around 555 nm. Therefore, two sources of light which produce the same intensity (W/m2) of visible light do not necessarily appear equally bright. The photometry units are designed to take this into account, and therefore are a better representation of how "bright" a light appears to be than raw intensity. They relate to raw power by a quantity called luminous efficacy, and are used for purposes like determining how to best achieve sufficient illumination for various tasks in indoor and outdoor settings. The illumination measured by a photocell sensor does not necessarily correspond to what is perceived by the human eye, and without filters which may be costly, photocells and charge-coupled devices (CCD) tend to respond to some infrared, ultraviolet or both.

Read more about this topic:  Light

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