The measurement of light has several different aspects, to wit:

380px-EM_spectrum.svg

Visible Light Spectrum in the overall EM Spectrum ref Wikipedia: 380px-EM_spectrum.svg

  • Physical: Wavelength, Intensity, Polarization and Power
  • Photometric: Brightness, Color etc.
  • Sources
  • Other

Light is generated by a wide variety of sources, manipulated by optics, captured by another variety of sensing devices, including the human eye, and it represents but a small portion of the overall Electromagnetic (EM) Spectrum of radiation that is described by the physical models underlying of it.

By the CIE International Lighting Vocabulary, the definition of light is: “Any radiation capable of causing a visual sensation directly.”

CIE (1987). International Lighting Vocabulary Number 17.4. CIE, 4th edition. ISBN 978-3-900734-07-7.

An excellent article on Wikipedia, http://en.wikipedia.org/wiki/Light, provides a very comprehensive definition of the term along with a set of table that offer a complete description of the units and major terms used to characterize measurement of light’s properties.

The article also points out that:

“In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.[12][13] This article focuses on visible light. See the electromagnetic radiation article for the general term.”

The following tables, and their descriptions shown below and between the double dashed lines, are from the reference Wikipedia article page last modified on 4 July 2014 at 02:32 with very slight editing.

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Units and measures

Main articles: Photometry (optics) and Radiometry

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

 

Quantity Unit Dimension Notes
Name Symbol[nb 1] Name Symbol Symbol
Radiant energy Qe[nb 2] joule J ML2T−2 energy
Radiant flux Φe[nb 2] watt W or J/s ML2T−3 radiant energy per unit time, also called radiant power.
Spectral power Φ[nb 2][nb 3] watt per metre W⋅m−1 MLT−3 radiant power per wavelength.
Radiant intensity Ie watt per steradian W⋅sr−1 ML2T−3 power per unit solid angle.
Spectral intensity I[nb 3] watt per steradian per metre W⋅sr−1⋅m−1 MLT−3 radiant intensity per wavelength.
Radiance Le watt per steradian per square metre W⋅sr−1m−2 MT−3 power per unit solid angle per unit projected source area.
confusingly called “intensity” in some other fields of study.
Spectral radiance L[nb 3]
or
L[nb 4]
watt per steradian per metre3
or
watt per steradian per square
metre per hertz
W⋅sr−1m−3
or
W⋅sr−1⋅m−2Hz−1
ML−1T−3
or
MT−2
commonly measured in W⋅sr−1⋅m−2⋅nm−1 with surface area and either wavelength or frequency.
Irradiance Ee[nb 2] watt per square metre W⋅m−2 MT−3 power incident on a surface, also called radiant flux density.
sometimes confusingly called “intensity” as well.
Spectral irradiance E[nb 3]
or
E[nb 4]
watt per metre3
or
watt per square metre per hertz
W⋅m−3
or
W⋅m−2⋅Hz−1
ML−1T−3
or
MT−2
commonly measured in W⋅m−2nm−1
or 10−22 W⋅m−2⋅Hz−1, known as solar flux unit.[nb 5]
Radiant exitance /
Radiant emittance
Me[nb 2] watt per square metre W⋅m−2 MT−3 power emitted from a surface.
Spectral radiant exitance /
Spectral radiant emittance
M[nb 3]
or
M[nb 4]
watt per metre3
or
watt per square
metre per hertz
W⋅m−3
or
W⋅m−2⋅Hz−1
ML−1T−3
or
MT−2
power emitted from a surface per unit wavelength or frequency.
Radiosity Je watt per square metre W⋅m−2 MT−3 emitted plus reflected power leaving a surface.
Spectral radiosity J[nb 3] watt per metre3 W⋅m−3 ML−1T−3 emitted plus reflected power leaving a surface per unit wavelength
Radiant exposure He joule per square metre J⋅m−2 MT−2 also referred to as fluence
Radiant energy density ωe joule per metre3 J⋅m−3 ML−1T−2
See also: SI · Radiometry · Photometry · (Compare)

 

Table 2. SI photometry units

600px-Linear_visible_spectrum.svg

Linear view of the visible light spectrum with colors annotated and wavelengths in nanometers
Ref: Wikipedia http://en.wikipedia.org/wiki/Light

Quantity Unit Dimension Notes
Name Symbol[nb 6] Name Symbol Symbol
Luminous energy Qv [nb 7] lumen second lm⋅s TJ [nb 8] units are sometimes called talbots
Luminous flux Φv [nb 7] lumen (= cd⋅sr) lm J [nb 8] also called luminous power
Luminous intensity Iv candela (= lm/sr) cd J [nb 8] an SI base unit, luminous flux per unit solid angle
Luminance Lv candela per square metre cd/m2 L−2J units are sometimes called nits
Illuminance Ev lux (= lm/m2) lx L−2J used for light incident on a surface
Luminous emittance Mv lux (= lm/m2) lx L−2J used for light emitted from a surface
Luminous exposure Hv lux second lx⋅s L−2TJ
Luminous energy density ωv lumen second per metre3 lm⋅sm−3 L−3TJ
Luminous efficacy η [nb 7] lumen per watt lm/W M−1L−2T3J 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.(Ed. emphasis added)

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.

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