Radiometry
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Radiometry is a set of techniques for measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power in space, as opposed to photometric techniques, which characterize the light's interaction with the human eye. The fundamental difference between radiometry and photometry is that radiometry gives the entire optical radiation spectrum, while photometry is limited to the visible spectrum. Radiometry is distinct from quantum techniques such as photon counting.
The use of radiometers to determine the temperature of objects and gasses by measuring radiation flux is called pyrometry. Handheld pyrometer devices are often marketed as infrared thermometers.
Radiometry is important in astronomy, especially radio astronomy, and plays a significant role in Earth remote sensing. The measurement techniques categorized as radiometry in optics are called photometry in some astronomical applications, contrary to the optics usage of the term.
Spectroradiometry is the measurement of absolute radiometric quantities in narrow bands of wavelength.<ref>Template:Cite book</ref>
Radiometric quantitiesEdit
| Quantity | Unit | Dimension | Notes | ||
|---|---|---|---|---|---|
| Name | Symbol<ref group="{{#if:|{{{3}}}|nb}}" name="note-suffix-e">Standards organizations recommend that radiometric quantities should be denoted with suffix "e" (for "energetic") to avoid confusion with photometric or photon quantities.</ref> | Name | Symbol | ||
| Radiant energy | Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-alternative-symbol-radiometric">Alternative symbols sometimes seen: Template:Mvar or Template:Mvar for radiant energy, Template:Mvar or Template:Mvar for radiant flux, Template:Mvar for irradiance, Template:Mvar for radiant exitance.</ref> | joule | J | M⋅L2⋅T−2 | Energy of electromagnetic radiation. |
| Radiant energy density | Template:Math | joule per cubic metre | J/m3 | M⋅L−1⋅T−2 | Radiant energy per unit volume. |
| Radiant flux | Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-alternative-symbol-radiometric"></ref> | watt | W = J/s | M⋅L2⋅T−3 | Radiant energy emitted, reflected, transmitted or received, per unit time. This is sometimes also called "radiant power", and called luminosity in astronomy. |
| Spectral flux | Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-suffix-nu">Spectral quantities given per unit frequency are denoted with suffix "ν" (Greek letter nu, not to be confused with a letter "v", indicating a photometric quantity.)</ref> | watt per hertz | W/Hz | M⋅L2⋅T −2 | Radiant flux per unit frequency or wavelength. The latter is commonly measured in W⋅nm−1. |
| Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-suffix-lambda">Spectral quantities given per unit wavelength are denoted with suffix "λ".</ref> | watt per metre | W/m | M⋅L⋅T−3 | ||
| Radiant intensity | Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-suffix-omega">Directional quantities are denoted with suffix "Ω".</ref> | watt per steradian | W/sr | M⋅L2⋅T−3 | Radiant flux emitted, reflected, transmitted or received, per unit solid angle. This is a directional quantity. |
| Spectral intensity | Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-suffix-nu"></ref> | watt per steradian per hertz | W⋅sr−1⋅Hz−1 | M⋅L2⋅T−2 | Radiant intensity per unit frequency or wavelength. The latter is commonly measured in W⋅sr−1⋅nm−1. This is a directional quantity. |
| Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-suffix-lambda"></ref> | watt per steradian per metre | W⋅sr−1⋅m−1 | M⋅L⋅T−3 | ||
| Radiance | Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-suffix-omega"></ref> | watt per steradian per square metre | W⋅sr−1⋅m−2 | M⋅T−3 | Radiant flux emitted, reflected, transmitted or received by a surface, per unit solid angle per unit projected area. This is a directional quantity. This is sometimes also confusingly called "intensity". |
| Spectral radiance Specific intensity |
Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-suffix-nu"></ref> | watt per steradian per square metre per hertz | W⋅sr−1⋅m−2⋅Hz−1 | M⋅T−2 | Radiance of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅sr−1⋅m−2⋅nm−1. This is a directional quantity. This is sometimes also confusingly called "spectral intensity". |
| Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-suffix-lambda"></ref> | watt per steradian per square metre, per metre | W⋅sr−1⋅m−3 | M⋅L−1⋅T−3 | ||
| Irradiance Flux density |
Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-alternative-symbol-radiometric"></ref> | watt per square metre | W/m2 | M⋅T−3 | Radiant flux received by a surface per unit area. This is sometimes also confusingly called "intensity". |
| Spectral irradiance Spectral flux density |
Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-suffix-nu"></ref> | watt per square metre per hertz | W⋅m−2⋅Hz−1 | M⋅T−2 | Irradiance of a surface per unit frequency or wavelength. This is sometimes also confusingly called "spectral intensity". Non-SI units of spectral flux density include jansky (Template:Val = Template:Val) and solar flux unit (Template:Val = Template:Val = Template:Val). |
| Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-suffix-lambda"></ref> | watt per square metre, per metre | W/m3 | M⋅L−1⋅T−3 | ||
| Radiosity | Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-alternative-symbol-radiometric"></ref> | watt per square metre | W/m2 | M⋅T−3 | Radiant flux leaving (emitted, reflected and transmitted by) a surface per unit area. This is sometimes also confusingly called "intensity". |
| Spectral radiosity | Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-suffix-nu"></ref> | watt per square metre per hertz | W⋅m−2⋅Hz−1 | M⋅T−2 | Radiosity of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅m−2⋅nm−1. This is sometimes also confusingly called "spectral intensity". |
| Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-suffix-lambda"></ref> | watt per square metre, per metre | W/m3 | M⋅L−1⋅T−3 | ||
| Radiant exitance | Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-alternative-symbol-radiometric"></ref> | watt per square metre | W/m2 | M⋅T−3 | Radiant flux emitted by a surface per unit area. This is the emitted component of radiosity. "Radiant emittance" is an old term for this quantity. This is sometimes also confusingly called "intensity". |
| Spectral exitance | Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-suffix-nu"></ref> | watt per square metre per hertz | W⋅m−2⋅Hz−1 | M⋅T−2 | Radiant exitance of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅m−2⋅nm−1. "Spectral emittance" is an old term for this quantity. This is sometimes also confusingly called "spectral intensity". |
| Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-suffix-lambda"></ref> | watt per square metre, per metre | W/m3 | M⋅L−1⋅T−3 | ||
| Radiant exposure | Template:Math | joule per square metre | J/m2 | M⋅T−2 | Radiant energy received by a surface per unit area, or equivalently irradiance of a surface integrated over time of irradiation. This is sometimes also called "radiant fluence". |
| Spectral exposure | Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-suffix-nu"></ref> | joule per square metre per hertz | J⋅m−2⋅Hz−1 | M⋅T−1 | Radiant exposure of a surface per unit frequency or wavelength. The latter is commonly measured in J⋅m−2⋅nm−1. This is sometimes also called "spectral fluence". |
| Template:Math<ref group="{{#if:|{{{3}}}|nb}}" name="note-suffix-lambda"></ref> | joule per square metre, per metre | J/m3 | M⋅L−1⋅T−2 | ||
| See also: Template:Hlist | |||||
{{#if:||Template:Reflist}} Template:Radiometry coefficients
Integral and spectral radiometric quantitiesEdit
Integral quantities (like radiant flux) describe the total effect of radiation of all wavelengths or frequencies, while spectral quantities (like spectral power) describe the effect of radiation of a single wavelength Template:Mvar or frequency Template:Mvar. To each integral quantity there are corresponding spectral quantities, defined as the quotient of the integrated quantity by the range of frequency or wavelength considered.<ref name="ISO 2013 i869">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> For example, the radiant flux Φe corresponds to the spectral power Φe,Template:Mvar and Φe,Template:Mvar.
Getting an integral quantity's spectral counterpart requires a limit transition. This comes from the idea that the precisely requested wavelength photon existence probability is zero. Let us show the relation between them using the radiant flux as an example:
Integral flux, whose unit is W: <math display=block>\Phi_\mathrm{e}.</math> Spectral flux by wavelength, whose unit is Template:Nobreak: <math display=block>\Phi_{\mathrm{e},\lambda} = {d\Phi_\mathrm{e} \over d\lambda},</math> where <math>d\Phi_\mathrm{e}</math> is the radiant flux of the radiation in a small wavelength interval <math>[\lambda - {d\lambda \over 2}, \lambda + {d\lambda \over 2}]</math>. The area under a plot with wavelength horizontal axis equals to the total radiant flux.
Spectral flux by frequency, whose unit is Template:Nobreak: <math display=block>\Phi_{\mathrm{e},\nu} = {d\Phi_\mathrm{e} \over d\nu},</math> where <math>d\Phi_\mathrm{e}</math> is the radiant flux of the radiation in a small frequency interval <math>[\nu - {d\nu \over 2}, \nu + {d\nu \over 2}]</math>. The area under a plot with frequency horizontal axis equals to the total radiant flux.
The spectral quantities by wavelength Template:Mvar and frequency Template:Mvar are related to each other, since the product of the two variables is the speed of light (<math>\lambda \cdot \nu = c</math>):
- <math>\Phi_{\mathrm{e},\lambda} = {c \over \lambda^2} \Phi_{\mathrm{e},\nu},</math> or <math>\Phi_{\mathrm{e},\nu} = {c \over \nu^2} \Phi_{\mathrm{e},\lambda},</math> or <math>\lambda \Phi_{\mathrm{e},\lambda} = \nu \Phi_{\mathrm{e},\nu}.</math>
The integral quantity can be obtained by the spectral quantity's integration:
<math display=block>\Phi_\mathrm{e} = \int_0^\infty \Phi_{\mathrm{e},\lambda}\, d\lambda = \int_0^\infty \Phi_{\mathrm{e},\nu}\, d\nu = \int_0^\infty \lambda \Phi_{\mathrm{e},\lambda}\, d \ln \lambda = \int_0^\infty \nu \Phi_{\mathrm{e},\nu}\, d \ln \nu.</math>
See alsoEdit
- Reflectivity
- Microwave radiometer
- Measurement of ionizing radiation
- Radiometric calibration
- Radiometric resolution
ReferencesEdit
External linksEdit
- Radiometry and photometry FAQ Professor Jim Palmer's Radiometry FAQ page (The University of Arizona College of Optical Sciences).