Radiometry is the science and technology of quantifying and measuring properties of electromagnetic radiation. That includes visible, infrared and ultraviolet light as well as radio waves and X-rays, for example. In contrast to photometry, the visibility of the radiation and its perceived brightness is not of interest in this field; one is dealing with purely physical quantities, not involving properties of the human eye.
Various terms used in radiometry are not identical to those which are common in optics and laser technology. The following table also specifies such alternative terms which are used particularly in optics:
|radiant energy||Qe||optical energy, pulse energy||joule (J)||total radiated energy, e.g. of a light pulse|
|radiant energy density||we||optical energy density||J/m3||applied e.g. to blackbody radiation|
|radiant flux||Φe||radiant power, optical power||watt (W = J/s)||radiant energy per unit time|
|spectral flux||Φe,ν or Φe,λ||optical power spectral density||W/Hz or W/nm||radiant flux per unit frequency or wavelength|
|radiant intensity||Ie,Ω||W/sr||radiant flux per unit solid angle|
|spectral intensity||Ie,Ω,ν or Ie,Ω,λ||W sr−1 Hz−1 or W sr−1 nm−1||radiant intensity per unit frequency or wavelength|
|radiance||Le,Ω||brightness (not recommended)||W sr−1 m−2||radiant flux per unit area and unit solid angle|
|spectral radiance||Le,Ω,ν or Le,Ω,λ||W sr−1 m−2 Hz−1 or W sr−1 m−2 nm−1||radiance per unit frequency or wavelength|
|irradiance||Ee||flux density||W/m2||received radiant flux on a surface|
|spectral irradiance||Ee,ν or Ee,λ||W m−2 Hz−1 or W m−2 nm−1||irradiance per unit frequency or wavelength|
|radiosity||Je||W/m2||radiant flux per unit area, leaving a surface (by emission, reflection or transmission)|
|spectral radiosity||Je,ν or Je,λ||W m−2 Hz−1 or W m−2 nm−1||radiosity per unit frequency or wavelength|
|radiant exitance||Me||W/m2||like radiosity, but counting only emitted radiation|
|spectral exitance||Me||W m−2 Hz−1 or W m−2 nm−1||radiant exitance per unit frequency or wavelength|
|radiant exposure||He||J/m2||received radiant energy per unit area, equal to the time-integrated irradiance|
|spectral exposure||He,ν or He,λ||J m−2 Hz−1 or J m−2 nm−1||radiant exposure per unit frequency or wavelength|
|hemispherical emissivity||ε||radiant exitance relative to that of a black body at the same temperature|
|hemispherical absorptance||A||fraction of absorbed radiant flux on a surface|
|hemispherical reflectance||R||fraction of reflected radiant flux on a surface|
|hemispherical transmittance||T||fraction of transmitted radiant flux on a surface|
|hemispherical attenuation coefficient||μ||m−1||fraction of absorbed or scattered radiant flux per unit length|
The subscript “e” of many of those quantities indicates that they refer to physical energies rather than to visual impressions (“v”) as in photometry.
Spectral and Integral Quantities
Some of those quantities are spectral quantities, referring to some unit frequency a wavelength interval. Their symbols contain “ν” or “λ” in the subscript. By integration of those over all optical frequencies or wavelengths, respectively, one obtains the corresponding integral quantities. For example, the radiant intensity equals the frequency- or wavelength-integrated spectral radiant intensity.
Note that it is not correct e.g. to integrate Φe,ν (a quantity referring to optical frequencies) over all wavelengths, simply using ν = c / λ. Even the resulting units would not be correct. One also needs to take into account the conversion from frequency to wavelength intervals. As we have
we must conclude that
where the conversion factor is wavelength-dependent.
Quantities Related to Solid Angles
There are also various quantities like Ie,Ω which refer to unit solid angles, and their integration over all solid angles (often only over a hemispherical region, i.e., a total solid angle of 2π) one obtains the corresponding integral quantities.
For some of the listed quantities, e.g. for the hemispherical absorptance, the corresponding spectral quantities or angle-resolved quantities are not listed in the table above; they are defined in completely analogous ways.
Various types of instruments can be used for measuring radiometric quantities:
- Optical power meters can be used for measuring a radiant flux, e.g. generated by a laser, and optical energy meters for a radiant energy.
- For highly divergent light, such devices are normally not usable. Here, one may need to use an integrating sphere in conjunction with a suitable detector, e.g. a bolometer or a pyroelectric detector.
- A photodiode can be used for measuring an irradiance, if it is calibrated for the wavelength of some quasi-monochromatic light and its active area is small enough for the required spatial resolution. The irradiance is the radiant flux divided by the active area. For measuring the irradiance over a larger area with high spatial resolution, one may use a kind of image sensor.
Related quantities such as the radiance of a light source are usually calculated from other measured values, e.g. from the irradiance at a location in some distance from the source.
The term radiometer can be understood as referring to any instruments measuring radiometric quantities. However, it is most common for instruments measuring quantities of invisible radiation.
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