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Radiance is used to characterize diffuse emission and reflection of electromagnetic radiation, and to quantify emission of neutrinos and other particles. The SI unit of radiance is the watt per steradian per square metre ( W·sr−1·m−2 ). It is a directional quantity: the radiance of a surface depends on the direction from which it is being ...
For an ideal absorber/emitter or black body, the Stefan–Boltzmann law states that the total energy radiated per unit surface area per unit time (also known as the radiant exitance) is directly proportional to the fourth power of the black body's temperature, T : The constant of proportionality, , is called the Stefan–Boltzmann constant.
Irradiance. In radiometry, irradiance is the radiant flux received by a surface per unit area. The SI unit of irradiance is the watt per square metre (W⋅m −2 ). The CGS unit erg per square centimetre per second (erg⋅cm −2 ⋅s −1) is often used in astronomy. Irradiance is often called intensity, but this term is avoided in radiometry ...
The cgs units of spectral radiance B ν are erg·s −1 ·sr −1 ·cm −2 ·Hz −1. The terms B and u are related to each other by a factor of 4π / c since B is independent of direction and radiation travels at speed c. The spectral radiance can also be expressed per unit wavelength λ instead of per unit frequency. In addition, the law may ...
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". Radiance of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅sr −1 ⋅m −2 ⋅nm −1.
Brightness temperature or radiance temperature is a measure of the intensity of electromagnetic energy coming from a source. In particular, it is the temperature at which a black body would have to be in order to duplicate the observed intensity of a grey body object at a frequency ν {\displaystyle u } . [2]
Formally, the wavelength version of Wien's displacement law states that the spectral radiance of black-body radiation per unit wavelength, peaks at the wavelength given by: where T is the absolute temperature and b is a constant of proportionality called Wien's displacement constant, equal to 2.897 771 955... × 10−3 m⋅K, [1] [2] or b ≈ ...
Emissivity of a body at a given temperature is the ratio of the total emissive power of a body to the total emissive power of a perfectly black body at that temperature. Following Planck's law, the total energy radiated increases with temperature while the peak of the emission spectrum shifts to shorter wavelengths.