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The SI unit of spectral radiance in frequency is the watt per steradian per square metre per hertz (W·sr −1 ·m −2 ·Hz −1) and that of spectral radiance in wavelength is the watt per steradian per square metre per metre (W·sr −1 ·m −3)—commonly the watt per steradian per square metre per nanometre (W·sr −1 ·m −2 ·nm −1).
Radiant intensity is used to characterize the emission of radiation by an antenna: [2], = (), where E e is the irradiance of the antenna;; r is the distance from the antenna.; Unlike power density, radiant intensity does not depend on distance: because radiant intensity is defined as the power through a solid angle, the decreasing power density over distance due to the inverse-square law is ...
Spectral radiance Specific intensity L e,Ω,ν [nb 3] 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 radiance Specific intensity L e,Ω,ν [nb 3] 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 ...
The measurement can be made directly with an instrument (such as a pyrgeometer) that collects the measured radiation all at once from all the directions of the imaginary hemisphere; in this case, Lambert-cosine-weighted integration of the spectral radiance (or specific intensity) is not performed mathematically after the measurement; the ...
The spectral flux density or monochromatic flux, S, of a source is the integral of the spectral radiance, B, over the source solid angle: = (,). The unit is named after pioneering US radio astronomer Karl Guthe Jansky and is defined as
Mathematically, for the spectral power distribution of a radiant exitance or irradiance one may write: =where M(λ) is the spectral irradiance (or exitance) of the light (SI units: W/m 2 = kg·m −1 ·s −3); Φ is the radiant flux of the source (SI unit: watt, W); A is the area over which the radiant flux is integrated (SI unit: square meter, m 2); and λ is the wavelength (SI unit: meter, m).
A common application would be measurement of a galaxy's surface brightness profile, meaning its surface brightness as a function of distance from the galaxy's center. For small solid angles, a useful unit of solid angle is the square arcsecond , and surface brightness is often expressed in magnitudes per square arcsecond.