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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 ...
The divergence of a vector field which is the resultant of radial inverse-square law fields with respect to one or more sources is proportional to the strength of the local sources, and hence zero outside sources. Newton's law of universal gravitation follows an inverse-square law, as do the effects of electric, light, sound, and radiation ...
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 ...
Such a surface has a constant radiance/luminance, regardless of the angle from which it is observed; a single human eye perceives such a surface as having a constant brightness, regardless of the angle from which the eye observes the surface. It has the same radiance because, although the emitted power from a given area element is reduced by ...
The SI unit of irradiance is the watt per square metre (symbol W⋅m −2 or 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 where such usage leads to confusion with radiant intensity .
Etendue is the product of geometric extent and the squared refractive index of a medium through which the beam propagates. [1] Because angles, solid angles, and refractive indices are dimensionless quantities, etendue is often expressed in units of area (given by dS). However, it can alternatively be expressed in units of area (square meters ...
The radiant exitance (previously called radiant emittance), , has dimensions of energy flux (energy per unit time per unit area), and the SI units of measure are joules per second per square metre (J⋅s −1 ⋅m −2), or equivalently, watts per square metre (W⋅m −2). [2] The SI unit for absolute temperature, T, is the kelvin (K).
Radiance: L e,Ω [nb 5] 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 L e,Ω,ν ...