11/22/2023 0 Comments Brilliance definition physics![]() ![]() Radiant flux emitted, reflected, transmitted or received by a surface, per unit solid angle per unit projected area. The latter is commonly measured in W⋅sr −1⋅nm −1. Radiant intensity per unit frequency or wavelength. Radiant flux emitted, reflected, transmitted or received, per unit solid angle. The latter is commonly measured in W⋅nm −1. Radiant flux per unit frequency or wavelength. This is sometimes also called "radiant power", and called luminosity in Astronomy. Radiant energy emitted, reflected, transmitted or received, per unit time. However, étendue may not decrease and radiant flux may not increase and, therefore, basic radiance may not increase. In real systems, the étendue may increase (for example due to scattering) or the radiant flux may decrease (for example due to absorption) and, therefore, basic radiance may decrease. Radiance of a surface, denoted L e,Ω ("e" for "energetic", to avoid confusion with photometric quantities, and "Ω" to indicate this is a directional quantity), is defined as L e, Ω = ∂ 2 Φ e ∂ Ω ∂ ( A cos θ ), This factor is obtained from the solid angle 2π steradians of a hemisphere decreased by integration over the cosine of the zenith angle. Its surface is Lambertian, so that its radiance is uniform with respect to angle of view, and is simply the Stefan–Boltzmann integral divided by π. For radiation emitted by the surface of an ideal black body at a given temperature, spectral radiance is governed by Planck's law, while the integral of its radiance, over the hemisphere into which its surface radiates, is given by the Stefan–Boltzmann law. Radiance is the integral of the spectral radiance over all frequencies or wavelengths. Spectral radiance expresses radiance as a function of frequency or wavelength. The light at the image plane, however, fills a larger solid angle so the radiance comes out to be the same assuming there is no loss at the lens. As an example, if you form a demagnified image with a lens, the optical power is concentrated into a smaller area, so the irradiance is higher at the image. For real, passive, optical systems, the output radiance is at most equal to the input, unless the index of refraction changes. ![]() This is sometimes called conservation of radiance. This means that for an ideal optical system in air, the radiance at the output is the same as the input radiance. The radiance divided by the index of refraction squared is invariant in geometric optics. The nonstandard usage of "brightness" for "radiance" persists in some fields, notably laser physics. This usage is now discouraged (see the article Brightness for a discussion). For this reason, radiance and luminance are both sometimes called "brightness". Since the eye is an optical system, radiance and its cousin luminance are good indicators of how bright an object will appear. In this case, the solid angle of interest is the solid angle subtended by the optical system's entrance pupil. Radiance is useful because it indicates how much of the power emitted, reflected, transmitted or received by a surface will be received by an optical system looking at that surface from a specified angle of view. "Intensity" has many other meanings in physics, with the most common being power per unit area.ĭescription Comparison of photometric and radiometric quantities It is especially dominant in heat transfer, astrophysics and astronomy. Historically, radiance was called "intensity" and spectral radiance was called "specific intensity". The related quantity spectral radiance is the radiance of a surface per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength. It is a directional quantity: the radiance of a surface depends on the direction from which it is being observed. ![]() The SI unit of radiance is the watt per steradian per square metre ( W Radiance is used to characterize diffuse emission and reflection of electromagnetic radiation, and to quantify emission of neutrinos and other particles. In radiometry, radiance is the radiant flux emitted, reflected, transmitted or received by a given surface, per unit solid angle per unit projected area. For other uses, see Radiance (disambiguation). ![]()
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