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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).
Relative spectral power distributions (SPDs) of CIE illuminants A, B, and C from 380 nm to 780 nm. A standard illuminant is a theoretical source of visible light with a spectral power distribution that is published. Standard illuminants provide a basis for comparing images or colors recorded under different lighting.
Within the visible spectrum, wavelengths of light are weighted according to a function called the "photopic spectral luminous efficiency." According to this function, 700 nm red light is only about 0.4% as efficient as 555 nm green light. Thus, one watt of 700 nm red light is "worth" only 2.7 lumens.
When launching light into the Fabry–Pérot resonator, by measuring the Airy distribution, one can derive the total loss of the Fabry–Pérot resonator via recalculating the Lorentzian linewidth , displayed (blue line) relative to the free spectral range in the figure "Lorentzian linewidth and finesse versus Airy linewidth and finesse of a ...
The specific (radiative) intensity is a radiometric concept. Related to it is the intensity in terms of the photon distribution function, [5] [24] which uses the metaphor [25] of a particle of light that traces the path of a ray. The idea common to the photon and the radiometric concepts is that the energy travels along rays.
The reflective and transmissive cases are very similar to the emissive case, with a few differences. The spectral radiance L e,Ω,λ is replaced by the spectral reflectance (or transmittance) S(λ) of the object being measured, multiplied by the spectral power distribution of the illuminant I(λ).
There are two luminous efficiency functions in common use. For everyday light levels, the photopic luminosity function best approximates the response of the human eye. For low light levels, the response of the human eye changes, and the scotopic curve applies. The photopic curve is the CIE standard curve used in the CIE 1931 color space.
The Eddington approximation can be used to obtain the spectral radiance in a "plane-parallel" medium (one in which properties only vary in the perpendicular direction) with isotropic frequency-independent scattering. Extracting the first few moments of the spectral radiance with respect to yields