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The Planckian locus is derived by the determining the chromaticity values of a Planckian radiator using the standard colorimetric observer. The relative spectral power distribution (SPD) of a Planckian radiator follows Planck's law, and depends on the second radiation constant, c 2 = h c / k {\displaystyle c_{2}=hc/k} .
Planck radiation has a maximum intensity at a wavelength that depends on the temperature of the body. For example, at room temperature (~ 300 K), a body emits thermal radiation that is mostly infrared and invisible. At higher temperatures the amount of infrared radiation increases and can be felt as heat, and more visible radiation is emitted ...
(In contrast with Balfour Stewart's, Kirchhoff's definition of his absorption ratio did not refer in particular to a lamp-black surface as the source of the incident radiation.) Thus the ratio E(T, i) / a(T, i) of emitting power to absorptivity is a dimensioned quantity, with the dimensions of emitting power, because a(T, i) is dimensionless.
The solid curve with dots on it, through the middle, is the Planckian locus, with the dots corresponding to a few select black-body temperatures that are indicated just above the x-axis. The figures on the right show the related chromaticity diagram. The outer curved boundary is the spectral locus, with wavelengths shown in nanometers. The ...
The even spacing of the isotherms on the locus implies that the mired scale is a better measure of perceptual color difference than the temperature scale. The notion of using Planckian radiators as a yardstick against which to judge other light sources is not new. [6]
A black body radiator used in CARLO laboratory in Poland. It is an approximation of a model described by Planck's law utilized as a spectral irradiance standard. As the temperature of a black body decreases, its radiation intensity also decreases and its peak moves to longer wavelengths.
The Planckian locus on the MacAdam (u, v) chromaticity diagram. The normals are lines of equal correlated color temperature. The CIE 1960 color space ("CIE 1960 UCS", variously expanded Uniform Color Space, Uniform Color Scale, Uniform Chromaticity Scale, Uniform Chromaticity Space) is another name for the (u, v) chromaticity space devised by David MacAdam.
The Planckian locus is depicted on the CIE 1960 UCS, along with isotherms (lines of constant correlated color temperature) and representative illuminant coordinates By the time the D-series was formalized by the CIE, [ 12 ] a computation of the chromaticity ( x , y ) {\displaystyle (x,y)} for a particular isotherm was included. [ 13 ]