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color temperature

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color temperature (In French température de couleur) measure Tx in Kelvin [K] or Mired [M] (M = 1000000/K) for the color impression of a light source compared to the luminescent color which the ideal “blackbody” (Planck radiator) gives off at a certain temperature. Three cases must be distinguished: 1) the Planckian color temperature PCT TP (température de couleur de Planck) itself, which shows this ideal body with the full absorption of incident heat radiation, whereby it passes through the merging stadia of red heat, yellow heat, white heat and blue heat; this constant passage is represented as the Planck curve in x;y-, u*;v* and a*;b* chromaticity charts; for each TP a concrete spectral radiation distribution can be determined, characterized by a continuous (gap-free) and constant (peak-free) curve characteristic; TP corresponds to the true glow temperature T. 2) the color temperature of relative spectral power distribution (DCT) TD (température de couleur de la répartition spectrale relative d’énergie) of a glowing technical radiator (e.g. light bulb) which, while having the same chromaticity as the Planck radiator, also has a continuous spectral radiation distribution curve; TD is lower than the true glow temperature T of this technical radiator. 3) the nearest or correlated color temperature (CCT) Tn, Tc (température de couleur de corrélée) of a random light source (e.g. sun, fluorescent lamp, quartz lamp, LEDs), which agrees approximately with the Planck radiator only in the light coloring (that is, whose chromaticity approaches the Planck curve), but which has to show neither a continuous nor constant radiation distribution curve – see Tables C-2 “Correlated color temperature” on page 32; Tn,c can be very much lower than the true temperature T of this light source. In the simulation of the standard illuminant D50 in the media industry, the color temperature Tn = 5003 K = 199.9M is less authoritative than the color rendering index on account of the approximation to the desired relative spectral radiation distribution (metamerism!).