Color Temperature Calculator

Color temperature describes a light source by naming the temperature, in kelvin, of an ideal black body radiator whose emitted color matches it. The underlying physics is Planck radiation law, which traces a curved path called the Planckian locus across the CIE 1931 chromaticity diagram. Familiar anchor points are a candle flame near 1850 K, a tungsten incandescent lamp at 2700 K, studio tungsten at 3200 K, the D50 illuminant at 5000 K used for print viewing booths, the D65 display white point at 6504 K, and clear north sky above 10000 K. The mired figure, one million divided by kelvin, exists because kelvin steps are perceptually uneven, and photographic correction filters are still specified in mired shift, with a Wratten 80A rated at about minus 131 mireds to take 3200 K tungsten up to daylight balance. Most real sources are not black bodies, so what is quoted is a correlated color temperature, meaning the nearest point on the Planckian locus, and the perpendicular distance from that locus is reported as Duv. ANSI C78.377, the chromaticity standard for solid state lighting products, keeps nominal bins within roughly plus or minus 0.006 Duv, and a lamp outside that range looks visibly green or pink even though its CCT label reads normally. The kelvin to RGB conversion used here is a curve fit to black body data rather than a colorimetric calculation, and because it clips red at 255 below about 6600 K it shows the hue shift but not the fact that a 2000 K source is also genuinely dimmer.

The absolute temperature scale that gives the unit its name was proposed by William Thomson, later Lord Kelvin, in 1848. The physics that connects temperature to color came from Max Planck, whose 1900 radiation law resolved the ultraviolet catastrophe and gave an exact spectrum for a heated black body. Because equal kelvin steps produce very unequal visual shifts, researchers at the United States National Bureau of Standards introduced the reciprocal scale in the early 1930s, and the contraction mired for micro reciprocal degree came into general use among photographic scientists shortly afterwards. Cinema and still photography turned the theory into a working craft. Kodak sold gelatin correction filters under the Wratten name, after the London firm of Frederick Wratten that Eastman Kodak acquired in 1912, calibrated in mired shift so a cinematographer could move any source to a known balance. The CIE issued its test color sample method for color rendering in 1965 and revised it as CIE 13.3 in 1974 and 1995, defining the eight pastel samples behind an Ra score. The rise of white LEDs, whose spectra depart sharply from any black body, forced two further standards: ANSI C78.377 first issued in 2008 to bound acceptable chromaticity, and the Illuminating Engineering Society TM-30 method published in 2015 and revised in 2020. The compact kelvin to RGB approximation used by most software tools, including this one, was published by Tanner Helland in 2012.