A measure of how warm (yellow/orange) or cool (blue-white) a light source looks, usually given in Kelvin.
Correlated colour temperature (CCT) describes a light source's appearance relative to an idealised blackbody radiator; lower Kelvin values (around 2700K) look warm/yellow, higher values (5000K+) look cool/blue-white.
Counterintuitively, lower Kelvin numbers are 'warmer'-looking light and higher numbers are 'cooler'-looking, the opposite of everyday temperature intuition.
A 2700K bulb gives a warm, cosy light; a 5000K bulb gives a cooler, more clinical light.
The same wall colour can look noticeably different under a 2700K bulb versus a 5000K bulb — see the lighting guide.
The figure printed on a lamp is a correlated colour temperature: the temperature, in kelvin, of the ideal blackbody radiator whose colour is nearest to the lamp's on a uniform chromaticity diagram. A spectroradiometer or a chroma meter measures the light's chromaticity and the nearest point on the blackbody curve is then found by calculation. The CIE cautions that the idea stops being meaningful when the light's chromaticity lies far from that curve, which is why a green or magenta stage light has no sensible colour temperature.
| Illuminant | Chromaticity x | Chromaticity y | CCT |
|---|---|---|---|
| Incandescent tungsten, CIE A | 0.4475 | 0.4074 | 2860 K |
| Phosphor-converted white LED, CIE LED-B3 | 0.3758 | 0.3730 | 4110 K |
| Cool-white fluorescent, CIE FL2 | 0.3713 | 0.3731 | 4240 K |
| Daylight, CIE D50 (graphic arts viewing) | 0.3457 | 0.3586 | 5000 K |
| Daylight, CIE D65 | 0.3128 | 0.3291 | 6500 K |
Tungsten light (illuminant A) comes out at about 2860 K and D65 daylight at about 6500 K. The LED and the fluorescent tube are only 130 K apart and look alike as white light, yet their spectra are very different. A colour temperature describes the colour of the light and says nothing about how it is made up.
How this was calculated: Each illuminant's CIE spectrum is integrated against the 1931 2° observer at 10 nm to give a chromaticity, then converted with McCamy's approximation, which is reliable only close to the blackbody curve.
Replacement lamps of a different colour temperature are fitted in part of a room or along one shop aisle.
The eye adapts to one white at a time, so the areas lit by the other lamps look tinted, and a wall colour appears to change halfway along. Keep one colour temperature within a single view.
A paint colour is approved under the 4,000 K lighting of a showroom and lived with under 2,700 K lamps.
Blues and greys lose strength and creams gain it, as the spectra in the worked example for spectral power distribution predict. View the sample under the lamps the room actually has.