What Is Color Temperature?

Color temperature is the temperature of an ideal blackbody radiator whose emitted light has the same chromaticity as a given light source. It is expressed in kelvin, and it condenses the spectral character of a white light into a single number: sources near 2700 K appear warm and yellowish, daylight sits near 5500 to 6500 K, and higher values appear bluish. The convention is counterintuitive, since hotter radiators produce bluer light while everyday language calls blue light cool.

The concept comes from Planck's law, which fixes the spectral radiance of a blackbody at each temperature. Plotting the chromaticity of blackbody radiation across a range of temperatures traces a curve through the CIE chromaticity diagram known as the Planckian locus. Incandescent and halogen lamps track that locus closely because a heated tungsten filament is a reasonable approximation of a thermal radiator. Fluorescent lamps, light-emitting diodes, and gas-discharge sources do not, since their emission comes from atomic transitions and phosphor conversion rather than thermal equilibrium, which is why a second and less strict measure is needed for them.

Correlated Color Temperature

For a source whose chromaticity lies near but not on the Planckian locus, the quantity used is correlated color temperature, or CCT, defined as the temperature of the blackbody whose chromaticity is closest to that of the source in the CIE 1960 uniform chromaticity scale. The SPIE reference treatment of color temperature and CCT notes that perceived differences scale with the reciprocal of temperature rather than the temperature itself, so lighting practice often uses reciprocal megakelvin units, in which one unit is roughly one just-noticeable difference. It also makes the key caveat explicit: many quite different spectra share a single CCT value, so two lamps rated at 3000 K can render the same object very differently.

Distance From the Locus and Measurement

Because CCT collapses a two-dimensional chromaticity onto a one-dimensional scale, it must be paired with a measure of how far the source sits from the locus. That distance, denoted Duv, carries the sign of the offset, with positive values appearing greenish and negative values pinkish. A Department of Energy method for determining CCT and Duv documents a closed-form calculation that replaces the older practice of numerically searching the locus, and product specifications such as ANSI C78.377 define chromaticity quadrangles around nominal CCT values so that lamps sold at a given rating fall within a bounded region rather than merely reporting the same number. Traceable measurement rests on spectroradiometry and integrating-sphere photometry, and the NIST solid-state lighting metrology program provides calibration services for chromaticity coordinates, CCT, luminous flux, and color rendering index against national standards.

Practice in Lighting and Imaging

In lighting design, CCT is chosen alongside color rendering metrics and illuminance, and tunable white luminaires now vary it through the day to follow circadian patterns. In photography and video, the same number appears as a white balance setting: a camera estimates the illuminant color temperature, either from a reference target or through automatic color constancy algorithms, and applies gains so that neutral surfaces record as neutral. Display specifications state a white point in the same terms, with 6500 K corresponding to the D65 illuminant used by most broadcast and computer graphics standards, while digital cinema projection conventionally targets the DCI white point, which sits near 6300 K and slightly off the Planckian locus.

Applications

Color temperature is used across several fields, including:

• Architectural and horticultural lighting design
• LED and lamp manufacturing, binning, and quality control
• Camera white balance and automatic color constancy
• Display and projector calibration
• Photometric and radiometric metrology
• Astronomy, where stellar effective temperature is inferred from color indices