Light
Correlated Color Temperature
CCT
The temperature, in kelvin, of the blackbody radiator whose color most closely matches a light source. It describes appearance only: two sources at identical CCT can render colors very differently.
In more detail
CCT describes the apparent color of white light in kelvin, by naming the temperature of the blackbody radiator whose color it most closely matches. Lower numbers look warmer and more amber; higher numbers look cooler and bluer. Residential lamps commonly run 2200K to 4000K; overcast daylight is far higher.
The word correlated is doing real work. Most electric sources do not sit exactly on the blackbody locus, so CCT is the nearest match rather than a description. How far off it sits is reported as Duv, and a source with a noticeably positive or negative Duv can read green or pink at a nominally neutral CCT.
CCT is also silent about spectrum. Two lamps at the same 3000K can contain very different amounts of energy at different wavelengths, which is why they can render skin, wood and fabric differently, and why they can differ in the short-wavelength content relevant to circadian metrics. CCT describes appearance; it does not describe composition.
Why it matters
CCT is the lighting specification homeowners actually notice, because mixing color temperatures within one sightline is among the most visible mistakes in a finished house — 4000K recessed downlights beside 2700K decorative fixtures reads as a defect even to people who cannot name it. Consistency within a space, and a deliberate choice per room, does more for how a house looks than any individual fixture.
The common misunderstanding
That CCT indicates light quality or biological effect. It reliably predicts neither. Two 2700K sources can score very differently on CRI or TM-30, and warm-appearing sources can still carry meaningful short-wavelength energy. The quantities for those questions are CRI or TM-30 for rendering, and melanopic EDI for circadian-relevant content.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.