Colour temperature describes the hue of white light on a scale measured in kelvin (K). It comes from physics: heat a theoretical black body and it glows — first dull red, then orange, yellow, white, and finally blue-white as it gets hotter.
The counter-intuitive part: low kelvin looks warm, high kelvin looks cool. A candle sits near 1800K and feels cosy; an overcast sky is 7000K+ and feels clinical. Our language for colour ("warm", "cool") is the opposite of the temperature scale.
The numbers worth memorising
- 1800–2000K — candlelight, firelight. Deep amber.
- 3200K — tungsten. The classic theatre "warm" and the reference white for stage.
- 4000K — neutral white, common in architectural LED.
- 5600K — daylight. The reference for film and "cool" stage looks.
- 6500K+ — overcast / shade. Blue-white.
Why it matters on stage
Mixing fixtures of different colour temperatures in the same wash reads as a mistake — one unit looks green or pink against the others. When you spec a rig, keep your white reference consistent, or use tunable-white fixtures so you can match.
CRI, TM-30 and why cheap LED looks wrong
CRI (Colour Rendering Index) scores how faithfully a source renders colour against a reference, 0–100. Cheap LED can hit a pleasant colour temperature yet render skin and costume poorly because its spectrum has gaps. TM-30 is the newer, more honest metric, reporting fidelity (Rf) and saturation (Rg) separately.
Correction filters: CTO (Colour Temperature Orange) warms a daylight source toward tungsten; CTB (Blue) cools tungsten toward daylight. Many moving heads carry these as dedicated CTO/CTB channels so you can match a tungsten wash without swapping anything.