Color temperature

Color temperature measures the “warmth” or “coolness” of a light source by analogy to the temperature of an idealised black body that would emit similar light. The unit is the Kelvin (K).

Reference points:

• 1700 K: match flame, candlelight. Very warm.
• 2700 K: incandescent “soft white” bulb. Yellow-orange.
• 3000 K: warm-white LED. Slightly less yellow.
• 4000 K: “neutral white” LED, office fluorescent. Reads slightly cool.
• 5500 K: noon daylight. The reference for most photographic white-balance.
• 6500 K: overcast daylight, “D65” — the colour-space white point for sRGB.
• 10,000 K: blue sky on a clear day, distinctly cool blue.

Counter-intuitively, lower Kelvin values look warmer (more red/orange) and higher values look cooler (more blue). This is because a low-temperature black body emits mostly long-wavelength (red) light; a high-temperature body shifts toward shorter wavelengths.

Practical use:

• Light bulbs. Pick 2700-3000 K for warm residential, 3500-4000 K for kitchens and offices, 5000+ K for tasks needing daylight-accurate colour discrimination.
• Photography and video. White balance corrects the colour cast of the ambient light. Setting WB to match the actual colour temperature produces neutral colour reproduction; deliberately mismatching creates a warm or cool tone.
• Display calibration. Most graphic design uses D65 (6500 K) as the white point. Some print-prepress workflows use D50 (5000 K) to match the standard viewing booth.