Colour temperature

Colour temperature is used to describe the colour of conventional light sources, which are difficult to define in terms of light wavelength due to their colour spectrum. Colour temperature is a useful aid and is measured in kelvin. A colour temperature of approximately 3,000 K is equivalent to a warm, reddish light colour. 6,000 K is equivalent to a cold colour similar to that of natural daylight.

Colour temperature is defined as the temperature of an ideal black body, the precise radiance of which is known (Planckian radiation formula). The hotter this black body is, the shorter the wavelengths of the light it radiates. A cold body radiates light almost exclusively in infra-red wavelengths. As a body heats up, it glows first red then yellow, white and finally blue.

Conventional light sources are thermal radiators. Sources that have an identical colour temperature mostly also exhibit the same spectral wavelength intensity distribution.

Here are a few common colour temperatures for natural and artificial sources:

• 1,900 K: Candle flame
• Up to 3,300 K: Warm white
• 2,600 K to 3,000 K: Incandescent lamp (40 to 100 W)
• Over 3,900 K: Fluorescent lamp
• Up to 5,000 K: Neutral white
• 5,100 K to 5,400 K: Midday sun (summer)
• 6,500 K: Overcast northern sky
• 6,500 K: Xenon short-arc lamp (Standard illuminant C)
• 5,000 K to 6,800 K: Daylight
• 9,900 K to 27,000 K: Blue sky

Synyonme

• Warm white
• Neutral white
• Daylight