Colour temperature in Kelvin (K) determines the light colour of a lamp. Normal commercial lamps have colour temperatures ranging from less than 3,300 Kelvin (warm white) through 3,300 to 5,300 Kelvin (neutral white) to over 5,300 Kelvin (daylight white).

The colour temperature of a light source is determined by comparison with a "black-body radiator". This is an "idealised" solid body – e.g. made of platinum – that absorbs all light that falls on it and thus has a reflective radiance of zero.

When a "black-body radiator" is slowly heated, it passes through a range of colours – from dark red, red, orange, yellow and white to light blue. The higher the temperature, the closer the colour to white. The temperature at which the colour of the "black-body radiator" resembles that of the tested light source is the correlated colour temperature of the lamp.