Color temperature

Also called colour temperature, CCT, correlated color temperature, kelvin, light color

Color temperature is the number in kelvin that says whether a white light looks warm and yellowish or cool and bluish, and it describes appearance only, not how bright a light is or how far it reaches.

What is color temperature?

Heat a piece of metal and it glows: dull red, then orange, then yellow-white, then bluish-white. Color temperature borrows that scale. A light described as 3000K looks like metal at 3000 kelvin, which is warm and yellowish. 6500K looks like an overcast sky, which is cool and bluish.

The full name for the version quoted on LED products is correlated colour temperature, because an LED is not a glowing lump of metal and the figure is a match rather than a measurement. NIST puts the definition plainly: the temperature of the Planckian radiator whose perceived color most closely resembles that of the light source in question. The International Lighting Vocabulary states the same thing formally, as the temperature of the Planckian radiator whose chromaticity is nearest to the source on a modified 1976 uniform-chromaticity-scale diagram.

The scale runs backwards from how people talk about warmth. Higher numbers are bluer and are described as cooler. Lower numbers are yellower and are described as warmer.

What it does not tell you

Brightness. A 6500K lamp is not brighter than a 5000K one, and the perception that it is comes from blue light appearing harsher rather than from more of it arriving. Manufacturers know this, which is why a certain kind of cheap light bar is very blue.

It also tells you nothing directly about colour rendering, which is a separate measure of how faithfully a light shows the colours of what it hits. Two 5000K lamps can render a rock face very differently.

What the off-road lamp makers pick

Baja Designs make their forward emitters exclusively 5000K daylight white, and give their reasons as reduced driver fatigue and better terrain recognition. That is roughly the middle of the range and roughly neutral daylight, and it is where most serious off-road lighting has landed.

Very blue light is the one worth avoiding, and the reason is glare rather than scattering. Scattering by fog droplets and dust particles is only weakly dependent on colour, because they are much larger than a wavelength of light, but blue-heavy light is harder on the eyes over a long night at the wheel, which is the argument Baja Designs make for staying at neutral daylight. Amber is the other end of the same conversation, and it is a separate entry because the claims made for it are separate.