Kelvin colour temperature scale

On the product datasheet of LED lighting luminaires, you'll find the colour temperature in Kelvin. But how can colour have a temperature? We explain it to you simply here.

Reading time: 3'00"

White light from black emitters

When a blacksmith makes a piece of iron red hot, it begins to emit light. At about 500°C, the iron becomes "red hot," and above 1000°C it becomes "white hot."

This phenomenon in which an object begins to radiate as it gets hotter is called blackbody radiation. 

Thus, from the colour of a black emitter, one knows its temperature.

Conversely, one can also characterize white light by indicating the temperature of a black emitter to which it would correspond.

And to complicate matters completely, this temperature is not expressed in degrees Celsius, but rather in Kelvin.

The light colour of a black emitter tells us its temperature. At high temperatures, black emitters radiate blue-white, while at lower temperatures they radiate red. This is the reverse of the cultural association of colours, where red is "warm," and blue is "cold."

White light at an incandescent bulb

A traditional light bulb also used the effect of the black emitter. An electric current heats up the filament of the bulb, and like glowing iron, the filament begins to emit light.

The temperature of the filament of a brightly burning light bulb will be about 2500°C (or 2800 K). So by dimming the bulb, you change not only the amount of current, but also the light colour of an incandescent bulb.

White light from fluorescent and led

With fluorescent and LED lamps, light is generated in a different way than an incandescent lamp. In both light sources, blue light is generated and beamed onto phosphor. It is the fluorescent effect of phosphor that generates the white light.

By using the right mixtures of phosphors, one can influence the light colour. Thus, one can make LEDs that emit light with the same colour as a black emitter at 3000 K, 4000 K, 5000 K... and without the LED having to reach that temperature.

White light in nature

The atmosphere scatters and refracts sunlight, and that changes its colour. Ordinary daylight has a light colour around 6500 K. Morning red has a light colour of 3000 K, and a bright blue sky can have a light colour of 16000 K and more.

Colour temperature at ETAP

Most of ETAP's fixtures are available as standard in a version with 3000 Kelvin or 4000 Kelvin colour temperature. However, by selecting the right LEDs, we can also offer devices with a different light colour, or even devices where the light colour varies: "Tunable White". Ask your ETAP representative about this!

For ETAP's emergency lighting, we look for LEDs with the highest efficiency, so that the fixtures provide as much and as long-lasting a light as possible with the limited energy content of the battery. We then require LEDs with higher colour temperatures, typically 5000 K and 6500 K.

…what about coloured light?

Denoting light colour by a temperature only makes sense for white light that approximates the spectrum of a black emitter. For other colours there are other methods of indicating them (e.g., the CIE 1931 colour space), but that will be the subject of another article. ✍