Yellow light is hot

Color temperature

The higher the color temperature,
the "colder" the light is

  • Candle flame: ~ 2000 K
  • Classic light bulb:
    ~ 2700 K
  • Halogen light bulb:
    ~ 2900 K
  • "Energy saving" fluorescent lamps:
    from 2400 K to 6500 K.
  • Led Lamps:
    from 2400 K to 6500 K.


White color

In the lighting department there are Color temperature Information about the general color of the light that the lamp produces: from the so-called "warm" colors with a predominant red component, in which the objects are illuminated like at sunrise or sunset, to the "cold" colors with a predominant blue component, which corresponds to a light like that of the intense sunlight at noon.

The Color temperature is in Kelvin (K) indicated. Here are a few examples: 2700 K corresponds to the yellowish light of a classic light bulb, 2900 K corresponds to the slightly light yellow light of a halogen lamp, 4000 K corresponds to a neutral white and 6000 K to a light with a high proportion of blue, as is the case at noon when the sun is at its highest point outdoors. The higher the color temperature, the greater the blue component of the light. The light at noon in the tropics has a very large proportion of blue.

From red to blue by heating!

When a blacksmith gradually heats up a piece of metal, it first becomes red-hot, then yellow, white and finally blue: the light emitted by the metal shows its temperature. This is why a reddish candle flame is less hot than the blue gas flame on a kitchen stove. And the filament of a classic light bulb with yellow light is less hot than that of a halogen light bulb, which produces a brighter, pale yellow light. It is common practice to describe the light at noon as "cold" light, although it is much more energetic than the "warm" morning or evening light.

Compact fluorescent lamps and LEDs do not produce their light with a highly heated, incandescent filament. With these types of lamps, chemical substances containing phosphorus are set in motion, which enables the lamp to emit a whole spectrum of visible light rays, the mixture of which we perceive as white light. Depending on the phosphor mixture, a color tone of the lamp light can be selected, and in this way the entire range of color temperatures of the light can be reproduced: from sunrise to the midday sun in the tropics. This temperature is specified in Kelvin on the packaging of the light bulbs (and often also on the light bulbs themselves). The color temperatures are given in words as follows:

  • Warm white (warm white) ~ 2700 K
  • White (neutral white) ~ 3000-4000 K
  • Cool white (cold-white) ~ 5000 K
  • Daylight (Daylight, daylight white)> 6500 K

Code number of the color temperature

On the fluorescent lamps ("neon" and compact fluorescent or "energy-saving lamps") as well as on some LEDs, the color temperature is indicated with a code number (827, 840, 930 ...), which indicates the Color temperature and the IRC color rendering index combined. Only the last two digits concern the color temperature. So the code means830 an IRC of at least80 and a color temperature of3000 K. And the code927 means an IRC of at least90 and a color temperature of2700 K.

Color temperature and IRC color rendering index are two very different things that are closely related to one another: just because a fluorescent tube or an LED lamp is labeled "daylight" does not mean that it has a good color rendering capacity.

Our brain wants to see white

Our brain is so predisposed that it automatically brings the colors we perceive into a certain correspondence. For example, if a living room wall painted white is illuminated by a classic incandescent lamp, we see the walls in white - even if they are clearly yellow for an instrument with which the light is measured. With its automatic "white balance" function, a digital photo camera works according to the same principle as our brain: with this function, the camera determines the color temperature and compares it with the existing light conditions so that the living room walls still appear white in the picture. even if they are illuminated with lamps of different color temperatures.

If you want to illuminate a room, it is important to take into account the color temperature of the various lamps. If the color temperatures are matched to one another, our brain does not have to manage different types of "white". Conversely, lamps of different color temperatures can of course also be combined with one another to create a special ambience.


Lumens and Lux

IRC color rendering index

The light spectrum of lighting fixtures

Additive color mixing-RGB

Choosing lightbulbs - with the sun as a model