Many people may have already seen that in a room that is completely illuminated in red, red things can hardly be seen, if at all. Red is therefore not reproduced in red light, it absolutely does not stand out from its surroundings, with blue looking completely black. The whole thing has to do with the color rendering of a light source. This color rendering is described in an index. Often there is also the abbreviation “CRI”, which is simply the English term for it. CRI is simply the abbreviation for "Color Rendering Index". In German, CRI is translated as “color rendering index” (abbreviation “Ra”). However, both are the same.

All well and good, but what is the CRI value and what does it say?

What does the CRI value mean?

As I already indicated in the introduction, color perception depends very much on the light source. For example, sunlight has the best color rendering there is. Each color looks very natural and that is exactly our reference. So if we go back into the house with a colorful shirt and look at ourselves in the mirror, but only a cheap energy-saving lamp is hanging from the ceiling, it could happen that the colors are very dull, or the shirt, which is still nice in the sun looked, suddenly looks pretty stupid. But that has nothing to do with the fact that the shirt has changed. After all, we haven't changed anything on the shirt, just our location. And this is exactly where the CRI value comes into play. The CRI value characterizes every light source, how well it can reproduce the colors compared to the sun. The higher the CRI or Ra value, the more natural and pleasant colors are reproduced or perceived. The lower the CRI value, the more the colors are distorted by the light source.

What are typical values ​​of the CRI index?

Since the sun is our reference and it is the only light source on earth that will probably never change in the course of our existence, the sun gets the maximum CRI value of 100. Black light would therefore have a color rendering of almost 0, because UV light can well hardly reproduce red, green or blue. We'd rather take it all as black.

Typical values ​​are, for example:

Incandescent lamp up to 100

Halogen lamp up to 98

White LEDs 70-95

Fluorescent lamp 50-90

High pressure mercury vapor lamp 45-50

Metal halide lamp 60-95

High pressure sodium lamp 18-30

With LEDs especially in the high-power range, 100 is not yet achieved. The best values ​​are 93-95, e.g. the Osram SSL or the Ostar.

There is also a classification for the color rendering index. An Ra value of 95 is classified as excellent, one of 90 as fair and one of 80 as bad. The color rendering index differs from the color temperature, which distinguishes between cold and warm light. A warm white LED can therefore be just as good in terms of color rendering as a cold white LED.

Why do warm white LEDs usually have a higher CRI value than cold white LEDs?

This is actually more due to the fact that every high power LED actually only emits blue light. White or warm white is only created by a fluorescent layer that is applied over the glowing crystal. (This is also nicely illustrated in this video on LED production.) This layer converts the blue light into various other wavelengths (colors). Warm white LEDs have a thicker layer or even several such layers, which then also contribute to a higher CRI value. However, the efficiency of the LED decreases, because losses occur every time a photon is converted. The narrower the band and thus the more monochromatic the light radiation, the lower the CRI value.

How can you find out the CRI value of an LED?

Normally this value should be found on the packaging or in the data sheet. Here you just have to trust the manufacturer's information. If the value is not on it, the whole thing becomes a bit very complex or rather simple. Simply because you have to compare 14 different colors. Once in sunlight and once under the influence of the artificial light source. Complicated because it's not that easy at all.

To calculate the CRI, the 14 test colors with a standardized remission curve (the light reflected by a body) are used, which are defined by DIN 6169. If you now calculate the CRI for a light source, the deviations in the secondary spectra are compared with those of the 14 test colors and serve as a measure. Spectral ranges that are outside the visible range are not taken into account.

If that was too imprecise for you, you are welcome to take a look at this presentation from Minolta. It is kept very scientific, but if you are really interested in calculating the CRI value, you will get it here