Making light understandable, part 2, CRI

Today I will discuss with CRI. It stands for Color Rendering Index. This index will give you a measure on how good colors can be recognized when lighted by a specific light source. Why do we need an index giving us a measure for that.

When looking at the spectrum of daylight all colors (or wave lengths) in the spectrum are being represented. This makes that we can see the color when reflected from a colored surface. If the color of the actual surface is not present in the light spectrum it can also not reflect it and the color will look different.

An artificial light source should therefore sent out light in the whole spectrum to assure that the objects look the way they should look. The CRI was therefore invented. To make this work you need a reference light source and in that time the best light source that was available was the incandescent lamp. It sounds like daylight is better, it actually is but on a sunny day or a cloudy day the light will have a different spectrum whereas if you have a defined incandescent lamp you have the same spectrum every time again.

So what is done is to compare the color difference when looked at it with incandescent lighting and with the alternative light source. The differences are than calculated into a index. This index is 100% if the colors are the same. If the colors are different the index will go down.

In real life it means when the index is above 90 the color rendering is good. The CRI is under debate since it can be that the color rendering is better than with incandescent lamps but the measurement will give a lower percentage. This index therefore needs to be reinvented. This is being handled by the committees but it will take time before the committee will agree on the new standard. Hopefully it will be available soon.

For now the CRI index is the best we have and it gives a fairly good indication on how good colors will be recognized under the artificial light source. In the pictures you see some spectra of different light sources. For example the low pressure sodium lamp has basically only yellow light and has an low CRI. The incandescent lamp has a very complete spectrum and therefore has a good color rendering index (100).

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In the LED you can see a big peak at the blue spectrum. This is one of the spectra but there are many others. The technology is developing in such a pace that the color rendering and the spectra emitted are becoming better and better.

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