The Colour Rendering Index (CRI)

We felt it is high time to write a brief blog article about the so-called “colour rendering index“, or CRI as its abbreviation is called. The CRI is often quoted in “Ra” (general color rendering index).

 

What does Wikipedia have to say about the CRI: “The color rendering index (CRI) (sometimes called color rendition index), is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source. Light sources with a high CRI are desirable in color-critical applications such as photography and cinematography. It is defined by the International Commission on Illumination as follows: ‘Color rendering: Effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant’.”

 

As this sounds a wee bit complicated, we’ve prepared an article which (hopefully) explains a bit mor simple what the CRI is and does. First of all, please note that the CRI is different from the light colour or colour temperature of the lamp. Even so-called “daylight” bulbs may have a comparatively low CRI (as per full-spectrum standards anything below 90 is seriously low) and it is the CRI rather than the colour temperature (measured in “K” for “Kelvin” by the way) which makes the real difference when it comes to the light spectrum and ultimately how good you’ll see and be able to do visual tasks under such lighting.

 

Researchers in Sweden found out that with the CRI being around or above 96 Ra, it is quite possible that we see up to 30% better when compared to lower quality light sources. So what light sources are those which are preferrable for a good CRI which are those who are not:

 

Colour Reproduction Rating Class
(EN 124645)
CRI
(RA)
Typical Lamps
Very good 1A
1B
>90
80-89
Full & Natural Spectrum Lamps
Metal Halide and Daylight Lamps
Good 2A
2B
70-79
60-69
Normal Fluorescent Lamps (Colour 10/25)
Normal Fluorescent Lamps (Colour 30)
Accepted 3 40-59 Mercury-vapor Lamp
Not acceptable 4 <39 Gas-filled Tube

 

I light source with a CRI = 100 Ra would be perfect and balanced in terms of colour reproduction, shading, contrast etc. – therefore the lesser the difference from 100 Ra, the better will colours be reproduced under such light. The lower the Ra-value, the worse colours will reproduced and the more difficult it is to see and work under such light.

 

A last but important word on the note of colour reproduction:

 

When comparing different light sources, it is only recommendable to compare them in the same colour temperature. It makes therefore not much sense to compare an incandescent 2700K bulb with a 6500K daylight energy saving lamp. For a proper comparison – either the incandescent lamp should be also rated at 6500K colour temperature or the energy saving lamp should be in the 2700K range. Then – and then only – true colour reproduction comparison can take place.

 

We hope this helps to clear up some confusion around the terms colour reproduction and colour temperature. Thanks for reading :-)

 

 

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  2. Stop Flirting & Commit : Rules of Engagement With Colour
  3. Full-spectrum Light and Colour Perception
  4. Should you care what light your dentist is using?
  5. A funny Colour and Reading Speed Test

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