Colour Rendering Index (CRI)

Knowledge 18/04/2019
colouring rendering index

What is CRI

Colour Rendering Index (CRI), is used to measure how colours look to the human eye. It represents how faithfully colours are rendered under an artificial light source in comparison to a natural light source (the sun).

The CRI provides a scale of values up to 100, with 100 (the sun) being the best colour rendering light quality and a value below 60 representing very poor colour rendering.

The higher the CRI value (also called CIE Ra), the better the artificial light source is at rendering colours accurately.

CRI values help us determine the ability of an artificial light source at rendering colours accurately and faithfully to the human eye.

“Accurate colour matching” means that a colour under an artificial light will reveal as truthfully as if you were looking at it under the natural sunlight.

How is CRI calculated?
The value of CRI for a light source is calculated by testing colours. The “Commission Internationale de l’éclairage” (CIE) established a scale of 8 CIE Standard Colour Samples for the CRI method. There are an additional 7 CIE Special Colours that can be used, giving a total of 15 colour samples.

The general Colour Rendering Index looks at the first 8 colour indices (Ri). Ra being the average of the 8 Ri values and is stated as CRI with a maximum value of 100.

Special CRI refers to the remaining 7 colours (9 to 15) including the saturated colours and skin tones.

The test involves comparing either 8 or all 15 colour samples under the light source and then comparing it to a reference light source, usually sunlight. The average differences are then subtracted by 100 to get the CRI value. That is why light sources that show more “real” colours have higher CRI values, the average differences are smaller between the light source and the reference sunlight.

At Daylight, our lights are tested using the full 15 colour samples, which include pastels, saturated colours and skin tones for more accuracy.