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Seeing colours clearly – An introduction to the Colour Rendering Index

Ever wondered why a colour can look so different when viewed in natural sunlight compared to how it looks under some artificial light? This guide will help to explain.
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Seeing colours clearly

An introduction to the Colour Rendering Index

 

Ever wondered why a colour can look so different when viewed in natural sunlight compared to how it looks under some artificial light? This guide will help to explain.

 

What is the Colour Rendering Index

The colours we see in natural sunlight create a vivid picture of the world around us. So it is important that electric lights replicate those colours as closely and accurately as possible.

 

How an electric light enables us to see the true colours of the object it is illuminating, is referred to as its ‘colour rendering properties’. And how we measure these properties is through the Colour Rendering Index (CRI).

 

A light source with a CRI rating of 100 is a perfect colour rendering score – you see the object in its true colours.

 

What is the difference between CRI and CCT (Correlated Colour Temperature)?

 

While the Colour Rendering Index shows how colours appear under different light sources, Correlated Colour Temperature (CCT) refers to the ‘colour’ or tone of the light source itself – its warmth or coolness. This is measured in degrees of Kelvin.

 

The CRI value does not indicate the colour or tone of the light source.

 

How do we measure the CRI value?

 

Measuring CRI value is a highly complicated process but here is a simple explanation.

 

The International Commission on Illumination (CIE) devised a system which mathematically calculates the colour consistency value of a light. It is a quantitative measure of the ability of a light source to reveal the true colours of various objects in comparison with a test light source.

 

The testing involves the use of eight standard colour swatches. The most commonly used value of CRI is ‘Ra’, which is the General Colour Rendering Index average value of the eight colours (R1 to R8).

 

CRI (Ra) values are on a scale ranging from one to 100. The sun has a maximum value of 100, because it has the best colour rendering light quality. An example of a lower value would be a monochromatic light source, such as the low-pressure sodium previously used for old-fashioned street lighting.

 

Testing a lamp

 

To test a lamp, each of the eight colours (R1 to R8) is illuminated under the lamp, then compared with how they look under a light source with a perfect score.

 

The colours are then scored (using some very advanced maths) and the eight results are averaged to achieve the CRI (Ra) value.

 

What the CRI rating means

 

The value of a light source on the CRI (Ra) scale will give you a good indication of how good its colour rendering properties are.

  • A value of 90 to 100 is considered to be excellent. This is ideal for locations where colour accuracy is important, such as art galleries, museums or hospitals.
  • A value of 80 to 89 is considered to be acceptable for internal applications.
  • A value of 70 to 79 is considered acceptable for external applications.
  • A value below 60 represents very poor colour rendering. Light sources with this value (such as the old-fashioned streetlights) are generally no longer considered for use in illumination.

 

You can find the CRI (Ra) value of a lamp or light source on the information panel on its packaging.

 

 

 

 

 

 

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