Colour & light - understanding the Colour Rendering Index

23rd July 2013

Colour rendering index (CRI) values are fast becoming a hot topic within the LED lighting world, promising to bring out the best of interiors and environments. But what do CRI values mean, and how do they in turn reflect the colours and details that a light source can pick out?

Understanding the principles behind colour and light is central to grasping the relevance of CRI values. We’ve pulled apart the key theories at play that ultimately impact upon lighting choices.

Light wavelengths & the human eye

The human eye is made up of rods and cones – two very sensitive receptors that detect light intensity and colour. When different wavelengths of light enter the eye, they create the sensation of different colours. The average eye can perceive light wavelengths ranging from 400 – 750 nanometres and is so sensitive that it can differentiate between around 7 million colours.

White light is made up of coloured light wavelengths combined together in a phenomenon known as additive colour. A ‘perfect’ white light is made up of all of the wavelengths that span the visible spectrum. The image below shows the separation of white light into its different wavelengths using diffraction through a prism.

Defining CRI

The Colour Rendering Index is a method of measuring a light’s ability to illuminate different colours using a number from 0 – 100. There are 14 standard colours that are used when testing for CRI, as shown below.

A ‘perfect’ white light is made up of all possible wavelengths of visible light combined and is said to have 100CRI, whereas a light with 80CRI would be missing 20% of the light wavelengths that make up the visible spectrum.

How is colour generated?

Colour in interiors is most commonly generated by pigmented surfaces – whether these are painted walls, woven textiles or decorative objects. These surfaces create the sensation of colour by absorbing some wavelengths of light and reflecting others that then enter the eye. Because the reflection of light wavelengths is essential to generating the sensation of colour, the more wavelengths that make up a beam of white light, the better it will be at producing different hues.

Light sources that have a high CRI value will bring out the details and subtleties of any surface that they illuminate. This means that everything from skin tone to décor details will appear sharper and more vibrant. Light sources that have a low CRI value will make colours appear dull and not true to life. Depending on which area of the visible spectrum the white light is missing, colours will appear more blue, green or red than they should, and fine detailing such as the flecks in marble surfaces will be lost.

The image below shows the difference between colour vibrancy under a light source with 70CRI and a light source with 95CRI. The low CRI light source is missing warm red and orange wavelengths, causing colours to appear dull and washed out.

Comparing CRI values - what to look for

When choosing LED lighting, look for as high a CRI value as possible to maximize colour, tone and detail. Similar to lumen output, lights that do not offer any information about CRI are to be avoided as this often indicates a low rating.

It is also important to note that the CRI values of one light can only be compared to another if the bulbs are of the same colour temperature – lights with different colour temperatures will fundamentally emit different wavelength variations.  

At Brightgreen, we developed the world’s first LED bulb with a Colour Rendering Index value of 95CRI – the D900 Curve LED downlight (now called the D900 Classic) – at a time when the LED industry standard was 80CRI. Our unique Tru-Colour 95CRI technology is still unbeatable; look out for the Tru-Colour 95CRI symbol across our product ranges for lighting that will illuminate near-perfect colour.

Do you have a burning question about lighting that hasn’t been answered? Visit our Lighting Guide and Lighting Glossary for more information about the science and history behind LED lights.