Colour rendering index
The Colour Rendering Index (CRI) describes how faithfully colours appear under a light source compared to a defined reference light source. The general colour rendering index is denoted by Ra. Eight specified test colours are used to determine it. Their colour rendering under the light source under investigation is compared with their rendering under a reference light source. Which reference light source is used depends on the colour temperature of the light source under investigation.
What does the CRI value mean?
The smaller the colour deviations from the reference, the higher the colour rendering index. An Ra value of 100 indicates a very high degree of correspondence with the respective reference. The CRI is not expressed as a percentage. For example, a value of Ra 80 does not mean that colours are rendered correctly 80 per cent of the time.
A high value can be particularly important where colours and colour differences need to be reliably identified. Examples include quality control and inspection work, or work areas where colour differences are relevant for the assessment of materials and surfaces.
Limits of the Colour Rendering Index
The general Colour Rendering Index (Ra) is based on the average of the results for eight relatively unsaturated test colours. In addition, there are specific Colour Rendering Indices for other test colours.
A high Ra value therefore does not automatically mean that every single colour is rendered equally well. Additional evaluation parameters may be required, particularly for applications where certain colours are especially important.
In addition to the traditional CRI, more modern methods for assessing colour fidelity are now available. The CIE Colour Fidelity Index Rf, for example, uses 99 test colours and was developed to address some of the known limitations of the CRI. However, this does not fundamentally replace the traditional Ra, which remains a commonly used specification for light sources.
CRI and luminous flux
The Colour Rendering Index describes the quality of colour rendering, not the amount of light emitted. It therefore says nothing about the luminous flux in lumens or the illuminance on a surface.
Light sources with the same CRI can therefore have completely different characteristics in terms of luminous flux, light distribution and illuminance.