Colour rendering generally refers to the ability of a light source to render colours as "true to nature" as possible. Colour rendering is a property of light sources. It depends on the spectral composition of the light emitted by the respective light source and not on the light colour (colour temperature) itself. Common methods of evaluating colour rendering are the CRI and the TM-30 method.
Spectra of a QT lamp
(1) LED with very good colour rendering, (2) LED with good colour rendering
The spectrum of white LEDs is less uniform because of light generation by semiconductors and phosphors. It drops significantly at the edges of the visible area, with stronger components and gaps in between. The spectrum of common white LEDs is made up of the narrow component of a blue LED and the broader component produced by phosphors. LEDs with better phosphors and a more uniform spectrum show better colour rendering properties.
Spectrum 1 depicts an LED with good colour rendering, and spectrum 2 an LED with very good colour rendering.
R1 - R14 reference colours for CRI method
A weakness of the general colour rendering index CRI is the assessment of colour rendering over only 8 pastel colours with no consideration of saturated hues. Red hues in particular are relevant in many applications (e.g. fashion, cosmetics, food) and are not considered in the CRI. In the procedure for determining the colour rendering index, 6 additional test colours have therefore been defined for which colour rendering can additionally be determined and specified for more detailed assessment. Saturated red is test colour 9, and the colour rendering for red is therefore known as "R9" and is often given as a supplement due to its high relevance for applications. With white LEDs, values of R9 > 50 are already considered very good, and values of > 90 are excellent.
The CRI value should be at least 80 in indoor areas where there are people. The European standard DIN EN 12464-1 for the lighting of indoor workplaces specifies minimum requirements for different work areas and visual tasks. The more important the correct evaluation of colours is for the respective activity, the higher are the requirements for colour rendering – maximum requirements are found, for example, in the health sector.
TM-30 reference colours
TM-30 colour vector graphic
The Rf and Rg values are usually shown in a diagram. However, this alone does not provide definitive information about the colour rendering quality of a lamp. This is only possible with the colour vector graphic that displays the size and direction of colour shift for selected test colours compared to a reference light source.
If the determined curve is located outside the circle valid for the reference, colours in this area are rendered with oversaturation. If it lies within the circle, the corresponding colours are rendered with a lower saturation.
