Color rendering generally refers to the ability of a light source to render colors as "true to nature" as possible. Color 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 color (color temperature) itself. Common methods of evaluating color 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 color rendering properties.
Spectrum 1 depicts an LED with good color rendering, and spectrum 2 an LED with very good color rendering.
R1 - R14 reference colors for CRI method
A weakness of the general color rendering index CRI is the assessment of color rendering over only 8 pastel colors 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 color rendering index, 6 additional test colors have therefore been defined for which color rendering can additionally be determined and specified for more detailed assessment. Saturated red is test color 9, and the color 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 containing 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 colors is for the respective activity, the higher are the requirements for color rendering – maximum requirements are found, for example, in the health sector.
TM-30 reference colors
TM-30 color vector graphic
The Rf and Rg values are usually shown in a diagram. However, this alone does not provide definitive information about the color rendering quality of a lamp. This is only possible with the color vector graphic that displays the size and direction of color shift for selected test colors compared to a reference light source.
If the determined curve is located outside the circle valid for the reference, colors in this area are rendered with oversaturation. If it lies within the circle, the corresponding colors are rendered with a lower saturation.
