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The homogeneity of the colours is of particular importance for the combination of a larger number of LEDs. Adjacently arranged luminaires should not exhibit any perceptible difference in colour (especially when they have to fulfil vertical lighting functions). Another quality criterion for the colour perception of LED modules or arrays is its representation in the form of MacAdams ellipses, which are grouped around a defined chromaticity coordinate. The less the dispersion of the LEDs around the chromaticity coordinate, the greater the homogeneity. The number of MacAdams ellipses, meaning the deviation from the target chromaticity coordinate, is given in the SDCM (Standard Deviation of Colour Matching). Deviations of <2 SDCM are classified as being ‘imperceptible to the eye’.


The colour rendering index Ra documents the quality of the colour rendering of light sources on the basis of eight defined test colours (R1-R8). Saturated colours beyond this range, for example red, blue or green, are not represented in the Ra value. Separate values must be defined accordingly for these colours (e.g. R9=red). The colour reproduction index Ra is a dimensionless value and amounts to a maximum value of 100. The EN12464-1 standard predominantly requires a Ra >80 in the field of indoor lighting.

Definition Ra value

The colour rendering index Ra (= General Index) is calculated on the basis of eight defined test colours R1-R8, whose arithmetic mean determines the Ra value. The reference light source with a maximum value of 100 is a light with a daylight-like spectral range. The values for R9-R15 need to be considered for the reproduction of saturated colours or skin tones. They do not form part of the Ra value and are therefore stated separately.

Daylight quality colour rendering Ra98

The ALPHABET SPECTRA family of products offers two different qualities: in addition to the products with a colour rendering index of Ra83, a second line is now available with the highest colour rendering indices across the entire colour spectrum. These luminaires not only stand out with an excellent colour rendering index of 98 but also provide consistently high values for saturated colours.

With R9 (red) >90 or R15 (skin tone) >95 ALPHABET SPECTRA permits the implementation of high-quality lighting solutions in a daylight-like quality.