Colour rendering of LEDs

Process for a quantitative comparison of colour fidelity

Evaluation processes with the colour rendering of lamps, e.g. the colour rendering index Ra or TM-30, evaluate the colour fidelity of light sources by means of a calculation. Using predefined colour samples, the spectrum of a test light source is compared with that of a reference light source. The calculated value makes no assertion about the general rendering of colours. As a consequence, a high colour rendering index does not automatically indicate good colour rendering for all material colours. Because these indices are therefore not a measure of the performance capability of human colour perception, visual sampling is recommended, particularly for illuminating art and for lighting in the shop. Lighting designers can however use the index to define minimum standards with light quality.

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The colour rendering index Ra as defined in standards evaluates the colour fidelity of a test light source compared to a reference spectrum with use of eight unsaturated test colours. A value of 100 corresponds to compliance with the reference, but does not automatically indicate an authentic impression of colour. The method has six further, partly saturated colours (e.g. red) that are not used to calculate the index.

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The TM-30 process compares the colour fidelity (Rf) of the test and reference light source using 99 saturated and unsaturated test colours. The colour saturation (Rg) can also be shown by graphically overlaying the range of both lamps. In this way experienced lighting designers can judge the behaviour of individual object colours. According to the specific material and chromaticity of the illuminated object, the saturation of individual colour tones can be experienced as either visually pleasant or alienating.

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