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.