Colour rendering for museum lighting

Light sources' colour rendering is particularly important in museums. Good lighting serves the reason why people go to a museum and look at exhibits. Visitors want to be stimulated and inspired by the form, colour, and message of works of art. Appropriate illumination of at least CRI > 90 is essential for this. The international lighting standard EN 12464-1 only specifies general lighting for museums. For circulation areas and waiting zones, >300 lx with a colour rendering value of Ra≥80 is required. However, Re≥95 is more suitable for evaluating high colour rendering qualities. For exhibit lighting we find this specific condition in the standard: "The lighting is mainly determined by the exhibition requirements." It is, therefore, up to the system operator which lighting quality should be present.

Assessment by means of Ra

In the colour rendering evaluation according to CIE 13.3, the colour fidelity of eight test colours from a light source is determined in relation to a reference light source. The average value is formed from the eight individual evaluations and given as Ra (general).

R1 Light greyish red

R2 Dark greyish yellow

R3 Strong yellow green

R4 Moderate yellowish green

R5 Light bluish green

R6 Light blue

R7 Light violet

R8 Light reddish purple

For a light source to achieve a colour rendering of Ra≥80 or Ra≥90 only the average value needs to match it. The colour fidelity rating of, for example, two of the eight test colours can be quite poor. See measurement result of R3 and R8 of a Ra≥80 light source.

The CIE 13.3 (1995) rating is not suitable for high colour rendering requirements.

Assessment by means of Re

The Re-value is better suited for the evaluation of high colour rendering qualities. CIE 13.3 requires the evaluation of 15 test colours. To determine the Re value, seven test colours are added to the already mentioned eight. The Re-value is the mean value of the colour fidelity rating of the 15 test colours.

R9 Strong red

R10 Strong yellow

R11 Strong green

R12 Strong blue

R13 Light yellowish pink

R14 Moderate olive green

R15 Asian skin

The mean value of these 15 test colours is called the Re value. Since the colour fidelity of four saturated colours (R9–R12) is also considered, the Re assessment represents a good light quality classification. Light sources with an Re value of ≥95 are particularly suitable for museums.

If the Re value is ≥95, then the Ra value is naturally also similarly high. Using light sources with an Re≥95 is good for museum lighting. For the highest demands, XAL offers light sources with an Re≥98 under the name XPECTRUM.

Example of a colour rendering measurement of a very high-quality light source.
Example of a colour rendering measurement of a very high-quality light source.

TM-30 Method

An even more precise colour rendering evaluation is offered by the American TM-30 method. Again, the highest theoretical value that light sources can achieve for Rf and Rg is 100. Light sources with an Rf and Rg ≥ 98 offer excellent colour rendering and colour saturation and are ideally suited for museum lighting.