CRI - Colour Rendering Index

Colour rendering index can be used for all light sources, both natural and artificial. It shows how accurate a light source is at rendering colours.

Light source with Ra=0 is a monochromatic light, i.e. single colour. This type of light does not allow to recognize the colour of the illuminated object. Ra=100 corresponds to daylight (sunlight). Under daylight, the colours of the observed object seem most natural. The daylight spectrum is continuous for all emitted colours. Some artificial light sources including incandescent light sources, i.e. light bulbs and halogen lights have CRI close to 100. In practice, they are used as a reference for all other light sources.

CRI for a specific light source is determined based on 14 test colours referred to as the TCS (Test Colour Samples); however, the manufacturers often use just the first 8 colours (Fig. 1).

Fig. 1. 14 test colour samples (TCS): 8 main and 6 additional colours selected by the International Commission on Illumination (CIE)

Test colour samples TCS1 to TCS14 are illuminated in turn with a tested light source. The colour rendering index is determined based on the degree in which the light reflected from the test colour is similar to the reflection of the reference light.

Colour temperatures of the tested light and the reference light should be the same. An average value for 14 indices is a CRI value of the light source. For the reference light, R1-R14 are 100. The higher the difference between the reflection of compared lights for a specific test colour sample the lower the index and the lower the overall CRI of the light source.

Images illuminated with a light source at different CRI levels are shown below. The image loses depth and the colours become less natural (Fig. 2).

Fig. 2. Image illuminated with light sources with different CRI levels. From the left: CRI > 80, CRI > 60, CRI > 40

Table 1. Approximate CRI levels for each light source

An average LED light (Table 1) has CRI between 60-80 and 80-95 for high quality light sources. Unlike incandescent lamps, white LED spectrum is not complete, since due to its design, it consists of the spectra of individual colours. In budget LEDs, white is a combination of red, green and blue. Modern LEDs use luminophores and the fluorescence phenomenon to reach a CRI of 90-95.

A noteworthy fact is that the very low colour rendering index of the sodium lamps, they are commonly used to illuminate streets, car parks etc. due to a low cost of obtaining a high luminous flux. Also, the yellow colour of the sodium lamps positively affects the visibility in difficult atmospheric conditions.

The European standards on lightning used in human-occupied areas require CRI of at least 70 outside the buildings and at least 80 for home and office use. The higher the index the better. Low CRI of the light source means that the object are rendered in unnatural colours which creates a discomfort to the person using this type of light source.