Colour rendering index (CRI) – is a ratio colours of objects, which illuminated by standard light source, to illumination of these objects by tested light source. In other words – it’s relative from 0 to 100 quantity, that shows how good are visible another colours in the light of this source: for example, in yellow colour will be worse visible yellow stripes, in blue – blue stripes.

For a standard (100) is accepted sun light, but sometimes as a standard use incandescent lamps (because their CRI is near 97-99). For comparing usually take eight basic colours [Ra-8] and calculate an average. The obtained value is marking by symbol Ra and accepting as CRI – the smaller this quantity, the worse is the colour rendering.