Color rendering index is rather simple in principle and expression. The color rendering index, or CRI, of a light source is expressed ad a number ranging from 0 to 100, where 100 is a spectral output that contains the entire visible spectrum and, therefore, renders all colors accurately. If a light source emits every wavelength in the visible spectrum, then the materials in an environment have source emits every wavelength in the visible spectrum, then the materials in an environment have the opportunity to reflect all of those wavelengths to the eye, and, thus, express all of the potential color in the environment.
The lower the CRI value, the fewer distinct wavelengths the source emits. Therefore, fewer potential color expressions can be reflected from materials in the environment. When we go on to investigate various electrical light sources, we will see that color rendering capabilities (and corresponding CRI values) vary greatly from source to source. This variety is of huge significance to the designer. Since designers are responsible for making many critical material and color decisions, it is imperative that they are aware of the possible shortcomings of the light source under which they are making their decisions. Many a person has been shocked to find that two materials that look to be the same color under a specific electric light source look utterly different under daylight. Daylight represents a complete spectrum and, therefore, has a color rendering index (CRI) of 100.
Incandescing sources like the common “light bulb” and halogen sources are engineered to also emit the entire visible spectrum, so they also have a CRI of 100. A bad fluorescent source may have a CRI of 60. Suffice it to say general understanding that the following guidelines can get us through the system:
Color rendering indices (CRI) in the 60’s and 70’s are relatively incomplete and are unacceptable for making critical color decisions.
CRI values in the 80’s do a reasonable job of revealing colors
CRI values in the 90’s render colors very accurately and are appropriate for color critical environments.
Figure 1: Color Rendering lndex (CRI) of different light sources expressed from 0 to 100.