Second, a comparison between the given light source and its reference light source is established. This comparison is expressed as an R factor, which is a ratio or percentage indicating how closely the given light source matches the color rendering ability of the reference light source. Some typical CRIs appear in the accompanying table. When calculating R, it is important to recognize that each light source has a specific color temperature reference. Furthermore, two lamps may have the same reference source and CRI yet render colored materials very differently.
R is an average of the color rendering ability of eight or more specific colors.
Better performance in some areas may be concealed when averaged with poorer performance in other areas. The color of incandescent light is warm in tone, with energy concentrated in the red and yellow range. Most fluorescent lamps are white when not lighted. Variations in phosphor composition produce differences in the color of emitted light. Fluorescent lamps also fall into two groups with regard to their efficiency and color rendition properties.