In photometry, a light's color rendering capacity is a recently constructed measure of how broad a color space can be perceived under that light's illumination.

For example, under very bright, white illumination, we will be able to see all objects vividly as whatever color they tend to reflect. On the other hand, under dim red illumination, all objects will look dimly red, no matter what color they seemed to have under the bright white light. By the rendering capacity terminology, bright white light therefore has high rendering capacity, while dim colored lights have low rendering capacity.

To put this in the context of a standard, simple color science model, refer to HSV color space. The colors perceivable under a particular illumination will form a continuous solid in the HSV color space (and in all the other transformations of color space, e.g. RGB, CIE, etc.). For a bright white light, this solid fills nearly the entire color space, while for a dim red light, the solid is a small, squished segment of the total space. The rendering capacity is thus usually defined as the volume of the solid representing all the colors perceivable, divided by the total volume of the color space.

Other measures of color rendering properties of a light source are:

• How closely the color appearances of colored samples lit by a given light resemble those of the same samples lit by a reference Planck radiator.
• How different the color appearance of eight colored samples lit by a given light look in comparison with each other. This is called the Color Discrimination Index (CDI)

References

• CIE Publication 13.3. Method of measuring & specifying colour rendering properties of light sources. CIE,1995.
• Thornton WA. Colour-discrimination index. J. Opt. Soc. Am. 1972, 62(2), 191-194. PMID 5009385
• Xu H. Colour rendering capacity and luminous efficiency of a spectrum. Lighting Res. Technol. 1993, 25, 131-132
• Xu H. Colour rendering capacity of illumination. J. Opt. Soc. Am. 1983, 73(12), 1709-1713. PMID 6663375