Human perception of material colors depends heavily on the nature of the light sources used for illumination.
One and the same object can cause highly different color impressions when lit by a vapor lamp or by daylight,
respectively. Based on state-of-the-art colorimetric methods we present a modern approach for calculating
color rendering indices (CRI), which were defined by the International Commission on Illumination (CIE) to
characterize color reproduction properties of illuminants. We update the standard CIE method in three main
points: firstly, we use the CIELAB color space, secondly, we apply a Bradford transformation for chromatic
adaptation, and finally, we evaluate color differences using the CIEDE2000 total color difference formula.
Moreover, within a real-world scene, light incident on a measurement surface is composed of a direct and
an indirect part. Neumann and Schanda1 have shown for the cube model that interreflections can influence the
CRI of an illuminant. We analyze how color rendering indices vary in a real-world scene with mixed direct and
indirect illumination and recommend the usage of a spectral rendering engine instead of an RGB based renderer
for reasons of accuracy of CRI calculations.