It was shown that the color of materials in a closed space can be measured from the outside of space under diffuse illuminations by using a concave glass as a partition. The measurement accuracy is ΔE*ab = 0.9 in average for 24 colors of the standard color chart, where ΔE*ab is the color difference in L*a*b* (D65/ 2-deg.) color space of JIS Z8781-4. It is lower than that of the conventional colorimetry without a glass partition, in which the measurement accuracy is ΔE*ab = 0.7 in average; it is, however, high enough compared to that of visual colorimetry.
