Stability of ceramic color reflectance standards

This article reviews the long-term repeatability of spectrophotometric color measurements of ceramic color standards calibrated by Hemmendinger Color Laboratory (HCL) since it was founded 25 years ago. A set of twelve BCRA tiles was measured at HCL in 1977, and a few months later at NBS. The CIELAB color-difference, averaged over the twelve tiles, between HCL and NBS was 0.25 units. Measured at HCL about every four months since that time, the repeatability of measurement, averaged over the twelve tiles, has been better than 0.15 CIELAB units. The difference from the NBS measurements has remained at about 0.25 units over two decades. From these data the conclusion can be drawn that the tiles have not changed color during this period of time by an amount exceeding the limits of instrumental repeatability, namely about 0.15 units.