Thermochromism in color measurement

Accurate color measurements have become more and more important during the past few decades. This is valid not only in physical research but also in industrial production, where the importance of accurate measurements is mainly due to increased quality requirements set by the customers of various goods. The development of technology enables more and more accurate measuring systems. While the accuracy has improved one has noticed, that many unexpected factors affect the color of an object. One of these factors is the temperature of the sample. It is known that for example the reflectance of the ceramic reference tiles used for calibration of colorimeters and spectrophotometers is temperature dependent. This phenomenon is called thermochromism, which is a reversible change of color of the sample as a function of temperature. It may be noticed already at room temperature if the temperature varies few centigrades. Red and orange samples are especially sensitive to temperature variation and may cause difficulties in precise color measurements. We show, how the phenomenon is based on physical processes and not only reflects the instability of red color pigments. We derive simple formulas, which are shown to explain the experimental data. We also discuss the meaning of thermochromism for color measurements, measure the magnitude of it and propose the experimental conditions to avoid this effect.