Measurement of Wavelength and Temperature-Dependent Optical Properties of Thermochromic Pigments

Thermochromic material is a substance that is capable of changing reversibly the color as the temperature rises. Therefore, the optical spectrum of thermochromic material is responsive to the environmental temperature. In this study, the temperature-dependent optical constants of thermochromic pigments over the wavelength of 350–1800 nm were investigated. Three kinds of thermochromic pigments featured with black, blue, and red colors at room temperature were suspended in water and the light reflection and transmission of the suspensions at different temperatures were measured by a multifunctional spectrophotometer. It was found that below the transition temperature of thermochromic material, the refractive index was 2.1–2.5, 2.2–2.6, and 2.0–2.4 over the wavelength range of 350–1800 nm for black, blue, and red thermochromic pigment, respectively, while above the transition temperature it reached 2.3–2.7, 2.4–2.9, and 2.4–2.7, respectively. It was also observed that the relationship between refractive index of thermochromic pigment and wavelength follows the cubic polynomial function. Furthermore, the extinction coefficient is in the range of 1 × 10⁻⁵–1.2 × 10⁻⁴ for all thermochromic pigments and remains approximately stable at different temperatures. The determination of optical constants of thermochromic pigments provides essential parameters in the modeling of light scattering and absorption by pigment particles to further fine-tune the optical properties of thermochromic coating.