La0.7Ca0.3−x SrxMnO3 (0 ≤ x ≤ 0.3) (LCSMO) is a thermochromic material. The material can show a phase transition from ferromagnetic-metal to paramagnetic- insulator under the appropriate doping level (x). In this paper, LCSMO
compounds were synthesized by a conventional solid-state reaction method. The normal emissivity and the spectral reflectance of LCSMO with different doping levels (x) are experimentally investigated. Based on the spectral reflectance data, the optical constants (refractive index n and extinction coefficient k) are calculated by using Kramers–Kronig (K–K) relations. Then, the emissivity changing mechanism is discussed. The experimental results show that the emissivity of LCSMO increases with increasing temperature. For the samples with x = 0.1 and 0.15, their emissivity has a sudden change in the vicinity of the phase transition temperature Tp.