In-situ high-temperature measurements of optical constants (refractive indices n and extinction coefficients k) for each layer of solar selective absorbing multilayers were carried out to simulate solar-thermal conversion efficiency at high temperatures. Conventional Mo layers showed a significant rise in infrared emissivity and a degradation of selective absorbing behavior with the increasing temperature. In contrast, thermally stabilized Ag layers maintained low emissivity even at temperatures below 700 degrees C. In these layers, agglomeration and vaporization were suppressed by SiNx nano-particles dispersed in the Ag matrix and interfacial W adhesive layers, respectively. The interband absorption of beta-FeSi2 layers at higher temperatures revealed that larger n and kin the infrared region due to thermally excited electrons induced no rise in emissivity of the multilayers, and the narrower band gap based on Einstein's model shifted the absorptance cut-off wavelength closer to the ideal value and hence resulted in a higher solar absorptance. The solar-thermal conversion efficiencies of the multilayers consisting of low emissivity Ag layers with beta-FeSi2 absorbers were found to be greater than 75.9%, as estimated from the temperature dependence of n and k spectra.