In one aspect, a spectrometer insert is provided. The spectrometer insert includes: an enclosed housing; a first transparent window on a first side of the enclosed housing; a second transparent window on a second side of the enclosed housing, wherein the first side and the second side are opposing sides of the enclosed housing; and a sample mounting and heating assembly positioned within an interior cavity of the enclosed housing in between, and in line of sight of, the first transparent window and the second transparent window. A method for using the spectrometer insert to locally heat a sample so as to measure temperature-dependent optical properties of the sample is also provided.