A seal assembly for an optical sensor employed in a high temperature environment is disclosed which includes an elongated metal guide tube having a distal end portion defining an open distal end and an inner rear wall, a sapphire window disposed within the distal end portion of the metal guide tube, the sapphire window having a rear end surface abutting the inner rear wall of the distal end portion of the metal guide tube, and a platinum mounting sleeve for securing the sapphire window to the metal guide tube, the platinum mounting sleeve having one portion joined to the metal guide tube and another portion joined to the sapphire window.