Abstract: An apparatus comprises an optical pane transparent to infrared radiation. The pane includes first and second sides, a periphery, and a retention region. A frame defines an opening that can accept the pane and includes a pane retention region. Additionally, a retainer is configured to secure the optical pane in the opening and includes a retaining surface mated to the pane support surface of the frame and configured to support the optical pane at the retention region. The pane, frame and retainer are configured to have mating features which can include a bead, a retaining ridge, a labyrinth configuration and protrusions, recesses, channels, or steps or a combination of these features.