Abstract: An optical system is provided comprising an entry window, an exit window, a radiation transmissive membrane, and first and second lenses. The entry window and the exit window define a radiation path extending from the entry window to the exit window. The radiation transmissive membrane is positioned along the radiation path between the entry window and the exit window. The membrane defines a first chamber between the entry window and the membrane and a second chamber between the membrane and the exit window. The first fluid is included within the first chamber so as to intersect the radiation path and the second fluid is included within the second chamber so as to intersect the radiation path. The first lens operative to focus radiation directed through the entry window at a focal point located within the second fluid. The second lens is operative to focus radiation passing through the exit window.