another part of an article from sony.........

A cooling medium is located in the space between window sections 38 and 48. The medium is preferably a mixture of the anti-freeze ethylene glycol and water, which has a refractive index similar to that of the adjacent window sections 38 and 48. The cooling medium can be injected through an orifice (not shown) in the housing 42 which can then be welded closed, or otherwise stoppered.