Quartz viewports are manufactured from the best grade synthetic fused silica giving excellent optical homogeneity, freedom from bubbles and transmission in the ultra-violet to below 180mm.  There are, however, strong absorption bands in infra-red wavelengths above 1000mm.  The use of kovar transitions produces a window hwihc is entirely satisfactory for most applications having a maximum operating temperature of 200⁰C.  Windows for operation at temperatures up to 250⁰C are also available.  All standard windows are polished to a minimum spectrosopic quality (flatness 5 fringes/cm, parrallelism 3 arc minutes, polish 80/50)  Other specifications may be available on application.  These viewports should only be used with annealed copper gaskets.