These KF40 demountable vacuum fittings have a flange size of NW40 with a Type C Coated Quartz Fused Silica viewport, view diameter of 1.19 in and a glass thickness of 0.158". Demountable Viewports are used for ultra-high vacuum applications and molecular spectroscopy. These fittings use a Viton O-ring gasket, operate down to 10-8 torr (10-11 Pa), and have a max bakeout temperature of 150° C. The operable wavelength of the Type C Coated Quartz is between 1050nm to 1620nm. Other coated quartz fused silica is available. Fused silica is for all but the most demanding optical applications. They are preferred over glass viewports because of their transparency to UV ultraviolet radiation. We carry Uncoated and Coated UV viewports and replacement fused silica lenses. See middle graphic chart above for the different light transmissions for each coated fused silica. These vacuum fittings are made of corrosive resistant stainless steel. North American flange sizes are given by flange outer diameter in inches: 1-1/3 ("mini conflat"), 2¾, 4½, 6, 8, 10, 12, 13¼, 14 and 16½. In Europe and Asia, sizes are given by tube inner diameter in millimeters: DN16, DN40, DN63, DN100, DN160, DN200, DN250.