Sightglasses are primarily used to observe the interior of a vacuum chamber, including during the process. Therefore, borosilicate glasses of corresponding thickness are generally used and installed with elastomeric seals into a sight flange (ISO-KF, ISO-K and ISO-F viewports). For UHV applications and for high temperatures, the glasses are metalized and soldered (fused silica or sapphire glass) or fused (Kodial glass) with welding lips to compensate for the thermal expansion. They are welded and hermetically sealed to the weld lips of the sight flange. In order to minimize tension, CF viewports should only be fitted with annealed copper seals. In addition, the heating and cooling must be smooth and not too fast.
If viewports also serve the transfer of electromagnetic waves, their transmission range and optical quality must be considered. Glass holders with FKM gaskets for one-inch and two-inch sight glasses allow the installation of optical glasses in vacuum flanges. Viewports shutter prevent or reduce the pollution of viewports, for example during coating processes.
Figure 3.19: CF viewport with glass-metal fusing