Chapter 4.4 - Viewports

Abstract

The three most common viewport materials used in high- and ultrahigh vacuum systems are glass, quartz, and sapphire. Glass viewports, the most widely used and least expensive of the three choices, are available in a wide variety of diameters. The transmission is approximately 90% between 0.32 and 2.6 microns. Quartz viewports have a transmission of approximately 90% in the range of 0.3 to 2.5 microns and 0.2 to 2.0 microns for UV-grade materials. They are available in both zero length and standard length. Quartz viewports are sealed to stainless steel frames with a low-temperature brazing alloy. The bakeout temperature is limited to 200°C. Sapphire viewports have the broadest transmission range. The transmission is above 80% at 0.3 to 4.0 microns and 0.25 to 4.5 microns for UV-grade sapphire. Viewports of all three types are available with a variety of mounting flanges, and, with the exception of the glass and quartz zero-length types, without flanges. In use, it is important not to induce stress into the seal, which could cause cracking or breakage. Viewports are designed for vacuum use. The zero-length glass viewports should be mounted in the normal configuration. Nonmagnetic glass viewports up to 4 inches in diameter, and quartz and sapphire viewports can be mounted with vacuum on either side.