Quartz viewports 
All MDC quartz viewports are constructed from UV grade fused silica. Sleeve and flange materials are manufactured from high purity 304 stainless steel and vacuum tube grade braze alloys are designed and rated for high and ultrahigh vacuum applications. Fused silica is a polycrystalline, isotropic material with no crystal orientation. The physical, thermal, dielectric and optical properties are uniform in all directions of measurement. Synthetic fused silica is manufactured by flame hydrolysis of silicon tetrachloride. These viewports are available up to 7.78 inch view diameters and are preferred over glass viewports because of their transparency to ultraviolet radiation. Other advantages include a higher abrasion resistance and a low coefficient of thermal expansion, making them very resistant to thermal shock. The main limitation with fused silica viewports is that they cannot be heated to temperatures over 200°C. The lead-silver alloy used to seal and bond the window to a stainless steel sleeve melts at 305°C.

Sapphire Viewports
Sapphire is a synthetic, hexagonal single crystal anisotropic material which displays substantially different physical, thermal, dielectric and optical characteristics when measured along different axes. MDC sapphire viewports are constructed using 90° crystal orientation, where the crystal orientation is parallel to a window’s surface. Viewports with crystal orientation parallel to the optical axis, or 0° orientation, can be custom ordered from Insulator Seal. MDC offers a wide selection of sapphire viewport assemblies suitable for high and ultrahigh vacuum applications. The use of high purity sapphire and nickel-iron low expansion alloys enables these viewports to be repeatedly baked to temperatures as high as 450°C. All sapphire viewports up to 1 inch in diameter use ultraviolet grade sapphire material. Those exceeding 1 inch diameters are supplied with regular grade sapphire. Please reference the transmission curves on this page for transmission characteristics and comparisons. Also note that limitations in current sapphire crystal growth technology make it difficult to guarantee constant transmission performance at wavelengths below 0.25µm.
In general, poor surface finish can contribute as much as 10% to a window’s total transmission losses. The sapphire viewports offered in this catalog are supplied with flat faces polished to a 50-20 scratch-dig while quartz viewports are 80-50 scratch-dig. Although these finishes are suitable for most optical applications, the optics industry recommends a 20-10 scratch-dig finish for wavelengths below 0.25µm.

Pyrex® Viewports
MDC Pyrex® viewports provide a convenient and economical solution for high vacuum systems requiring periodic glass substrate replacement. Some applications like vacuum coating will deposit coating materials on system viewports which will eventually render them useless. The Pyrex® substrate is sealed against an elastomer gasket and clamped in place with an aluminum retaining flange. It is important to note that most glass materials are susceptible to point stressing if clamped directly against a metal surface. For this reason MDC Pyrex® viewport retaining flanges are fitted with a cushion gasket that prevents metal to glass contact. These viewports are offered in a weldable 4 inch view diameter half-nipple geometry. They are constructed with Corning type 7740 glass and are suitable for service to 1x10^-8 Torr. 

Viewport Shutter Assemblies
VPS Series viewport shutters provide quick and efficient shielding for all standard viewports. These shutters are ideally suited for service in high and ultrahigh vacuum coating applications. These swing type pivoting shutters feature a double-sided sealing design and are designed to mount between a port flange and the viewport. The shape of each shutter takes advantage of the maximum aperture of the inside of a standard port tube. The actual amount of shutter swing is dependent on the mating chamber’s tube port diameter. The table below gives the maximum swing angle for each shutter when mounted on a standard Del-Seal™ flanged half nipple. To provide maximum shielding of viewports each shutter body has a step machined to match the shape and contour of the shutter’s flapper plate. Actuation of the shutter is provided through an MDC bellows sealed rotary drive. Improved design features include all stainless steel body allowing for bakeout up to 230°C. A positive click-stop action has also been added to the rotary driver that allows for partial opening of the shutter’s flapper plate between the fully opened and fully closed positions.