Observing Real-Time Process Conditions in a UHV Using a Standard Glass Viewport

In both manufacturing and research and development (R&D) applications, it is crucial to view the real-time process conditions in high-vacuum and ultra-vacuum (HV/UHV) chambers.

The addition of process control equipment and high-precision thermocouples into the vacuum system is enabled by hermetically sealed feed-throughs to carry out in-situ modifications or to gather data without limiting the highly depressurized atmosphere.

However, it is essential to use a transparent viewport with a high-performance flange to directly observe the process setting with the naked eye.

sapphire window for vacuum chambers

Allectra’s expertise lies in the areas of development and supply of several HV/UHV components, including standard and specialized viewports ideal for complex and difficult pressure procedures. This article analyzes some of the typical viewports available in Allectra’s series of HV/UHV products.

An Overview of Standard Viewpoints

Each of Allectra’s standard viewpoints is completely equipped with high-performance Kodial glass. This glass is called borosilicate because its basic constituents include boron trioxide (B2O3) and silica.

This glass material exhibits exceptional thermodynamic stability and is recognized for its specifically low coefficients of thermal expansion (CTE). These features make it ideal for heat-treatment applications under vacuum settings. The exact optical properties of borosilicate glass are maintained across its entire range of operating temperatures. This is because it is highly resistant to thermally induced physical variations.

For each of Allectra’s standard viewpoints, the window diameter and peak operating temperature have substantial variation. The diameters range from narrow apertures (16 mm) to considerably wide fields of view (152 mm). But this geometric range does not have any impact on the optical transmissivity, with each regular viewport created by Allectra covering a considerable portion of the ultraviolet-visible and near-infrared (UV-Vis-NIR) spectrum (300–2500 nm).

The glass viewport that has the largest viewing diameter at present is the 120-CPGO-ISO160, with a viewing diameter of 152 mm. The glass viewport’s body is made of grade 304 stainless steel, with a Viton O-ring vacuum seal. This component offers the widest field of view for direct observation of the process conditions. But its sealing component restricts its use to HV applications, with a leak rate of 1 × 10−8 mbar liters per second (L/second).

The CF glass viewports from Allectra range from highest viewing diameter of 135 mm, for example, on the 120-VPG-C200, to the tiniest glass viewport on the 120-VPG-C16, with a viewing diameter of 16 mm. Annealed copper (Cu) gaskets are used as a combination in this case, with a vacuum leak rate of 5 × 10−10 mbar L/second and increasing the highest use temperature from 150 °C (302 °F) to 400 °C (752 °F).


Each standard viewport from Allectra is made from superior quality materials to guarantee they meet the unique and essential customer specifications in HV and UHV processing applications. Moreover, Allectra provides a wide range of specialized viewports, which are designed for increasingly difficult processing environments.

This information has been sourced, reviewed and adapted from materials provided by Allectra Limited.

For more information on this source, please visit Allectra.


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