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  Resources / Pressure Loading

Pressure Loading

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It’s not unusual to use a lens or window as the port between a vacuum chamber and the outside, or to encounter a situation where an optic must withstand pressure loading. Given the cost of most infrared optics, as well as the potential safety issues, it’s important that the optic under pressure be sufficiently thick to withstand the loading without breaking. On the other hand, since increasing thickness reduces optical transmission, it’s desirable to minimize thickness for optical considerations.

The formulae equations given in the following text show how to calculate the necessary thickness for an optic under pressure. It is assumed that the window is unclamped and supported by a flat flange around its edge. Other important factors which may affect the required thickness for a given application, but which are not included in this treatment, include:

  • Mounting flange size
  • Stress resulting from mounting or sealing
  • Flange clamping stresses
  • Mounting flange flatness
  • Stress due to thermal expansion
  • Vibration effects
  • Pressure cycling or surges
  • Thermal shock/cycling
  • Mounting surface rigidity
  • Mounting surface roughness
  • Optic edge roughness
  • Desired optical specifications

Since it’s not possible to include all these factors in our analysis, it’s common practice to include a “safety factor” in the equation which increases the predicted thickness to an amount which should be adequate for most applications. Doing this yields the following equations.For a circular window the minimum thickness is:

For a rectangular window, the minimum thickness is given by:

M Values for Common II-VI Materials

8,000 psi
10,000 psi
15,000 psi
13,500 psi
20,000 psi


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