Pressure window design

Posted by Steve Rowe on

Custom pressure windows 

The design of pressure windows is a multi-faceted task. When planning a pressure window for a vacuum chamber, detection equipment, underwater viewport, or other application your material selection should be your starting point. Optical properties are always a consideration, but when dealing with pressure forces, the focus shifts to the mechanical properties of glass. The mechanical properties of glass determine the amount of stress the glass can withstand. Stress is defined as the perpendicular force per unit area applied to an object, in a way that compresses (compressive stress) or stretches (tensile stress) the object. 

 Directional strength of glass, mechanical properties of glass

All glass has defined mechanical properties which are measured in a variety of ways:

  • Modulus of Rupture (M): bending or flexural strength of a material.
  • Shear Modulus (also known as modulus of rigidity): twisting force glass can withstand.
  • Knoop Hardness (K): penetrative force calculated from indentation produced by a diamond tip pressed onto the surface.
  • Young's Modulus: the stiffness of glass.
  • Modulus of Elasticity (MoE): an object or substance's resistance to being deformed elastically (i.e., non-permanently) in the direction of the applied force (i.e. MoE = stress / strain).
  • Poisson’s ratio: dimensional response to a force applied at right angles to the dimensional measurement (e.g. “bulge” of a cylinder when compressed axially). The signed ratio of transverse strain to axial strain.

So where do you start? First, you'll need to know if your application will have the window clamped or unclamped. Unclamped (figure 1) is the window resting on a shelf similar to a manhole cover or will it be secured on both sides in a clamped fashion (figure 2).

Pressure window formulas, pressure window fixture design

The next step is to gather information for our equation. Circular and rectangular windows have slightly different parameters. With any pressure application, it is important to understand potential failures. Glass just like many other materials can experience material fatigue, scratches, sub-surface damage which can lead to failure. It is critical to understand your application's risk to determine a “Safety factor” value. The safety factor scale is a guide from 1 to 7 with 4 as a modest value sufficient for many applications and 7 as the highest-rated for safety.

 pressure window formula, vacuum window formula, thickness of material for pressure window

Below are the mechanical properties of the most common materials for pressure windows.  


Sapphire Mechanical Properties
Modulus of Rupture 65,000-100,000 psi  ~ 448-689 MPa
Knoop Hardness 2000 kg/mm2 (9 Mohs std)
Young’s Modulus 400 GPa @ 20°C (63 x 106 psi)
Compressive Strength 2.0 GPa Creep @ 100 MPa
Fracture Toughness 2.0 MPa (m½)
Flexural Strength 900 MPa
Bulk Modulus 240 GPa
Shear Modulus 175 GPa (26 x 106 psi) (Rigidity Modulus)
Tensile Strength 300 to 400 MPa
Poisson's Ratio 0.29
Density 3.97 gm/cm3


UV Fused Silica Mechanical Properties
Modulus of Rupture 7,600 psi ~ 52 MPa
Knoop Hardness (100g load) 522 kg/mm
Young’s Modulus 73 GPa
Compressive Strength 1.14 GPa
Bulk Modulus 35.9 GPa
Shear Modulus 31 GPa
Tensile Strength 54 MPa
Poisson's Ratio 0.16
Density 2.20 gm/cm3


Quartz Mechanical Properties
Modulus of Rupture 7,000 psi ~ 48 MPa
Knoop Hardness HK 0.1/20 600 kg/mm
Young’s Modulus 7.2 x 1010 Pa (10.5 x 106 psi)
Compressive Strength 160,000 psi
Bulk Modulus 3.7 x 1010 Pa (5.3 x 106 psi)
Rigidity Modulus 3.1 x 1010 Pa (4.5 x 106 psi)
Tensile Strength 4.8 x 107 Pa (N/m2) (7,000 psi)
Poisson's Ratio 0.17
Density 2.2 x 103 kg/m3


N-BK-7 Mechanical Properties
Modulus of Rupture  9210 psi ~ 63.5 MPa
Knoop Hardness 610 kg/mm
Young’s Modulus 82 GPa
Compressive Strength 1103 MPa
Bulk Modulus 34 GPa
Tensile Strength 48 GPa
Poisson's Ratio 0.206
Density 2.51 g/cm3

1 psi = 6,894.75729 Pa | 1 Pa = 0.006895 MPa


In many cases, we've had customers specify sapphire as their default material due to its hardness and strength. While sapphire can be a good choice for some applications there are alternative materials that will perform just as well for a fraction of the cost. Pressure window applications can be both large and small and many involve gaskets to properly seal between housings. We are here to help alleviate the high-pressure and stress of your custom machined window.

Contact our sales team today to get started.

Disclaimer: Esco Optics is a manufacturer of precision optics; data and formulas shown above are to be used as reference. Esco Optics assumes no liability for performance failures of goods as designed and/or specified by our customers.
Custom pressure windows, vacuum windows, custom vacuum window, prototype pressure window

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