How to Select Commercial Quality Circular Windows
By Esco Optics, Inc. | Published: 2026-10-22
Category: Buying Guides
Learn how to choose commercial quality circular windows based on material, tolerances, and surface specs. Compare fused quartz, UV fused silica, and Borofloat 33.
Quick Answer
For most sight glass and instrument window applications, choose a commercial quality circular window that balances material cost with the required optical and mechanical tolerances. Start with Borofloat 33 for budget-friendly general use, fused quartz for higher thermal resistance, and UV fused silica when deep UV transmission is essential.
Material Matters: Fused Quartz vs. UV Fused Silica vs. Borofloat 33
The material determines the window's transmission range, thermal stability, and cost. Fused quartz (G1) is a cost-effective choice for visible and near-IR applications, offering good thermal shock resistance. UV grade fused silica (S1-UV) extends transmission into the deep UV and has higher purity, making it critical for UV spectroscopy and laser systems. Borofloat 33 is a borosilicate glass that provides decent transmission in the visible and near-IR at a lower price point, but it has lower thermal resistance and is not suitable for deep UV.
Trade-off: UV fused silica costs more than fused quartz, and Borofloat 33 is the least expensive. If you don't need UV transmission, choosing Borofloat 33 can save money without sacrificing performance in the visible spectrum.
Tolerances and Surface Quality: What the Numbers Mean
Commercial quality windows have specified tolerances for diameter, thickness, surface accuracy, and surface quality. Diameter and thickness tolerances of +/-0.125 mm are typical, ensuring a consistent fit in your mount. Surface accuracy of ≤2λ per inch indicates the flatness, while surface quality of 60-40 scratch-dig means minimal scratches and digs. Borofloat 33 has looser tolerances (thickness +/-0.250 mm, parallelism <15') and lower surface quality (80-50), which is acceptable for less demanding applications.
Trade-off: Tighter tolerances and better surface quality cost more. If your application requires precise alignment or low scatter, invest in fused quartz or UV fused silica. For simple sight glasses, Borofloat 33's looser specs are often sufficient.
Edge Beveling and Clear Aperture: Practical Considerations
All these windows feature fine ground and safety beveled edges, reducing chipping risk during handling and installation. However, parts under 25.4 mm in diameter may not have beveled edges, so handle small windows with extra care. The clear aperture is 90%, meaning the outer 5% on each side may have edge effects; account for this in your optical design.
Trade-off: Beveled edges are standard for larger windows but not guaranteed for smaller ones. If you need a small window, verify the edge condition before ordering and consider ordering a slightly larger diameter to allow for mounting tolerances.
Decision Rules for Choosing a Circular Window
- Define your wavelength range: If your application needs deep UV transmission, select S1-UV fused silica. For visible or near-IR, G1 fused quartz or Borofloat 33 are sufficient. UV transmission is the primary material differentiator.
- Assess thermal and mechanical demands: Fused quartz and fused silica handle higher temperatures and thermal shock better than Borofloat 33. If your window will be near heat sources or in vacuum, choose a fused material. Fused materials are more robust in harsh environments.
- Match tolerances to your mounting and optical needs: For precise alignment or optical systems, use windows with +/-0.125 mm tolerances and 60-40 surface quality. For simple sight glasses, Borofloat 33's looser specs are acceptable. Tighter tolerances reduce the need for shimming or adjustment.
- Check the clear aperture and edge bevel: Ensure your optical path is within the 90% clear aperture. For small windows under 25.4 mm, beveled edges may not be present, so handle carefully and consider mounting protection. Account for edge effects in your design.
- Balance cost with performance: Borofloat 33 is the lowest cost but with trade-offs in precision and thermal resistance. Fused quartz offers a middle ground, while UV fused silica is the premium choice for UV applications. Don't over-specify; choose the material that meets your needs without excess cost.
Product Comparisons
- Commercial Quality Windows, Circular, G1 Commercial Grade Fused Quartz: G1 Commercial Grade Fused Quartz has diameter and thickness tolerances of +/-0.125 mm, surface accuracy ≤2λ per inch, and surface quality 60-40 scratch-dig. This provides a good balance of precision and cost for general-purpose windows in visible and near-IR applications.
Commercial Quality Windows, Circular, G1 Commercial Grade Fused Quartz - Commercial Quality Windows, Circular, S1-UV Grade Fused Silica: S1-UV Grade Fused Silica offers the same tight tolerances and surface quality as G1 but with enhanced UV transmission. Choose this when your system requires deep UV transmission, such as in spectroscopy or UV laser applications.
Commercial Quality Windows, Circular, S1-UV Grade Fused Silica - Commercial Quality Windows, Circular, Borofloat 33: Borofloat 33 has looser tolerances (thickness +/-0.250 mm, parallelism <15') and surface quality 80-50, but is the most budget-friendly option. It's a cost-effective choice for sight glasses and instrument windows where extreme precision is not required.
Who Should Choose Which Window?
Best for
- General-purpose sight glasses and instrument windows in the visible spectrum
- UV spectroscopy and laser systems requiring deep UV transmission
- Applications with high thermal or vacuum demands
Not for
- Applications requiring extreme flatness or surface quality beyond commercial grade
- Very small windows where beveled edges are essential for safety
