Introduction
Selecting the right optical material is one of the most consequential decisions in optical system design. Among the dozens of available optical materials, UV fused silica (synthetic silicon dioxide) and BK7 (borosilicate crown glass) represent the two most widely used substrates in precision optics. Together, they account for the majority of lenses, windows, prisms, and mirrors manufactured worldwide. Yet their properties differ so dramatically that choosing the wrong material can lead to system failure, premature damage, or unnecessary cost.
This comprehensive guide provides optical engineers with a detailed, data-driven comparison of UV fused silica and BK7 glass across every relevant property: transmission range, refractive index, thermal behavior, laser damage threshold, mechanical strength, chemical durability, and cost. We also discuss which component types are available in each material, how application requirements drive the selection, and what common mistakes to avoid when specifying these materials.
What Is UV Fused Silica?
UV fused silica (also called synthetic fused silica or UV-grade fused quartz) is an amorphous silicon dioxide (SiO2) glass produced by flame hydrolysis of silicon tetrachloride (SiCl4) or by electric fusion of high-purity quartz crystals. The synthetic process yields a material with extremely low metallic impurity content (<1 ppm total transition metals), making it transparent deep into the ultraviolet spectrum. The most common commercial grade for optical applications is equivalent to Corning 7980, Heraeus Suprasil, or SCHOTT FSQ-U.
Key attributes of UV fused silica include:
- Broadband transmission from approximately 180 nm to 2.5 μm
- Extremely low coefficient of thermal expansion (CTE = 0.55 × 10-6/°C)
- High laser-induced damage threshold (LIDT)
- Low autofluorescence in UV-grade versions
- Excellent homogeneity (±1 × 10-6 refractive index uniformity)
- Superior resistance to thermal shock
What Is BK7?
BK7 (known as N-BK7 in SCHOTT nomenclature, or B270 in some regions) is a borosilicate crown optical glass that has been the industry-standard visible-light substrate for over a century. It is produced by melting high-purity silica sand with boron oxide and alkali metal oxides in platinum crucibles. BK7 offers an excellent combination of optical homogeneity, ease of polishing, and low cost, making it the default material for most visible and near-infrared optical systems.
Key attributes of BK7 include:
- Good transmission from approximately 350 nm to 2.0 μm
- Refractive index nd = 1.5168 (Abbe number Vd = 64.17)
- Easy to machine, polish, and coat
- Low cost and widely available in all standard sizes
- Good chemical durability (resistance to humidity and mild acids)
- Moderate hardness (Knoop 610) suitable for most handling environments
Comprehensive Properties Comparison
The table below provides a side-by-side comparison of all key properties relevant to optical system design:
| Property | UV Fused Silica | BK7 (N-BK7) |
|---|---|---|
| Transmission Range | 180 nm – 2.5 μm | 350 nm – 2.0 μm |
| Internal Transmittance (10 mm) | >99% from 250 nm to 2.0 μm | >99.5% from 400 nm to 1.8 μm |
| Refractive Index (nd) | 1.4585 | 1.5168 |
| Abbe Number (Vd) | 67.82 | 64.17 |
| Density (g/cm³) | 2.20 | 2.51 |
| CTE (10-6/°C, 20–300°C) | 0.55 | 7.1 |
| dn/dT (10-6/°C @ 633 nm) | 10.5 | 3.0 |
| Thermal Conductivity (W/m·K) | 1.38 | 1.11 |
| Strain Point (°C) | 1050 | 557 |
| Annealing Point (°C) | 1140 | 565 |
| Softening Point (°C) | 1585 | 820 |
| Knoop Hardness (kg/mm²) | 480 | 610 |
| Young's Modulus (GPa) | 72.7 | 82.2 |
| Poisson's Ratio | 0.17 | 0.206 |
| LIDT (1064 nm, 10 ns) | ~20 J/cm² | ~10 J/cm² |
| LIDT (355 nm, 10 ns) | ~10 J/cm² | Not recommended |
| Chemical Durability | Excellent (resists all except HF) | Good (ISO 79-3 Class 1) |
| Homogeneity (typical) | ±1 × 10-6 | ±2 × 10-6 |
| Autofluorescence | Very low (UV-grade) | Moderate |
| Relative Cost (same geometry) | 2–4× BK7 | 1× (baseline) |
Optical Transmission Analysis
Deep UV Performance (<350 nm)
This is the most decisive difference between the two materials. UV fused silica transmits efficiently down to approximately 180 nm, with internal transmittance exceeding 90% per 10 mm at 193 nm (ArF excimer wavelength) and 95% at 248 nm (KrF). BK7, in contrast, becomes opaque below approximately 300 nm and is completely unsuitable for any application below 350 nm. The iron oxide and other transition metal impurities in BK7 create strong absorption bands in the UV.
For applications requiring UV transmission below 350 nm — including excimer laser optics, UV lithography, UV fluorescence microscopy, UV curing systems, and UV spectroscopy — UV fused silica is not just preferred but mandatory. BK7 cannot be used under any circumstances in these wavelength ranges.
Visible Spectrum Performance (400–700 nm)
Both materials transmit excellently in the visible spectrum. BK7 actually has a slight edge in internal transmittance (99.5%+ per 10 mm from 400 nm to 1.8 μm) compared to fused silica (99%+ per 10 mm from 250 nm to 2.0 μm), though the difference is negligible for most practical applications. BK7's higher refractive index (1.517 vs 1.458) means less light is reflected per surface, but it also requires thicker AR coatings to achieve the same reflectance performance.
Near-Infrared Performance (700 nm–2.5 μm)
Beyond 1.8 μm, BK7 begins to show absorption from iron and hydroxyl impurities. By 2.0 μm, BK7 transmission drops below 50% for standard 10 mm thickness. UV fused silica maintains good transmission up to approximately 2.2 μm, beyond which OH absorption bands become significant. For applications between 2.0 and 2.5 μm, IR-grade (low-OH) fused silica is required. Neither material is suitable for mid-IR applications beyond 2.5 μm.
Thermal Properties Deep Dive
Coefficient of Thermal Expansion
The most dramatic thermal property difference is the CTE. BK7 expands at 7.1 × 10-6/°C, while UV fused silica expands at only 0.55 × 10-6/°C — roughly 13 times less. This has critical implications for any optical system operating across temperature ranges:
- Dimensional stability: A 100 mm BK7 lens heated by 50°C changes diameter by ~35 μm, while a fused silica lens of the same size changes by only ~2.8 μm. For precision systems requiring tight focus or beam position stability, fused silica is overwhelmingly superior.
- Thermal shock resistance: Fused silica's combination of low CTE and high strain point (1050°C vs 557°C) means it can withstand extreme temperature gradients without cracking. BK7 is more susceptible to thermal shock, especially in large optics with uneven temperature distributions.
- Outdoor and aerospace applications: Systems exposed to temperature ranges from -40°C to +70°C (110°C swing) will maintain far better alignment and focus with fused silica optics.
Thermo-Optic Coefficient (dn/dT)
While BK7 has a lower dn/dT (3.0 × 10-6/°C) than fused silica (10.5 × 10-6/°C), the overall thermal optical path difference (OPD) must account for both dn/dT and CTE effects. The thermal defocus power depends on the combination of refractive index change and surface deformation. For fused silica, the very low CTE means surface figure is preserved even though dn/dT is higher. The effective thermal focal length change is typically smaller for fused silica in most practical configurations.
Laser Damage Threshold
Laser-induced damage threshold (LIDT) is critical for any high-power laser application. UV fused silica consistently outperforms BK7 across all wavelengths and pulse durations:
- At 1064 nm (Nd:YAG, 10 ns pulses): Fused silica ~20 J/cm², BK7 ~10 J/cm² (2:1 ratio)
- At 532 nm (frequency-doubled Nd:YAG): Fused silica ~12–15 J/cm², BK7 not recommended for high-energy pulsed applications
- At 355 nm (frequency-tripled Nd:YAG): Fused silica ~8–10 J/cm², BK7 unsuitable
- At 193/248 nm (excimer lasers): Fused silica 15–40 J/cm², BK7 completely opaque and unsuitable
The higher LIDT of fused silica comes from its superior purity (no metallic impurities that serve as damage nucleation sites), higher band gap energy (~9 eV vs ~4 eV for BK7), and better resistance to UV solarization. For pulsed laser systems above approximately 100 mJ/cm² fluence, fused silica should be considered the minimum standard regardless of wavelength.
Mechanical and Chemical Properties
Hardness and Polishability
BK7 is harder (Knoop 610) than fused silica (Knoop 480), which makes BK7 slightly more resistant to surface scratching during handling and cleaning. However, fused silica's lower hardness also makes it easier and faster to polish to very fine surface finishes, partially offsetting the higher material cost during manufacturing. Both materials can be polished to 10-5 scratch-dig surface quality and λ/10 flatness or better.
Chemical Resistance
Both materials offer excellent resistance to water, most acids (except hydrofluoric acid), and atmospheric corrosion. BK7 meets ISO 79-3 Class 1 for climate resistance. Fused silica, being pure SiO2, is even more chemically inert and is the standard material for laboratory ware and semiconductor process chambers. Neither material should be exposed to hydrofluoric acid (HF), which attacks both rapidly.
Young's Modulus and Weight
Fused silica has a lower density (2.20 g/cm³ vs 2.51 g/cm³ for BK7), making it approximately 12% lighter for the same geometry. This is advantageous for aerospace applications and systems where weight is critical. BK7's higher Young's modulus (82.2 GPa vs 72.7 GPa) makes it slightly stiffer, which can be beneficial for large optics requiring minimal deflection under gravity.
Component Availability
Both materials are available in a wide range of standard optical component types. Here is a summary of commonly available components:
| Component Type | UV Fused Silica | BK7 |
|---|---|---|
| Plano-Convex Lenses | Available | Available |
| Bi-Convex / Laser Lenses | Available | Available |
| Optical Windows | Available | Available |
| Optical Prisms | Available | Available |
| Optical Mirrors | Available | Available |
Fused silica is also available in specialized forms including cylindrical lenses, ball lenses, and rod lenses. BK7 additionally comes in meniscus lenses, C-lenses, and ball lenses. For custom geometries, both materials can be ordered as blanks and machined to specification.
Application Selection Decision Matrix
Based on the property comparisons above, here is a practical decision framework for selecting between UV fused silica and BK7:
Choose UV Fused Silica When:
- Your system operates below 350 nm (UV laser, fluorescence, spectroscopy)
- High-power pulsed laser fluence exceeds ~100 mJ/cm²
- Excimer laser compatibility is required (193 nm, 248 nm)
- Thermal stability across wide temperature ranges is critical (space, outdoor, interferometry)
- Thermal shock resistance is needed (rapid temperature cycling)
- Ultra-low autofluorescence is required (Raman spectroscopy, single-molecule fluorescence)
- Superior homogeneity is needed for high-precision wavefront applications
Choose BK7 When:
- Your system operates entirely in the visible to near-IR (400 nm – 1.8 μm)
- Laser power levels are moderate (CW < 100 W or low-energy pulsed)
- Cost is a primary driver, especially for large-diameter optics (>50 mm)
- The optical system operates at or near room temperature with small thermal excursions
- Standard catalog optics meet your requirements
- Volume production where per-unit cost differences compound significantly
Cost-Performance Tradeoffs
The cost differential between fused silica and BK7 is real and significant. For equivalent geometries and surface finishes, fused silica optics typically cost 2–4× more than BK7. The raw material cost is higher because high-purity synthetic fused silica blanks are more expensive to produce. Additionally, fused silica's lower hardness can increase polishing time slightly (though this is partially offset by faster stock removal).
However, the cost picture changes when you consider system-level economics:
- Failure cost: A BK7 window damaged by UV laser exposure may cost $50 to replace, but the system downtime could cost $5,000+ per hour. Fused silica prevents this failure mode entirely.
- Performance cost: Using BK7 in a UV application results in significant transmission loss, reduced signal, and potential solarization. The system underperforms, requiring more gain, longer integration times, or reduced throughput.
- Thermal recalibration: Systems using BK7 in variable-temperature environments may need periodic recalibration to maintain focus and alignment, adding operational cost.
Rule of thumb: if your application is within BK7's comfort zone (visible, moderate power, stable temperature), use BK7 and save 60–75% on optic cost. If you are near the edge of BK7's capabilities, the fused silica premium is almost always justified by improved reliability and performance.
Common Material Selection Mistakes
Based on our experience manufacturing optics for thousands of customers, these are the most common mistakes we see in material selection:
1. Using BK7 for Near-UV Applications (350–380 nm)
At the boundary of BK7's transmission range, internal transmittance begins to drop. For a 20 mm thick BK7 lens at 350 nm, transmission is only ~90%, and it drops further at shorter wavelengths. If your system operates at 355 nm (frequency-tripled Nd:YAG), BK7 is marginal at best. Always verify transmittance data at your exact operating wavelength and thickness.
2. Ignoring dn/dT in High-Power CW Laser Systems
Even in continuous-wave (CW) laser systems where LIDT is not a concern, absorbed laser power causes local heating and thermal lensing. BK7's higher CTE means the optic deforms more under thermal load, degrading beam quality. For CW laser systems above ~100 W, consider fused silica for all transmissive optics.
3. Over-specifying Fused Silica for All Applications
Conversely, specifying fused silica for a simple visible-light imaging system where BK7 performs perfectly wastes 2–4× the optic budget. Material selection should match the application requirements, not default to the most expensive option.
Frequently Asked Questions
Can BK7 transmit UV light at all?
BK7 transmits some near-UV light from 350–400 nm, but its transmittance drops rapidly below 380 nm due to iron oxide absorption. At 300 nm, standard BK7 is essentially opaque. For any application below 350 nm, you must use UV fused silica or another UV-grade material. BK7 should not be used in UV laser, UV spectroscopy, or fluorescence applications.
Is UV fused silica the same as fused quartz?
They are related but not identical. Fused quartz typically refers to material made by fusing natural quartz crystals, which contains more impurities than synthetic fused silica. UV-grade fused silica is made synthetically (from SiCl4) and has much lower metallic impurity content, resulting in deeper UV transmission. For UV applications, always specify UV-grade or synthetic fused silica.
Can I use BK7 optics in a Nd:YAG laser system at 1064 nm?
Yes, for low-to-moderate power levels. BK7 transmits well at 1064 nm, and many low-power Nd:YAG systems use BK7 optics. However, for pulsed Nd:YAG systems with fluences above ~10 J/cm², you should switch to fused silica for better LIDT and reliability. For CW systems above ~100 W, fused silica is also recommended to minimize thermal lensing.
What about fused silica for large optics over 100 mm diameter?
Large fused silica blanks are available but significantly more expensive and have longer lead times than BK7. For large windows or mirrors in visible-light applications where UV transmission and extreme thermal stability are not required, BK7 is the pragmatic choice. The cost of a 150 mm fused silica blank can be 5–8× that of BK7.
Does the choice of material affect AR coating performance?
Yes, indirectly. The refractive index difference between the substrate and coating materials affects AR coating design. BK7 (n=1.517) and fused silica (n=1.458) require different coating designs for optimal performance. Additionally, UV applications require special coating materials (like Al2O3 and MgF2) that work in the UV, while visible coatings typically use Ta2O5/SiO2 or TiO2/SiO2 pairs. Always specify the coating for your exact wavelength and substrate material combination.
How does fused silica compare to BK7 for outdoor optical systems?
Fused silica is far superior for outdoor applications due to its extremely low CTE (0.55 vs 7.1 × 10-6/°C). Outdoor optical systems experience daily temperature swings of 30–50°C and seasonal swings of 60°C or more. Fused silica optics maintain their figure and alignment through these temperature changes, while BK7 optics can shift focus and alignment significantly. Solar exposure also adds UV radiation that can cause BK7 solarization over time.
Conclusion
UV fused silica and BK7 are both excellent optical materials, but they serve fundamentally different application spaces. BK7 is the cost-effective workhorse for visible and near-IR optical systems operating under moderate conditions. UV fused silica is the premium material required for UV transmission, high-power lasers, thermally demanding environments, and applications requiring the highest optical homogeneity. The key to optimal material selection is matching the material properties to your specific wavelength, power, temperature, and environmental requirements rather than defaulting to either material.
PhotonEdge manufactures precision optical components in both UV fused silica and BK7, including fused silica plano-convex lenses, fused silica windows, fused silica laser lenses, BK7 plano-convex lenses, BK7 windows, and BK7 prisms. All components are available with custom coatings, tight tolerances, and full inspection reports. Contact our engineering team for material selection guidance on your next project.