A practical engineering comparison of the two most common optical materials — helping you avoid costly material selection mistakes in lens, window, and substrate design.
BK7 (N-BK7, borosilicate crown glass) and UV Fused Silica are the two most frequently specified optical materials worldwide. Together they cover the majority of lens, window, prism, and substrate applications. Yet optical designers still over-specify fused silica for visible-light systems (paying 3–5× more than necessary) or mistakenly choose BK7 for UV applications (causing transmission failures at wavelengths below 350 nm).
This guide provides a direct, property-level comparison so you can make the right choice for your application — without over-engineering or under-performing.
The following table compares the critical engineering properties of N-BK7 and UV Fused Silica (Schott/Ohara equivalents). All values are representative at room temperature unless otherwise noted.
| Property | N-BK7 | UV Fused Silica | Impact on Design |
|---|---|---|---|
| Transmission Range | 350 – 2,000 nm | 185 – 2,500 nm | FS reaches deep UV; BK7 blocks below ~350 nm |
| Refractive Index (nd) | 1.5168 | 1.4585 | BK7 bends light more; fewer surfaces needed for same power |
| Abbe Number (Vd) | 64.17 | 67.82 | Both low dispersion; BK7 pairs well with SF glasses for achromats |
| CTE (α) | 7.1 × 10−6 /K | 0.55 × 10−6 /K | FS ~13× lower expansion — critical for athermal designs |
| Thermal Conductivity | 1.11 W/(m·K) | 1.38 W/(m·K) | FS dissipates heat better; matters for laser window stability |
| dn/dT | 3.0 × 10−6 /K | 9.6 × 10−6 /K | BK7 lower thermo-optic; FS dn/dT matters in precision optics |
| Softening Temperature | ~557 °C | ~1,580 °C | FS withstands far higher temperatures without deformation |
| Laser Damage Threshold (CW, 1 μm) | ~10 MW/cm² | ~40 MW/cm² | FS preferred for high-power laser optics |
| Hardness (Knoop) | ~580 kg/mm² | ~470 kg/mm² | BK7 slightly harder; easier to polish to fine finish |
| Stress Birefringence | Class 1–2 (low) | < 2 nm/cm (very low) | FS preferred for polarization-sensitive systems |
| Available Blank Sizes | Up to Ø300 mm | Up to Ø400+ mm | Both available in large formats; FS excels for large windows |
| Relative Cost | Baseline (1×) | 3–5× baseline | BK7 is the cost-effective default for non-UV, non-laser use |
N-BK7 is the default workhorse optical material. It should be your first choice unless specific application requirements rule it out. Choose BK7 when:
UV Fused Silica becomes necessary (not just preferable) when one or more of these conditions exist:
Use this step-by-step framework to select between BK7 and fused silica:
| Application | Recommended Material | Reason |
|---|---|---|
| Camera lens / imaging optics | BK7 | Visible range, low cost, excellent achromat pairing |
| Laser marking head (532 nm, 20 W) | Fused Silica | High power at visible wavelength |
| Microscope coverslip (brightfield) | BK7 (standard) or FS (UV fluorescence) | Depends on excitation wavelength |
| Spectrometer window (UV-Vis) | Fused Silica | Must transmit below 350 nm |
| Beam expander for HeNe (633 nm) | BK7 | Low power, visible wavelength |
| Excimer laser window (248 nm) | Fused Silica | Deep UV + high energy per pulse |
| Optical flat / reference surface | Fused Silica | Thermal stability preserves flatness |
| LED collimation lens | BK7 | Visible range, cost-sensitive, high volume |
BK7 contains iron impurities that cause strong absorption below 350 nm. At 300 nm, BK7 transmission is essentially zero for any meaningful thickness. Designers new to UV systems sometimes specify BK7 because it is the “default” optical glass, then discover their optics are opaque at the operating wavelength. Solution: Always check the transmission curve. For any wavelength below 350 nm, specify fused silica or CaF₂.
Some engineers specify fused silica for all optics “because it’s better.” But in visible imaging systems, fused silica’s lower refractive index (1.458 vs 1.517) means you need steeper curvatures to achieve the same optical power, potentially increasing aberrations. It also disrupts achromatic doublet designs optimized for BK7/SF-glass pairs. Solution: Use fused silica only when UV, laser power, or thermal stability demand it.
While fused silica has much lower CTE than BK7, its dn/dT (9.6 × 10−6/K) is actually higher than BK7’s (3.0 × 10−6/K). The total thermal optical power shift depends on both CTE and dn/dT. In some athermal designs, BK7 can actually outperform fused silica when both effects are considered together. Solution: Calculate the total thermo-optic coefficient, not just CTE.
AR coatings designed for BK7 (n=1.517) will not perform optimally on fused silica (n=1.458), and vice versa. The layer thicknesses in multi-layer coatings are calculated for specific substrate indices. Always specify the substrate material when ordering coated optics. Solution: See our AR Coating Selection Guide for details on matching coatings to substrate materials.
Understanding the cost structure helps you make rational tradeoffs:
| Factor | BK7 | UV Fused Silica |
|---|---|---|
| Raw blank cost (Ø25 mm) | Low (baseline) | 3–5× BK7 |
| Raw blank cost (Ø100 mm) | Moderate | 4–8× BK7 |
| Polishing difficulty | Standard | Slightly harder (higher softening point) |
| Typical lead time (standard catalog) | 1–2 weeks | 2–4 weeks |
| Coating cost | Standard | Similar (may need UV-rated coating materials) |
| Cost of wrong material choice | System failure in UV | 3–5× unnecessary cost |
The real cost question is not “which material is cheaper” but “what is the cost of getting the material wrong?” Choosing BK7 for a UV system means complete optical failure — no amount of cost savings compensates. Conversely, specifying fused silica for a simple visible LED collimator means paying 3–5× more for no performance benefit.
Our optical engineers can review your application requirements and recommend the optimal material — balancing performance, lead time, and cost.
Request a Quote Browse All MaterialsBK7 has marginal transmission at 365 nm — typically 60–70% per 10 mm thickness depending on the specific melt. For demanding UV-A applications, fused silica is strongly recommended. If you must use BK7 near 365 nm, request a transmission curve from the manufacturer for your specific batch and keep thickness minimal.
Only if your application requires its specific advantages: UV transmission, high LIDT, or thermal stability. For standard visible-light imaging, collimation, or beam steering, BK7 provides equivalent performance at a fraction of the cost. The cost premium is justified for laser systems, UV instruments, and precision references.
UV-grade BK7 (with tighter iron content control) improves transmission near 350 nm but does not reach true deep-UV performance. It can be a reasonable choice for 340–365 nm applications where fused silica cost is prohibitive, but it is not a substitute for fused silica below 330 nm.
Yes, but be aware that their different refractive indices (1.517 vs 1.458) and dispersion properties (Vd 64.2 vs 67.8) will affect the optical design. Achromatic doublets are typically designed with BK7 paired with high-index flint glasses, not fused silica. Hybrid designs are possible but require optimization from the start.
In a word: sometimes. The total thermal shift of optical power depends on both CTE (which favors fused silica) and dn/dT (which favors BK7). For systems where the optical path length through air dominates (long focal lengths), BK7’s lower dn/dT can actually give better thermal stability. Always calculate the full thermo-optic effect for your specific design.
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