A comprehensive comparison of optical materials — from BK7 to Germanium — with properties, applications, and a practical decision framework for choosing the right material for your optical system.
The optical material you choose determines virtually every aspect of your system's performance: what wavelengths can pass through, how the optic behaves under thermal stress, what laser power it can handle, how it survives in harsh environments, and ultimately how much it costs.
A wrong material choice can manifest as: unexpected UV absorption, thermal lensing in high-power lasers, birefringence artifacts in polarized systems, window fracture under thermal shock, or simply excessive cost for a performance level you don't need. Getting the material right from the start avoids costly redesigns and delays.
When evaluating optical materials, these are the properties that matter most:
| Property | Symbol | Unit | Why It Matters |
|---|---|---|---|
| Transmission Range | λ | μm | Defines usable wavelength window |
| Refractive Index | nd | - | Determines lens power, reflection loss |
| Abbe Number | Vd | - | Dispersion / chromatic aberration |
| Thermal Expansion (CTE) | α | ×10-6/K | Dimensional stability with temperature |
| Thermal Conductivity | k | W/m·K | Heat dissipation, thermal shock resistance |
| dn/dT | dn/dT | ×10-6/K | Refractive index change with temperature (thermal lensing) |
| Knoop Hardness | HK | kgf/mm² | Scratch resistance, durability |
| Density | ρ | g/cm³ | Weight-critical applications |
| Laser Damage Threshold | LIDT | J/cm² or W/cm² | Maximum laser power/energy handling |
| Birefringence | Δn | - | Critical for polarization-sensitive systems |
The following table compares the most commonly used optical materials across key properties:
| Material | Range (μm) | nd | Vd | CTE | Hardness | Best For |
|---|---|---|---|---|---|---|
| N-BK7 | 0.35-2.0 | 1.517 | 64.2 | 7.1 | 560 | General visible optics |
| UV Fused Silica | 0.18-2.1 | 1.458 | 67.8 | 0.55 | 460 | UV, high-power lasers |
| Sapphire | 0.15-5.5 | 1.768 | 72.2 | 5.0 | 2000 | Harsh environments, windows |
| ZnSe | 0.5-20 | 2.40 | - | 7.1 | 120 | CO2 lasers, thermal imaging |
| Germanium | 2.0-14 | 4.02 | - | 6.1 | 780 | LWIR thermal imaging |
| Silicon | 1.2-7.0 | 3.42 | - | 2.6 | 1150 | MWIR, weight-sensitive IR |
| CaF₂ | 0.13-7.0 | 1.433 | 95.3 | 18.9 | 158 | UV-IR spectroscopy, excimer |
| MgF₂ | 0.11-7.0 | 1.380 | 95.0 | 13.7 | 415 | Deep UV, durable windows |
N-BK7 is a borosilicate crown glass and the most widely used optical material worldwide. If your application operates in the visible to near-IR range (350-2000nm) with no extreme requirements, BK7 is almost certainly the right choice.
Imaging lenses, beam steering, laboratory optics, educational setups, machine vision, general-purpose windows and prisms.
UV Fused Silica (Synthetic Fused Silica) is amorphous SiO₂ produced from high-purity silicon compounds. It is the go-to material when BK7 falls short — especially for UV transmission, high-power lasers, and thermally stable applications.
Sapphire is the second hardest natural material (9 Mohs), making it ideal for optics exposed to harsh environments, abrasion, or high-pressure cleaning. It transmits from 150nm to 5.5μm — covering UV through mid-IR.
Zinc Selenide (ZnSe) is the dominant material for CO2 laser systems (10.6μm) and thermal imaging. It offers excellent transmission from 0.5μm to 20μm with relatively low absorption at CO2 wavelengths.
Germanium has the highest refractive index of any common optical material (n≈4.0 at 10μm), enabling compact lens designs with strong optical power from nearly-flat surfaces. It is the standard material for LWIR (8-14μm) thermal imaging systems.
Silicon is widely used for MWIR (3-5μm) optics, offering a cost-effective alternative to Germanium in the mid-IR. It has lower density than Ge (2.33 vs 5.33 g/cm³), making it preferred for weight-sensitive applications.
Calcium Fluoride (CaF₂) is a crystalline material with exceptional transmission from deep UV (130nm) through mid-IR (7μm). Its very low dispersion (Abbe number 95) makes it irreplaceable for apochromatic lens designs and UV laser systems.
Magnesium Fluoride (MgF₂) transmits from 115nm (deepest UV of any common optical material) to 7μm. It is harder and more durable than CaF₂, though with slightly narrower overall spectral range.
Use this decision framework to narrow down your material choice:
Within your viable material set, choose the most economical option that meets all requirements. BK7 is almost always the lowest-cost choice for visible optics. For IR materials, Silicon is generally lower-cost than Germanium. Fused Silica is 2-3x the cost of BK7 for visible applications.
Our engineering team will analyze your application requirements and recommend the optimal material for performance and cost.
Request Material Consultation Ask AI Optical EngineerN-BK7 (borosilicate crown glass) is the most widely used optical material for visible to near-IR applications. It offers excellent homogeneity, good transmission from 350nm to 2μm, and low cost. It is the default choice when no special requirements (UV, IR, high temperature, or high power) exist.
Choose UV Fused Silica when you need: (1) deep UV transmission below 350nm, (2) higher laser damage threshold, (3) lower thermal expansion for temperature-sensitive applications, or (4) better transmission uniformity in the UV. Fused Silica transmits from 180nm to 2.1μm and has 5x lower CTE than BK7.
ZnSe (Zinc Selenide) is the standard choice for CO2 laser systems. It offers >67% transmission at 10.6μm with excellent thermal conductivity. For lower-power applications (<50W), single-crystal Silicon provides a cost-effective alternative. Germanium works but has higher absorption at 10.6μm than ZnSe.
Sapphire excels when you need extreme hardness (9 on Mohs scale), superior scratch resistance, and good transmission from 150nm to 5.5μm. It has higher thermal conductivity than Fused Silica. Choose Fused Silica when you need better UV transmission, lower birefringence (sapphire is birefringent), or lower cost for large optics.
For MWIR (3-5μm) thermal imaging, Germanium is the standard with highest refractive index (n~4.0) enabling compact lens designs. For LWIR (8-14μm), Germanium or ZnS (multispectral grade) are typical choices. ZnSe works across both bands but is softer. Chalcogenide glasses offer moldable aspheric IR optics for volume production.
BK7 transmission drops significantly below 350nm due to iron impurity absorption. For UV applications below 350nm, use UV Fused Silica (down to 180nm), CaF₂ (down to 130nm), or MgF₂ (down to 115nm). If you must use BK7 near 350nm, specify UV-grade BK7 with tighter iron content limits.
Among common optical materials, ULE (Ultra-Low Expansion) glass and Zerodur have near-zero CTE (<0.05 × 10₊/K). UV Fused Silica has very low CTE (5.5 × 10₊/K). For most practical applications where extreme stability isn't needed, Fused Silica provides excellent thermal performance at much lower cost than ULE or Zerodur.
For high-power CW lasers: UV Fused Silica (for VIS-NIR) or Silicon/Cu (for IR). For high-power pulsed lasers: UV Fused Silica with IBS coatings for VIS-NIR, CaF₂ for UV and deep-UV. The key factors are low absorption coefficient and high LIDT. Always specify IBS-deposited coatings for high-power applications.