Optical Materials Selection Guide
Glass, Fused Silica, Crystal & Infrared Substrates
Material choice decides optical performance, part cost, and service life more than any other decision on a drawing. This guide compares the substrates we work with and shows how wavelength, environment, and performance requirements narrow the field to one or two candidates.
Material Categories
Optical Glasses
Borosilicate and heavy glasses with controlled homogeneity and low bubble content. The default choice for visible imaging, where cost and delivery dominate.
Typical materials: N-BK7, B270, SF11, LaSF9
Fused Silica & Quartz
Synthetic silica for UV transmission, low thermal expansion, and high laser damage resistance. Crystalline quartz adds birefringence control.
Typical materials: UV Fused Silica, IR Grade, Crystalline Quartz
Crystal Materials
Single-crystal substrates spanning deep UV to long-wave IR, including birefringent, nonlinear, and infrared-transmitting crystals.
Typical materials: CaF2, MgF2, Sapphire, ZnSe, Ge, Si
Infrared Materials
Substrates for thermal imaging and CO2 laser optics, selected by band coverage, index, thermal behaviour, and erosion resistance.
Typical materials: Germanium, Silicon, ZnSe, ZnS, BaF2
Material Comparison
Transmission bands are for 2 mm thickness. Refractive index is quoted at 589 nm for visible materials and at the design wavelength for infrared substrates, which are opaque in the visible.
Showing 12 of 12 materials.
Order as drawn up on this page, grouped by material family.
N-BK7
Band: 0.31-2.5 µm (UV – SWIR)
Index: 1.517
Abbe: 64.2
Applications: Imaging lenses, windows, prisms
B270
Band: 0.32-2.5 µm (UV – SWIR)
Index: 1.523
Abbe: 58.5
Applications: Covers, filters, display optics
UV Fused Silica
Band: 0.185-2.5 µm (DUV – SWIR)
Index: 1.458
Abbe: 67.8
Applications: UV windows, lithography, laser optics
CaF2
Band: 0.13-10 µm (DUV – LWIR)
Index: 1.434
Abbe: 95.1
Applications: Excimer optics, UV-IR windows, achromats
MgF2
Band: 0.12-8 µm (DUV – MWIR)
Index: 1.380
Abbe: 106
Applications: DUV windows, polarizers, waveplates
Sapphire
Band: 0.17-5.5 µm (DUV – MWIR)
Index: 1.768
Abbe: 72.2
Applications: Harsh-environment windows, domes
ZnSe
Band: 0.6-21 µm (Visible – LWIR)
Index: 2.40 at 10.6 µm
Abbe: n/a
Applications: CO2 laser optics, thermal imaging
ZnS
Band: 0.4-12 µm (Visible – LWIR)
Index: 2.25 at 10.6 µm
Abbe: n/a
Applications: FLIR windows and domes
Silicon
Band: 1.2-7 µm (NIR – MWIR)
Index: 3.42 at 4 µm
Abbe: n/a
Applications: MWIR windows, laser mirrors
Germanium
Band: 1.8-23 µm (NIR – LWIR)
Index: 4.00 at 10.6 µm
Abbe: n/a
Applications: LWIR imaging lenses
PMMA
Band: 0.4-1.6 µm (Visible – SWIR)
Index: 1.491
Abbe: 57.4
Applications: Molded lenses, light guides
COC
Band: 0.35-1.6 µm (UV – SWIR)
Index: 1.530
Abbe: 56.0
Applications: Molded optics, microfluidic covers
Note: Abbe numbers describe dispersion in the visible, so infrared substrates show n/a. Index values for zinc selenide, zinc sulfide, silicon, and germanium at their design wavelengths, together with band-by-band transmission and diamond-turning guidance, are listed on the crystal optics page.
Choosing by Wavelength
The band you need removes more candidates than any other requirement. These are the practical shortlists for each region.
UV & DUV (below 400 nm)
Below 300 nm, absorption and solarisation rule out ordinary glasses. UV fused silica covers 193 nm and above with good homogeneity, CaF2 reaches deeper with less solarisation under excimer exposure, and MgF2 serves the shortest wavelengths. Sapphire adds hardness where a window must survive repeated cleaning.
Deep-UV substrates and their polishing limits are covered on the crystal optics page.
Visible (400-700 nm)
N-BK7 remains the default: low cost, low bubble content, and predictable polishing. B270 suits volume windows and display covers, fused silica is chosen when the design must also work in the UV or hold figure through thermal cycling, and PMMA or COC takes over when thousands of identical elements are moulded.
Glass elements made from these substrates are described under custom spherical lenses.
NIR & SWIR (700-2500 nm)
Glasses remain usable into the near infrared, with fused silica and sapphire preferred once transmission has to extend past 2 µm. Silicon becomes transparent from about 1.2 µm, which makes it attractive for SWIR windows and mirrors where weight is not the limiting factor.
Windows and flats for these bands are listed under flat optics.
MWIR & LWIR (3-12 µm)
Germanium covers 8-12 µm imaging with a high index that keeps element counts low, silicon suits 3-5 µm systems, zinc selenide is the standard for 10.6 µm CO2 optics, and zinc sulfide trades a little transmission for thermal shock and erosion resistance. Sapphire stops at roughly 5.5 µm.
Infrared substrates and diamond-turned IR optics are detailed on the crystal optics page.
Choosing by Application
Once the band is fixed, the duty cycle and environment decide between the remaining candidates.
Imaging optics
Prioritise homogeneity, low bubble content, and a well-characterised index across the band. N-BK7 or fused silica usually win; CaF2 enters when an apochromatic design needs a low-dispersion partner.
Ground and polished singlet designs are specified on the spherical lenses page.
Laser optics
Absorption and inclusion density set the damage threshold. Fused silica, CaF2, and sapphire cover most pulsed and CW lasers, while ZnSe handles 10.6 µm CO2 systems.
Laser windows, mirrors, and prisms are quoted on the flat optics page.
Thermal imaging
Germanium and silicon carry LWIR and MWIR designs, zinc sulfide is preferred where a dome must survive rain and sand erosion, and chalcogenides help when athermal behaviour matters.
IR windows, lenses, and domes are fabricated in house — see crystal optics.
High-power applications
Low absorption, high thermal conductivity, and low dn/dT decide the outcome. Silicon and sapphire conduct heat away from the beam, while fused silica resists thermal shock in pulsed systems.
Damage-threshold data and coating options are listed under optical coatings.
Harsh environments
Sapphire windows handle abrasion and pressure, zinc sulfide handles airborne domes, and any substrate becomes viable with a hard AR coating when salt fog or cleaning abrasion is in the specification.
Hard-coated sapphire and ZnS windows are covered on the coatings page.
Once the material is fixed, send the drawing or the prescription and we will confirm substrate availability, coating, and lead time — request a material recommendation.
Five Steps to a Material Decision
The order matters: each step removes options before the next one adds cost.
- 1
Define the wavelength range
Start from the full band the optic must pass, not only the design wavelength, and include alignment and inspection wavelengths used during test.
- 2
Set the index and dispersion budget
Decide whether one material can meet the aberration target or whether a low-dispersion partner such as CaF2 is needed to correct colour.
- 3
Assess the environment
Temperature range, humidity, abrasion, pressure, and cleaning regime eliminate candidates faster than optical performance does.
- 4
Weigh manufacturing and cost
Mouldable plastics win at volume, glasses polish cheaply, and crystals cost more per part but often remove elements from the optical design.
- 5
Confirm availability and lead time
Check that the grade, size, and homogeneity you need are held in stock before the design is frozen, since specialty melts add weeks.
Surface quality from 40/20 to 10/5 scratch-dig to MIL-PRF-13830B applies to every material listed, subject to the limits on the tolerance chart.
Frequently Asked Questions
N-BK7 is a borosilicate glass with an index of 1.517 and an Abbe number of 64.2; it polishes easily, costs little, and covers 0.31-2.5 µm, but it absorbs below roughly 350 nm. Fused silica is synthetic, transmits from 185 nm, has lower thermal expansion and far higher laser damage resistance, at two to three times the material cost.
Work down from the band: keep the materials that transmit the full range, compare index and dispersion against the aberration budget, then eliminate candidates on environment, coating, and cost. The wavelength sections above give the practical shortlist for UV, visible, NIR, and IR work.
Germanium for 8-12 µm imaging, silicon for 3-5 µm, zinc selenide for 10.6 µm CO2 laser optics, and zinc sulfide where a dome must resist thermal shock and erosion. Barium fluoride and calcium fluoride cover spectroscopy windows where broadband transmission matters more than index.
Yes for moulded elements at volume: PMMA gives good visible transmission and low birefringence, while COC adds low moisture uptake and better dimensional stability. Neither suits service above about 100 °C, UV below 350 nm, or surface quality tighter than 40/20 scratch-dig.
Absorption and inclusion density drive the damage threshold, followed by thermal conductivity and dn/dT, which set thermal lensing and depolarisation. Homogeneity controls transmitted wavefront, and surface quality to 10/5 with a matched coating completes the chain.
Need help selecting materials?
Talk to our engineers about band coverage, damage thresholds, and lead times before you fix a substrate.
Talk to our engineers