Engineering Knowledge, Shared

All three materials transmit at 10.6 µm, but they fail differently — and that's what should drive your choice.

ZnSe has the lowest bulk absorption of any practical CO₂ material (<0.0005 cm⁻¹ in laser grade), which means minimum thermal lensing and the most stable focus at multi-kilowatt power. It also transmits visible light, so a red aiming beam passes straight through. Its weakness is mechanical: at Knoop 120 it scratches easily, and aggressive lens cleaning in a dirty shop shortens its life.

GaAs is the ruggedness play: roughly six times harder (Knoop ~750), stronger, and more thermal-shock tolerant. In dusty, high-spatter cutting environments a GaAs focusing lens often outlives ZnSe several times over. The trade: slightly higher absorption and no visible transmission.

Germanium is generally the wrong answer for CO₂ power delivery. Its absorption rises steeply with temperature — above roughly 70–100 °C it runs away thermally and can self-destruct in a high-power beam. Reserve Ge for imaging, not power transmission.

A thermal camera that focuses at 20 °C and blurs at 45 °C usually has one root cause: Germanium's enormous thermo-optic coefficient (dn/dT ≈ +396 × 10⁻⁶/K). As the optic heats, its index — and therefore the system focal length — shifts dramatically.

Three practical strategies:

1. Material substitution. Chalcogenide glasses cut dn/dT roughly ten-fold (+30–60 × 10⁻⁶/K). An all-chalcogenide or Ge/chalcogenide hybrid design often holds focus passively across a military temperature range with no moving parts.

2. Optical passive athermalization. Combine materials whose thermal defocus cancels — CaF₂ is the classic tool because its dn/dT is negative (−10.6 × 10⁻⁶/K). A Ge/CaF₂ or chalcogenide/CaF₂ pairing can be designed so thermal focus shifts cancel to near zero.

3. Mechanical compensation. Let the housing do the work: mount materials with matched CTE so barrel growth moves an element exactly enough to re-focus. Cheapest at volume, but it must be designed in from day one — send us the operating temperature range with your RFQ.

Uncoated IR semiconductors are terrible transmitters — Fresnel reflection alone costs Ge 53% of the light. The AR coating is not an accessory; it is half the optic. The question is bandwidth versus depth.

V-coats concentrate all performance at one wavelength: reflectance below ~0.25% per surface at the design line (10.6 µm, 1064 nm), with the reflectance curve shaped like a "V". Use them for laser lines — power delivery, rangefinders, designators. Off the design wavelength, performance degrades quickly.

BBAR (broadband AR) spreads performance across a band — e.g. 3–5 µm or 8–12 µm at ~0.5–0.75% average reflectance per surface. Every imaging system should specify BBAR over its full sensor band, not just the center wavelength.

What to put on the drawing: the band (or laser line), average and maximum reflectance per surface, angle of incidence range, and the durability spec (adhesion, humidity, abrasion per MIL test methods). If the optic sees weather or cleaning, say so — it changes the coating design.

Diamond-like carbon (DLC) is an amorphous hard-carbon film applied to the exposed surface of Ge and Si optics — the standard front-element treatment for military FLIR, vehicle sights, and any thermal window that meets sand, rain, salt spray, or a soldier's glove.

DLC serves double duty: it is a genuine single-layer AR at LWIR on Germanium (index ~2.0, near-ideal for n=4 substrates) and an armor layer tested to severe abrasion, salt-fog, and humidity methods per drawing class. A typical fielded configuration is DLC on the outside surface and a high-efficiency BBAR on the protected inside surface.

Trade-offs to know: DLC absorbs slightly more than a conventional BBAR, so total transmission drops a point or two — a fair price for a window that survives a decade of field cleaning. It is not recommended as the primary AR for high-power CO₂ transmission optics.

Three different specs control three different things — and over-tightening any of them buys cost, not performance.

Scratch-Dig (e.g. 60-40, 40-20, 10-5) is cosmetic surface quality per MIL-PRF-13830B. It matters most near focal planes and in high-power laser optics (a dig is a damage-initiation site). For a protective window far from focus, 60-40 is usually invisible in the image; specifying 10-5 there can double the polish cost.

Surface irregularity (λ/2, λ/4 … λ/10) is the deviation of the surface from its ideal shape, measured interferometrically. It controls aberration added by each surface. Remember it scales with the test wavelength — λ/4 at 633 nm is a very different requirement than λ/4 at 10.6 µm.

Transmitted wavefront error (TWE) is the spec that actually predicts imaging performance, because it sums both surfaces plus material homogeneity. For demanding systems, spec TWE on the finished optic and let us allocate the per-surface budget.

Rule of thumb: spec the tightest number where the beam is small or the flux is high; relax everywhere else. Our engineers will flag over-specified surfaces at quote time — it's often 30% of the cost.

Germanium is a strategic material: byproduct-refined (mostly from zinc smelting), concentrated in a handful of countries, and subject to export restrictions that have whipsawed prices. For a thermal-imaging supply chain, Ge risk is schedule risk.

American Photonics' domestic reclaim program takes in Ge scrap — grinding swarf, edge trim, rejected blanks, decommissioned optics — and routes it back into the U.S. supply chain. Participants receive credit against new material purchases, effectively hedging their Ge exposure.

What we accept: clean Ge solids and optics, coated or uncoated (we strip coatings), and segregated grinding sludge. What you get: documented chain of custody, weight-based credit at current reclaim rates, and certificates for your sustainability and supply-chain reporting.

To enroll or price a lot, contact sales@americanphotonics.com with approximate weight and form factor.

Also recycling ZnSe: our online store runs a standing ZnSe Buy Back program for used and damaged zinc selenide optics.