1. What Is an Infrared Optical Coating?

An infrared optical coating is a deposited surface structure designed to control reflection, transmission, or another surface property at specified infrared wavelengths. It may reduce reflections, form a mirror, select a spectral band, or add environmental protection.

The coating and substrate work together. A suitable coating cannot make a strongly absorbing substrate transparent, and a transmitting substrate does not guarantee that a particular coating will perform well on it.

Specify the function first: a protective thermal-camera window, a CO₂ laser lens, and a gas-analysis filter require different coating designs.

2. Define the Working Band Explicitly

Infrared band names are useful shorthand, but their boundaries vary by convention. Use numerical limits in the drawing and purchase specification.

Atmospheric transmission is not uniform across every named band. Define the optical requirement from the complete system rather than treating an atmospheric window as a guaranteed flat passband.

3. Why Reflection Matters for IR Optics

Some infrared materials have relatively high refractive indices, making reflection a significant loss mechanism. At normal incidence between nonabsorbing media, a single-interface approximation is R = [(n₁ − n₂)/(n₁ + n₂)]².

For an illustrative air interface and substrate index of 2.4, the calculated surface reflectance is about 17.0%. For an index of 3.4, it is about 29.8%. These are rounded-index examples, not material specifications across an infrared band.

Refractive index varies with wavelength and temperature. Absorbing materials require a model that includes the complex refractive index. Use measured material data and a suitable optical model for the actual design.

4. AR, BBAR, Mirrors, and Filters

BBAR is a type of AR coating, not a separate physical principle. Manufacturer coating families demonstrate the importance of specifying spectral and angular conditions. [1]

5. How Thin-Film Designs Work

Thin-film interfaces produce reflected waves with different phases and amplitudes. Choosing the refractive indices and thicknesses of the layers controls how these waves interfere.

A quarter-wave optical thickness, nd = λ₀/4, is a useful starting concept for simple designs at normal incidence. It is not a universal rule for every layer in a practical multilayer stack.

Infrared designs must account for both the real refractive index and absorption of each film material in the operating band. A film that works well in visible optics may absorb too strongly at a longer wavelength.

More layers do not automatically improve the result. Bandwidth, residual reflection, absorption, stress, uniformity, environmental durability, and manufacturing repeatability must be balanced.

6. Match the Coating to the Substrate

This table provides screening considerations, not universal transmission guarantees. A coating design qualified on one substrate cannot simply be transferred to another with a different index, surface chemistry, or thermal behavior.

7. What DLC Adds—and What It Does Not

Diamond-like carbon (DLC) describes a family of carbon coatings. Optical DLC designs can provide a hard, low-friction surface and improve resistance to specified wear or environmental exposure. Their optical behavior depends on composition, thickness, and the complete layer structure.

A DLC-coated surface may also provide an antireflection function over its design band, but it is not automatically equivalent to a high-throughput BBAR coating. Manufacturer guidance describes a potential tradeoff between durability and transmission. [2]

DLC compatibility must be confirmed for the substrate. Jenoptik identifies special hybrid solutions for ZnS, illustrating why “apply DLC” is not a complete manufacturing instruction. [3]

Do not interpret DLC as scratch-proof, erosion-proof, or suitable for every wavelength. Require a qualified optical and environmental specification for the actual stack.

8. Specify External and Internal Faces Separately

An exposed infrared window may use a protective optical stack on its external face and BBAR on its protected internal face. A sheltered component may instead use BBAR on both faces. The correct arrangement depends on exposure and the allowable optical loss.

Label the faces on the drawing using physical datums or a clear orientation view. Identify coated areas, edge exclusions, bonding zones, and any masking requirements.

A requirement such as “S2: T ≥ 95%” is ambiguous unless it defines what is being measured. Clarify whether the target is a modeled interface value, a witness-sample result, or finished-component transmission through both surfaces and the substrate.

For a DLC/BBAR window, require a complete-part transmission target as well as the necessary coating acceptance data.

9. Separate Reflection, Absorption, and Transmission

Low reflection does not by itself prove high transmission. The substrate and coating may absorb light, and surface defects or contamination may scatter it outside the receiver’s useful aperture.

If each surface reflects 1% and internal transmission is 98%, the estimated direct transmission is 0.99 × 0.99 × 0.98 = 96.05%. This is a calculated illustration, not measured Felix Glass data.

For thermal imaging, absorption and temperature can also contribute to the component’s thermal emission. Include the window in system calibration and thermal analysis when those effects matter.

10. Angle, Polarization, Temperature, and Curvature

Thin-film response changes with incidence angle, and s- and p-polarized light can behave differently. Coating performance specified at normal incidence should not be assumed valid across a wide-angle lens or dome.

For curved parts, calculate incidence relative to the local surface normal. Define the useful aperture and ray-angle envelope from the actual optical layout.

Film thickness uniformity across a dome is a separate manufacturing concern. Agree on how coating witness samples represent the finished curved surface and what component-level checks are required.

Temperature can affect layer optical properties, substrate behavior, and mechanical stress. State whether optical limits apply at room temperature only or throughout operation. Supplier coating specifications show why band and angle conditions must accompany performance values. [4]

11. Deposition and Process Control

Infrared coatings may use vacuum evaporation, sputtering, ion-assisted processes, or other deposition methods. DLC may involve plasma-based processes. The appropriate route depends on the film materials, substrate, geometry, and requirements.

Critical controls include substrate cleanliness, surface condition, deposition temperature, layer-thickness monitoring, film composition, stress, and adhesion. Process names alone do not establish optical quality or environmental performance.

Define pre-coating surface quality and post-coating inspection. For sensitive substrates or precision surfaces, assess whether the process changes surface figure or creates unacceptable stress.

Prototype approval should cover the complete production route. Changes to film materials, fixtures, substrate grade, or deposition conditions may require review.

12. Verify Optical Performance and Durability

• Spectral testing: Use suitable IR spectrophotometry or FTIR methods and define wavelength range, resolution, aperture, angle, polarization, and reference method.
• Acceptance curves: Distinguish theoretical, typical, witness-sample, and finished-part data. State minimum transmission and maximum reflection where needed.
• Band averages: Define averaging or spectral weighting; an average can conceal a narrow transmission dip.
• Environment: Specify adhesion, abrasion, humidity, temperature cycling, salt exposure, or cleaning tests relevant to service.
• After exposure: Recheck optical performance and physical condition using agreed acceptance criteria.
• Laser use: Specify wavelength, pulse or CW operation, beam size, power or fluence, repetition rate, and relevant damage-test conditions.

Do not claim compliance from a coating name alone. If a customer names a standard, obtain the edition, test methods, severity, and acceptance criteria before committing to it.

13. What to Include in an Infrared Coating RFQ

A useful requirement structure is: “Finished-component transmission ≥ [target] over [band], at [angles], for [polarization], across [aperture], measured under [conditions], before and after [tests].” The fields must be agreed for the project; there is no universal target for every IR coating.

14. Frequently Asked Questions

15. Discuss Your Infrared-Coated Optical Component

Felix Glass supports custom precision infrared components, including ZnSe and ZnS optics and selected silicon applications. Share your drawing, substrate grade, working band, coating-face requirements, environment, and quantities for a project-specific review.

Coating feasibility, transmission targets, durability, inspection scope, and delivery conditions should be agreed before manufacture. Distinguish component supply from complete-system qualification responsibilities.

Dongguan Felix Glass Co., Ltd.
Email: james@felixglass.com