How to Choose the Right Anti-Reflective Coating
Optical Design Guide
How do you choose the right anti-reflective (AR) coating?
To choose the right anti-reflective coating, match the coating design to your system’s wavelength range, angle of incidence, substrate material, and operating environment. AR coatings are optimized for specific spectral bands and angles, so performance will degrade if these conditions are not aligned. Engineers must also balance optical performance with durability, especially for applications exposed to temperature changes, humidity, or abrasion.
Anti-reflective (AR) coatings are critical to your optical system performance by their ability to minimize surface reflections and increase transmission. However, selecting the correct AR coating best suited for your application is not a one-size-fits-all decision. This may come as a surprise to many people searching for an AR coating for their project for the first time. To avoid a poor result, this guide outlines how to evaluate and specify the right AR coating from Omega for your application.
The optimal solution in this case depends upon your required wavelength range, application angle of incidence, substrate material selection, operating environmental conditions, and design-critical system tolerances. A poor AR coating selection can result in reduced transmission, spectral shift, durability failures, or system-level inefficiencies – all things you want to avoid.
What Does An Anti-Reflective Coating Do?
Anti-reflective coatings reduce Fresnel reflections at optical surfaces by using thin-film interference.
With each uncoated interface, a percentage of that source light is reflected back to the light source. Using an AR coating introduces carefully designed layers that cause destructive interference of that reflected light, allowing more of your source light to transmit through the optic to the sensor.
Noted outcomes when AR coatings are applied are:
- Increased transmission and efficiency of the coated optic
- Reduced stray light and ghosting (especially important in imaging systems)
- Improved signal-to-noise ratio
Key Factors When Selecting Your AR Coating
Key factors when choosing an anti-reflective coating:
- The Required Wavelength range (UV, visible, IR)
- Angle of Incidence (on-axis vs. off-axis)
- Substrate material (glass, silicon, polymers)
- Polarization/ Brewster’s angle
- Environmental conditions (temperature, humidity, abrasion)
- Durability requirements (hard vs. soft coatings)
Knowing Your Wavelength Range
The most important parameter is the wavelength or spectral band of operation.
- UV (200–400 nm): Requires specialized materials with low absorption
- Visible (400–700 nm): Broadband AR coatings are common in the visible range
- NIR/SWIR (700–2200 nm): Material selection and layer design may shift
- MWIR/LWIR: Requires entirely different coating materials and processes
Application Insight:
AR coatings are optimized for a specific spectral band and the performance will drop off outside that wavelength range. This is especially critical to understand when requesting AR coatings on filters intended for use in LEO space conditions.
Understanding Angle of Incidence (AOI) and Optical System Type
AR coatings are highly sensitive to angle to perform at their maximum and as a result, there are factors that engineers need to keep in mind. As AOI increases, the effective wavelength shifts toward shorter wavelengths, which can degrade the performance if not accounted for in the coating design stage. When designing, the rule of thumb is: at normal incidence, one lens will lose roughly 4%, for two uncoated surfaces, the estimate is ~8% loss.
- Normal incidence (0°): Simplest design, best performance
- Off-axis systems: Will experience spectral shift and reduced effectiveness
- Wide-angle systems: Require advanced coating designs to perform correctly
Substrate Materials Which Typically Require AR Coatings
The different substrates hold distinct variations with AR coatings applied and have known identities for selection. Each material has a different refractive index, which directly impacts coating design. Your AR coating must be matched correctly to the substrate to minimize reflection at the interface. Omega’s experts will assist you in determining the best material selection and coating option for your intended use and specifications. Weight and dimension limitations may also play into the final choice.
- BK7 (fused silica glass): n ≈ 1.5168 @ 587.6 nm (d-line) Common range: ~1.515 (visible) Slightly decreases into NIR. BK7 is the industry standard for coated lenses and windows. It offers low cost with moderate dispersion.
- Silicon (Si): n ≈ 3.4 @ 3–5 µm (MWIR), Typical range: ~3.42 (MWIR). Good selection for MWIR, and offers high index when uncoated ~30%
- Germanium (Ge): n ≈ 4.0 @ 10 µm (LWIR), Very high uncoated index ~36%, excellent LWIR transmission (8–12 µm), the index will change with temperature, which may make coating challenging.
- PMMA Polymers (for diamond-turned optics): n ≈ 1.49 (visible). Lower index than IR crystals, which offers lower Fresnel loss (~4-6%). Material is very suitable for diamond-turned optics or injection-molded parts.
Environmental Conditions of Application
Your operating environment can determine whether an applied optical coating succeeds or fails. Knowing this in advance is one of the important factors engineers will take into consideration, and also plan for final MIL-SPEC or other custom environmental testing. A typical coating optimized for lab use may often fail in aerospace or field-deployed systems.
Environments to Consider:
- Temperature extremes and thermal cycling (space applications)
- Humidity and moisture exposure (ships, traffic monitoring)
- Environmental Abrasion (dust, sand)
- Cleaning requirements (chemical)
- Radiation (space applications)
Knowing Durability and Adhesion Requirements
Not all AR coatings present equal durability. Again, this is a conversation with your design and engineering team at Omega during the scoping process. There is typically a balance between optical performance vs. mechanical durability and there may be more than one option to offer.
With our multiple geographic locations, we can offer customers custom AR coatings on substrates tailored to their final use requirements.
Single Layer Coatings – Typically MgF2 deposited as a ¼ wave thickness optimized to the central wavelength. These traditional AR coatings are a good fit for low-cost solutions but only offer a moderate reduction in reflection.
Considerations for Surface Geometry and Size
Large or complex-shaped optics introduce additional challenges during the application of AR coatings. Designing for manufacturability is essential as the coating performance and must be achievable at scale.
Uniformity across large apertures – large optics have additional coating factors.
Edge effects are unwanted non-uniformities that cause changes in optical performance. Omega mitigates this issue by using masking strategies, deposition modeling and multi-axis filtering.
Curved or freeform surfaces will change the local angle of incidence during coating deposition, resulting in shadowing zones and may lead to edge failure. Our Ayers, Hopewell Junction, and Ithaca locations specialize in IR coatings on curved surfaces and can add an AR coating to your order.
Common Types of Anti-Reflective Coatings Offered at Omega Optical
Single-Layer AR Coatings are optimized for a narrow wavelength, are typically lower in cost and have more limitations to performance.
Multi-Layer Broadband AR Coatings are designed for use with wide spectral ranges that need higher transmission across those bands. Multi-layer broadband coatings are more complex and sensitive to the AOI.
V-Coatings (Narrowband AR) are so named for the V-shaped plot of reflectance vs wavelength. Ideally used for laser applications, the performance of a V-coating will drop significantly outside the target wavelength. These offer extremely low reflectance.
Why You Need to Engage a Coating Partner Early in the Project Build
Early project collaboration can prevent costly redesigns, and we’ve heard those stories of frustration from customers who come to us for solutions and support after attempting to work internally without expertise. We ask your team to consider engaging during the optical design phase for optimal material selection, tolerance definition, and prototyping development so your project stays within scope.
Reduce the risk associated with reduced time to market by requiring fewer adjustments or post-coating “fixes.”
Benefit from the maximum yield from each coating run.
Choosing the right anti-reflective coating requires balancing optical performance, environmental durability, and manufacturability needs. By understanding how wavelength, angle, substrate, and environment interact, all involved engineers can make informed decisions that will improve your system performance and the long-term reliability of the optics, system and life of the project.
Have your specific AR coating questions answered by one of our experts today.
