Optical coatings shape how light interacts with a surface.
They improve transmission, reduce reflections, protect substrates, filter specific wavelengths, and in many cases make high-performance optical systems physically possible. From medical imaging and industrial automation to emerging AR/VR platforms, coatings determine whether an optical system performs at an acceptable level or fails to meet real-world demands.
This article walks through the fundamentals of optical coatings with a focus on their behavior on polymer substrates. You’ll find clear explanations, practical considerations, and insight into where polymer coatings differ from coatings on traditional glass.
Where helpful, we include perspectives from Apollo Optical Systems (AOS), a long-standing polymer optics manufacturer with decades of coating experience. These references are educational—not promotional—to give context around real engineering constraints encountered in production environments.
What Optical Coatings Do and Why They Matter
At their core, optical coatings are engineered thin films deposited onto an optical surface. These layers are measured in nanometers, and even a slight deviation can shift the way light behaves. Coatings can:
- Increase transmission through an optical surface
- Reduce unwanted reflections
- Create highly reflective mirror surfaces
- Split or combine beams
- Shape spectral behavior, such as bandpass or notch filtering
- Protect the substrate from environmental exposure
Without coatings, many modern optical systems would lose efficiency, struggle with stray light, or require bulkier designs that are impractical for today’s applications.
Optical coatings exist across the UV, visible, and NIR ranges, and their design must reflect the optical, environmental, and mechanical realities of the system they’re serving. For complex applications—especially those using polymers—this requires a thoughtful engineering approach.
How Optical Coatings Work: A Quick Primer
Most optical coatings are thin-film interference coatings. They consist of alternating layers of materials with different refractive indices. When light passes through or reflects off these layers, constructive and destructive interference shape the resulting transmission or reflection curve.
Three factors determine performance:
1. Material selection – High/low index pairings define the basic behavior.
2. Layer thickness – Even nanometer-level variations shift the spectral response.
3. Deposition method – Vacuum deposition is the most common; the method influences stress, uniformity, and adhesion.
For systems built around precision optics, these variables must be tightly controlled. Polymer optics add another layer of complexity, which we’ll come back to shortly.
Where Optical Coatings Are Used
Coatings appear throughout the optical industry. Common applications include:
• Medical & Life Sciences – Endoscopes, diagnostic imaging, surgical optics
• Industrial Imaging & Automation – Machine vision lenses, illumination modules
• Aerospace & Defense – Rugged imaging, target acquisition, aircraft-mounted sensors
• AR/VR and Wearables – Lightweight polymer assemblies, projection systems
• Consumer Electronics – Cameras, embedded sensors, display optics
• Automotive Sensing – ADAS cameras, LiDAR supporting optics, cabin monitoring
Across these markets, polymer optics continue to gain ground for certain wavelengths and mechanical needs. Their lightweight nature, geometric flexibility, and scalability make polymers an attractive substrate—but only when coatings are engineered correctly.
Polymer vs. Glass: Why Coating Behavior Changes
It’s a common misconception that coatings behave the same on polymer and glass surfaces. They don’t.
Apollo Optical Systems, which manufactures both molded polymer optics and coated components, notes that the glass transition temperature (Tg) of polymer materials is typically two to three times lower than that of glass. This single difference changes the entire coating strategy. Three critical distinctions arise:
Types of Optical Coatings for Precision Optics
This overview covers the most common coating categories and how they typically interact with precision optical systems.
How Coating Quality Is Measured
No coating discussion is complete without metrology. Accurate measurement ensures a coating performs as intended—not only when it leaves the coating chamber, but also under real-world conditions.
The most common measurement tools include:
Spectrophotometry
Used to measure transmission and reflectance across wavelengths. Precision systems, like the PerkinElmer Lambda 1050+ used at Apollo, provide high-resolution spectral data vital for tight tolerance coatings.
Surface inspection
Evaluates defects such as pinholes, scratches, and non-uniformity. This is particularly important for polymer substrates.
Environmental durability testing
Some industries require testing for humidity, temperature cycling, or abrasion.
When evaluating coating providers, engineers should look for clear, repeatable measurement processes and evidence of controlled production conditions.
Production Scale Matters—and It Influences Coating Design
Optical coatings behave differently depending on batch size, fixture design, and chamber configuration. A coating that works beautifully in prototype quantities may shift during volume production if not engineered for scalability.
Key considerations include:
• Fixture geometry
• Part orientation
• Uniformity across multiple components
• Deposition rate and stress control
• Repeatability from run to run
We emphasize the importance of custom fixture fabrication, noting that polymer optics often require specialized mounting to prevent deformation during coating. This capability—whether handled internally or through a trusted partner—is essential for consistent results.
Choosing a Coating Partner: What Engineers Should Look For
Whether you work with Apollo or another provider, the evaluation framework remains the same. A strong optical coating partner should demonstrate:
When Polymer Coatings Are the Right Choice
Polymers are not a universal replacement for glass, but they offer compelling benefits in the right contexts:
• Lightweight optical assemblies
• Integration of optical and mechanical features
• Faster prototyping cycles
• Lower cost at scale
• Complex freeform geometries
• Applications in the visible and NIR ranges
For these systems, coatings act as the final performance-enabling step. Well-engineered polymer coatings unlock the true capabilities of molded precision optics.
Apollo’s experience underscores this: polymer coatings are most successful when developed with the optical design, molding process, mechanical constraints, and environmental expectations in mind.
A Consultative Approach to Optical Coating Engineering
Optical coatings are easy to overlook but foundational to system performance. Engineers evaluating coating options should consider not only the spectral requirements but the underlying physics, substrate limitations, and manufacturing pathways.
A consultative coating partner—whether Apollo Optical Systems or another specialized provider—helps teams:
• Understand trade-offs
• Refine realistic specifications
• Prioritize manufacturability
• Predict long-term stability
• Ensure coating behavior aligns with system-level performance
This engineering-first approach prevents redesign cycles and cuts down development time, especially for polymer-based precision optics.
Building Reliable Optical Systems Through Better Coating Choices
