Coating Plastic Substrates – Part 2 of 2
In Part 1 of this series, we talked about the specialized cleaning, handling, and deposition processes required to deposit coatings on plastics to MIL-SPEC standards. But why choose plastic optics? In Part 2, we explore the many coatings we can offer in the transition from glass to plastic. We’ll also share case studies of some successful projects, from laser sighting optics and mylar sheets to hot mirrors for virtual reality.
Why Choose Plastic Optics?
Weight, cost, and durability are the three most common reasons to design an optic in plastic. Plastic is 1/10th the weight of glass, yet offers comparable transmittance. It can also be inexpensively molded in volume, often in configurations that simplify mounting and system assembly. Though more sensitive to scratching, plastic offers greater impact resistance than glass, and tends to break into fewer, less dangerous fragments rather than splintering.
We frequently work with customers who want to transition a design from glass to plastic due to damage or weight, but without compromising optical performance or the ability to meet a specific MIL-SPEC standard. The conditions that must be met can be very severe – including extreme temperature cycling, shock, and chemical resistance to materials like insect repellent, antifreeze, and rifle bore cleaning compound. In new optical designs, the basic needs are the same – a coating that performs well optically and is durable.
AccuCoat has spent nearly 20 years developing processes specifically for coating on a wide range of plastic substrates, at wavelengths from the visible to the IR. The volume of experience we bring to each new plastic coating project allows us to identify the right cleaning, handling, and deposition parameters to meet the most challenging optical and environmental requirements.
Coating Options on Plastic Optics
While standard and wide-angle anti-reflection (AR) coatings are no doubt the most popular request we get on plastic optics, we routinely provide many other coating types, including mirrors, beamsplitters, and filters. In fact, we can offer comparable optical performance to the same coating on glass in many cases, particularly in the visible wavelength range.
An air-plastic interface reflects approximately 4% of light, which can easily be reduced to below 0.5% with an AR coating, and even down to 0.25% at a single wavelength on a high-quality substrate like Zeonex®. Wide-angle AR coatings are often needed for lenses and elements with a high degree of curvature; we created a 1064 nm AR coating for one military customer that performed at up to 45° AOI.
We have developed robust processes to apply both dielectric and metal mirror coatings to plastic substrates, including silver (Ag), gold (Au), and aluminum (Al). While many vendors have issues with adhesion when coating metal on plastic, we understand how to adapt our process to the specific substrate type to achieve adhesion that easily passes a standard tape test and meets the same optical specifications as on glass. We’ve even been able to adapt our processes to apply ITO coatings to plastic substrates with very good performance.
Case Study: Laser Sighting
The OEM that came to us had been using a small triangular coated glass window at the end of a gunsight but was seeing frequent cracking in the field due to the rough handling. They needed a drop-in replacement window in plastic that met both the needed optical performance and all environmental requirements (including chemical exposure and severe temperature cycling). Of the three suppliers who provided prototype optics in plastic, AccuCoat was the only one to pass the full suite of environmental tests.
Case study: Hot mirrors for Virtual Reality
Hot mirrors are used in virtual reality (VR) systems to reflect the IR light used by eye or positional tracking systems while passing the visible light. These heads-up display optics need to be lightweight and thus are coated on plastic. Optical requirements for VR vary but can include 80-90% reflection of 800-830 nm light or nonpolarizing beamsplitting of the laser and visible, both of which are possible using AccuCoat coatings on plastic substrates.
We can successfully apply many filter coatings to plastic substrates, including some quite challenging curves. Broadband filters, longpass filters, and hot mirrors are all possible with somewhat less edge steepness than on glass – even simple cold mirrors can be done.
Our beamsplitter coating offerings include standard splitting ratios from 50/50 to 70/30, as well as polarizing beamsplitters with up to a 300:1 extinction ratio. We routinely work hand-in-hand with vendors to create finished plastic beamsplitter cubes: the prisms are shipped to us to apply beamsplitting coatings, after which the vendor assembles the prisms into a cube at their facility for final AR coating at AccuCoat. The beamsplitter deposition processes we’ve developed are designed to maintain prism flatness without compromising optical performance and are gentle enough to enable the application of AR coatings to assembled optics.
Case Study: AR coated mylar sheets
The laser instrumentation and diagnostics markets have a need for very thin (2 mil) mylar coated sheets. By adjusting coating processes and developing custom tooling to accommodate the thin, flexible material, current coating technology can achieve a uniform AR V-coat across the sheet without compromising pliability; the coated sheet can even be rolled up and flexed without cracking or crazing the coating.
Make AccuCoat your partner for coated plastic or polymer optics
While coating on plastic substrates to create environmentally durable optics is not without its challenges, AccuCoat has developed specialized cleaning, handling, and deposition processes to meet even the most demanding MIL-SPEC standards. In fact, we can match the performance of many optical coatings available on glass, from AR and mirror coatings to beamsplitters and filters. We can work with you to create a drop-in replacement for a glass component to reduce weight, cut cost, or allay safety issues associated with cracking and splintering in the field.