Silicone Optics: Optimal Control, Low Cost
When LED light manufacturer Smart Vision Lights (Muskegon, Michigan, USA) joined forces with optical engineers at LumenFlow (Wyoming, Michigan, USA) to create prototypes for a 5-million-lux LED linear light, they expected great results. What they didn’t count on was being able to manufacture them for a fraction of the normal tooling cost of $100,000 per mold. They did it by using silicone, the malleable compound that produces a host of benefits for applications that require cameras with premium optics.
Enter Silicone Optics: The Perfect Match for LEDs
LEDs offer a trifecta of benefits prized in the machine vision industry: greater spectral control, low cost and maintenance, and high efficiency. However, these solid-state emitters are similar to their incandescent predecessors in that they emit light in all directions. In the machine vision industry, where controlling light is critical to success, the lamp needs to collect all available light and direct it to its target.
Silicone optics overcome this and many other challenges when compared to acrylic, for example, primarily because they allow end users to control light with the precision previously only available with complex, cost-prohibitive glass optics.
What Sets Silicone Apart?
The benefits of the new optical-grade silicone are many. It offers high transmission across a broad spectrum, with 95 percent transmission or better from 365 nm (UV) to 2000 nm (IR). It is also robust, maintaining optical function over its lifetime. During the manufacturing process, optical-grade silicone holds fine structure patterns and possesses reverse curves in a single molding tool. And silicone’s ability to form complex optical elements using multiple shots in a single injection mold allows for a lower total cost solution for complex, multipart optics.
What it doesn’t do is age or yellow over time, like polycarb, vinyl, or acrylic, or react to UV light or harsh chemicals. Crazing due to heat is a nonissue in temperature ranges from from –115°C to 200°C.
The Beauty of the Silicon Molecule
The advantages of silicone optics are rooted in the unique properties of the silicone molecule. Its long, spaghetti-like structure results in a liquid that slowly cures into a flexible solid with a low index of refraction. This characteristic lowers light losses at the interface between optic and air or multipart optics.
Despite retaining a flexible semi-rigid shape, silicone’s liquid origin means it can be formed into very fine structures below 10 nm to create diffractive, holographic, Fresnel, and other optical structures with minimal loss. And because the silicone maintains this flexibility for up to a year or more, injection-molded optics easily can be blown out of the mold without sacrificing fine structures.
Discovering the Benefits of Silicone Optics
Matt Pinter, Smart Vision Lights’ head of engineering, had looked at extruded acrylic rods to focus the light from the water-cooled LEDs, but they would need to be upward of 5 cm in diameter. Poor surface quality compromised the integrity of the light line, and the material was vulnerable to high temperatures.
The engineering team then designed a 40-mm-diameter silicone complex optic that could handle the 5 million lux and associated heat while maintaining the focus of the light line. The silicone optic comprised two 40-mm molded large-aperture silicone lenses placed back-to-back.
Each cylinder included a complex conic structure that reduced induced aberrations common to rod optics. Since silicone remains a “living material,” the lenses could be made in 6-inch (15.4 cm) segments and butted together. Due to the material’s fluidity, the molecules flowed back together. This allowed manufacture of the light at lengths up to 9 feet (2.74 m).
A larger object distance also was possible, which meant less heat stress on the optics and the LEDs. Finally, the back-to-back placement allowed the inclusion of a wire polarizer, which was unheard of in LED lights of that intensity.
Smart Vision Lights’ 5-million-lux linear light is a promising development in molded optical materials and technology. By showcasing the unique chemical properties inherent in optical-grade silicone, Smart Vision Lights has opened the door for LED manufacturers to explore the development of new, low-cost applications.