Using Silicone Optics for Maximum Light Control with Minimal Cost

The solid-state lighting industry is about to be transformed by optical-grade silicone. It enables injection molding of complex optical designs which combine a number of glass and plastics’ best properties to offer low cost thermal resistance and flexibility, as well as brilliant light transmission.

In terms of the high-quality optics required for cameras, the majority of the component costs are due to lenses. Either the highest quality glass or other optically transparent materials are required for multipart complex optical designs. As a result of these and the need for precise design and manufacturing processes, the cost of the lens is significantly increased.

However, while cameras require lenses to collect and focus light onto a focal plane, they are capturing light emitted from many different sources. Those light sources, for today’s industrial and architectural imaging applications, are increasingly high-efficiency solid-state LED lights.

Silicone Optics: Tailor-Made for LEDs

There are multiple benefits of LEDs, including their low cost and maintenance, high efficiency, and the greater spectral control they offer. Yet, in terms of one crucial criteria, solid-state emitters are reminiscent of their incandescent ancestors: they emit light in all directions.

After spending two years acquiring equipment and proving competency, Smart Vision Lights began testing its own LED products in September 2013 in order to legally apply marks of conformity, such as Europe’s CE mark, to their products.

The traditional solution if an application is in need of more light is to increase the number of bulbs, and the same is the case for LED lights. The down side of this solution is that more power is consumed and more heat generated, neither of which is desirable or efficient.

Gas discharge (halogen) lamps, fluorescent, and incandescent lights are significantly less efficient at converting electrical power into light than LEDs. However, to increase the efficiency of LEDs still further, the lamp must be able to collect all of the light available and direct it to the intended area. For any lighting application, this goal is important.

However, controlling light is pivotal to success in the machine vision world, whether that is generating bright field, dark field, polarized, structured, diffuse, or some combination of any of these types of lighting systems.

Standard glass molding and grinding methods would, unfortunately, produce micro lenses whose cost far exceeded the chip. Another option is provided by plastic molded optics, however plastics are liable to yellow over time, particularly in the presence of ultraviolet (UV) light. Another issue is that they are unable to hold the fine features required of complex optical design.

Furthermore, they are unable to resist the high temperatures produced in lighting applications, which leads to crazing (developing microscopic cracks) as well as other adverse conditions. Limited success has been achieved via the addition of phosphors to the plastic materials, in order to improve plastic optical performance.

These challenges, as well as others, can be overcome by silicone optics. These allow end users to control light with precision which can otherwise only be found in complex glass optics, which are significantly too expensive for most architectural or machine vision applications.

How are Silicones Special?

An exciting alternative available to lighting manufactures is offered by new optical-grade silicones from Dow Corning. Unlike glass and plastics, silicone:

  • will not age like polycarb, vinyl, or acrylic.
  • will not yellow with time.
  • will not craze due to heat, exhibiting no material changes in temperature ranges from –115 °C to 200 °C.
  • will not react to UV light.
  • will not react with most harsh chemicals.
  • offers high transmission across a broad spectrum, with 95% transmission or better from 365 nm (UV) to 2000 nm (IR)

In addition to these material benefits when compared with plastic molded lenses, silicone also provides advanced manufacturing benefits, such as:

  • Silicone is very robust, maintaining its optical function over its lifetime. It is also resilient to environmental changes.
  • Unlike conventional plastics, optical-grade silicones are capable of holding fine structure patterns. They can also possess reverse curves in a single molding tool.
  • A lower total cost solution for complex, multi-part optics is allowed by silicone’s ability to form complex optical elements using several shots in a single injection mold.

More Light, Fewer Lamps, Better Control

There are multiple benefits of silicone optics, all of which are rooted in the unique properties of silicone molecules. Their long, spaghetti-like structures result in liquids which cure gradually into flexible solids with low indexes of refraction. This reduces light loss at the interface between air and optic, or multi-part optics.

Although it retains a flexible semi-rigid shape, due to its liquid origin it is possible to change silicone into extremely fine structures below 10 nm in order to create diffractive, Fresnel, holographic, and other optical structures with minimal loss.

Furthermore, the injection-molded optics can be easily blown out of the mold without sacrificing fine structures, because silicone can maintain this flexibility for over a year. The majority of optical materials which have rubber-like properties do not revert to their original shape following the stretching which occurs when blowing an injection-molded part out of a mold.

It is also possible to mold silicone at significantly lower temperatures than plastic or glass. Consequently, polyethylene and polyester resins can be used to create prototype molds. These resins are able to generate as many as 3,000 prototype optics with good repeatability.

Consequent upon its ability to be molded at relatively low temperatures, other materials with low melting-points – such as seals, O rings, and snap fixtures for attaching the silicone lens to the LED – can be included in the mold of the optical design.

Characteristics of Silicones

Example of Applications (LED Package)

Example of Applications (Lamps)

Example of Applications (LED Luminaires)

Making use of this feature recently, optical engineers as LED light manufacturer Smart Vision Lights (Muskegon, MI) and LumenFlow (Wyoming, MI) created numerous prototypes for a five million-lux LED linear light at a fraction of the standard $100,000-per-mold tooling cost.

Silicone and the 5M-Lux LED Light

One of the first optical companies which began working with Dow Corning in order to develop silicone optics for LED lights was LumenFlow. Fortunately, LumenFlow is located close to Smart Vision Lights – a leading LED light manufacturer. Working together, they aimed to create the first ever 5 million-lux LED linear light.

As a means of illuminating products for high-speed machine vision camera-based quality inspection systems, linear lights are used frequently on large area production lines. The camera is able to acquire images of the product quicker when the light is brighter, and as a result the production line is more profitable.

Matt Pinter, the head of engineering at Smart Vision Lights, had experimented with extruded acrylic rods in order to focus the light from the water-cooled LEDs. However, they would have needed a diameter in excess of 2 inches.

The line’s integrity was also being compromised by poor surface quality. In addition, the acrylic material was put at risk by high temperatures, leading it to eventually craze and misshape over time.

The silicone complex optic designed by the two companies had a 40 mm diameter and was able to cope with the five million lux and associated heat, while still maintaining the focus of the light line. The silicone optic was comprised of two back-to-back 40 mm molded large-aperture silicone lenses.

Included within each cylinder was a complex conic structure which reduced induced aberrations which are common to rod optics. As silicone is a “living material”, it was possible for the lenses to be made in six inch segments and butted together.

As a result of the material’s fluidity, the molecules simply flowed back together – within a short time, there was not a discernible difference between the joint material and the optic. This allowed Smart Vision Lights to manufacture the light at lengths as long as nine feet.

As a result of the back-to-back lenses, a larger object distance (So) was allowed. This meant that both the optics and the LEDs were subjected to less heat stress. Finally, by placing the lenses back-to-back, Smart Vision Lights was able to include a wire polarizer. This was hitherto unheard of in LED lights of that intensity.

Conclusion

Optical-grade silicone - courtesy of its molecular geometry, unique chemical make-up, and exceptional optical properties – provides LED manufacturers with the ability to enhance their products to new performance levels. They are able to open new applications while saving considerable money for end users as they fill their illumination needs.

As the development - at a fraction of normal development costs - of a five million-lux linear light with outstanding optical performance shows, silicone is evidently the new leader in optical materials and technology.

Thermal stability of silicone

200 h thermal aging test (4-mm thickness)

200 h thermal aging test (4-mm thickness)

Typical organic materials used for optical systems in lamps and luminaires, and silicone resin aged at 200 ºC for 24 hours

Typical organic materials used for optical systems in lamps and luminaires, and silicone resin aged at 200 ºC for 24 hours

Injection molding optical parts-benefits

 

This information has been sourced, reviewed and adapted from materials provided by Smart Vision Lights.

For more information on this source, please visit Smart Vision Lights.

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