Minimal Cost, Maximum Control, a Guide to Silicone Optics

Lenses frequently take up the majority of the component costs when it comes to cameras and the requirement for high-quality optics. Multipart complex optical designs must have precision design and manufacturing processes in addition to the highest-quality glass or other optically transparent materials, All of these requirements add to the cost of the lens.

Whilst a camera requires a lens to collect and focus the light on a focal plane, it is also capturing the light emitted from numerous different light sources. The light sources are increasingly solid-state, high-efficiency LED lights for today’s industrial and architectural imaging applications.

Silicone Optics: Tailor-Made for LEDs

LEDs provide a number of advantages: low cost, greater spectral control, low maintenance, high efficiency, etc. Yet, in one critical way, solid-state emitters are similar to their incandescent ancestors: they emit light in every direction.

Smart Vision Lights began testing their own LED products in September 2013, after two years of proving competency and gathering equipment. This was part of the process for legally applying marks of conformity, for example Europe’s CE mark, to Smart Vision Lights’ products for sales and operation in certain countries. The typical solution is to add more bulbs if an application requires more light, the same goes for LED lights.

More bulbs generate more heat and consume more power, which is unfortunate as neither of these outcomes are efficient or desirable. LEDs are a lot more efficient at converting electrical power into light than fluorescent, incandescent, and gas discharge (halogen) lamps.

The lamp needs to gather all of the light available and direct it where it is required in order to make it even more efficient. This is key for any lighting application, but controlling light is critical to success in the machine vision world, whether it be generating bright field, diffuse, polarized, dark field, structured, or a combination of these different variations of lighting systems.

Unfortunately, standard glass molding and grinding techniques would result in micro lenses that surpass the chip in cost. Plastic molded optics are an alternative, but plastics yellow over time, particularly if ultraviolet (UV) light is present; cannot hold fine features required of complex optical design; and are not resistant to high temperatures found in lighting applications, which leads to crazing (developing microscopic cracks) and other adverse conditions, amongst other flaws.

Attempts to add phosphors to the plastic materials to enhance the plastic optical performance have resulted in limited success. Silicone optics can overcome all of these challenges, they permit end users to control light with precision known only in complex glass optics, which are far too costly for most machine vision or architectural.

How are Silicones Special?

New optical-grade silicones from Dow Corning (now part of Dow Chemical Company) provide an exciting alternative for lighting manufacturers. Unlike glass and plastics, silicone:

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

Whilst silicone provides these material advantages compared to plastic molded lenses, it also supplies advanced manufacturing advantages. For example:

  • 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, multi-part optics.
  • Optical-grade silicones are capable of holding fine structure patterns and can possess reverse curves in a single molding tool; this cannot be done in conventional plastics.
  • Silicone is very robust. It maintains its optical function over its lifetime and is resilient to changes in the environment.

Characteristics of Silicones

Table 1​

Material High Temp Flexible to
Silicones +150 to 200 °C -45 to -115 °C
Epoxies +150 to 180 °C
Urethanes +115 to 125 °C -60 °C
Acrylics +85 to 125 °C

 

Fewer Lamps, More Light, Better Control

There are many advantages of silicone optics, and they all have their root in the unique properties of the silicone molecule. Its spaghetti-like, long structure results in a liquid that gradually cures into a flexible solid with a low index of refraction, which lowers light losses at the interface between optic and air or multi-part optics.

Silicone’s liquid origin means it can be made into extremely fine structures, below 10 nm, in order to create Fresnel, diffractive, holographic, and other optical structures with minimal loss, despite maintaining a flexible semi-rigid shape.

The silicone keeps this flexibility a year or longer, so the injection-molded optics can be easily blown out of the mold without sacrificing fine structures. (Most optical materials with rubber-like properties do not revert to their original shape after the stretching that happens when blowing an injection-molded part out of a mold.)

Silicone can also be molded at significantly lower temperatures than glass and plastic. This means prototype molds can be made from polyester and polyethylene resins that can create up to 3,000 prototype optics with excellent repeatability. It also allows the optical design to incorporate other low-melting-point materials in the mold, like O rings, seals, and snap fixtures for attaching the silicone lens to the LED.

By employing this feature, for just a fraction of the usual $100,000-per-mold tooling cost, the optical engineers at LumenFlow (Wyoming, MI) and LED light manufacturer Smart Vision Lights (Muskegon, MI) recently created numerous prototypes for a 5 million-lux LED linear light.

Example of Applications (LED Package)

Example of Applications (Lamps)

Example of Applications (LED Luminaires)

Silicone and the 5M-Lux LED Light

LumenFlow was one of the first optical companies to work with Dow Corning to develop silicone optics for LED lights. LumenFlow is fortunately located close to Smart Vision Lights, a leading LED light manufacturer. They worked together to create the world’s first 5 million-lux LED linear light.

Linear lights are frequently utilized on large scale production lines to illuminate products for high-speed machine vision camera-based quality inspection systems. The brighter the light, the quicker the camera can capture product images – and the more successful the production line will be.

The head of engineering at Smart Vision Lights, Matt Pinter, thought about extruded acrylic rods to focus the light from the water-cooled LEDs, but the diameter would need to be over two inches. The integrity of the light line was also compromised by poor surface quality and high temperatures put the acrylic material at risk. Eventually the material would craze and mis-shape.

Together, the two companies created a 40 mm diameter silicone complex optic that could handle the 5 million lux and the heat associated with it, whilst still preserving the focus of the light line. The silicone optic was made up of two 40 mm molded large-aperture silicone lenses placed back-to-back. Each cylinder was comprised of a complex conic structure which decreased induced aberrations typical to rod optics.

The lenses could be created in six inch segments and butted together, since silicone is a ‘living material.’ Within a short time, there was no difference between the joint material and the optic due to the fluidity of it. The molecules flowed back together easily, which permitted Smart Vision Lights to create the light at lengths up to nine feet.

The back-to-back lenses also permitted a bigger object distance (So), which resulted in lower heat stress on both the LEDs and the optics. Plus, the back-to-back placement permitted Smart Vision Lights to incorporate a wire polarizer, which was unheard of in LED lights of that intensity up until then.

Thermal Stability of Silicone

200 h thermal aging test (4-mm thickness)

ypical organic materials used for optical systems in lamps and luminaires, and silicone resin aged at 200 °C for 24 hours.

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

Injection Molding Optical Parts-Benefits

Conclusion

Optical-grade silicone gives LED manufacturers the opportunity to bring their products to new levels of performance, thanks to its molecular geometry, unique chemical make-up, and exceptional optical properties. This opens up new applications while saving end users considerable costs meeting their illumination requirements.

Silicone is clearly at the forefront of molded optical materials and technology, as clearly shown by the development of a 5 million-lux linear light with exceptional optical performance for a fraction of the usual development costs.

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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