21 Dec 2007
What factors should be taken into account when considering plastic versus glass optics? Andreas Maahs of JENOPTIK Polymer Systems discusses the merits of using plastic optics and explains the criteria that optical systems designers should bear in mind.
The perception of plastic (or polymer) materials has changed significantly over the past five years. Once thought suitable only for applications with low optical requirements or demands, plastic optics are now found in a wide range of everyday products. These include barcode scanners, DVD players, optical computer mice, rear-view cameras in cars, LED illumination, security systems and mobile cameras to name just a few. Beyond these consumer products, plastic optics are also used in specialized applications such as endoscopes in hospitals, solar-cell technologies and industrial sensors.
Factors to consider
One significant advantage that plastic materials have over glass is their ability to support high-volume production at low cost. Plastic optics offer design freedoms that are simply not achievable or economical with glass optics, such as aspheres, lens arrays, Fresnel and diffractive surfaces. It is also possible to add mounting features within the optical design of plastic components, which reduces the cost of the overall system. This flexibility is not possible when working with glass and the mechanical mounting hardware is separate.
Another factor to consider is that all lenses produced by injection moulding are made from the same tool, which assures consistent quality. This quality is constantly monitored during the moulding process by modern statistical-control techniques. A third important advantage is the low weight of the material. Plastic optics can be five times lighter than glass optics – an important issue for the wearers of instruments such as head-mounted displays.
However, certain disadvantages arise when using plastic materials instead of glass. First, plastic is sensitive to environmental influences and can be scratched easily compared with glass, which is more durable and stable over a wider temperature and humidity range. Maximum continuous service temperatures for plastic optics range from –40 to 130 °C. Using plastic optics above these temperatures can cause unacceptable variation of the focal length and aberrations in some applications.
Humidity is another environmental factor that can influence the form and the refractive index of plastic optical components. However, moisture uptake can be reduced by using materials that have been specially designed to handle these circumstances.
It is also worth noting that the refractive index of glass is higher than that of plastic. The refractive index is a measure of the refraction of light as it passes through the material. In glass, the refractive index can be up to 2.0, but with most commercially available plastics, only up to 1.7. To overcome this, purpose-engineered materials have been developed that are hybrids of both glass and plastic.
A final point is that plastic is sensitive to the ultraviolet content of sunlight as well as to the acids that are present in polishing and scouring preparations, which can ultimately destroy the optical function of the component over extended periods of exposure.
Glass components are made by grinding and polishing processes, and there is usually little or no cost for tooling. Plastic components are made by injection moulding, injection compression moulding or through ultraprecision diamond turning (which is also referred to as single-point turning). The first two methods are used for mass production and are efficient ways of reproducing complex surfaces at low cost. Components can be produced in a wide range of quantities with high degrees of repeatability. With injection moulding, the moulds can have one or more cavities and the technique can produce several parts per shot in multicavity tools. These tools must be produced prior to production and therefore entail upfront, non-reoccurring costs.
During injection compression moulding, the material is pressed between heated platens with accurately defined temperature cycling during pressing. This is an effective way to manufacture small structures with high aspect ratios and tight angular and positioning tolerances.
Ultraprecision or single-point diamond turning machines have a long production time and high machine costs, so are therefore mainly used for prototyping, small lot sizes (if the tools for mass production are more expensive) or for parts that cannot be produced by injection moulding.
Although the selection of materials for plastic optical components is limited, the range is growing every year. Today, the choices include crown-like materials such as acrylic (PMMA) and polyolefin or flint-like materials such as polystyrene and polycarbonate. The hurdle with these is that the range of refractive index is still limited in comparison with glass.
The differences between the materials are in their reaction to environmental influences such as water, sunlight or the acids and chemicals in cleaning preparations – especially organic solvents. The limited variety of available plastic materials for these manufacturing processes can be a restriction on optical design freedom.
The physical properties of plastics that must be taken into account for production are weight, impact and abrasion resistance, temperature resistance and thermal expansion. The thermal expansion of plastic is 10 times higher than that of glass, and the optical design or mounting must compensate for this. Polycarbonate, for example, has the highest impact resistance of all optical plastics and is used for windscreens and crash helmets. Acrylic, on the other hand, has the best abrasion resistance. Each material has its advantages and disadvantages, and should be tailored towards specific applications.
Additives and colours can be used to improve the stability of plastic materials in sunlight. Colour filters, such as infrared colouring, appear black in the visible spectral range but transmit in the infrared range, and are used primarily for sensors, optical scanners or remote-control front windows. Additives can also influence the temperature stability of plastic materials.
Thin-film surface technology is another option that can change the properties of plastic. Plastic optics can be coated in a similar way to glass optics, however plastics present different challenges. The coatings are generally applied at lower temperatures and are not as durable as those coated on glass. This is because of the specific properties of the component (base) material. Typical coatings used on plastics are antireflective coatings or metallization (gold, silver and aluminium).
Customers choose plastic over glass primarily to drive down mass production costs. Plastic is also the material of choice in applications that involve a high number of components, those that require a combination of optical and mechanical features or if the optic has a complex surface.
There are several important criteria that a buyer, or optical designer, should consider when deciding to use plastic optics. These include costs, weight, the operating/storage environmental conditions in which the optic is used, the application and the function that the optic will perform. The designer also has to take the properties of the available materials into account to include the mechanical, thermal and humidity limitations mentioned previously.
All designers must work closely with the manufacturer to understand the requirements as well as the manufacturability of their components. To provide accurate manufacturing costs, the supplier of the plastic optics needs to know the projected production volume requirements, any weight requirements, the tolerances and the target costs. Fundamental optical properties include transmission, refractive index, dispersion and birefringence. In cases where the optical design is not finalized, the manufacturing company can consult with the customer and help design an optic that is aligned with its manufacturing capabilities.
Lighting applications and outlook
An important application area for plastic optics is lighting technology, and LED systems in particular. Such systems provide a means of generating high-power illumination for a variety of applications and markets in a well-controlled and specified manner. Customers seek flexible, low-cost solutions for lighting applications that may require a variety of angular distributions of LED arrays to mix light and avoid colour-banding. In some cases, different LED systems may be combined in an appropriate manner.
The design of an optical system to extract the light bridges the gap between the output of the LED arrays and the customer's illumination surface. The design process should include a detailed discussion between the plastic optics supplier and the customer.
Although the manufacturing of plastic optics is limited by material properties, the design and assembling freedoms will ensure many new applications in the future.
• This article originally appeared in the December 2007 issue of Optics & Laser Europe magazine.