Well.......sometimes anyway! And when you combine them, as in the picture above, you can achieve some pretty amazing results!
Glass has some advantages over plastic. It is far more resistant to heat, does not outgas, has a significantly larger range of refractive index and Abbe values, does not exhibit “knit lines”, is able, with care, to hold better surface power and irregularity specifications than plastic in certain surface forms, and has less haze in very thick sections. Now this may sound like a long list of advantages, but in many instances, the disadvantages of plastic can all be managed quite readily, thereby unlocking the key advantages of polymer optics.
Polymer optics do not exhibit significant haze except in very thick sections. Even in sections centimeters thick, the haze can be acceptable with proper material selection and processing. Knit lines and “sink” resulting in loss of power and irregularity control can be reduced or eliminated at the design stage by managing section thicknesses and curvatures. Heat does eliminate many polymers from the design when operational temperatures vary extensively – and “extensively” depends on the optical requirements of the system. The key benefit of polymer optics is that of cost, and often the cost/performance advantage. This is true because polymer optics enables replication of an optical quality surface by molding the plastic rather than requiring generation of the optical surface through grinding and polishing. The replication process also means that the optical surfaces can have bizarre shapes, and that those shapes generally don’t require any more time to replicate than an ordinary, simple surface. This in turn means that aspheric surfaces and multiple components such as prisms combined with lenses, lens arrays, and diffraction surfaces can all be combined using polymers with a single “shot” of plastic into the mold. In addition, precision mounting features that set air spaces, concentricity, and wedge between adjacent elements can be combined with the element itself reducing assembly costs, eliminating parts, and improving positioning all at the same time as enabling the bizarre shapes and element combinations mentioned above!
Bizarre shapes in optics are enabled with polymers!
Now to be fair to glass, there are now means to replicate aspheric shapes in glass that expand the repertoire of glass significantly. However the restrictions on the shapes and sizes permissible still limit the possibilities far more for glass than for plastic. Injection molding, clearly possible for polymers, is still out of the reach for glass. In general, glass pressing is the preferred molding method, and while pressing can be very successfully done, it often incurs the cost of requiring carefully prepared preforms. And while injection molding of some glasses may be done in the laboratories of some of the glass research centers, there is currently no one molding glass this way with anything like the freedom of size and shape, much less the costs of comparable plastic parts.
Molded (pressed) Glass Aspheres
Many optical coatings can be applied to plastic surfaces to achieve performance comparable to those placed on glass. The key differences occur again if high heat is required in the deposition process, or if the mechanical stresses built up in the stacked layers of an optical coating cause failure of the under-laying polymer surface.
In hard vacuum applications, most polymers will begin to out-gas and break down due to the comparatively weaker molecular bonds between the chains making up the polymer. Optical coatings can slow this process considerably however.
Polymer Optics Often Employ Refractive, Diffusive, and Absorbtive Properties
Most polymer optics are compatible with dyes, both absorptive and fluorescent. Some of these enable calibration plaques to be fabricated with long lifespans and resistance to photobleaching due to volume absorption.
The true advantages of plastic optics is realized when they are used in combination with glass elements. These hybrid lens systems, permitting the variety of glass indices and dispersions together with the inexpensive benefits of plastic aspheres, offer some remarkable advantages over strictly glass or plastic designs. The amazing thing is that so few companies have historically blended the two material types to gain the advantages of both cost savings and performance enhancement. That is changing however, and cell phone cameras and DVD players tend to be leading the market in achieving these benefits.
An Extremely High Precision, High Performance Hybrid Lens System
SMA enjoys both a history and current relationships that provide the ability to develop and design hybrid systems. The close relationships with a plastic optics molding partner is very important to successful implementation, since the quality of a lens element, and therefore the performance of it, is highly dependent on gating, venting, overflow cavities, process temperature profiles, flange configurations, and fill pressures. Birefringence plagues some polymers, but can be remedied by substitution of polymers as well as by careful control of process parameters. These issues need to be addressed by the lens designer and the molder jointly prior to manufacture to ensure optimal system performance. And since process parameters for plastic optics are so different from opaque plastic molding, utilizing “conventional” opaque molding experience will very often produce disappointing results.
What are some of the rules of thumb when employing plastic aspheres in a lens system? What kind of performance can be achieved? What methods of prototyping are available? What are some of the unusual size limitations unlocked by plastics? We’ll examine these and other interesting capabilities of polymer optics in an upcoming blog.