Materials

For centuries, glass proved to be the material of choice for all optical applications. During the 20^th century, applications for glass elements and complex optical systems greatly expanded. Early uses of plastic molded elements included toy objectives, gauge windows and watch crystals. Throughout the 1970' and up to the present, high-grade polymers were developed specifically for optical applications. These advancements in materials, coupled with improved mold design have enabled plastic optics to replace glass optics in a wide and growing number of applications.

The range of optical grade plastics continues to grow. An abbreviated list of some of the most important families of optical polymers and co-polymers includes the following:

Acrylic is one of the most commonly used optical plastics. This material is very hard and has very good mechanical stability. It has an index @588nm of 1.49, and an Abbe number of 55.3. Acrylic is also known for its very good clarity and excellent transmission properties throughout the visible portion of the spectrum.

Polystyrene has a higher index @588nm of 1.59 and lower Abbe number (30.87) than acrylic. It is generally less expensive than acrylic. The material also tends to absorb somewhat in the deep blue spectrum. Polystyrene has a lower resistance to UV than acrylic and is more easily scratched than acrylic. A styrene lens, when paired with an appropriate acrylic lens offers an effective achromatic solution.

Polycarbonate is known for its very high impact resistance and for the ability to perform over a wide range of temperatures (-137 to +124 ^oC). Polycarbonate is similar to polystyrene in index (@588nm, 1.586) and Abbe number (29.9). Because of its high ductility, polycarbonate is not easily machined. The resin is more expensive than acrylic.

Cyclic Olefin Polymer (COP) is a family of plastic resins with low native stress bi-refringence properties. Zeonex is a resin developed by Nippon Zeon. One grade, E48R, has a heat distortion temperature of about 122^o C, an index of refraction of approximately 1.530. Zeonex has a remarkably low water absorption value of less than 0.01% (compare with Polycarbonate 0.2% and PMMA 0.3%). G-SPO has developed considerable expertise in molding optics with this resin. Zeon Chemicals has another class of polymers called Zeonor that can serve as a lower cost alternative to Zeonex.

Cyclic Olefin Copolymer (COC), with a trade name of Topas, is another high temperature alternative to acrylic. It has an index of about 1.5350 and an Abbe value of 56. Its heat distortion temperature is rated at about 123^o C. Generally, it is a lower cost alternative to COP.

NAS, is a copolymer of 70% polystyrene and 30% acrylic, having a refractive index @ 588nm of 1.564. This material is less expensive than acrylic.