For centuries, glass proved to be the material of choice for all optical applications. During the 20th century, applications for glass elements and complex optical systems greatly expanded. Early uses of molded plastic elements included toy objectives, gauge windows and watch crystals. Throughout the 1970s and up to the present, high-grade optical 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. Acrylic options are very hard and have very good mechanical stability. Acrylic 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 optics are 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 122o 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%). GS Plastic Optics has developed considerable expertise in injection 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 123o 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 optics material is less expensive than acrylic.