Material selection for polymer optics is driven by several factors; optical performance is only one of them. Designers should also be aware of the mechanical features of different optical polymers and how various environmental conditions could affect system performance. Cost and manufacturability issues are other important factors.
Materials for polymer optics require careful consideration of all properties of the thermoplastic resins and their compatibility with the target application and system performance requirements.
A general overview of the most popular materials is presented below.
One of the most commonly used optical plastics. Acrylic polymers are very hard and have very good mechanical stability. Acrylic is also known for its very good clarity and excellent transmission properties throughout the visible portion of the spectrum. However, it has low operating temperature.
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 polystyrene lens, when paired with an appropriate acrylic lens, offers an effective achromatic solution. High contamination is a potential issue for molded polystyrene optics.
Known for its very high impact resistance and for the ability to perform over a wide range of temperatures (-137°C to +124°C). Because of its high ductility, polycarbonate optics are not easily machined. The resin is more expensive than acrylic.
A family of plastic resins with low native stress birefringence properties and relatively high heat distortion temperature. Zeonex®, a resin developed by Nippon Zeon, has a remarkably low water absorption value of less than 0.01% (compare with Polycarbonate 0.2% and PMMA 0.3%). G&H has developed considerable expertise in injection molding optics.
A special polyester for optical use with a high refractive index, extremely low birefringence, and high fluidity. It is significantly more expensive than other thermoplastics used for optical molding.
To assure selected materials match the product application optical designers should note and consider:
Index of Refraction – Thermoplastic materials have a fairly narrow range of index of refraction: from 1.49 for Acrylic to 1.61 for Polyester. Based on the index and Abbe number, acrylics and COP behave more like crown glass types. On the other hand, styrene and polycarbonate behave more like flints.
Impact of temperature – polymers will exhibit a change in index of refraction relative to temperature. The thermoplastic dn/dt is fairly large (about 20 times that of glass) and negative (i.e., as the temperature goes up, the refractive index goes down).
H2O absorption of the selected materials to make sure they match product application.
The amount of birefringence in an optic can be exacerbated by poor part design and/or poor gate location in the mold. Some materials, such as polycarbonate and polystyrene, have a higher amount of stress to begin with because of their polymer properties.
Mechanical Properties for Commonly Used Thermoplastics
Optical Properties for Commonly Used Thermoplastics
Material transmission charts
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