The most common type of LED illumination optics are freeform lenses, including TIR lenses, and lightpipes. These are generally made with injection molding and use plastic. Glass materials can be used but this is less common and used mostly with sphere lenses or asphere for short range illumination tasks such as optical instruments with Abbe or Köhler illumination.
The 'top 5' listing below is based on our experience of frequently used plastics. The 'best' material for a given application is highly dependent on technical requirements.
First Place: PMMA
The most widely used plastic material is polymethylmethacrylate, also called PMMA, acrylic or Plexiglass and Acrilite brand names. Advantages of this material is low cost, easy mold-ability into any shape, high light transmission, "green" material characteristics and high UV transmittance. Disadvantages of this material are reported yellowing under prolonged UV exposure, higher water absorption, that it is easy scratched and less heat resistant than other plastics.
Second Place: Polycarbonate
Second place plastic material is polycarbonate, also called PC or Macrolon and Lexan brand names. This material has more scratches resistant than pmma, is harder, have higher refractive index, easy for molding, lower water absorbtion, and higher softening temperature than pmma (130 C). Disadvantages of PC are - yellowing under prolonged UV exposure, not as "green", some toxicity and not suitable for several biomedical applications, have high birefringence, higher price. Polycarbonate have very low UV transmission under 400 nm so is widely used for sport outdoor safe-eye glasses.
Third Place: Zeonex
Third place material is Zeonex materials specially designed by Zeon company. Zeonex is kind of Cycloolefin copolymer. Advantages of these materials are high light transmission, very low yellowing under UV exposure, very low water absorbtion significantly less than PC and pmma, high termal resistance, softening temperature is 130-150 C, "green" material suitable for medical applications, low birefringence and high Abbe number, good chemical resistance. Main disadvantage is much higher cost.
Fourth Place: PMMI
Fourth place material is polymethacrylmethylimid called pmmi or trademark is Pleximid. This is kind of pmma like plastic, but it has very high softening temperature about 170 C. It is used for for applications that involve maximum thermal loads. The key optical properties are similar to pmma.
Fifth place is optical silicone resins. These are new kind of optical material which have some advantages over PC and pmma like higher UV yellowing stability, higher temperature resistance up to 150 C, low viscosity cure which allow it to be easily molded in smaller parts like microarrays. But for now they are not wide spread as pmma or PC.