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Jan 29, 2024

Plastic Optics: Specifying Injection

Change in refractive index with temperature (dn/dt) — Polymers will exhibit a

Change in refractive index with temperature (dn/dt) — 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). Transmission — Most optical polymers have high clarity in the broadband visible portion of the spectrum (~400 to 650 nm). For example, acrylic and some grades of Zeonex have transmission properties of about 92%. Materials such as polycarbonate and polystyrene have slightly lower transmission. Ultem, which has good transmission in the near-IR, may not be suitable for broadband visible applications because it absorbs in the blue end of the visible spectrum. Birefringence — The amount of birefringence a system can tolerate is an important design consideration and should be discussed with the manufacturer early in the process. Some polymers such as polycarbonate and polystyrene have a higher amount of stress to begin with because of their polymer properties. The amount of birefringence in an optic can be exacerbated by poor part design and/or poor gate location in the mold. Table 2 summarizes the amount of stress inherent in the different optical polymers. Because birefringence can be difficult to measure, a qualitative rating system of 0 to 10 was used. In this scale, 0 is the lowest amount of stress, and 10 is the highest amount of stress.

It is critical that the molder have the metrology capability in-house to perform all of the necessary measurements for the components it manufactures. It is safe to say that you cannot manufacture what you cannot measure. Creating precision polymer optics is a highly specialized discipline requiring a detailed knowledge of optical design, mold construction techniques, state-of-the-art mold processing capability, and optical metrology expertise. The design considerations outlined in this article will provide the designer with the fundamental knowledge required to begin a successful program using precision polymer optics.