What Birefringence Means
Where Birefringence Comes From
For some materials, notably crystals, the birefringence is intrinsic to the molecular structure of the material itself.
For other materials, the birefringence is caused by residual stress inside the materials.
With optical polymers, the main contributor to birefringence is stress.
Residual stress introduced during molding or processing can change the optical behavior of the material, producing polarization-dependent refractive indices.
Why Birefringence Is Usually Undesirable
Whatever the source, birefringence is usually undesirable, as the nominal system design is typically based on the assumption of isotropic optical materials.
For common imaging systems, birefringence leads to ray splitting, resulting in loss of contrast and the potential for multiple images.
In imaging systems that require control of the state of polarization, for example, the so-called “pancake optics” that are increasingly common in AR/VR applications, birefringence control is essential to optimal performance.
Birefringence in Injection-Molded Polymer Optics
For injection molded polymer optics, keeping birefringence below unacceptable levels requires attention to several factors:
- material selection
- mold and gate design
- processing technique
Expertise in all of these areas at the design for manufacturing stage will help the system engineer avoid unexpected performance issues that can result from potential birefringence in polymer optical materials.
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