In addition to the material properties (e.g., index, dispersion, manufacturer, and material type) discussed in Chapter 1, there are material tolerances that are key to specifying optical component substrates. These additional essential material tolerances are stress birefringence, bubbles and inclusions, homogeneity, and striae. While these material tolerances are usually thought of in terms of optical glass, they can also be present in components made from other materials. Each of these tolerances can impact the overall optical performance, albeit they require an understanding of the material fabrication process.

Stress within optical materials is a mechanical stress that is typically a function of the type of material and the annealing process, creating a birefringent effect in transmissive optical elements. This stress in the refractive element leads to an anisotropic optical retardation between the electric field component directions. Stress birefringence variation with temperature can cause thermal instability in both the refractive and reflective optical elements. This is especially true for optics made from plastics and some crystal materials.

During the melting and annealing processes multiple types of imperfections can arise. Pockets of air and small contaminants, typically toleranced together and referred to as bubbles and inclusions, are produced from the raw materials in the melting and thermal cycling processes. These imperfections in the optical material lead to scattering effects within the optical material.4 In the melting and annealing cycles, both occurring over a thermal cycling process, variations of refractive index continuity are created. The variations are known as striae or homogeneity. The distinction between these two types of refractive index variations is attributed to the range length through the optical material.