The introduction of aspherical lenses into optical designs has generated a significant impact on the size, weight, and performance of optical systems. However, because the same key optical properties are as important as for spherical optics, material selection and the development of materials for aspheres is not driven by new material requirements but rather by cost issues. As differences between spherical and aspherical lenses are mainly caused by production, new development is driven by the production technology used, such as precision molding, diamond turning, and computerized numerical control (CNC) machining.
Although costs are the driver, materials for aspheres are still specified by their main properties according to their applications and production or processing requirements. The most important physical parameters of any optical material are the refractive index n, the Abbe number Ï
d , and different partial dispersion values P. Note that the Abbe number Ï
d =(n d â1)âD , where D is the dispersion term D=n f ân c . Thus, small Abbe numbers describe high-dispersive materials.
The values of these parameters are material dependent and thus vary in a wide range. This allows the designer to properly select and combine different materials to optimize optical systems. Other important material parameters to be considered are the transmission values and the scattering characteristics in the ultraviolet (UV), visual (VIS), and infrared (IR) parts of the spectrum. Stress optical coefficients K and birefringence are of secondary relevance but clearly must be considered by the designer. In some high-end applications like UV wafer steppers for microlithography, the intrinsic birefringence can play a significant role.
Chemical resistance of materials against water, acids, and bases are of relevance, not only for a specific application, but also with respect to processing steps like grinding, polishing, and cleaning.
Mechanical properties, such as hardness (Knoop HK) and Young's modulus (E), are important for grinding and polishing, but they also determine the scratch and stress resistance of optical devices.
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