ABSTRACT
Optical glass is the predominant material for photographic lenses, being highly transparent to the visible spectrum and having suitable refractive properties. An outline of the principal methods of producing glass for optical use. Optical design in plastics requires techniques of athermalization to reduce the thermal focal shift of a lens. Another optical effect is one of birefringence from residual stresses in manufacture. This property is utilized in techniques of photo-elastic stress analysis. The properties of glass and plastic have been combined in a number of optical systems. The fabrication and weight advantages of plastics are utilized in many optical and camera systems. Prior to the 1930s, selected crystals of natural quartz were used in some optical components, but this material has birefringence, the emergent light generally being elliptically polarized. Most optical materials such as glass, plastics and crystals are solid and have to be ground or moulded into shape.
