The term homogeneity generally refers to the volume constancy of properties. This may be any property of a material, even the material itself. In local imperfections, different materials may be present, such as gas-filled bubbles or stones in glass. In, by far, the most cases, however, homogeneity means refractive index homogeneity in optical materials. So refractive index homogeneity, otherwise known as optical homogeneity, will be covered in the majority of this chapter. Bubbles and inclusions as material homogeneity defects will be discussed at the end of the chapter.
General optical homogeneity can be subdivided into two aspects: global optical homogeneity and striae. Global optical homogeneity describes the refractive index changes over longer glass volume ranges, which means typically above several millimeters and extending to the total diameter of the optical element. The term striae refers to short-range changes from 1 mm down to 0.1 mm and below. This differentiation not only applies to the geometrical scales of refractive index changes but is also found in the variations that originate during production, the different measurement methods used for inspection, and the different effects these variations have on image quality.
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