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Crown glass is a type of opticalglass used inlenses and other optical components. It has relatively lowrefractive index (≈1.52) and lowdispersion (withAbbe numbers between 50 and 85). Crown glass is produced from alkali-lime silicates containing approximately 10%potassium oxide and is one of the earliestlow dispersion glasses.
The term originated fromcrown-glass windows, a method of window production that began in France during the Middle Ages. A molten blob of glass was attached to a pole and spun rapidly, flattening it out into a large disk from which windows were cut. The center, called the "crown" or "bullseye", was too thick for windows, but was often used to make lenses ordeck prisms.[1][2]
Theborosilicate glassSchottBK7 (glass code 517642) is an extremely common crown glass, used in precision lenses. Borosilicates contain about 10%boric oxide, have good optical and mechanical characteristics, and are resistant to chemical and environmental damage. Other additives used in crown glasses includezinc oxide,phosphorus pentoxide,barium oxide,fluorite andlanthanum oxide. The crown/flint distinction is so important to optical glass technology that many glass names, notably Schott glasses, incorporate it. AK in a Schott name indicates a crown glass (Krone in German). TheB inBK7 indicates that it is a borosilicate glass composition.
BAK-4 barium crown glass (glass code 569560) has a higher index of refraction than BK7, and is used for prisms in high-end binoculars. In that application, it gives better image quality and a round exit pupil.
Aconcave lens offlint glass is commonly combined with aconvex lens of crown glass to produce anachromatic doublet. The dispersions of the glasses partially compensate for each other, producing reducedchromatic aberration compared to asinglet lens with the samefocal length.