Crown glass (optics)
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Crown glass is a type of optical glass used in lenses and other optical components. It has relatively low refractive index (≈1.52) and low dispersion (with Abbe numbers around 60). Crown glass is produced from alkali-lime silicates containing approximately 10% potassium oxide and is one of the earliest low dispersion glasses.
As well as the specific material named crown glass, there are other optical glasses with similar properties that are also called crown glasses. Generally, this is any glass with Abbe numbers in the range 50 to 85. For example, the borosilicate glass Schott BK7 (Schott designates it as 517642. The first three digits tell you its refractive index (1.517) and the last three tell you its Abbé number (64.2)) 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 include zinc oxide, phosphorus pentoxide, barium oxide, fluorite and lanthanum oxide.
BAK-4 barium crown glass (Schott designates it as 569560. The first three digits tell you its refractive index (1.569) and the last three tell you its Abbé number (56.0)),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.
A concave lens of flint glass is commonly combined with a convex lens of crown glass to produce an achromatic doublet. The dispersions of the glasses partially compensate for each other, producing reduced chromatic aberration compared to a singlet lens with the same focal length.
- History of the achromatic telescope
- John Dollond, who patented and commercialised the crown/flint doublet
- The crown/flint distinction is so important to optical glass technology that many glass names, notably Schott glasses, incorporate it. A K in a Schott name indicates a crown glass (Krone in German — Schott is a German company). The B in BK7 indicates that this is a borosilicate glass composition.