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Glass Designation |
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The most common types of glasses used in optics are classified into crown glasses and flint glasses with designations based on their refractive indices and dispersions. Crown glasses have a bigger Abbe value with an nd>1.60and an vd>50, as well as an nd<1.60and an vd>55. In SCHOTT designation, Crown glasses have a designation letter K, while flint glasses have a designation letter F. To differentiate the various types of optical glasses, a letter/letters and number designation are used. The letter identifies the basic chemical composition, while the number is to further differentiate glass types within a group. Table 1 gives an overview of optical glasses from SCHOTT and HOYA.
A more uniform method of identifying optical glass types is to use a six-digit code. The first three digits represent the nd after the decimal point, and the last three digits indicate the vd. If the fourth digit is bigger than 4, 1 is added to the first three digits. For example, N-BK7, the most widely used optical glass, the nd is 1.51680, the vd is 64.17, which yields 517642. Properties:
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