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There are several different glass classification schemes in use, most based on the catalogue systems used by glass manufacturers such as Pilkington and Schott Glass . These tend to be based on the material composition, for example ''BK7'' is the Schott Glass classification of a common Borosilicate Crown Glass . The international glass code is based on U.S. Military Standard MIL-G-174, and is a six-digit number specifying the glass according to its Refractive Index ''n''d at the Fraunhofer d- (or D3-) line, and its Abbe Number ''V''d also taken at that line. The resulting glass code is the value of ''n''d-1 rounded to three digits, followed by ''V''d rounded to three digits, with all decimal points ignored. For example, BK7 has ''n''d = 1.5168 and ''V''d = 64.17, giving a six-digit glass code of 517642. The following table shows some example glasses and their glass code. Note that the glass properties can vary slightly between different manufacturer types. |
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