A glass code is a method of classifying glasses for optical use, such as the manufacture of lenses and prisms. There are many different types of glass with different compositions and optical properties, and a glass code is used to distinguish between them.
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.
Technical definition 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 at the Fraunhofer d- (or D3-) line, 587.5618 nm, and its Abbe number also taken at that line. The resulting glass code is the value of rounded to three digits, followed by rounded to three digits, with all decimal points ignored. For example, BK7 has and giving a six-digit glass code of 517642.
Consequently, a linear approximation for the refractive index dispersion close to that wavelength is given by:
where is the wavelength in nanometers.
The following table shows some example glasses and their glass code. Note that the glass properties can vary slightly between different manufacturer types. Glass Glasscode Manufacturer name Schott Pilkington Hoya Ohara Borosilicate crown 1.5168 64.17 517642 BK7 BSC517642 BSC7 S-BSL7 Barium crown 1.5688 56.05 569561 BaK4 MBC569561 BaC4 S-BAL14 Dense crown 1.6204 60.32 620603 SK16 DBC620603 BaCD16 S-BSM16 Lanthanum flint 1.7439 44.85 744448 LaF2 LAF744447 LaF2 S-LAM2 Dense flint 1.7847 25.76 785258 SF11 DEDF785258 FD11 S-TIH11
References