# Glass code

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A '''glass code''' is a method of classifying [glass](/source/glass)es for [optical](/source/optics) use, such as the manufacture of [lenses](/source/lens_(optics)) and [prisms](/source/prism_(optics)). 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](/source/Pilkington) and [Schott Glass](/source/Schott_Glass). These tend to be based on the material composition, for example ''BK7'' is the Schott Glass classification of a common [borosilicate](/source/borosilicate_glass) [crown glass](/source/Crown_glass_(optics)).

== Technical definition ==
The '''international glass code''' is based on [U.S. military standard](/source/U.S._military_standard) MIL-G-174, and is a six-digit number specifying the glass according to its [refractive index](/source/refractive_index) {{mvar|n}}{{sub|d}} at the [Fraunhofer](/source/Fraunhofer_line) d- (or D<sub>3</sub>-) line, 587.5618&nbsp;nm, and its [Abbe number](/source/Abbe_number) {{mvar|V}}{{sub|d}} also taken at that line. The resulting glass code is the value of {{nobr| {{mvar|n}}{{sub|d}} − 1 }} rounded to three digits, followed by {{mvar|V}}{{sub|d}} rounded to three digits, with all decimal points ignored. For example, BK7 has {{nobr|{{mvar|n}}{{sub|d}} {{=}} 1.5168}} and {{nobr|{{mvar|V}}{{sub|d}} {{=}} 64.17,}} giving a six-digit glass code of 517642.<ref>{{cite web |title = How the generic optical glass code works |website=newportglass.com |url=http://www.newportglass.com/GeneCd.htm }}</ref>

Consequently, a linear approximation for the refractive index dispersion close that wavelength is given by:

::<math>n(\lambda) = \frac{ 1 - n_\mathsf{d} }{ 170.2 V_\mathsf{d} }(\lambda - 589.3) + n_\mathsf{d}\ ,</math>

where <math>\lambda</math> 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.<ref>{{cite report |title=Cross reference |year=2010 |website=OharaCorp.com |url=http://www.oharacorp.com/pdf/cross-ref-2010.pdf |access-date=2010-11-30 |archive-date=2011-10-08 |archive-url=https://web.archive.org/web/20111008041651/http://www.oharacorp.com/pdf/cross-ref-2010.pdf |url-status=dead }}</ref>
{| class="wikitable" width="80%" style="text-align:center;"
!rowspan=2| Glass
!rowspan=2| {{mvar|n}}{{sub|d}}
!rowspan=2| {{mvar|V}}{{sub|d}}
!rowspan=2| Glass<br/>code
!colspan=4| Manufacturer name
|-
! Schott !! Pilkington !! Hoya !! Ohara
|-
| [Borosilicate crown](/source/borosilicate_glass) || 1.5168 || 64.17
|bgcolor="#FFEBAD" style="text-align:center"| 517642 || BK7 || BSC517642 || BSC7 || S-BSL7
|-
| [Barium crown](/source/barium_oxide) || 1.5688 || 56.05
|bgcolor="#FFEBAD" style="text-align:center"| 569561 || BaK4 || MBC569561 || BaC4 || S-BAL14
|-
| [Dense crown](/source/crown_glass_(optics)) || 1.6204 || 60.32
|bgcolor="#FFEBAD" style="text-align:center"| 620603 || SK16 || DBC620603 || BaCD16 || S-BSM16
|-
| [Lanthanum flint](/source/lanthanum_oxide) || 1.7439 || 44.85
|bgcolor="#FFEBAD" style="text-align:center"| 744448 || LaF2 || LAF744447 || LaF2 || S-LAM2
|-
| [Dense flint](/source/flint_glass) || 1.7847 || 25.76
|bgcolor="#FFEBAD" style="text-align:center"| 785258 || SF11 || DEDF785258 || FD11 || S-TIH11
|}

==References==
{{reflist|25em}}

{{DEFAULTSORT:Glass Code}}
Category:Optical materials
Category:Glass engineering and science

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Adapted from the Wikipedia article [Glass code](https://en.wikipedia.org/wiki/Glass_code) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Glass_code?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.