Glass code

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, 589.3 nm, 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.[1]

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

${\displaystyle n(\lambda )={\frac {1-n_{\mathsf {d}}}{170.2V_{\mathsf {d}}}}(\lambda -589.3)+n_{\mathsf {d}}\ ,}$

where ${\displaystyle \lambda }$ 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.[2]