The different kinds of inhomogeneities that may occur in glasses are discussed. Inhomogeneity of glhsses depends upon: (a) the melting process; (b) the density variations due to thermodynamic imbalance; (c) the permanent strains resulting from temperature gradients in the annealing range. If the higheist possible optical homogeneity of glass must be achieved, it is necessary to know the influence of (a), (b), and (c). By eliminating the inhomogeneities resulting from (b) and (c), the inhomogeneity resulting from the melting process can be measured. To achieve reliable test data on the inhomogeneity of a specific glass, two different measuring methods should be used. The best homogeneity that means the smallest variation of refractive index Δn = ±3 × 10-7 was achieved with a BK7 disk having a diameter of 400 mm and a thickness of 47 mm. Further examples are also discussed.
F. Reitmayer and E. Schuster, "Homogeneity of Optical Glasses," Appl. Opt. 11, 1107-1111 (1972)