The Christiansen filter - A centennial retrospective review Part 2. Application in the chracterization of the homogeneity of glass

Abstract

The only application of the Christiansen filter which was not foreseen by its inventor - the characterization of glass homogeneity - is reviewed in this part of the paper. First of all Shelyubskh's method for the evaluation of σn^2 (the variance of the refractive index of glass) is analyzed on the basis of the theories which were discussed in part 1. Later, the experimental evidence of the correlation between the actual values of σn^2 and those determined by Shelyubskh's method or its variations are reviewed, as well as the influence of the experimental parameters of the method on the latter values. Thus, it was verified that both magnitudes are correlated over the usual range of values, but the influence of some of the experimental parameters — particularly the grain size, the washing process and the characteristics of the optical system - remains still unclear. Finally, the applications of this method in glass technology are discussed, arriving at the conclusion that by means of the σn^2 values determined by this method it is possible to foresee general trends of the glass homogeneity and in many cases it is also possible to determine ranges of values of σn^2 within which one can work without homogeneity problems. But beyond these ranges, the method is of limited utilty in order to determine if the inhomogeneities which are present in a glass ware may be harmful because of their intensity and/or distribution.