Optical Glass – Proposal to remove ambiguity from definition of the deviation of relative partial dispersion from normal line

The design of high quality imaging lenses strives for best color trueness over wider wavelength ranges. Glasses suitable for sharp images at more than two wavelengths must differ in their dispersion from the classical crown and flint glass types which gather along a straight line in a plot of the relative partial dispersion against the Abbe number. Glasses suitable for multi-color correction can be recognized by a considerable deviation of their relative partial dispersion from this normal line. Originally the normal lines for different relative partial dispersions were defined by using the SCHOTT glass types K7 and F2. Today’s data sheets of all glass manufacturers contain numerical values for deviations of relative partial dispersions from the normal lines. Comparison of almost identical glasses shows differences between these deviations being too large, coming from different versions of K7 and F2 dispersion curves used. For preselection in designs and for direct comparison of glass types it is necessary to agree on common dispersion curves each for K7 and for F2 in order to obtain comparable values for deviations of the relative partial dispersion from the normal line.