Optical EngineeringOptical glass: deviation of relative partial dispersion from the normal line—need for a common definition
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The design of high-quality imaging lenses continues to strive for the best color trueness over wider and wider wavelength ranges such as for multiwavelength fluorescence microscopy or hyperspectral imaging. 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 multicolor 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. A comparison of almost identical glasses shows differences between these deviations being too large, obviously 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 really comparable values for deviations of the relative partial dispersion from the normal line.