Abstract The advances in optical waveguides have permitted a clear measurement of many glass properties normally obscured by impurities in conventional glasses. Among these is the intrinsic optical absorption in both the infrared and ultraviolet regions. The wavelength dependence and the statistical fluctuations of another intrinsic property, the refractive index, have also been determined in detail. Knowing accurately the foregoing characteristics of “pure” glasses, deviations due to additives are easier to detect. This has led to absorption determinations for atomic defects and selected impurities as well as for deliberate additions of other oxides.