With deliveries of optical glass lots measurement data are given for the visible range usually from 436 nm (g-line) to 656
nm (C-line). Sometimes the question arises if refractive index values in the near infrared can be calculated from these
data. With near infrared we mean the range from the C-line up to 1700 nm in this publication. The reason is that up to
1700 nm most optical glasses have hardly any reduction in their transmission.
On the basis of a large amount of production data obtained over more than ten years with precision v-block refractometer
evaluations are possible up to 1014 nm. The precision spectrometer URIS developed by SCHOTT enables to analyze the
refractive index with measurement uncertainty fairly below 10-5 for even longer wavelengths up to 2325 nm, however
on a much smaller data basis.
The variability of the IR dispersion is shown for selected glass types. Frequency distributions for the different deviation
shapes give information how reliable extrapolations are from the visible range to the near IR. The precision refractometer
data were used to simulate such extrapolations employing partial dispersion data from catalog data sheets and to check
the consistency of simulated with real data.
For some glass types extrapolations seem to be possible. However, there are also glass types, where the method using
catalog partial dispersions leads to significant deviations from reality. So if extrapolations are intended to be done, a
general check should be performed if this is justified for the glass type of interest.