Bruker OPUS is a proprietary file format, so we do not know its structure exactly.
One problem is, given only a few series, how to decide which are absorption spectra?
Our solution (empirically developed) is:
1. Remove broken series (ones with FXV > LXV, missing NPT information, etc.)
2. Remove interferograms. (See https://github.com/philipp-baumann/simplerspec) Interferrograms have a starting value of 0.
3. If after these two steps we still have more than one series left, we can choose the one with the highest average value. We empirically checked that other series are usually random noise with values near 0.