I've been schooled by my mentor, R. Juergens, for using the term "Abbe number" for an IR doublet. The term "Abbe" denotes a visible solution. Therefore when discussing dispersion in a non visible material one should use the term "V number" Invert the therm optic coefficient, β, and you get the thermal V number or VT. Predicting thermal defocus based on the thermal V number is analogous to calculating axial color defocus.
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