I have a note that the 1 1/2 mile figure can be found in Infra Red Systems Engineering by Hudson but a quick skim though my copy hasn't located it. I shall have to find time to sit down and read it properly again. The longer estimate would have been derived from data in either or both the Handbook of Military Infra Red Technology or its multi volume successor The Infra Red Handbook (title?). Most likely the latter given the period when I did the work. Unfortunately I don't have a copy of The Infra Red Handbook so I can't check (worst £50 I ever saved was turning down an offer of the last stand copy about 5 minutes before an exhibition ended!) and the data in my copy of the Handbook of Military Infra Red Technology doesn't look sufficiently familiar. although there is at least one suitable graph from which the extinction co-efficient can be derived and hence absorption path length estimated.
I managed to get a sight of the current Infra Red & EO Handbook but couldn't see any simple statements there. Sorry.