When spectrophotometry is conducted in the presence of unavoidable stray radiation, the usual expression for the absorbance [A =log(100/T)] becomes log[(100-k)/(T-k)], where T is the percent transmittance (100I/I0) and k is the percent stray radiation. To meet the demands of routine infra-red spectrophotometry in the 15–22µ-region, a table of values of this function was prepared for 1<T<99 and 0.5<k<20. The calculation of these values was performed by punched card machines.
ASCHER OFLER, "Spectrophotometry in the Presence of Stray Radiation: A Table of Log[(100 - k)/(T - k)]," J. Opt. Soc. Am. 40, 401-401 (1950)