We have developed a modified approach to the extraction of the real and imaginary parts of the refractive index (optical constants) of a thin film from a single transmission spectrum measured in the infrared spectral range. Our algorithm is similar to those implemented by previous authors, with some major changes that yield results for strong absorptions where previous approaches fail: (1) an adaptive k-correction step size, (2) the use of a root-finding algorithm to obtain a more accurate k-correction at each iteration, and (3) a k-correction step that prevents non-physical results such as negative n-values that prevent convergence in the calculation algorithm. The source code provided is written in Python 3.8. An executable version, compiled for Windows 10, is also included. 

Version 1.01a includes everything from version 1.01 plus the data from Figures in the associated journal article. (Note that the filenames for the CH3OH data have been mistakenly swapped.)