What is the true Kramers-Kronig transform?

The results of Kramers-Kronig transformation obtained from three different software packages (Omnic, Perkin Elmer and Kluger Nielsen) are discussed. Although identical testing input data (reflectance spectra of yttria-aluminosilicate glasses) were used for the calculations of absorption spectra, the transformations yield qualitatively different results. As the exact algorithms used in commercially available software are unknown to us, it is not possible to give sufficient explanation of described phenomenon. The main aim of the present contribution is to call the attention to this problem and to initiate discussion among the spectroscopists community.