of tested surfaces, and those of the luminescent background due to the remaining penetrant on the surface.
Traditional methods of the threshold processing, however, cannot separate these two components of the
background. This problem can be solved using several methods. One of them is the threshold processing
with several levels. In the general case, however, these levels must be determined empirically for each specific case. For this reason, one cannot be sure that comparison between two different processed images
will yield reliable results.