This technique was developed by Alan Whanger to compare areas of images, very detailed and superimposed.
This was done by using two slide projectors on top of each other and projecting two superimposed images on a lenticular screen (one with tiny vertical grooves; non-lenticular screens do not work with polarizing filters).
Whanger used then polarizing filters. These are crystalline filters that transmit light in a single plane only. If two of them are positioned so that their planes are in the same direction, you can see through them. But if one is positioned in the horizontal plane and the other in the vertical plane, light is blocked almost totally.
So, using two projectors and the lenticular screen you can, for example, project the Shroud face image through a polarizing filter in the horizontal plane and an Icon face image through a polarizing filter in the vertical plane. Then, using a third polarizing filter which can be rotated in front of your eyes, you will be able to observe the two images fade in and out of one another.
This technique has proven to be extremely useful, as it allows minute, exacting comparisons between the two images. These comparisons can be diagrammed and counted so that anyone who wishes can check them for accuracy and can repeat the same process and compare results. This was published in the peer-reviewed journal APPLIED OPTICS “Polarized Image Overlay Technique,” 766-72.
What you are looking for are points of congruence (PC). In a court of law, fourteen points of congruence (PC) are sufficient to determine the same source for simple images such as fingerprints; for more complicated images such as a face, forty-five to sixty PC are enough to declare the faces to be the same.