How do we know? No directionality of light.
Clarence in Chillicothe suggests posting daily facts that are commonly accepted and ask how it is that we know it to be so. He has provided a list that will keep us busy until Christmas. (Skipping a couple of them for now, here is the first one):
#1 - There is no directionality of light in the image on the shroud.
To artists, accurate duplication of the light falling on their subjects is the primary basis for realism in their results. The history of art clearly documents the attempts made by artists at achieving this through the centuries. It is this relationship of highlights and shadows on a subject that provides the modeling that allows depth, shape and form to exist in a two dimensional plane. Artists must first discipline themselves to "see" the effects of light on their subject, then perfect the techniques for incorporating these effects into their artwork. Without doubt, this task is much simpler for photographers since it is the light itself that creates the result that is captured on the film.
Maybe we should say ‘no sense of directionality of light’ since we have no solid reason for thinking the image represents reflected light on a man’s body.
Picture: Head of Christ (detail), Rembrandt van Rijn