Raking light: By placing a light source close to a surface at a low angle, the light creates an exaggerated series of bright peaks and shadow filled valleys. This is useful for examining for surface distortions such as creases and particles attached to a surface. Raking light is often used to examine paintings to reveal brush marks and palette knife marks, as it was to study details in Salvador Dali painting, "Forgotten Horizon."
In 1978, John Jackson, a physicist from the Air Force Academy in Colorado used raking light to examine the surface of the Shroud of Turin. He found a number of persistent crease, some especially distinct that spanned the width of the cloth. By following the folding patterns of persistent creases, Jackson discovered that the cloth could be folded, first, inhalf where the heads of the two body images met and then such that the cloth could be folded into a stack of material roughly 3½ feet wide and 1¾ feet high. The face from the front body image, and only the face, he discovered, was centered right in the middle of this stack of cloth.
The University of Cambridge, Hamilton Kerr Institute, Fitzwilliam Museum has an excellent web-based demonstration of raking light.