Some interesting information about the photographic techniques used by Jim
Henderson of Oregon City, Oregon to bring out faint and virtually invisible
(to the human eye) inscriptions on historic rock art along the Oregon Trail
can be found in the Northwest OCTA website at:

http://nwocta.com/newsletters/99jul/kelton.html

He is currently carrying out work for the Park Service at City of the Rocks
in Idaho.  His technique, using polarized lighting and computer enhancement
to bring out inscriptions on historic rock inscriptions, has wide ranging
possibilities for both historic and prehistoric rock art panels, especially
where it's difficult to discern images.

Mike Polk
Sagebrush Consultants, L.L.C.
Ogden, Utah