Nicholas Allen, of the Port Elizabeth Technikon, South Africa, has also suggested that the image on the Shroud was produced as a photograph. He has conducted a series of experiments to show that it was possible. However, he fails to meet many of the image criteria.
The late Dr. Alan D. Adler, Emeritus Professor of Chemistry at Western Connecticut University, in an article entitled, "The Nature of the Body Images on the Shroud of Turin," comments:
Allen has proposed a variation of the method just examined [the da Vinci polemic] except that his charging photosensizers are silver salts. The receiving cloth is a crude photographic plate. It is still an albedo image and will fail a VP-8 test [3D encoding] and there is no microscopic, chemical, or spectroscopic evidence for silver species or the expected products of their chemical reaction on the Shroud body image areas or sticky tape samples. He does not really deal with the blood image problem, either.The Shroud is not a photograph".
Historian Dan Scavone also writes:
When South African scholar Dr. Nicholas Allen, Chair of Fine Arts at the University of Port Elizabeth, published his Shroud-as-photograph theory in 1993 that, consonant with the radiocarbon dating of the Shroud, his hypothetical photo was made before 1356, he overcame some of the palpable weaknesses of the Leonardo theory. Today we know the ingredients and requirements for making a photo. We can read a children's handbook and make rudimentary home-made pictures. Dr. Allen notes that all the ingredients were available in the 14th c., and all one had to do was suspend the corpse for three to four days in sunlight, at the proper focusing distance from the fourteen-foot cloth that has been treated with silver nitrate or silver sulphate, outside a large camera obscura whose aperture contains a double convex quartz crystal lens fifteen centimeters in diameter and seven milimeters thick, then fix the negative image with ammonia or with urine.
We know enough about the image chemistry from spectral analysis to know that the image was not produced with photosensitive chemicals.
