The Coin in the Eye of the Shroud of Turin

      This classroom experiment is one of the best ways I know to demonstrate to students the power of suggestion and the necessity for blind and double-blind experimentation, even when the experimenter thinks that he can maintain an unbiased attitude. Of course, it also serves an a good introduction to polarization of light.
      Extensive analytical work was performed on the Shroud of Turin, claimed by some to be the burial shroud of Christ. Scientists were allowed to extensively study the relic, including the removal of samples for carbon-14 dating, which provided an origination date between 1260 and 1390 A.D. Techniques used prior to the carbon-14 dating included x-ray and ultraviolet fluorescence, mass spectrometry, and visible and electron microscopy, and the work, in general, was of good quality, although some incorrect conclusions were drawn because of inappropriate sampling methods. An interesting sidelight to the main investigation was a study of the claim that button-like images in the eye areas of the Shroud were lepton coins of Pontius Pilate, placed over the eyelids. This claim was made by Chicago theology professor Francis Filas, who noted that under high-magnification, the image on the right eye appeared to show the letters UCAI and a shepherd's crook, which were characteristics of a coin in existence at the time of the crucifixion. This inspired Alan and Mary Whanger (Whanger, A. D., and Whanger, M. (1985). Polarized image overlay technique: a new image comparison method and its applications. Applied Optics, 24, 766-772) to invent a technique called "Polarized Image Overlay", in which two projectors were fitted with oppositely polarized filters. One projected the image from the eye of the Shroud, and the other the image of a lepton coin. A third polarizer was used by the observer to switch from one image to the other so as to note congruencies between the coin the the image of the Shroud eye. Was this really any more than a complex Rorschach test that misinterpreted scorch marks on the cloth? The Whangers attempted to alleviate that concern by doing the image analysis of other coins and with other observers, although there is no evidence that the tests were done blind. In any event, the question of Shroud authenticity was settled by the carbon-14 dating results, so you can be the judge of the reality of the coins in the eyes of the Shroud image.
      The classroom experiment can be done with two overhead projectors rather than with the 35-mm slide projectors used in the original studies. Large polarizers, available from various educational supply companies, can be used to cover the projection lenses of the overhead projectors, and small polarizers are then distributed to the class for use as analyzers. Images of various ancient coins and the image of the Shroud eye are scanned into a computer and digitally-modified to account for angular differences upon display. The experiment has been done two ways. First, the Polarized Image Overlay experiment is performed in a manner similar to the original work, except that the students are not informed which coin is the lepton of Pontius Pilate. Results from this experimental design have resulted in a chance distribution, with approximately 20% of students choosing the lepton on Pontius Pilate from a selection of 5 coins. A more interesting way to perform the experiment is to attempt to bias the student selection by providing them with false information regarding the coin that "should" be selected. Not surprisingly, the distribution of coin choice in this experiment tends to be biased towards the coin that "should be" but "really isn't".
      The experiment can also be done using a computer graphics program that allows one image to be superimposed over another image and the transparency of each image to be modified. No matter how the experiment is done, it is definitely a wakeup call for the students.

Figure 1. The coin in the eye of the Shroud of Turin
(A) Images of the lepton coin of Pontius Pilate and an enhanced image of the cloth in the area of the Shroud eye. Numbers are for screen alignment.
(B) An image of the face on the Shroud of Turin

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Page prepared by: Mike Epstein
Last Modified: 30 April 1999