When talking about the image on the Shroud of Turin, one must recognize its superficiality. Even the darkest part of the image (the nose) is only on a microscopic surface layer. To this day, there is no known process which can make such a superficial image.
So how was the image on the shroud formed? In 1978, a research team of 33 US scientists were given 120 hours of access to the shroud in order figure out this question. 1981, this research team officially reported that they were unable to conclusively say how the image was formed; but they could give 3 conclusive facts of how it was not formed. First, there were no added paints, pigments or dyes on the cloth. They also concluded that the shroud image was not the work of an artist. And, there are no chemical or physical methods which can account for the totality of the image.
The 1981 conclusions, however, have not stopped theorists from coming up with hypotheses concerning the image’s creation. In total, there are 10 major hypotheses which fit into 3 categories: dead body, artistic creation, and radiation. Under the dead body category, there are two hypotheses: physical contact and gas. Artistic creation boasts six different hypotheses including paint, dust, bas relief, frottage, photography, and shadow image. The only serious area of research concerning the creation of the shroud’s image is radiation, which contains two hypotheses: fall-through and coronal discharge. Only the fall-through radiation theory is consistent with all 17 characteristics of the shroud image.
One of the characteristics which only the fall-through theory is consistent with is the 3-D information contained within the image of the shroud. The 3-D information was found by using the same technology for mapping out the surface of the moon, which measures light intensity. So, the 3-D aspect of the image had to come about by some sort of light event. The fall-through theory takes it a step further due to the image itself. It states that, at the time of the light event, the body contained within the shroud acquired a mechanical transparency which allowed the shroud to fall through the body. This would account for the lack of side images on the shroud. As gravity pulls the cloth downward, the cloth would flatten and be unable to capture the side image.
Resources: our two-part Virtual Pilgrimage on the Shroud of Turin (coming soon to YouTube)