What is the second face and what does it mean?
Using new photographs of the reverse side of the Shroud, two researchers at the University of Padua, Giulio Fanti and Roberto Maggiolo, discovered the faint indications of a second facial image that corresponded to the face on the front of the cloth. Their findings were reported in the scientific Journal of Optics published by the Institute of Physics in London (April 14, 2004). This image, like the image on the front side of the cloth, is completely superficial to the topmost fibers of the cloth.
Both images are superficial. There is no image producing colorant between them. This rules out a liquid such as a paint or a dye. But it does not rule out a reactant gas as an agent in the image formation.
Some have argued that the second face rules out photography. It does not. But it makes it implausible. It would have required that the photosensitive emulsion would have needed to have been superficially on both sides of the cloth and not soaked in and that the light used to make the image was strong enough to shine through the cloth. There are plenty of other scientific reasons to know that the image on the Shroud is not a photograph.
The second face is an important criteria in any attempt to explain how the images were made.