How do we know the images are not a photographic negative?
Since the picture is a negative image, some have speculated that the Shroud of Turin might be a life-sized medieval photograph. At first glance this seems reasonable. But common sense should prevail. How likely was it that photography was invented in the Middle Ages, used once to make a single fourteen-foot long fraud, never exploited for fame and fortune, never mentioned, never even used again until it was "reinvented" in an age of science.
But science also shows us that it cannot be a photograph. The image contains height-field data (3D) and a photograph does not. There is no evidence of any photosensitive material, neither silver nor the resulting products of other photosensitive compounds. Image analysis shows that the image is not produced by light as a photograph would be.
See Negative Images