Most scholars accept the gory implications of the forensic story of the bloodstains on the Shroud. It's Real Blood. However, the faint shadowy images of a man on the Shroud are more controversial. No one knows how these images, one of a man's back on the lower half of the Shroud, and a man's front on the upper half, were formed. They could not have been painted, as some suppose. The chemistry and physics of the image chromophores that which gives visible images are now well understood by researchers, but the method by which the images were created remains a mystery.
The images are the result of a selective, color producing chemical change at the surface of discreet lengths of some cellulous fibers of the linen. These chemical changes could not have been produced with paint, dye, stain or liquid chemical; there is no evidence of any matting, capillarity, wicking, or penetration expected from liquids. Also, numerous tests including visible, ultraviolet and infrared light spectrometry, x-ray fluorescence spectrometry, and direct microscopic viewing of the Shroud confirm that the images were not painted.
The images, closely examined with the aid of microscopes and microphotography, are similar to halftone images, in which shades of color are derived from pixels of a single color, like a picture in a newspaper. The images on the Shroud are not black and white, but they are monochromatic. That is, they are of a single color often described as sepia or straw yellow. The color produced by the chemical change to the fibers is constant and the various darker and lighter tones of color we perceive are the result of the density of the altered fibers. It is interesting to note that on a high quality inkjet printer (1200 dots per inch), the ink droplets are about 60 microns across, whereas on the Shroud, the image-bearing fibrils are only about 15 microns thick or about one fifth the thickness of typical human hair.
Another interesting attribute of the images is that they are negatives; that is, the darker and lighter tones of color are reversed. In that sense, the images on the Shroud are like photographic negatives. This was discovered more than a century ago, when in 1898, a photographer named Secondo Pia took the first-ever photographs of the Shroud with a large box camera. When developing his photographs, he discovered that images that appeared on the glass plate negatives were positive images, startling in clarity and realistic appearance. For the first time people could see the amazing detail in the Shroud's images, detail people had not previously been able to observe. The detail is there in the negative images, but the human mind is not well adapted to interpreting negative images. With Pia's discovery, what for centuries had appeared only as ghostlike images were shown to be graphically clear front and back pictures of a man.
But they may not be pictures at all. At least they are not pictures of a human face or body in a traditional sense. Like any picture that tries to convey a sense of dimensionality, by showing how light is reflected from objects, faces and bodies, the images on the Shroud look like pictures with reflected light. However, image analysts tell us they are not.