A densitometer measures the differences in density in a photograph. In a black and white photograph, black areas are dense with information and white areas are void of information, The in-between areas are many shades of gray, from almost white to almost black. You can assign a different number or value to each shade of gray, for example on a scale of 1 to 10, where 1 represents black and 10 represents white. All numbers between 1 and 10 represent then the various shades of grays. This is a gray-scale or gray map, where different numbers represent different densities in the image. Now you can analyze a photograph, with for example a densitometer, that measures these before-mentioned differences in density, and give each density a different number. When in 1976 John Jackson and Eric Jumper from the Air Force Academy performed this test on a photograph of the Shroud and then used the VP-8 Analyzer to translate this information in vertical height, they found to their big surprise, that this resulted in a perfect 3D model, with an undistorted anatomy of the face and the body. It could even be turned sideways, so that the face could be seen in profile. In the Shroud photographs you will notice, that the parts of the body that were close to the cloth show darker (denser) than the parts of the body that were further away from the linen cloth, with very subtle gradations of gray in-between (in the positive image). This is a very unique inverted distance to image relationship. There is no existing photograph of a face or a human body that contains this information and when converted in the VP-8 Analyzer will always give distortions, like the nose pressed in the face and the arms pressed into the body, with a flattening of the whole relief. The importance for the Shroud image is, that we used this gray-scale, present in the ENRIE photographs from which we know that every pixel contains the cloth-to-body information, to translate this mathematically into vertical height on the Z-axis. This is called the conversion process from 2D to 3D.
If you look at the photograph of the positive and the negative image of the Shroud face you will detect the different densities present in the image. The positive image is like you will see it on the real Shroud and the negative is the reversed version (reversal of dark/light and left/right) as shown by a photographic negative, where we see the image like in real life.