Like any painting or photograph of a face or an entire human body (or for that matter a vase, apple or any three dimensional object) brightness represents light. Look at a full frontal picture of a man. The tip of his nose approaches white and the depth of the recesses of his eyes are darker. The roundness of his face from his cheeks towards his ears is progressively darker. At first glance, the face on the Shroud of Turin appears to be such a picture. It isn't.
How do we know this? All regular pictures, be they paintings or photographs, represent light coming from some direction and being reflected towards our eyes. The eye of the painter or the camera lens is a proxy for our own eyes. The reason the recesses of a man's eyes are darker than the tip of his nose is because less light gets to into the recess. Image analysis shows us that this is not so with the facial image on the Shroud. There is no direction to what seems like light. Something else is causing the lighter and darker shades. That is looks like light to us is an optical illusion.
Look at the black and white picture that looks like a smoke ring (see http://www.shroudofturin4journalists.com/3d.htm). We might think that this is light reflected off of the smoke. It is not.
This is an analog data file of elevation, sometimes called a bump map in the world of computer graphics. With special computer software we can plot the data, the brighter and darker tones, as an elevation. That is exactly what we can do with the image on the Shroud of Turin: plot it as an elevation.
Let's be clear: You can not plot a regular photograph this way. Nor can you do so for a painting, even a brown and white painting. You can do so with a precise copy of the Shroud, however.
Not only does this show that the image on the Shroud is not a photograph or painting, it shows that something extraordinary occurred to form the image.