It
is important to realize that the image we are accustomed to seeing is a
negative of the image on the cloth (the lower image here). That is, lighter and darker
visually-blended colors are reversed. For instance the tip of the nose
contains many color bearing bits of coating close together. This produces, as
our eyes see it, a dark area. (Incidentally, cameras, depending on the size
and granular characteristics of the film also blends the pixels). But when
reversed, the tip of the nose appears white. This phenomenon was not
discovered until 1898 when an amateur photographer, Secondo Pia, first
photographed the Shroud and saw the positive image in his glass-plate
negatives.
It is this visual characteristic of the Shroud's image
that has led some to believe that the image is a form of medieval
proto-photograph. Chemically, it is not so. The carbohydrate layer is not
photosensitive and this is where the image resides.