* "Cellulose undergoes auto-oxidation in the presence of UV radiation leading to bleaching of the surface. Cellulose itself does not absorb UV, however, lignin, hemi-celluloses and some dyes and pigments act as photo-sensitizers. (Photo-sensitizers absorb UV radiation and transfer the energy to the cellulose, initiating a reaction.) As a result some of the long molecular chains break up, lowering the degree of polymerisation, and weakening the material."
Taken from: link
It is important to keep this is mind when one reads of so much emphasis, possibly over-emphasis on the Shroud image being superficial, exceedingly thin, or present on opposite faces of cellulose fibres/fibrils but not in between. Any photodynamic action involving visible or uv light is likely to be confined to a very small distance from the interface between putative photosensitizer and cellulose.
In my recent previous posts (see link to posts with the keyword thermo-stencilling), I have described the use of "charcoal paint", subsequently washed out after heat-irradiation, to produce a thermo-stencil. It has just occured to me that the charcoal might be described as a "thermosensitizer", producing a highly localised scorch mark. A thermosensitizer would be to the infrared region of the em spectrum what a photosensitizer would be to the visible or uv regions.
Did the production of the Shroud image require the presence of sensitizer? If so, was it a photosensitizer or thermosensitizer?