Intense sources, e.g from a laser, may simply target a trace component that wouldn’t normally be sufficiently energized to produce coloration.
Hopscotching over to his other specialized blog, recently renamed “The Shroud of Turin: medieval two-stage imprint? The blog that separates the science from the pseudo-science…” Colin Berry presents us with …an updated version of [his] ‘iconoplastic’ modelling of that Turin so-called “Shroud”.
It is "probably a misnomer," he adds.
BUT THE BEST PART is what he has to say "about those gee whizz ‘radiation’ models”:
The First Law of Photochemistry states that light must be absorbed for photochemistry to occur. This is a simple concept, but it is the basis for performing photochemical and photobiological experiments correctly. If light of a particular wavelength is not absorbed by a system, no photochemistry will occur, and no photobiological effects will be observed, no matter how long one irradiates with that wavelength of light.
Anyone proposing a radiation-based theory MUST (a) state the wavelength of the radiation and (b) the chemical species (chromophore) that is capable of absorbing that particular wavelength.
Be wary of those who try to sidestep the First Law by telling you that their radiation source is hugely intense and monochromatic, or a type of radiation unknown to physics. There is no escaping the First Law. No absorption means no photochemical reaction, no localized heating, no coloration. That applies to ALL electromagnetic radiation, from long wavelength radio waves though microwaves, infrared, visible, uv, x rays to the highest frequency/energy short wavelength gamma radiation.
Intense sources, e.g from a laser, may simply target a trace component that wouldn’t normally be sufficiently energized to produce coloration. Trace components of linen that come to mind as normally overlooked chromophores, but more readily energized molecule for molecule than cellulose, would be lignin and other phenolicss with aromatic ring structures, absorbing moderately in the blue end of the visible spectrum and the near uv.