Hi,I'm not sure about this UV-light method. My theory about it:

If one takes into account, that a sporeprint contains many spores of different fungi and bacteria, and of course even bacteria (vegetative form), then you can define the purity of the print as the quotient of the amount of contaminant spores and the amount of the "good" spores:

Purity = Ngoodies / Ncontam.

The problem is that the spores of different organism have different susceptibility (Delta) to UV, and of course the spores have different sizes (Sigma), and obviuosly the goodies are also susceptible to UV. The product of these two factors give the probability that a spore is hit by a UV-light quantum and killed. If you irradiate a Sporeprint with UV, then the amount of Spores of one of the organism, say goodies, will follow a radioactive decay law:

N = No Exp(- Delta Sigma J t).

Where t is the time you irradiate the print, No is the number of spores before you irradiated, J is the currentdensity (proportional to the Intensity) of the UV light.

Now say that our print has only one contam then we get for the purity:

P(t) = Po Exp{- (Deltagoodies*Sigmagoodies - Deltacontam*Sigmacontam) J t}.

Taking a look at that equation you see that a radiative process can also make the purity worser, by having:

Deltacontam*Sigmacontam < Deltagoodies*Sigmagoodies

And this is easily done. For example Bacillus spores in a Psilocybe cubensis print. See page 249 of TMC, Stamets & Chilton, for a photo of Bacillus on a Psi. cub. spore. Now a Bacillus spore is even smaller than a Bacillus. So for this case Sigmacontam is much smaller than Sigmagoodies, and this is not all: Bacillus spores are very UV resisting, while Psi. cub. are relative susceptible. So without looking for weird not uncommon bacteria or mold, we found a case were the above unequality is given.

The susceptibility of molds like Aspergillus or Penicilium could be the same as for Psi. cub., but the spores are only 5 microns big, so the above inequality could also be fulfilled and using UV-light would make the purity smaller.

I hope this text didn't get to weird, should someone be interested in the complete calculation of the above, then e-mail me and I'll send/post a Postscript.

Elektrolurch