Thanks for your timely response to my earlier question.

I guess my questions still remain. If the spectral response of Azo is in the UVA or near band UV range, as Kodak indicates, then the reflector flood lamps that I and others use must be emitting some UVA energy (or so it would seem to me). Perhaps the percentage is small in relation to total light output and that would seem to bear out the problem with short exposure times using a purer uv source for Azo exposure.

That still leaves me with the question of the differing density effects as you have described them insofar as Pyrocat developer is concerned. If, as information available would seem to indicate, then the relative UV effects on exposure by a stained negative would seem to apply to both Pt-Pd and to Azo. The relative "speed" of the two materials would seem to have no bearing on the fact that they both expose to UV.

Bob Herbst in his tests ( seems to show with step tablet exposure comparisons on Azo and Pt-Pd materials/processes that there is only 1/2 step greater density range recorded on Pt-Pd materials. That, if I am reading the results of his conclusions correctly, the silver chloride emulsion is capable of greater dmax then Pt-Pd and that Pt-Pd seems to show a greater ability to record and differentiate higher tonal values when compared to Azo.

Would this ability to distinguish high negative densities by Pt-Pd be the sole reason for the greater negative contrast range used by Pt-Pd photographers? However, I still question why a seeming differing effect on the transmission of UV should not apply to Azo as you seem to indicate that it does.

Thanks for any insight that you can share.