Quote Originally Posted by Michael R 1974 View Post
Any reason NOT to use UV glass in a negative carrier (Saunders VCCE diffusion head - quartz halogen)?...Is there even a meaningful amount of UV-range light in the system?...
No reason NOT to. In fact, I suspect doing so would be beneficial. Of all enlarger light sources I'm familiar with, halogen lamps are the only ones that emit UV.

Quote Originally Posted by Michael R 1974 View Post
...I'm just wondering because I suppose if there was, and you filtered it out, and further assume enlarging paper sensitivity extends into that range...
See page 81 of Ctien's Post Exposure


for the full explanation of why you might experience focus shift with certain enlarging lenses in this situation.