They are interference patterns apparently caused by relative perfect/imperfect contact. A while ago I experimented with several different types of coated optical glass but nothing really worked well.
The problem is normally easier to solve in contact printing than in projection, because one can increase contact pressure, which sometimes helps.
I would not recommend sprays. They also require careful cleaning.
Elevated humidity is said to increase the likelyhood of getting Newton rings. One method some people use is a quick wipe-down of the glass, the negative base, or both with alcohol. This may or may not work, but is a much lower risk procedure than using A-N sprays.
Assuming the rings are occuring between the glass and the negative, one quick fix is to put a sheet of unexposed, fully fixed Tri-X 320 (TXP) between the negative and the glass. The base side of TXP has just enough "tooth" to prevent Newton rings. Actually this reminds me, I was going to send Thomas a few of these TXP A-N spacers a while back and completely forgot!
Last edited by Michael R 1974; 07-11-2013 at 04:36 PM. Click to view previous post history.