Michael, there are many solutions to your problem. One, suggested by EK years ago was to use Opal glass which is frosted and prevents Newton rings. Also, there are methods to prevent them.

Even in the most critical applications, we did not find glass carriers significant. In fact, oil immersion carriers with glass were of more use. We had some contact with that as far back as 1962 when I was at Cape Canaveral.

Also, with a mixed workflow which includes scanning (the most significant change yet) this helps imaging. So, there were solutions and tradeoffs. It seems to work for most.

PE