For some time now I had the idea I could faintly discern a wavy pattern in my prints, not always but on evenly grey areas and with certain films.

First my set-up

Durst L1200 Laborator.
Condensor head.
Photocrescenta 150 watt Tungsten bulb.
Ilford multigrade filters in drawer between lamp and condensors.
Negative holder with only Anti Newton glass (from Durst) on top, no glass below for formats upto 6*6, larger with normal clear glass at the bottom.

Film HP5+ 120 format

As I said I sometimes noticed a wavy pattern which could be caused by the AN wavy pattern, so the obvious thing to do was print the same negative (a portrait with smooth mid gray tones)with AN glass and normal clear glass (both original Durst glass)

And quite clearly, side by side comparison showed that the wavy pattern was caused by the AN glass!
It's not a major difference, but side by side it is clear.

Is this a known phenomena ?

Is there a way around? I switched to AN glass because Newton rings are quite ugly. They were not frequent, humidity is mostly in the 60-80% range in my darkroom.

Best,

Cor