I have a Beseler 23CII dichro head I use to print color negatives. I had this suspission bothering for a while but now I'm completely sure. The exposure changes from one day to the next. Here is an example. Last night I already had my test strip made. Had my exposure set giving me a good print. I made about four(4) 11x14 prints of this negative exposed for 26 sec @ F/11. I had some minor additional burning to do to make it a "perfect" print. But it was late and decided to stop and continue the next day. Today about 24hrs later I picked up where I left off. Absolutely Nothing changed, nothing was moved. With the same negative at the same height, I made another print with exact same exposure. To my annoyed surprise the print is now much lighter for approximately a stop, to the point that I have to run a new test strip ( I shouldn't have to ). Same batch of chemicals, and Again no variables were changed except that 24hrs have past. What is going on ? And how can I avoid this problem ? The lamp is new. And I believe this was happening with old lamp as well.