I've two anecdotes to contribute:
First, my Philips PCS130/PCS150, which uses three MR11 halogen bulbs (for red, green, and blue light), blew a bulb a while back. I replaced it with a spare bulb that came with the enlarger, but I found that I needed to radically up the output from the replacement bulb. When I looked more closely, I found that the replacement bulb was cloudy, as Richard notes. I don't know if the bulb was defective new or if it had been used, deteriorated, and set aside for emergency use only by the previous owner. In any event, it's worth checking the bulb for obvious discoloration/darkening.
Second, I mostly do 8x10-inch and smaller prints, but occasionally I do larger, so I keep some 11x14 sheets in my freezer. After about two years, I pulled these out to make some prints, but I did some test strips on newer 8x10 sheets of the same brand and type (Foma Variant 311 VC RC). When I went to make the final enlargement, I found that it was much lighter than my test strips on the same setting. I suppose this could have been due to emulsion-to-emulsion differences, but my suspicion is that it was the age of the paper -- even cold-stored, it seems to have lost speed over a 2-year period. As I recall, I needed to roughly double the exposure to get matching results. |