Dichroic Lamphouse Uniformity & Light Fall-Off
I conducted careful tests on my Omega 10x10 Chromega F lamphouse to determine light fall off and discovered three phenomena:
a) The difference in light fall off between the upper right hand corner of the print and the lower left corner is 1 full f/stop (0.3 density units) at all magnifications (1X, 2X, 3X, 4x). The upper right hand cornet exhibits almost no light fall off (as compared to the center) whereas the lower left hand corner exhibits the worst fall off.
b) At magnification = 1X, where there should be no fall off, the difference between the upper right hand corner and the lower left hand corner is 0.3 density units = 1 full f/stop.
c) There is a "white" line right down the middle of the print.
I am positive that the enlarger is aligned (lamphouse, lens, baseboard) as I spent the better part of a day aligning these three surfaces using a high-accuracy machine level. I cleaned the two glasses in the negative carrier and the enlarging lens has no scratches.
My guess is that the lack of uniformity is coming from the lamphouse itself. I know that the Omega F lamphouse is not famous for its unifotrmity buy 1 full f/stop is a bit much, IMO. What can I do to improve the situation?
It sounds like the inside of the mixing box is screwed up, check inside one side of the white baffeling may have fallen over.
as well make sure the bulbs are securley placed in their proper position.
If this is a Dichroic head also look at the filters and make sure all three are doing what they are supposed to do.
Alignment is not the issue here.
The Culprit was found
After a battery of tests I found the mixing chamber and the bulbs to be inocent. The culprit was the fiffusing plate. A zone V print through the diffuser (using another enlarger I know has uniform illumination) revealed horrific lasck of unifortmity.
I have not noticed light fall off with my 10x10 enlarger. However recently, I rebuilt my 10x10 color head, and I noticed that the white diffuser was thicker in the middle than at the edges. This was very easy to see once you took a close look. You may want to check your diffuser.
It also may be the design of the machine itself. My Fotar 10x10 uses a 2000 watt head with 8 250W bulbs. Perhaps Fotar realized that this was the minimum requirement needed to provide even lighting over a 10x10 area. Despite the number of bulbs, they still varied the thickness of the diffuser glass.