I purchased an Aristo cold light head for my Besler 23CII XL enlarger and have used it successfully on my 35mm negs. I recently upgraded to medium format (6x7) and have gotten uneven illumination on my paper. I did a test print by exposing a blank negative onto the paper to about a middle grey and after developing it found the grey tones were completely uneven across the print. Not even in any regular way, but a little lighter here, a little darker there - but for the most part definitely a darker spot toward the middle. I tried this test with 35mm film and got a little variation but nothing compared with the 6x7. I am using a 50mm enlarging lense for the 35mm and a 105mm for the 6x7. Also have a cold light stabalizer. Has anyone heard of this problem before and are there any solutions? Thanks in advance Alexandra

-- Alexandra de Steiguer (Alexdes@concentric.net), March 09, 1999

Answers

I had the same problem when I first started using cold light. Is your diffusor seated properly underneath the light head? I would suspect that the uneven light wouldn't show up in smaller formats due to the fact that the area of light coming through them is much smaller. In larger formats the area of light they are exposed to from the head is much larger. It was a nightmare printing 4x5 with uneven light and is actually impossible if you are looking for high quality prints. Check your diffusor (the white, semi-opaque piece of glass or plastic that goes directly beneath the cold light head) and see if there are any abnormalities there. Also, are you letting the head warm up? Most cold light heads require approximately 10 minutes to produce even light. There is a three-prong plug which comes directly from the head and this must be plugged in several minutes before printing. It's purpose is to warm the tubes inside the head so they perform correctly. You may have already done all of this but these are just a few of the problems I encountered and thought it might be of some help.

Brian

-- Brian Jefferis (jefferis@erols.com), March 09, 1999.

You may have racked out the upper bellows on your 23C. when you use a cold light head, you want to keep the surface of the white diffusion disc on the business side of the cold light right next to the neg stage. Set your upper bellows to bring the light head as close to the neg as possible and leave it there. That adjustment is only for when you are using the condensers.

-- Tony Brent (ajbrent@mich.com), March 10, 1999.

Thanks alot for the advice. I will check out those possibilities. It's real nice of you both to take the time for my question. Alex

-- Alexandra de Steiguer (Alexdes@concentric.net), March 12, 1999.

I encountered the same problem with my 23CIII and Zone VI cold light head. It was more apparent when using higher contrast paper or filters. Aristo and Zone VI 23C lamp/diffuser assemblies are essentially identical, and I have replaced the Zone VI tube with an Aristo V54.

Primary cause of this uneven illumination is the lack of space in such a small "can" to make the fluorescent tube cover the diffusion disk without twists and turns. If you look at the diffuser with the lamp on (wear sunglasses) you can actually see a projected pattern of the lamp. The solution appeared to be incorporating more diffusion.

I obtained a second translucent round plexiglas diffusion disk identical to the one that came with the cold light head and once again donned sunglasses to collect some empirical data. When the two diffusers were separated by 1/4" the tube pattern disappeared. Any more space and things just got darker. I fabricated four spacers from 1/8" thick plex, each one 1/4" by approximately 3/8" and, using an appropriate solvent adhesive, created a two-diffuser sandwich. Getting everything back into the 23CIII required removal of the original condensor head's white reflector cone, since the now-taller Zone VI equipment needed to go further up into the Beseler's volume. I added some aluminum foil around the edge of the diffuser sandwich to increase intensity there.

The good news is that now all formats through 6x6 are receiving absolutely even illumination. The bad news is that, since the tube physically cannot go far enough out towards the head's edge, there is some corner falloff with 6x7. Corner burning is required. This is unrelated to the diffusion spacers, since they present only a 1/8" obstacle to light in four small areas, and 6x7 corner brightness is the same whether the spacers are aligned or offset 45 degrees. Fred Picker was right when he said that a cold light one size larger than the original being printed should be used; it just can't be done with the largest medium format negatives in a 23C. At least my 8x10 contacts are now perfect when the head is raised high enough to use only light coming through a 645 carrier!

-- Sal Santamaura (bc_hill@qwestinternet.net), July 29, 2000.