# UV Lamps for exposure



## unpopular

Would I be better off using germicidal UV-C or regular blacklight (UV-A/B) bulbs for dichromated colloids? I know that the germacidal bulbs are more expensive, but if they provide significantly more exposure, they'd be worth it. I hate waiting.

I know that sodium dichromate has a peak sensitivity below 300nm, but the glass to hold the negative will absorb much of the radiation below about 400nm. Without using UV transmitting glass, will any of the UV-C radiation reach the print? Do germicidal bulbs emit more UV-A/B radiation than blacklight bulbs? Am I over thinking the issue, like I usually do?


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## Proteus617

I've never done gum bichromate, but I have done plenty of cyanos.  The disadvantage of BLB bulbs is that they don't really provide enough light in the proper wavelength.  The advantage is that they are cheap.  If your exposure times are too long, put more lights in the bank.  The ultimate (cheap) solution to long exposure times is more wattage.  CFL BLBs are $5-$10 each, so you can afford alot of wattage without spending cash on bulbs and ballast.  Lots of advice on the Flickr group concerning light sources and exposure times.  Also, my contact frames for cyanos are made from cheap picture framing glass.  I have no issues with the UV transmission.


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## unpopular

According to my research in college, there is a bit of a peak around 360nm before dropping off entirely at 500nm.

I did some poking around, and it's hard to tell. The 254 spike is so huge on germicidal bulbs, I'm having a tough time reading the output at around 360nm as it compares to BL-B bulbs.

Xenon strobes produce about as much radiation from 300nm to 400nm as it does from 600nm to 800nm, and about 10x the amount from 240nm to 300nm, but again, I don't think I can use that spectrum. I tried using a low wattage xenon strobe in the past at 20 flashes/sec, with not-so-good results. Maybe if I had one at 500w that could do more flashes it would work, but these are expensive.

Mercury vapor might work. Mercury short arc definitely would but I have no idea how to ballast them - my bet is everything about them is going to be expensive.


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## Proteus617

unpopular said:


> my bet is everything about them is going to be expensive.



Here was my approach to artificial exposure to UV.  I went to Home Depot and bought a single BLB in a can for $10.  I set up my printing frame and hung the can over it with a tripod.  Got a good exposure at X minutes.  Plugged the height of the bulb and the exposure time into the inverse square law.  Figured out how high the bulb should be to give me a reasonable exposure.  Tested again just to be sure my calculations were correct.  A quick visual check of the test print regarding the falloff from the center of the print will give yu a very good idea of how many bulbs you need in the lightbank and how far apart they need to be.  Total cost of the test = $10 plus printing materials and chemistry (negligible).  If your results suck, it's still $10 well spent as you then know why you are spending the big bucks on an expensive UV source.


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## unpopular

Your prob right, I have a tendency to reinvent the wheel and over complicate matters. Just curious, what kind of exposure times are you getting with CFLs verses full sunlight?


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## Proteus617

A cheap spiral BLB 4 inches from a 6x9 negative gives me about 15 min for cyano.  Sunlight is 10 min depending on the season.  How many BLBs depends on the size of the print.  Bichromate should be much faster than that.  I just run the lights through a darkroom timer and walk away, go to work, whatever.  I am seeing a difference in density (if that's the right word).  The prints seems to lack a certain intensity that comes naturally with the sun and you have to work at to achieve with artificial.


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## unpopular

15 minutes isn't bad at all.

Though my dichromates take about 20 minutes in full sun at 3500', depending on pigment selection. Still though, at the most I'd be looking at 40 min, i'd imagine.


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## unpopular

Well. I got a U-Line florescent fixture at Salvation Army for 10 cents. Everything in teh store was ten cents.

Picked up a couple biff rose albums, too


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