Spectral Range of a Cold Light Head

[Bruce Pollock]

I've been experimenting with 4x5 contact prints made on Gum Dichromate. Like many other alt.photo processes, gum needs a good UV light source to print well. I've tried using my enlarger with a Zone VI cold light head as a light source with limited success (actually, not much success at all).

I had been under the impression that a cold light head was a fairly rich source of UV -- am I mistaken? Can anyone tell me what the spectral range of a cold light head is?

Before I rush out and try to find some other light source, I thought I should get other opinions. Perhaps I just need to make a significant increase my exposure time.

[Craig Wactor]

depends on the bulb. Aristo makes different bulbs for the head I use, the one bought is optimized for vc silver paper. I would think any cold head would put out a lot of UV though.


Ok don't quote me on this, I don't print alt processes, but I seem to recall reading a million years ago that the glass in many enlarger lenses cut out a lot of UV light, and that a special lens is needed, or that you should not use a lens at all.

[Donald Miller]

depends on the bulb. Aristo makes different bulbs for the head I use, the one bought is optimized for vc silver paper. I would think any cold head would put out a lot of UV though.


Ok don't quote me on this, I don't print alt processes, but I seem to recall reading a million years ago that the glass in many enlarger lenses cut out a lot of UV light, and that a special lens is needed, or that you should not use a lens at all.


According to the information that I have, an El Nikkor is the best lens to pass UV outside of a pure quartz lens(very rare and expensive if you can find one)...but even an El Nikkor cuts down about 1/2 to 1 stop of UVA transmission. Rodenstock and Schneider will shut off a lot more UVA transmission than the El Nikkor.

The bulb of a cold light head may not be very rich in UV at all....that depends on the bulb phosphors. Alt process requires a lot more UVA than most any silver based process.

[Jan_6568]

Donald,
I have been printing with gum bichromate for a couple of years now and I am using a regular 500W halogen lamp as a light source. I am almost sure you can get it in Home Depot. It is cheap. Depending on the pigment I am using the exposure times are ranging from 2 up to 10 minutes. The distance to the printing frame is some 2 ft. The lamp is getting really hot when printning - be carefull.

I know, I do not answer your question on a sepctral range, but I hope it helps with finding an appropriate light source for gum printing.

BTW - in my experience (and taste) 4x5 is a way too small for gum printing. It is very hard to obtain small details with gum process so I think something between 5x7 and 8x10 is a minimum. However I must say - it is a wonderfull process,

hope it is of some help,

cheers,

Jan

[sanking]

I've been experimenting with 4x5 contact prints made on Gum Dichromate. Like many other alt.photo processes, gum needs a good UV light source to print well. I've tried using my enlarger with a Zone VI cold light head as a light source with limited success (actually, not much success at all).

I had been under the impression that a cold light head was a fairly rich source of UV -- am I mistaken? Can anyone tell me what the spectral range of a cold light head is?

Before I rush out and try to find some other light source, I thought I should get other opinions. Perhaps I just need to make a significant increase my exposure time.

You really need a prodigious amount of light to expose a gum dichromate or other alternative processes that need UV light. I know one fellow who had an 8X10 unit built for enlarging platinum and it used a very powerful light source, but that was a very expensive unit. Then there is the problem of absorption of UV light by the enlarger lens that Donald mentioned. So there are many issues that make enlarging for UV sensitive processes impractical, though not completely impossible.

The cheapest and easiest solutin in my opinio if you don't want to make large prints is to get a small contact printing frame and purchae one of the spirl BL tubes. I believe they come now up to 27 watts. These tubes work like other spiral fluroescents and do not need ballast. Just put it in a small reflection and hand it about fifteen inches over the printing frame and your exposures should be reasonably short, say less than thirty minutes.

Sandy King

[erie patsellis]

another option would be to find an exposing unit used by printers to expose plates, they typically go for less tna $100 on Ebay and work well. I used one quite a bit for exposing screenprinting frames, in addition it has an integrating timer, based on light output.


erie

[Ed Richards]

Or do what the old timers did - use the sun.