Believe it or not - I have actually started work on my experimental UV enlarger (yep - the DIY and the Christmas shopping is finally all done
)
Not got anything worth showing anyone yet, just a load of soldered up LEDs.... but here are my thoughts and findings so far:
UV Light:
UV is a wide spectrum of light, like Infrared. Visible light is just a tiny slice in the middle of around 720 nM to around 405nM This is where UV is usually considered to start. It goes all the way up to around 10nM - after which point we are talking X-rays.
Now - it seems that most of the photographically actinic rays that are useful to us are around 400 to 350nm - that is, the area just outside the visible band. These are in what is called the UV-A band. This extends up to 315nM After this is UV-B, then C, then far UV and eventually X-rays.
Sunburn, skin damage, cataracts etc. are mostly caused by UV-B. UV-C are 'germicidal' - used for sterilization and to be avoided at all costs - as are anything else of shorter wavelengths.
Lenses:
It appears that crown glass is pretty transparent to UV-A and B. Flint glass is the problem, this starts to fall off in transmission fairly sharply after about 390 nM. UV also seems not to like cemented lenses either.
Consequently, people wanting to take UV pictures require special UV lenses, because they are interested in frequencies well into the UV-B band for photographing flowers and for forensic testing.
BUT.... we are only just dipping into the UV-A region, so if we choose carefully and select a lens without balsam and with a minimum of flint glass, we may get quite a good transmission. I found an interesting article on a site dedicated to UV photography that actually recommended an APO Nikon enlarging lens as a cheap UV macro lens as it had fairly low UV light loss and no UV focus shift, at least at the longer wavelengths.
LEDs:
UV LEDs usually emit around 400nM to 385nM. Some are available up to around 365nM, but these are rare and low output.
So - the good news is that LEDs are working in the 'safe' area of UV (UV-A isn't completely safe, but it is at least the 'least dangerous' and at the lowish levels we are talking about shouldn't burn anyone or fry anyones eyeballs unless we do something
really silly) - and also we might be fairly near the required frequencies for some alternative processes.
LEDs have a fairly low light output, compared to a fluorescent tube or an arc light... but these light sources are omni-directional. LEDs have a tiny output - but it is in a narrow directional beam, usually 20 degrees. They are also cheap. I am experimenting with a bank of 24 LEDs, covering an area just slightly larger than a 35mm neg - all carefully aligned to project on the same spot. Hopefully this means most of the light goes where we want it - through the enlarger optics.
Very roughly the diameter of the circle of light produced is about a third of the distance from the LED. So, if the LEDs are 10mm above the neg and the lens is 50mm below it, then the patch of light is about 20mm across. That is a lot bigger than the pupil size of the enlarging lens - and as the lens is lowered to focus it gets bigger, so more light is lost.
Would a simple condenser lens, maybe in CR39 (plastic) increase the light intensity by improving the focus more than it attenuates it by light loss? I don't know - but that is an experiment I hope to try.
Another big advantage of LEDs is they are very efficient, so we get a lot more light to heat than from most other sources.
It is very difficult to compare light intensities from LEDs and other sources. Light intensity is usually measured in Lumens, which is a measure of luminous flux and this isn't frequency specific and most sources produce light over a range of frequencies.
LED output is usually measured in mC - (milli candelas). In this case it will be at a specific frequency (or very narrow range) and candelas are measures luminosity of light per unit of angle - so we aren't comparing the same things. Trying to get my head around this drove me nuts, my IQ isn't high enough.
Easiest and most practical will be to contact print a cyanotype in the sun and compare it with the LEDs.
Cyanotypes are notoriously slow, but I have the chemicals to hand. At least if it works with this - it ought to work with the other alternative processes.
So, the plan:
Step 1: Contact print a (small) negative with the sun and with the LEDs and see if the light source is viable.
Step 2: Try shining the light source through some lenses and see how much light is lost (I'm lucky enough to have a laser power meter to hand - I can take some measurements).
Step 3: IF it looks like there might be enough light available to get a print before frying the negative, try mounting the LEDs and lens in an enlarger.
Step 4: See whether a condenser makes it better or worse.
I have a hunch that the LEDs simply won't be bright enough - mainly because if it was that easy someone would else would have done it by now. But, nothing ventured, nothing gained...
