LED RGB Additive Enlarger Head - Building One?
I've built a variable contrast LED head for my Durst 138S. It uses green and royal blue LEDs for contrast control. It dawned on me that I'm 66% of the way to an additive color head; all I need is a red channel. This would let me decommission the color enlarger and reclaim some space in the darkroom. But I don't know much about additive enlarging. So I have a few questions:
1) Is this even feasible? I know the Chromira uses LEDs to exposure paper, and the Lightjets use RGB lasers to expose papers so it seems like it should work, but neither go through a negative first.
2) What would the ideal frequency ranges be for the three colors? How much tolerance is there in being off. For example I can get a couple of different frequencies in blue and red LEDs, but very little choice in the green. It seems they would need to be matched to the color absorption parameters of the paper.
Larry, I'm extremely skeptical that the technology is quite there yet. But I'd go ahead and do some tentative thinking about it, since this kind
of lighting is evolving rapidly. Don't believe everything you read from marketing people. Getting a good spectrum is impossible at this point
in time. I've tried to keep myself updated frequently, and given the sources of supply and the shady reputation of certain distribution channels, take things with a grain of salt. More legit advances are being made in true architectural lighting at dramatically higher price levels. Some of this
has to be computer controlled, which means potentially obsolete software as well as hardware if something goes wrong or bankrupt. The hypothetical advantage would be low heat. The downside would be trying to get complicated circuits to get along with one another. I know how and currently use two additive halogen enlargers of my own design (employing all kinds of cannibalized components too). But if the
spectral characteristics are proper, and you can design some way to control the channels and the diffusion, the color quality will be at least
equal to what you can get with a laser printer or Chromira at a fraction of the operating expense. I will admit that ZBE helped me out with
some of the special devices that helped me psychoanalyze my own printers, similar to what the NSA uses in their additive 9x9 enlargers.
But they're out of business. The electronics side of all this can get pretty complicated unless you're an electrical engineer yourself. The way
Durst themselves did it (yet never marketed the results) resulted in a very very hot colorhead, so you'd end up having to replace the filters
every six months of so, at terrible expense. But I know who will sell you the guts of the system, if you don't mind dropping tens of thousands
of bucks. That's why I started from scratch and built a relatively cool system which is far more economical to operate. Old school rheostatic
controls or dimmers won't allow you to have enough muscle for serious color printing. I won't go into xenon flashtube systems. The Minolta one was weak, to put it mildly. And a few old Durst sequential additive systems might still exist in storage somewhere from back in pre-scan
graphics days - but they will almost melt your original negs or trans, ideally need water cooling, and won't allow you to dodge and burn...
but like I say, keep an open mind, because the picture might change pretty soon. This control aspect will remain tricky unless you go to
computer control on the cheap.... I'd look for some used Mac setup from a rock concert lighting crew, that can handle at least twenty circuits simultaneously, plus some backup components. Otherwise, that alone will hit you around six grand new.
Matching the paper sensitivity is the key and is way beyond my scope.
I do know that Agfa Classic works in a Lambda but Ilford Warmtone does not..
I believe the Adox Classic paper would work as well.
Very fine line you are reaching for, way beyond my knowledge level , good luck would be interested on how you do.
But you are using the LED for exposing through a negative right? You are not contemplating designing a system to work form digital files?
Drew, thanks for the input. I think I have control aspect down well. With the two color LEDs I'm using in the variable contrast head I control them with an Arduino (small programmable microprocessor). Combined with a constant current power supply that dims with PWM (pulse width modulation) I can control the brightness of the LEDs from off to full power fairly precisely. One concern is it's only 8bit control, so only 255 possible between on and off. That may not be precise enough, but with a newer Arduino I could get 12bit precision on two of the channels.
From what I can determine the CREE LEDs are produced to a high standard and closely match the documentation. They are also available "binned" where they are tested for color. So I could be assured of getting ones with the characteristics that I request. Still they have a distribution of frequencies around a strong peak. Between the blue and green absorption curves for both Endura and Crystal archive the green layer could still be sensitive to a small bit of light from the blue LED, but less so to the Royal Blue (but the blue is also less sensitive to the Royal Blue). I think the Red end is OK. I can't get LEDs that match the absorption peak of the paper perfectly, but they won't cross expose the green.
The diffusion aspect is also working well. Currently the individual color LEDs are mounted a bit farther apart than I would like, but they still produce an even light given a few inches of space between the LEDs and the diffusion screen. I have since found I can get three LEDs mounted on a single "star". This would allow me more freedom to position them for evenness, and would get all three LEDs as close as practically possible. I think this might be required for color, whereas in black and white you wouldn't notice the slight contrast unevenness (if it's there - I can't see or measure it).
So how do you print with an additive head? I'm used to a subtractive head where if I want more red I need to add cyan filtration. I assume with an additive system I would need to subtract red?
You can study the spectral characteristics of various paper and film profiles. Each of the three color of light need to pretty much hit the nail on
the head in terms of peak sensitivity. Otherwise you need some distinct overlap on each side, which can be further restricted with dichroic
"trimmer" filtration. This get pretty involved since it is operating temperature affected. Remember, you're aiming for a much high color quality
than ordinary subtractive colorheads, so the rules get a lot stricter. It also takes much more light intensity when exposing additive versus
subtractive. If supplementary filters are involved, you need to mechanically incorporate them at the correct angles of incidence. You can't
just partially overlap them like in subtractive work. So things get real complicated real fast.... or you could just wait things out awhile and
see if someone does come up with appropriate RGB narrow-band LED bulbs. The parameters need to be far stricter than what will work with
VC paper if you want quality results. It might require a very different head design too. But don't let me discourage you! I could be a fun
and worthwhile project if you're patient with it.
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That's interesting that you have issues with different papers in the Lambda. I am surprised you have issues since I thought all papers were approximately the same in there spectral response (but not their sensitivity). I've tested the VC head on Ilford MG IV, Adorama RC (no idea who makes it), and Forte Polywarmtone FB. In all cases it worked as it should. I'll need to test the Ilford Warmtone.
Originally Posted by Bob Carnie
Yes, I'm looking to expose through a negative. Currently I use a ZBE Starlight (I must have the only good pair of them in existence from what I've read). But this can only do 4x5, and it would be cool to be able to enlarge 5x7 color. I would also like to keep only one enlarger with the ability to quickly switch heads.
We posted at the same time, so here's some more tidbit. The color theory is basic, so in printing color neg material like Crystal Archive, your
primary additive sources affect the direct complement of CMY dyes in the paper. I have the added luxury of controls programmed to CMY,
even though the actual lights are RGB. There are ways to cannibalize feedback controls from other pro enlargers like Durst to make this simpler, but you could certainly print without either feedback loops or subtractive math. The less the number of light sources the better. Once you get into more than three or so, EMI becomes a very serious headache, and can potentially wreak havoc on anything solid state within the vicinity. So one more super top-secret trick to getting even diffusion: something called a linear-array fresnel (not to be confused with fresnel lenses in the conventional sense). These can be used in tandem with conventional ground diffusers, and placed in mirror boxes or whatever - but oddly, I think I'm the first person who every used these in enlargers - and what a difference!
Keep in mind that printers like the Lambda are designed to print from RGB digital files and they don't have to worry about film dye curves; just the paper spectral sensitivity curves. Your task is to thread the needle of both the color negative dye curves and the color paper spectral sensitivity curves. Good luck.
So I guess the question is what's the ideal frequency (and how wide can the band be) for each channel that works well with the film dyes and the paper absorption? If I can solve that I think I can build an even head. I can also probably use the same Arduino based controller to drive it, but worst case I control the brightness of each channel with a very fine potentiometer (manual dial setup).
Also, assuming I get this working perfectly, is there an advantage to an additive head vs a subtractive one?
Larry do you still make negs with your LVT recorder?
Originally Posted by L Gebhardt