LED RGB Additive Enlarger Head - Building One?

L Gebhardt · Jun 27, 2013

EdSawyer said:
Wow, some good info in this thread. I am impressed with the depth of knowledge from all parties.

Larry, I am in Dover. Drew is right, the large-print capability of the minolta/beseler is probably not great - I haven't tried really huge prints yet, but it's ok up to 16x20 or so at least. They are less obnoxious than strobes like you would use to take a picture. Many multiple high-frequency/low-intensity flash firings, makes for almost continuous light in a way. It's not too bad from an annoyance standpoint.

color printing with this system is easy - additive is more "logical" than subtractive. The head controller can be used either way, but basically if you need it more red in the print, you add more red. Density can be changed independently. Density and color can be changed in 1-cc increments from 0-255. There's a small probe that can be used to sample different areas of the negative at the easel, and you can do sophisticated averaging across multiple points, etc. The analyzer itself is quite impressive, allows automated color ring-arounds, etc. Pretty expensive in its day but uber-cheap nowadays on ebay.

I used one professionally as a photojournalist years ago and liked it, so standardized on it for home use when I set up my current darkroom.

-Ed

Yes, lots of good info here. Probably the best thing to do is to order a few LEDs and do some testing. I want to get a few more to build another light source so I can use the condensers, but with variable contrast. I'll order a few of the RGB stars so I can build that, and do a few simple tests with color printing. Worst case I'll be out a few bucks.

I'm over in Amherst, by the way.

L Gebhardt · Jun 27, 2013

Using filters over the individual LEDs would be difficult. It would be hard to fit tiny dichroic filters at the right angle given the design constraints of the head. Securing the Wrattens would also be difficult, but might be possible. I suspect fading would be an issue, since at least on the blue there is some UV emitted. So if I can't build it with straight LEDs it's probably not worth the effort to me. In truth, it's possibly not the worth the effort now, but I do enjoy an engineering challenge.

Prof_Pixel · Jun 27, 2013

EdSawyer said:
Drew is right, the large-print capability of the minolta/beseler is probably not great - I haven't tried really huge prints yet, but it's ok up to 16x20 or so at least. They are less obnoxious than strobes like you would use to take a picture. Many multiple high-frequency/low-intensity flash firings, makes for almost continuous light in a way. It's not too bad from an annoyance standpoint.

The Kodak RFS film scanners used multiple high-frequency/low-intensity flash firings as their light source.

DREW WILEY · Jun 28, 2013

Time is on your side, Larry.... just keep thinking this out and wait awhile until the correct bulbs appear on the market. Having the lower heat
of LED's would certainly be a real advantage. But exactly what form any allegedly ideal bulbs might take is impossible to know at this point in
time, so it's difficult to contemplate any specific design. But you could also adopt the old Salthill model and work with remote bulbs and
trimmer filters, then distribute the light to the mixing chamber using fiber-optic bundles, plus some general diffusion. I built my colorheads back
when the suitable computer controls were not on the horizon yet, so ended up with a bunch of tempermental triacs etc typical of the feedback
circuitry of the era, which is progressively sensitive to EMI as the number of channels increases. That can be solved with sinewave computer
control, but then you've got a whole new paradigm of expense and potential rapid obsolescence. So it's really the electronic end of this that
someone needs to rethink, and not just the mechanical logistics.

DREW WILEY · Jun 28, 2013

Prof Pixel - xenon flashtubes are nothing new. They're incredibly inefficient from a heat/light standpoint; so to get enough muscle to punch
large color prints using deep separation filters, the energy has to be pretty well dammed up before the burst. I once had a fairly strong
xenon flashtube spectrophotometer in use here at the company, and those capacitors would cut loose pretty hard. One time this not only
knocked out every internal barrier, but every intline surge protector on the other side. In effect, every dam broke and the EMI echo ended up
ruining an entire room full of office computers on a sidebranch circuit. That's why most industrial spectrophotometers now used interpolated
readings at maybe a dozen set points, rather than calculating off the full spectrum. A lot less energy is required. The powerful old xenon additive enlargers were of course sequential, because they were just intended for color separation work on registered easels, and not for typical color printing.

Prof_Pixel · Jun 28, 2013

DREW WILEY said:
Prof Pixel - xenon flashtubes are nothing new. They're incredibly inefficient from a heat/light standpoint; so to get enough muscle to punch large color prints using deep separation filters, the energy has to be pretty well dammed up before the burst.

Luckily, the RFS film scanners didn't need nearly as much light as you need in an enlarger.

pitagoras · Feb 14, 2014

I became interested in making a led based additive RGB head. Anyone did some experiments?

DREW WILEY · Feb 14, 2014

It's really premature to build a true color enlarger this way because the correct bulbs are just not available yet. You'd have to equip every one
of them with appropriate dichroic "trimmer" filters to cut off unwanted bandwidth, and that would get pretty damn expensive as well as be
a serious engineering problem. But it might be an appropriate era to do mock ups and figure out your electronic controls, or build a semi-color
unit as a prototype, suitable for printing VC papers. That seems realistic.

kuparikettu · Feb 15, 2014

I'm about to try to build one with the following LED parts (got them already):
http://www.ebay.com/itm/10PCS-3W-RE...981?pt=LH_DefaultDomain_0&hash=item3a863d23d5
http://www.ebay.com/itm/3W-High-Pow...Domain_0&var=650098090345&hash=item58a08353f2 (green)
http://www.ebay.com/itm/12pcs-3W-Ro...435?pt=LH_DefaultDomain_0&hash=item416ec7f533

The color head will include a control box with microcontroller to enable exposures of different lengths for different colors and of saving the exposure settings.

L Gebhardt · Feb 15, 2014

kuparikettu said:
I'm about to try to build one with the following LED parts (got them already):
http://www.ebay.com/itm/10PCS-3W-RE...981?pt=LH_DefaultDomain_0&hash=item3a863d23d5
http://www.ebay.com/itm/3W-High-Pow...Domain_0&var=650098090345&hash=item58a08353f2 (green)
http://www.ebay.com/itm/12pcs-3W-Ro...435?pt=LH_DefaultDomain_0&hash=item416ec7f533

The color head will include a control box with microcontroller to enable exposures of different lengths for different colors and of saving the exposure settings.

Take a look at the variable contrast head I built (link in my signature), which is based off of Polyglot's fstop timer. It wouldn't take much to convert the code to handle color. It's arduino based. Keep us informed as to how it goes. I'm also interested in the quality of the LEDs you bought. They are about 15% of the price of the CREE units I used, so they are both tempting and worrying.

LED RGB Additive Enlarger Head - Building One?

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