The Big Ugly, or building a LED color printing light source. Part 1: the beginnings

Previously, I bashed compound/integrated RGB LEDs such as RGB COB LEDs for the application of a color enlarger for RA4 prints. That was basically a lesson learned the hard way when building a LED-based color enlarger light source. In this series, because one post is probably not going to be enough, I’ll highlight some aspects of the system I’ve built. Or I should say: I’ve built so far, because it’s really just a prototype that will likely never be really finished. Hence the title, The Big Ugly. Because there’s one thing it isn’t, and that’s beautiful. But hey, it works! Sort of. Most of the time. Within reasonable limits. If I’m careful.

This post is mostly going to be about things like scope, starting points and context. It’s not going to be very technical, yet, and mostly serves to explain what you can and cannot expect from the remainder of the series. If you want to skip ahead to the dirty pictures, feel free to do so, but I feel that this post helps you see that information in its proper context. If you can spare the time, skim through it for a bit, at least.

Let’s start with the caveats, and we’ve touched on the first one already: I have never engineered and built a market-ripe system that looks decent or that even is constructed in a decent way mechanically. Mechanics are not my forte to begin with; frankly, I usually get very, very bored and frustrated whenever I need to actually box up an electronics project in a decently looking or durable way. I cut so many corners on this system that I can’t even begin to point them out; most of the construction is actually unpainted MDF held together with glue, or not even that – some tape and a quick prayer.

Alright, another one: I’m not a trained electrical engineer. Despite having spent a few weeks studying electrical engineering at university, whatever I know I learned through hobbies, tinkering, listening to my father (also an autodidact), reading lots online, studying other people’s schematics, doing simulations and of course building something from time to time. As they say, I know just enough to be dangerous, and that’s it.

All good things come in threes, so let’s extend that to the bad ones as well: to top it off, I’m not going to pretend that the contraption I built actually works. Alright, that sounds odd, because it does work, and I make prints with it all the time, which look good enough to me. But I’m pretty sure that if you were to torture test this thing using a decent mix of systematic testing and color densitometry, I’m pretty sure you’ll hit some rather fundamental problems. I didn’t do this (another caveat) and instead rely on the odd color checker chart and just making lots of prints and examining those for apparent problems. And in that informal way, I arrived at something that works for me, at this point in time.

So, in short, all I’ve got is a sort-of-working prototype of a concept that may or may not work, but that gets the job done that I ask of it. And the lessons I learned building this thing. The value of these writings will mostly be in making you aware of design considerations and issues you may run into when doing this yourself (or bribing someone to do it for you). By now, you probably have realized that this is not going to be a ‘how to build your own color enlarger’ guide.

Still here? Hm, you’re the tenacious type. That’s good; only those stand a chance with this to begin with!

I’d also like to explain a little about how this project came to be. It started when I was generously gifted an old and somewhat rusty, but entirely functional Durst 138 enlarger. If you’re not familiar with this: it’s probably something from the early space age, and as enlargers go, these were built to apparently survive a trip to the Moon and back again. Without any parachutes for a soft landing, either. It’s a 5×7″ enlarger of the condensor type, and highly configurable due to the presence of zillions of accessories, options etc. The kit I got was very basic: just the chassis with a baseboard and the frosted light bulb head, and two 240-type condensors. I somehow also ran into an Ilford 500 head to do variable contrast printing with it, rooted around until I found a complete set of condensors for this enlarger, acquired some lenses here and there and in the end I had a kit that worked very nicely for enlarging B&W negatives from 35mm up to 4×5″. I don’t do 5×7″, but I suppose I could have done that as well. I just never tried.

There’s one thing I could not do with it, which was color. And it so happens I acquired a taste for RA4 printing a few years back, so that was a bit of a bummer. I did have one or two additional Durst enlargers for smaller formats; an M305 and an M605, both of which are very nice pieces of kit, but with the evident drawback that the 305 will only do 35mm, and the 605 will only go up to 6x6cm. And I wanted to be able to do 4×5″ in color as well. Not that I had or have all that much 4×5″ color film…Besides, I’m not the kind of guy who likes to keep a handful enlargers round the place, because…you see, I really don’t know why people would do that, unless you’re doing fancy stuff like multiple exposures etc. And I don’t, or very rarely, do that. Having one enlarger to do all my work with appealed to me.

I just couldn’t find a suitable color head for it.

Yes, there’s the Durst CLS heads in various power levels that would nicely suit this enlarger. I even tracked one down, second hand, which some guy from Germany would bring over at some point. Which never materialized…After searching (not very hard, but at least monitoring some channels pretty consistently) for a year or so, I decided that if I wanted to get this show on the road, I might as well do it myself.

Plan A was to take a smaller dichroic head and just fit it to this chassis. A friend of mine did pretty much the same thing, I think with a Beseler medium format dichroic head, which worked OK for him. But like I said, not being one of the mechanically inclined, it wasn’t the most obvious thing to do for me (you might opine differently and be quite right at it!) Plus, I don’t like waste. I didn’t see any color heads with the rest of a pretty much complete and functional enlarger attached to it, and apart from the higher cost of a functioning enlarger compared to a loose color head, I didn’t like scrapping a chassis just because I needed the head.

Long story short: if you can’t find it, build it. So I started thinking, with the unabating assistance of Google. This turned up a couple of promising things. The most important source was this series of posts from a guy called Larry on building a LED light source for B&W printing. Evidently far more mechanically gifted than I am, there were also parallels. The most important one: he also used a Durst 138. And purely by showing that his project could be done, he convinced me that I could do something similar. He showed in a compelling way that the Durst 138 is fairly easy to tinker with. It has room to spare in crucial places, essentially. And he showed that LEDs create light that is more than capable enough of exposing silver halides. Not that you could reasonably doubt this, but it helps if someone pushes your face against the glass from time to time.

There was also a forum post by some guy (sorry, don’t recall the details) who used an RGB COB LED module for more or less the same end. This was also for B&W printing and he used the red channel only for focusing and composition. But…what if we actually used the red channel along with the blue and the green to mix color? Wouldn’t that be essentially the same thing the commercial RA4 processing machines do, with their semiconductor lasers?

Anyway, the idea was born, it got stuck in my head, and it translated into a set of objectives or informal design principles for this project:

  • The device shall expose color RA4 paper as well as variable contrast B&W paper with the same light source.
  • It will cover all formats from 35mm up to 4×5″, so this one device will suit all my enlarging needs (I did not do 8×10″ at the time, and still only shoot it for contact printing).
  • The device will fit a Durst 138 chassis, which should moreover remain in its original state as much as possible so it can easily reverted back to its original state.
  • It should be no more difficult to use than a dichroic filter head or the Ilford 500 head I used for B&W. In fact, the device should leverage the easy access consumers like me have to microcontroller technology to make life easier in the darkroom.
  • The components should be easy to obtain and of low to reasonable cost. No exotic unobtainium parts that are special order from the forges of Hephaistos at the cost of a kidney and a firstborn. Otherwise I might have just gotten a nice Heiland LED head with accompanying controller and called it good. (Did those go down in price over the last few years, by the way? I remember distinctly higher prices?)

So that was the beginning of it all. A color light source for a Durst 138 chassis, using cheap(ish) and easy to get parts, so I could do 35mm up to 4×5″ on the same machine in B&W as well as color. Simple, right? Let’s have a look at how that panned out for me in the next installments.

Read the other parts here:

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