Kind of blue – A test with UV LEDs for DAS carbon transfer

So the 400nm LEDs I had been using since October or so worked really nice for dichromate carbon transfer, but I ran into trouble with DAS carbon with them. Some further testing with my bank of trusted Philips BL tubes and a little theoretical exercise suggested that at least some 365nm exposure is needed to get DAS carbon to harden reliably especially in the highlights. So I’ve been testing a bit with 365nm LED options – two in particular that seemed attractive. This is a small report on these tests.

First lets get the obvious option out of the way. I used cheap 400nm LED floodlights previously, so why not do the same for 365nm? I simply didn’t find any product to my liking. Whatever I found was either underpowered or overpriced, basically. I want something in the range of 100W for a unit that exposes 4×5″, so around 300-400W for an 8×10″ unit. I know this is the kind of power level that’s convenient in terms of exposure times with carbon transfer. And evidently, I wanted this in particular to be cheap, since I’m running a test initially to see how the 365 LEDs work in the first place. Yes, the UV tubes worked OK, but the 400nm LEDs didn’t work so well with DAS carbon, so I wasn’t feeling like wasting a lot of money on a LED option that may or may not work. So I was left with no other option but to DIY something.

I make no secret of my preference for cheap electronics components from AliExpress. So that’s where I headed in search of some 365nm LEDs. The one thing not so nice about shopping there for components is that the search function is kind of abysmal – you either get nothing, or you’re swamped with options. And most of the time it’s a bit of both: some relevant ‘hits’ among a massive number of totally different stuff that I’m not sure how they manage to match it to my search terms. Ah well.

In terms of 365nm LEDs, there’s a few options to choose from – but after whittling away the chaff, they boil down to a couple of options (but generally available from a multitude of suppliers). Cost is of course always a factor, so I decided to figure out how I could get the most bang (defined as power in Watts) for my buck (Euro, actually). After some copy-pasting, I could make a plot like this:

The vertical axis shows the effective price per Watt based on the nominal power as specified in the ads. The horizontal axis shows the total amount of Watts you purchase in one go within an ad. So for instance, if an ad offers 100 LEDs of 3W each and the total cost of this quantity would be around €170, the total power purchased would be 100 x 3W = 300W at an effective price of around €0.59 per Watt.

Furthermore, there are some colors in there, denoting the categories of LED elements I came across. I excluded large assemblies like floodlights and LED strips – mostly because they are barely offered, and in particular strips don’t generally offer that much bang for your buck at this wavelength, currently. They’re also somewhat inefficient, but that’s a personal gripe I have. I did include a couple of strip-like assemblies, but didn’t end up choosing them since they’re not very economical anyway.

It’s also pretty clear that the price per Watt is all over the place, but trends between roughly €0.20 and € 1.00 per Watt, with some exceptions in both directions.

The grey dots denote high-power COB LEDs, which technically are quite interesting since they are close to a point source. Calvin Grier uses these in his fancy dual wavelength exposure units, together with lenses to focus the light so he can get a nicely collimated beam over a long distance. You’ll also note that the COB LEDs are on the expensive end of the spectrum, all of them being over €0.55 per Watt. COB LEDs come in various sizes and power ratings, ranging roughly from 5W up to 100W for UV units – and even beyond, for regular colors and white.

One step down in terms of power concentration are beads, which come in sizes of 1W through 5W these days. Currently, for pure 365nm wavelengths, these are an attractively priced option. I got a bag of 100pcs 3W ones and these particular ones came down to around €0.23 per Watt.

Then, there’s a whole slew of smaller SMD devices, ranging roughly from a 3535 form factor up to 6565 or so, with 3535 devices being the more common and affordable ones. Many of these turned out to be significantly more costly than the beads – except for one type that piqued my interest in particular.

Turns out the cheapest option were 3535SMD LEDs that are dual-wavelength. There are actually two LED emitters within each device: one emitting at 400nm, and another one emitting at 365nm. Apparently, this combination is used for hardening UV nail polish and these LEDs see wide usage in nail studios as a result. Well, I don’t care much about what they’re intended for, but they sounded like an interesting option.

When I was doing initial tests with DAS carbon, one of the things I observed was that I could get decent transfers if I exposed a tissue using two light sources, dividing the exposure time between both of them. That way I determined that e.g. a 10 minute exposure, I could split up into 8 minutes under the cheap & cheerful 400nm LED floodlights and 2 minutes under the bank of UV tubes. This seemed to be the sweet spot in terms of getting decent contrast (using the 400nm LEDs) and sufficient hardening in delicate highlights (with the tubes). Is it possible that I stumbled upon a type of LED that essentially packages this in a ready-made solution? Interesting! Oh yeah, these little 3535SMD dual wavelength thingies worked out at a little less than €0.10 per Watt. Very interesting, indeed.

So I ended up buying some 3W pure 365nm UV beads as well as a bag of ridiculously cheap 1W dual-wavelength 3535 SMD LEDs. Well, very nice, but now this needed to be turned into an actual light source, so I also got myself a 24V power supply and some inductors, resistors and whathaveyou so I could get going. I still had a few pieces of aluminum-core PCB material lying around, so I went ahead and made a PCB for both types of LEDs.

Here’s some snaps of the 3535 version that I took as I was building it:

Then I had to do something similar for the beads. I decided to make them pin-compatible and fit for use with the exact same driver, so I only needed to make a single driver unit for A/B testing.

Both units dissipate a little over 100W. The 3535 board uses 96 LEDs to achieve this, while the beads are more powerful, so I only needed 48 of these and could run them significantly below their maximum rating for the same output power. On the 3535 board, I use strings of 6 LEDs each with two strings (so 12 LEDs total) being put in parallel for a single channel, for a total of 8 channels. The beads are not paralleled and are arranged in 8 channels of 6 LEDs each. Each channel is thus driven with around 500mA of current, or 12W at close to 24V.

Now for the driver. I actually wrote about making that PCB, because it was subject of a previous blog. Now you know what that was all about! The driver board is based on a couple of MP24894 LED drivers – the same ones I use in my color enlarger (I had a few leftovers anyway).

The driver board takes a 24V input from a switching power supply. The MP24894 are buck topology drivers, that use inductors (mounted to the bottom of the PCB) to step down the 24V to whatever the LEDs consume at the current set by a series sense resistor. They’re efficient and when implemented correctly, they are quite stable and reliable. I ended up with these in my enlarger project because they are one of the few types that combine a low price with good documentation and a voltage rating up to 60V, making them fit for fairly high-powered designs.

Finally, I had to somehow mechanically bolt all this together in a usable form. I’m more of an electronics guy than a mechanics guy, so I went for something I could actually wrap my peanut brain around and used some scrap wood, screws, tie wraps and other unfortunate objects lying about the place. Here’s the contraption I built:

And now, the million dollar question…does it work?

Yes, yes it does. And that’s not a surprise, because from experience with the previous two exposure systems, I had a pretty good idea of the kind of power range to aim for. But there was a surprise in there nonetheless.

My expectation was that the 365nm beads would offer superior performance, especially in terms of exposure speed. After all, DAS carbon is significantly more sensitive to 365nm wavelength light than to 400nm wavelength:

Well, my expectation proved to be wrong. It turns out that the pure 365nm wavelength light source is about 1.5 stops slower than the 400nm + 365nm combined light source. That’s right: the LEDs that cost less than half of the 365nm beads offer more than twice the gelatin hardening action in DAS carbon!

How about the whole highlight problem that I set out to fix in the first place? Well, I’ve made 20 or so prints so far, and it seems that the highlight problem is certainly no worse in the cheap dual wavelength LEDs than with the pure 365nm beads. I’d say it’s more or less the same, or perhaps it’s even a little better with the dual wavelength light source, which implies that the amount of 365nm wavelength light in the cheap dual-wavelength option is sufficient to circumvent the issue.

How about some examples? I made a couple of prints with both light sources, aiming at comparable overall densities, so compensating for the difference in speed/efficiency. I used a tissue with the following composition:

	% w/v in glop	% of dry gelatin weight
Gelatin	6.0%	100 %
Sugar	2.0%	33 %
Glycerin 86%	0.25%	4.17 %
DAS sensitizer	0.25%	4.17 %
Kremer Pbk7	0.25%	4.17 %
Kremer Pb15:3	0.005%	0.08 %
Kremer Pr122	0.16%	2.67 %
Kremer Py154	0.06%	1.0 %

The glop is a very high contrast one in terms of pigment load – see in particular the >4% pigment to dry gelatin ratio. Note that the pigments used here are dry pigments, hence the low quantities compared to using e.g. India ink as a ‘pigment’ (which is actually mostly water). For instance, a typical and still fairly high-contrast tissue that I use with dichromate will have a dry pigment to gelatin loading of 2% to 3%, with the latter being a decidedly ‘punchy’ tissue that will print negatives in the range of normal grade 1 negatives or thereabouts. We’ll come back to this issue at the end, but first let’s look at how this pans out with the two different light sources.

First up is the 365nm LED bead light source.

Next is a nearly identical print made with the 400nm + 365nm dual wavelength LEDs. Exposure time was less than half that of the print above to achieve the same overall density.

I haven’t printed any step wedges (yet). For now, I’m mostly concerned with how the light sources ‘feel’ when printing in-camera negatives, which is what I enjoy doing most at this point. Going by this informal test (and a couple dozen more along similar lines not shown here), I note that there’s no big difference in contrast between both light sources, nor does the highlight flaking issue differ all that much between them.

This leaves me with the preliminary conclusion that the dual wavelength LEDs work at least just as well as the pure 365nm ones, but they’re much cheaper and much more efficient / faster. For this reason, I’ve been mostly using the dual wavelength light source over the past week or so in further testing, and it seems to work very well indeed.

Now for that highlight flaking issue, because the prints above aren’t exactly perfect, yet. My suspicion so far is that DAS just has a much higher ‘tonal threshold’ as Calvin Grier calls it. For a brief explanation, see e.g. this pdf on Gum Printing, page 19. The same principle applies to carbon transfer, and my experience so far is that dichromate-hardened tissues exhibit much less fragile highlights, and this pushes down the tonal threshold.

Remember that the tissues used for the tests above are very high-contrast ones: they have a high pigment load. The main way to push down the tonal threshold is to simply ensure that there’s a thicker gelatin layer remaining in the delicate highlights, making them a little less delicate. To obtain the same low optical density, this means the tissue needs to have a lower pigment load. Well, easy enough to test this!

I mixed up some more glop and reduced pigment load, but also increased the DAS load a bit. I figured that a little more DAS wouldn’t hurt in making the exposed gelatin tissue a bit firmer. I also made the tissue a bit thicker – since it has lower contrast, there will have to be a bit more gelatin to render the shadow areas, and I wanted to prevent exposing through the entire tissue, which leads to transfer problems. Here’s the formulation of that lower contrast glop:

	% w/v in glop	% of dry gelatin weight
Gelatin	8.0%	100 %
Sugar	2.5%	31.25 %
Glycerin 86%	0.25%	3.13 %
DAS sensitizer	0.4%	5.0 %
Kremer Pbk7	0.175%	2.19 %

In the following examples, excuse the lack of proper calibration in the scanning process. The important thing here is how the highlights render. Also, you might find the negatives a bit odd in their lack of shadow detail. These are some of the negatives that were victim to my recent light meter failure. I ended up unleashing chromium intensifier onto them, followed by a healthy dose of pyrocat. After multiple rounds of poisoning them, they actually turned out to be…interesting. And they’re a lot nicer to print with than boring step wedges.

In the high-contrast glop, the lighter patches in the leaves would have bitten out in several places. In fact, I also printed this negative with the higher contrast tissue and that’s exactly what happened. In this print, however, the transitions remain continuous. No, I mean, really – look:

In the zoomed-in examples above, the curves are not adjusted to make paper white align with pure monitor/digital white in order to make the highlight gradations more easily visible. As seen, they retain density, which was not possible with the higher contrast tissue.

The same pattern in another print above; note once again the retained detail and density in the leaves, which in reality are extremely close to paper white.

As usual, scans do very little justice to the prints. While the negatives are evidently compromised, the compositions are somewhat gratuitous and the test prints are a bit of the quick & dirty persuasion, these little prints actually have something (in real life) to them that I quite like. Again, for me, this kind of testing beats printing step wedges, although the latter obviously allows for a more objective analysis and proper quantification. Perhaps I’ll put myself to that again one day.

For now, I intend to have some more fun with this new light source. I’ll mix up some more glop and make some more negatives to print with and to further my grip on DAS carbon. It feels like I’m getting closer to what I was able to do previously with dichromate – but with the improved ease of use of DAS-incorporated tissue. The only ‘sacrifice’ I’ll have to make is to really tailor the negatives a bit more to the process. This was a little less necessary with dichromate, but with DAS, there’s just no way around it. DAS just likes those really punchy, ‘classic’ carbon transfer negatives that you can use to armor a tank with.

Oh, and I’m already thinking about scaling up the nail-polish hardener LED light source to 8×10 size. Maybe I should order some more components and get cracking on that.