UV tube vs led: a quick comparison

I already announced in my previous journal entry, but it arrived even quicker than I anticipated: the 300W UV led source I ordered from China. Although it was also laundry-and-house-cleaning day, I couldn’t resist giving it a go. And in particular, seeing how it would pitch against the bank of UV tubes I’ve had around for years. Here are my expectations when I ordered it: (1) higher total luminous flux, so shorter exposures, (2) better collimation, so less bleed and better detail rendering. How does it pan out, you think?

Well, I was sort of right. Sort of. Let’s start with a little inconvenient truth: stuff you order from China is sold with rather optimistic specifications. I know this, of course. I have ordered thousands of Euro’s worth of stuff from China, and I sort of know what to expect. Yet, I wasn’t quite prepared for this degree of…well, optimism, let’s put it this way, on behalf of Chinese salespeople. These are the relevant specs from the sales ad:

  • Outdoor UV light, IP66
  • Wattage: 300W
  • Wavelength: 395-405nm
  • Angle: 120 degrees

And here’s what the little unit looks like:

Comes with a Euro plug and fancy switch. The exterior construction of the thing is actually quite decent.
Yes, it turns on alright.
Note the lens array in front of the 150 UV LEDs.
These look like typical low-end UV LEDs to me, probably rated for 1W dissipation.

Granted, I did not measure the wavelength. But the 300W thing is just…a lie. I can’t put it any other way. I plugged in the unit and the home power monitor (which overall is dreadfully accurate) shows a bump in power consumption of around 110W. Yep, that’s around 36% of the advertised power. You get what you pay for, I guess! The floodlight is in fact an array of 150 individual LEDs that look like they might be rated for 1W each. Running them conservatively would get us in the 100-125W ballpark. I’m not sure where the 300W rating comes from; this unit doesn’t get anywhere near that kind of power level. Good thing too, because there’s no active cooling or even a proper radiator / heatsink on this thing! Even at its meagre 100W power level, it gets darn hot.

Concerning the angle, I suppose it depends on how you measure it. If you define the angle as the angle in which any light is emitted, then I suppose 120 degrees would be accurate. But it sure is a very skewed profile, with a hot spot in the center and rapid falloff to the edges. It’s going to take a lot of distance to have this cover any meaningful size, like 8×10″.

Fancy colors. This is the unit held at something like 30cm/1ft from a white wall. Note the hot spot and rapid falloff.

I can whine and complain all day long, but all of that is meaningless in the absence of at least one real world test of what this thing actually does. To get a feel for how/if it works, I decided to make a small 4×5″ salt print on my old light source and one on the LED floodlight, both with the same exposure time and at the same distance between the light source surface and the print:

Old light source doing its thing; distance between the UV tubes and the print happens to be around 14cm.
The LED contraption can do it too! Same distance (perspective on the photo is somewhat misleading).

Exposure time for the print was 12 minutes; this is within the typical range of a salt print as I make them. Here’s how that panned out:

4×5″ salt prints; LED floodlight to the left, UV tubes to the right. Raw scan, both at the same time, with small corrections to approximate the real-life impression of the actual prints.

At first glance, I’d say the LED light is actually about a third to half a stop faster under these conditions. I didn’t measure it exactly, but the difference is actually not all that big. Which in effect is disappointing, because the bank of UV tubes I’ve been using consists of 8 Philips Actinic BL tubes rated at 18W each. The home energy monitor indeed shows around 150W power consumption when turned on. This makes the 100W LED unit rather disappointing, because the tube bank covers an area of around 50x45cm at close distance, whereas the LED floodlight will likely struggle to cover 20x25cm (8×10″) at this distance – in all likelihood with noticeable falloff towards the corners. Given the small difference in print speed, I just get much more bang for my UV tube Watts than with the LED bank. At least with salt prints!

Let’s clean that scan up a bit, while trying to keep them close to the real thing. In this digital side by side comparison, the difference in exposure is actually exaggerated. It’s by far not this distinct in the real, dry prints. It was a little more pronounced when still wet.

Let’s have a closer look at these prints though, because there are some good things about the LED source. For instance, the corners tell a promising story. The light indeed is more collimated, and this is evidenced by a lot less blooming around the edges of the print compared to the UV tubes:

Top left corner; LED source is on top. Note the sharper rendering of the notch with the LEDs and less blooming.
Bottom left corner; LED to the left, UV tubes to the right. Clearly visible is the sharper edge of the negative and less blooming around the negative’s edges with the LED source.

The immediate reason for giving these LEDs a go was actually the blooming issue I pointed out earlier on my carbon transfers. So in that respect, there’s something going on here that might be usable. In terms of rendering of details, the LEDs have the edge too. It doesn’t come across very clearly in the digital scan, but inspection of the prints with a loupe does show somewhat better depiction of small details. The overall impression of the naked eye is also slightly better overall clarity in the LED exposed print. It’s a bit of a futile attempt because especially on the digital version, the difference is rather subtle, but here’s the same area of the print put side by side:

LED to the left, UV tubes to the right.

Finally, there is an obvious difference in hue to these two prints, with the LED print being more purple. However, I don’t think it’s a meaningful difference. I coated both sheets at the same time, then exposed the UV tube print first, followed by the LED print. The LED print therefore had some time to adjust its internal moisture (both were initially dried with a hairdryer). I made some other prints that showed the same difference in speed between both light sources, but no meaningful difference in hue. Running a clothes iron at high heat over both prints will probably bring them pretty close to each other, neutralize the tone a bit and increase dmax somewhat. But I didn’t bother for this comparison.

So, what’s next? I’ll probably try an 8×10″ salt print to see how the coverage of the little LED source pans out. And then maybe do a carbon print or two to see how it behaves with that process. LEDs are evidently promising in this application, as many before have already found out, but this particular floodlight unit is not something I’m very likely to keep using for long. It’ll probably mostly serve to work out the design specs for a decent UV source, and making the choice between for instance COB LEDs or bead/SMD LEDs.

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