300W UV LED floodlight experience for alt. process printing

[koraks]

Lately, UV floodlights have emerged on the market, and they appear to be quite suitable for alt. process printing. Not my original idea; to be frank, I read about them on here I think. Apparently having nothing better to do at some point, I ordered one and gave it a go. I wrote two small articles about my findings so far.

This one is about a test print I made to put the LED unit side by side with the bank of UV tubes I've been using for the past few years: https://tinker.koraks.nl/photography/uv-tube-vs-led-a-quick-comparison/
And here's one investigating the intestines of the 300W unit I purchased: https://tinker.koraks.nl/photography/what-a-300w-uv-floodlight-is-not/

A brief summary:
* My unit consists of an array of SMD LEDs and is not a COB led based design. I got it from AliExpress. I think the ones I saw before (on Amazon?) were COB-based.
* Mine was advertised as 300W, but turns out to be....75 Watts. Apparently, the Chinese have different rules of arithmetic or something, or perhaps Ohm's Law changes as you move east.
* Print contrast and detail are pretty good. For the small amount of power it packs, it performs OK.
* The thing gets hot, so good thing it doesn't actually live up to its advertised rating.
* It's not a massive improvement over the bank of UV tubes I already have, and in some ways even the opposite. On a power-per-surface-area scale, the UV tubes actually perform (much!!) better. The UV thingy exposes about 200cm2 at short range with good evenness, while my UV bank easily covers 2000cm2. The UV tubes use 150W, while the LED unit consumes 75W, while offering at best half a stop more speed at this distance compared to the tubes.

It's hard to beat the price and simplicity of a LED unit like this one, but it does not quite live up to its expectations IMO. Notwithstanding, I'm going to put 4 of them in an array and see if that works well for 8x10". The one thing I do like is that the light is more collimated and hence, there's less blooming around negative edges etc. Also negative-print contact should be slightly less critical.

I would expect COB-led based floodlights to better live up to their specifications; in my experience, they generally dissipate around 75% of their rated power or thereabouts, so a 100W unit tends to be 75W in reality. That's still way better than a 300W unit that actually only does 75W.

 

[Ethan Brossard]

Could the 300 watt rating be meant to indicate an equivalance with 300 watt tradional bulbs, but got missed in translation? LEDs usually consume about 25% the power of incandescent bulbs with the same output, so that would match with 75 watts vs 300 watts.

Out of curiosity, how did you determine the 75% output number? That's around what I've heard for white LED bulbs, but I was under the impression UV LEDs are generally much less efficient (though still considerably more so than tradidional black light bulbs)

 

[koraks]

Could the 300 watt rating be meant to indicate an equivalance with 300 watt tradional bulbs, but got missed in translation?

Yes, that's what I also thought. It's actually the only somewhat reasonable explanation I can think of. Btw, dozens of sellers on AliExpress offer the same item with the same 300W spec. At least they're consistent in being optimistic!

Out of curiosity, how did you determine the 75% output number?

It's just a rough ballpark estimate; take for instance a typical 100W RGB COB LED (as you know from the other thread, I had a close look at those at some point). The ad will say '100W', but now look at the actual maximum current and forward voltage for each individual array (R, G and B). Add it all up and see how it pans out; you'll probably end up somewhere around 75-80W. They round it all up to 100W because it's a nice a round number. The situation may very well be different for white LEDs.

I was under the impression UV LEDs are generally much less efficient

Yeah, OK, but I think there's a small misunderstanding here. What I meant is if you buy a 100W (COB) LED, then in reality, its actual power dissipation at its max. permissible current may only be 75W or so in the real world, even though it's sold as a 100W unit. This says nothing yet about luminous efficiency!
Talking about efficiency, I think it's particularly important to note that all UV is not created equal, and efficiencies drop steeply once you get below 390nm or so.
Also, keep in mind that white LEDs are today nearly always blue LEDs with a few phosphors coated onto them that convert some of the blue light into red and green light. The quality of those phosphors is crucial in determining things like spectrum and thus CRI, but they will evidently also have influence on overall efficiency. Whether the average UV led of 400nm (to take an example) is more or less efficient in terms of emitted electromagnetic radiation, I don't know. Measurements will play a role here as well; if you look at luminous efficiency, a UV LED will always use because lumens by definition take into account (heavily!) the sensitivity of the human eye, and thus anything that emits only UV will always score ridiculously low on luminous efficiency tests. It says very little though about how well the device converts electrical current into UV radiation as opposed to heat.