[If you're reading this during the UK lockdown and have no access to tools or facilities note that it's entirely possible to buy the LED sets and just stick them inside any old box - no need for fancy stuff!]
There's nothing like necessity for driving projects. Both my UV exposure units are on semi-permanent loan to UCA. They spend most of the time in the finishing room where everyone can use them and I only bring one back home when I need it - only now, of course I can't! They are languishing in the UCA Photo dept, left there in the lockdown.
I want to make use of some of this time to do some serious testing of the carbon print process, particularly working out a useable contrast curve for digital negs. While it may seem odd in these days of glorious sunshine, testing processes like this requires an artificial, and therefore 100% consistent light source. - I need a UV box!
While mine use fluorescent tubes from home sun-tan units I've seen people online who've made them from long strips of UV LEDS. These are cheap and generate almost no heat. I've toyed with building one as an experiment for a while but now necessity means I'm finally getting round to it:
The light source.
Accepted wisdom online (see links to sources at the end of this article) says the LED strips to use must be
SMD 5050 type, 395-405nm wavelength. (Don't use the lower-powered SMD 2835 type) I bought two rolls of 5 metres with UK 240v power supplies. (NB: you don't need the versions with waterproof outdoor-spec LEDs or remote controls (!?) These cost me £16.99 each from China via Ebay.
Layout.
The strips are 10mm wide and carry 300 LEDs in each 5 metre length. The strips can be cut with scissors at every three LEDs at marked points where there are solder pads. Mine (possibly because they were cheap?) were made up from ten 30 LED / 500mm lengths, soldered together. As this was just the length I was going to use anyway it was easy to un-solder them at these points.
Twenty lengths of LEDs spaced 10mm apart means a light panel 500mm x 390mm. (around 20" x 16") This is a good size, roughly equivalent to my current ones. You could, of course build a smaller, high-intensity version by putting the strips closer together.
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| closeup of LED strips. Arrows show the cut points. |
Each strip is backed with double-sided tape. They should stick to most surfaces and as they don't get hot you can use things like plastic or cardboard boxes to support them. I wanted to make something a bit tougher (it'll probably end up at UCA like the others!) so I used Dibond. This is a sandwich of thin aluminium sheets with a plastic centre. Very flat and rigid, with the advantage of being reflective (every bit of brightness helps). I was slightly concerned the aluminium would short-circuit the LEDs but the sticky backing seems to insulate it OK. I added extra strips of electrician's tape at the ends to be on the safe side.
Power and current distribution.
LEDS are terrific things but they are a bit fussy about the way power is supplied to them. They need the right voltage of course but also a carefully controlled amount of current. Unlike old-fashioned lightbulbs, unless the power is regulated, LEDs will consume more current than is healthy for them and burn out. They therefore need the correct power supply for the application. This is why it makes sense to buy a matched power supply at the same time.
The strips are supplied with pre-soldered wires on both ends and a plug to connect to the power supply. While they do work with just one end connected, the resistance of 5 metres of strip means that the ones furthest from the supply are noticeably dimmer than the nearest: - Not good for making an even light source. The answer is to wire the strips in parallel like this:
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| one set of ten strips of 30 x LEDs wired to power supply |
It's a bit of a fiddle to solder lots of little jumper wires but it means that the current is much more evenly distributed.
As I was running two sets from two power supplies I arranged them so the second set was powered from the opposite end of the board. This ensures that even if there is a little drop-off in brightness it's evened out. Here's the first set of ten 500mm strips (total 5 metres) installed:
- And the second. It is possible to run just one set of lights, giving half-power but why you'd want to I don't know. - also the lighting is unlikely to be as even with wider spacing. I offset the second set a little to get the most even pattern of lights possible.
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| the finished array. See below for the schematic of the wiring. |
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the second set of ten strips of 30 x LEDs and power supply interlaced with the first.
Note no electrical connection between the two sets! |
- and it works! LEDs are hard to photograph as digital sensors seem to react oddly to them. In reality they look almost disappointingly low-powered but that's hopefully because they are mainly emitting invisible UV light.
The only way to assess the brightness is by testing. I propped the panel up on some scrap wood, made a couple of pieces of cyanotype paper and ran some quick test strips. Maximum density is achieved at around 5-6 minutes with the paper 120mm away. This is much the same as the fluorescent UV units.
Wiring and safety.
The LED power supplies come with standard 'kettle lead' wires with three-pin plugs. The problem with these is that the plugs have no fuses, which I think might actually be illegal in the UK. - At any rate it's a bad idea. The fuse is to protect you in case of an electrical fault in the power supply box, cutting the power before the thing catches fire or electrocutes you. For this reason I'm ditching the supplied cables and fitting proper leads with fused plugs. - I should have checked before I bought the cheapie sets!
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'kettle lead' fitting and un-fused 3-pin plug
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Making the box.
The panel can be used any way convenient. Gavin Lyons (see link below) put his into a plastic crate which is a super-easy and quick way to house it. I've built a simple box from scrap wood with a hinged lid and an opening flap on the front:
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| The finished box. the flap at the front hinges down and the top opens too. Note I'm running the lights off an enlarger timer. |
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| I like the way it looks like a Las Vegas casino in there! |
Verdict.
The LED option does require a bit of electrical expertise, soldering wires and ensuring the power supplies are connected safely. However, if you are confident in tackling that part, there are several benefits over the fluorescent tube tanning lamps: The light panel is compact, weighs very little and generates negligible heat. It uses very little power, can be left on for long periods and almost any size of panel can be made. I'm happy with this set. - I could add two more sets of LEDs and double the power - though I'll look for fused plugs next time.
Links:
Gavin Lyons excellent video on making a very similar lightbox using a plastic crate:
Large Format Photography forum article by Eric Biggerstaff:
Greg Brophy: a photographer in the US who has made his own UV box:
