I've tried to overcome this problem with things like LED spotlights, but quickly come to realise that they cast too many shadows and have intense reflections. What's needed is a distributed, even light source. Instead of reinventing the wheel I looked at professional photography solutions like soft boxes and light tents, and although they work well, they're rather bulky and cumbersome. It'd also be nice if I could use the lights for general electronics work as well. So I looked at what's recently become available due to the advancement of LED technology.
I came across these cheap ($30, AUD) LED panel lights from China and thought it was worth buying one to test out ideas. The one I bought is 200 mm x 200 mm and is only 14 mm thick. The construction is similar to how the back light in an LCD TV works, but we'll get to that later. I didn't have a way to mount the light for testing but as luck would have the edges are only a few mm thick and it fits perfectly into my Panavise PCB holder.
|LED Light Panel 200 mm x 200 mm in Operation|
The label on the back of the light says it's rated for 85 - 265 V AC which is pure B.S. The driver module that comes with the light will take that but the light itself takes around 60 Volts DC. I could understand if the two were integrated, but the driver unit is connected via a removable barrel jack, so the light should be labelled separately. The colour temperature isn't what I'd normally go for, it's way too orange, but for testing it'll be fine. The light is rated for 18 watt and will apparently output 1600 Lumen. I have to take them at their word on this as I have no way to test it, but for comparison those number aren't too different from the LED fluorescent tubes I have.
|Light Panel Back - 18 Watt - 3500 K - 1600 Lumen|
Teardown time. removing the back panel shows what I expected, a panel of optical material bordered by a strip of surface mount LEDs.
|Light Panel Interior|
The back panel has four foam blocks to keep the layers of optical material sandwiched together.
|Back Panel with Foam Compressors|
I mentioned before that the way the panel is lit is similar to how an LCD TV back light works. I'd wouldn't be surprised if the manufactures of these devices are use the same materials because of low cost due to volume. For something so effective it really is quite low tech. LEDs border the edge of a clear sheet of plastic and shoot light into the panel. The light bounces around in the clear sheet and leaks though the diffuser in a controlled manner to give an even light. There's a bit more to it than that, but that's the gist of it.
The image below show the construction of the optical materials. The thin white layer at the bottom is the reflector panel that's patterned to help the light escape the light guide. The clear thick section above it is the material that transports the light. The off white section above that is the diffuser panel and is also the outer face of the light.
|Plastic Reflector, Diffuser and Optical Waveguide|
The diffuser has the same look and rough texture as a milk bottle has near the handle.
The power is supplied to the LED strips by couple of wires that are soldered directly to pads on them. You can see in this image that the small amount of tension applied to the wires is causing the strips to come away from the frame. This is a problem, as the frame is also the heat sink for the device and poor bonding will lead to the LEDs getting hotter than they should, causing premature failure. I'd already made the decision to drive the panel to only 80% of its rated capacity, I think that was a good call.
|Power Connection to LED Strip|
The LED strips form a loop around the border and consist of two strips containing 45 type 2835 surface mount LEDs each for a total of ninety 200 mW LEDs. It's a very peculiar arrangement that I can't seem to find any information on anywhere. Looking at the strips it can be seen that the LEDs are in groups of five, and we know that each strip draws around 30 Volts (60 volts divided by two strips in series). This means that there's 30 Volts across 5 LEDs, giving a forward voltage of 6 volts, which is way too high for white LED. After a bit of searching I've been able to find dual junction LEDs that are basically two LEDs in series in one package. I assume this is done to get a higher power rating without needing a higher current, as this would increase the size of internal connections.
|LED Strip Specifications|
The two strips are soldered together on the other side of the panel.
|Join of the two LED Strips|
I hooked the panel up to my lab power supply and took some current and voltage readings at different operational points. This is a bit new for me, I've embedded the graph from a Google sheet into the page so you should be able to hover over the each data point and the see the accompanying details. Disconcertingly there seems to be a couple discontinuities in the graph. I have a feeling this is due to either the multimeter measuring the current or voltage automatically switching ranges. (I need better equipment)
For the hell of it I used my phone to measure illuminance. I'm not sure why, I think I just needed to know that there was a relationship. You can't actually read anything into this data There's no guarantee the sensor in the phone is linear and my test set-up wasn't very rigorous.
The drive module supplied with the device seemed flimsy and I wouldn't use it if you paid me to. First of all, it apparently only outputs 45 volts, but it managed to get to 59 Volts in the one test I did with it connected to the panel. So I'm not sure what's happening there.
|LED Driver Module|
The internal construction is average. The wires are barely soldered to the PCB, and the only way to get the wires out on the mains side of the box cause the active and neutral wires to be switched around according to the markings on the case.
|Top Side of LED Driver PCB|
There seems to be an attempt at isolating the mains from the output, and indeed the multimeter shows no connection, but I'd feel better if there were some isolation slots, and as for the feedback path, I couldn't see any suitably rated opto-couplers. It may be marginally fine if used in a roof space, but if I'm touching the thing I don't want the output connected to mains making the frame live. I like to live dangerously, but not stupidly.
|Bottom Side of LED Driver PCB|
So now down to business. What's it like at actually illuminating things? The image below is a quick test using a PCB from an old modem, and I'm quite happy with it. It looks pretty good and has an even illumination. Not withstanding the colour temperature and the camera quality I think this could work. A couple of panels lighting the scene from different directions would work quite well. The other advantage is that the exposure time for this image was less than a tenth of my usual photography.
|PCB Lighting Test|
As a final test I thought I'd see what it would look like as light table. Maybe you're trying to track down where an internal trace goes, who knows, but I think it works well allowing you to see features you'd not normally see. For instance, in the top left you can see a trace between two vias that connect two traces on the other side of the board.
|PCB Light Table Test|
I'm certainly going to do a bit more research on these products, I think they show promise and will allow me to create better content.