Backlighting a Stretched Canvas Painting

[Ellis Greensitt]

Heres a good one for you !!

I was contacted last week by an artist in Kidderminster who wishes to modify his artwork (stretched canvas paintings over a wooden frame) to include a option to have them illuminated from behind. He wishes the solution to be self-contained i.e. battery powered, and will just work "out-of-the-box" so to speak. After discussing in detail with him about the feasibility of this, he has done some preliminary testing using mains powered light sources to create the effect he is looking for. He is using some sort of polycarbonate sheet on the rear of the canvas to diffuse the light over the entire rear surface so the position of the light source is not critical. He requires the light source to be turned on/off via a small pull cord drilled through the centre of the frame bottom so that the painting may me turned on/off without removing it from the wall and without disturbing the hang hence the central position for the pull cord. Naturally he would like the solution to be as economical as possible. The intended donor paintings are approx 18-24" square using a frame thickness of 2". The only thing I can come up with so far is a battery powerd 12" mini fluorescent T8 Tube at 8W modified to use a pull cord. Does anyone have any solutions that are financially viable for a painting that may retail for £50-60 ? Can anyone think of a rechargable solution possibly using solar power through the daytime to trickle charge the batteries for use at evenings ?

Help !!!!

[Ian Stirling]

It's almost certain that this is impossible - a similar question was asked over on sci.engr.lighting.

Batteries will at best give weeks of service, if thousands of pounds are spent on them, and even exotica (tritium powered lights) are very dim. If he wants it to run for a few hours, that's possible. Every night for a year, impossible. In very few cases will solar be an option. Only if the painting is in direct sun for several hours a day, and the solar panel attached to it similarly. And even then, if it's cloudy, it's going to go out.

Specifications. How long does it need lit for? How bright? Can the artist get to it to charge/replace batteries? Is it for a sold work? For all but "a couple of hours, dimly", the vastly cheapest, and often only possible answer is going to be a wall-wart.

[BIG NIGE]

LEDs would use less battery power and create less heat, but may cost too much for initial outlay.

[Andy Dingley]

Fit a battery box, and a DC power inlet socket too. If anyone wants to mains power it for permanent use, then they can plug a suitable external wall-wart into it. As this is all LV inside the box, then it also avoids the CE marking issue.

Osram are selling lots of small 6V hand torches with fluorescent tubes these days (Lidl keep having cratefuls). They're only small, but they're very cheap.

[Ian Stirling]

For the same level of illumination, LEDs use about 2-5 times the power of fluorescants. And, getting close to 2 requires inefficient fluorescants, and bleeding edge LEDs.

[Steve Walker]

Good idea - it can be fast-charged between viewings, especially with NIMH cells?

[Dave Plowman (News)]

Not so - they are less efficient than fluorescents for this task.

[Dave Plowman (News)]

I'd go for caravan fittings. You should be able to get those in 18" Depending on spacing, you may need to use a thin sheet of white perspex etc between them and the display to 'soften' the light sources.

IMHO, anything run off dry cells will cost too much to run.

[John Rumm]

If wires are a problem then you could use a inductive loop to connect to the charger. You could even build the charging loop into the wall behind the frame.

[Ian Stirling]

Gonna be orders of magnitude easier just to put a FCU behind the piccy.

[Dave Plowman (News)]

Yup. Unless mains simply isn't available any battery solution is gonna cost. But then it's only stating the obvious. ;-)

[Calvin]

For the lamps you could consider using the cold cathode tubes found in scanners.

I recently did something very similar to that described by the OP although in my case I was happy to power it from a wall wart. Our local recycling centre gets tens of scanners per day and they were happy to set a few aside for me. Carefully rip the scanner apart and inside you will find a white cold cathode lamp which is extremely thin (about 3mm) along with a small inverter PCB to drive it. Such a thin tube can easilly be hidden in the frame of the artwork. To make life easy for yourself look out for the internal working voltage of the scanner, many are 12V but some are as high as 24V, 12V is easy to provide with a wall wart for a fiver, 24V is harder.

The trickier problem was diffusing the light enough so that the illumination was even and became a pleasing part of the whole rather than a distraction. In the end I routed a groove in each side of the frame and bounced the light off a rough surfaced white card backing. It works quite well but I still would like to get it more even if I could.

Good luck with your project.

[Martin Angove]

Haven't noticed anyone mentioning electroluninescent material. It's ages since I tried any out, and all you see in Maplins these days is EL "string", but you used to be able to get it in sheets which could be cut to size with a sharp scissors. The downside is that it needs a driver. In the days I was looking at it (for backlighting LCDs) it was only available in colours (bluish or greenish IIRC) but maybe there is a white nowadays or an "almost white"?

Hwyl!

M.

[Ian Stirling]

The problem is not the light emitter, for which fluorescants are probably a good match, if used appropriately, but the "no mains cable" requirement. IIRC, EL is much less efficient than fluorescants.

[Ian Stirling]

Much easier.

case modding. They used to be 1.50 for 1*12" white, but have about doubled in price IIRC. Run on 12V.

[Calvin]

I have a number of these and they're fun, especially the UV type. They now come in pairs driven from a single driver circuit so a doubling in price isn't too unreasonable.

For this application though they can be a bit bulky and hence more difficult to hide. You see, I think the OP is yet to discover that for artwork diffusing the illumination is critical and much harder then you imagine it should be. The only solution I've yet come up with is to mask the light source so that it is not viewed directly, you're then left with having to diffuse the reflected light which is lots easier as the contrast ratio is much lower. The tubes from scanners really are very thin and I was able to route a 3mm groove in the frame sides and set them back quite a way so that even from about 45 degrees off axis they were hidden.

[Dave Plowman (News)]

It's possible to get an even soft source using fluorescents direct but with a defuser between them and the eye. That's how most illuminated shop signs work. But to do it properly costs.

[Ian Stirling]

The tubes from these are the same ones as from scanners basically. You just need to rip off the plastic cover, which admittedly can be moderately tricky, but takes moments with an abrasive disk.