Evenly light an irregular shape from behind?

June 8, 2006 6:56 AM

I have an irregularly shaped metal object that I want to light up from behind. I want to the lighting behind it to be very even.

I'm making something that will eventually look like this (though I went with black coated steel instead of brass). The pieces will be mounted on some sort of backing.

Each piece needs to have lighting installed behind it controlled by a voltage that'll be hooked up to a pin on a BASIC stamp. (So, whatever light source I end up using must be able to be of variable intensity and roughly linear with respect to voltage - I'll be driving it with a transistor which in turn will be driven by the stamp.)

The easy way to do this is string a bunch of LEDs behind each of them - but if I do that, I'll get several pools of light - very uneven. How can I do this so the light is evenly distributed - so there's a smooth, even glow coming from under each piece?

posted by dmd to media & arts (11 comments total)

You could try an electroluminescent panel, like the backlight on a laptop screen. Some can be cut to shape.

Alternatively you could distress a piece of fiber optic by sanding it and run one or more strands around the perimeter.
posted by StickyCarpet at 7:05 AM on June 8, 2006

My parents use this clear tubing with Christmas lights inside it underneath and on top of their cabinets, and it seems to give a pretty even glow; could put that around the perimeter?
posted by librarina at 7:14 AM on June 8, 2006

Electroluminescent (EL) wire. This will give you a tighter bend than you can get with rope lights. Check out, say, Cool Neon. I don't know anything about adapting the EL-wire inverter to your needs.

I also recently ran across this LED flex stuff. I have no idea if it can bend to the radius you need, and it looks expensive, but it also looks very cool.

If you decide you need to go with point light sources like LEDs for some reason, perhaps creating a frame of frosted plastic would diffuse it enough to create the right effect.
posted by adamrice at 7:23 AM on June 8, 2006

Have you considered neon? Not sure if it would work with your specs, but it gives a very smooth and uniform glow.
posted by iconomy at 7:38 AM on June 8, 2006

Make a similar reverse cutout in metal that is a little larger and place this behind. Take a frosted plexiglass panel or even frosted mylar and place it behind the panel. Then place fluorescent lighting behind that. It won't be as flat as you might like but it is a worlkable solution.
posted by JJ86 at 8:17 AM on June 8, 2006

Nix the fluorescent lighting for the variable voltage - I guess that won't work. Instead use a tunsten or led behind the frosted mylar. If you want flatter, you may be able to find a flat light source from old copier parts.
posted by JJ86 at 8:19 AM on June 8, 2006

I've had pretty good results doing similar things by placing the LEDs behind the piece you wish to light, aiming away from it. The piece is then illuminated by the indirect light reflecting off the wall behind the object (or you could put a white panel behind it). This works best if the wall/panel is about an inch from the LEDs and you're willing to accept a fairly diffuse glow.
You might also be able to embed a fair number of LEDs in some milky plastic so that there's lots of scattering to smooth out the intensity distribution.
You could also try using plexiglas rod as a light guide and sanding one face to release the light.
EL wire is probably a good idea as well.

Finally, instead of adjusting the intensity by changing the voltage, you might want to use pulse-width modulation - it's easy to implement in software and requires no additional circuitry to implement.

For LEDs, you want to vary the current to adjust the intensity - while you can do that by changing the voltage it's kind of inefficient. Anyone know of a simple digitially-controlled current source for varying LED intensity? I have a project that could benefit from one.
posted by pombe at 9:13 AM on June 8, 2006

pombe - yeah, I know about pulse-width modulation. unfortunately I can't use it, because basic stamp can only pulsout one pin at a time, and I'm going to be varying 5 of them separately. so, instead, I'm just sending hi/lo to a op-amp circuit that gives me smooth rise&falls.
posted by dmd at 9:20 AM on June 8, 2006

What's the space like? Could you get away with a fresnel with a diffusion gel on it, or would that be too big? Aside from that, I'd work on balancing EL wire or reflecting an incandescent off a backstop through a diffuser.
posted by klangklangston at 9:31 AM on June 8, 2006

I helped my wife backlight a 4"x5" photograph in an assemblage piece she did awhile back. I bought nine "white" (bluish white) T1-3/4 LEDs from DigiKey. I think I wanted diffusor lenses, but I ended up buying 20 degree lenses due to availability.

I arranged the LEDs in a 3w x 3h grid at points equally spaced within a 4"x5" rectangle. To do this I (conceptually) divided the 4" x 5" rectangle into equal-sized mini-rectangles (4/3)"h x (5/3)"w and placed an LED in the center of each mini-rectangle. The LEDs were supported, along with a dropping resistor for each, by a piece of FR-4 prototyping board. The dropping resistors were on the opposite side of the board from the LEDs.

Now here's the secret sauce: I then pressed a 4"x5" piece of 1/2" open-cell Styrofoam onto the LEDs to act as a diffusor. The LEDs were embedded about half-way into the Styrofoam. Note it's gotta be open-cell; the pressed pellet stuff probably wouldn't diffuse as well.

We put (glued?) the photo to the foam and powered-up. We looked real hard and could not find any significant variability in the illumination across the photo. We did some temperature measumrents to make sure the thing wouldn't catch fire, and she finished the piece.

Maybe you could experiment with this technique.
posted by ZenMasterThis at 9:35 AM on June 8, 2006

Thanks everyone! I think I'm going to go with some LED-driven method, as the EL stuff seems to have a very short lifetime (the 'cool neon' apparently dims significantly after about 2000 hours). I'm going to play with some plexiglass for diffusion. I'd really like to use styrofoam - so easy to work with - but I'm afraid it won't hold up over years.
posted by dmd at 12:19 PM on June 8, 2006