Optics n00b question
May 15, 2007 6:36 PM   Subscribe

Optics n00b question: I'm working on an art project wherein I etch drawings onto glass and then project shadows of said srawings onto a wall. I've done this before using an overhead projector as a light source and it worked great. This time around I bought some lights that don't work quite as well...

I bought a couple of these, learning the hard way that what I need is some sort of point source lighting to make the kind of crisp shadows that the overhead projector made. What I'm wondering is if there is some sort of optical apparatus such as a lens or whatnot that I could mount to the front of these lights to change their properties. Or, failing that, some sort of cheap alternative to buying more overhead projectors? Any help is appreciated!
posted by garethspor to Media & Arts (7 answers total)
A lens probably wouldn't work, because in order to get a distinct source, you'd probably need to customize the lens' 'curve' and size for the lights.

Would basically sticking a piece of cardboard (or something) a while in front of the light work? You'd have to be careful of its placement, in order to to prevent burnination from constant use (if it's an installation piece).

Have you tried flashlights to see if they give you the crisp shadows?
posted by flibbertigibbet at 7:10 PM on May 15, 2007

Make a cone aka "snoot" out of paper (aluminium foil works too) and attach it to the front of your rig. That ought to narrow and intensify the beam of light.
posted by squeak at 7:15 PM on May 15, 2007

You don't have to have a point source for crisp shadows, a source with parallel rays (the Sun, for example) will also do.

Accordingly, I would try automobile headlights-- the kind that are designed to cast a beam a long way and have front glass as clear and flat as window glass, and I would get the kind that have a little shield in front of the filament so that all the light emitted is coming off the reflector.
posted by jamjam at 7:45 PM on May 15, 2007

Best answer: You are trying to use a gobo. The trick is to get a lamp that allows you access to the focal plane.
posted by ikkyu2 at 8:43 PM on May 15, 2007

What you want is light that's collimated (as jamjam says). You can probably do that with a Fresnel lens. Actual glass optics will be pricey at the diameters you are talking about. If you look at the overhead projector you will probably find one of those Fresnel lenses below the platen, where you put the art work.

Some of the PAR30L spot lights will cast quite distinct shadows, too.
posted by jet_silver at 8:47 PM on May 15, 2007

Best answer: You know, there's a reason why overhead projector manufacturers put in that Fresnel lens (the thing with all the circular grooves) and the other lens in the projector head, instead of just selling you a floodlight in a can and a few bits of cardboard. The lenses make it practical for a lamp of finite size to behave like a near-enough-to-perfect point source without wasting most of its brightness.

Unless the drawings you've etched onto glass are about the same size as the shadows you want to make, you're going to need either an overhead projector or its optical equivalent to do what you want.

If you're going to roll your own optics, you probably don't need the mirror you'd typically find in an overhead projector's head. Just get a cheap Fresnel lens the same size as the etched glass you want to project, and put it in the front of a box with a flat white interior and your PAR cans in the back, so the only way light comes out is through the lens. You will want some fairly aggressive fan-forced ventilation in this box, lest you turn your cheap plastic Fresnel into a useless warped lump of goo. Devise some suitably sculptural mounting to fix an objective lens (a big magnifying glass might do) close to the point where the Fresnel concentrates the light from the box down to a point, and fool around with distances until it all works.

Don't put your hand at the focal point when the lights are on. You don't want to know what a few hundred watts of incandescent radiation feels like when concentrated onto a tiny patch of skin. To get the focusing distances right, you'd probably be better off with a standard low wattage lamp bulb in the back of your box than unleashing the awesome power of two PAR cans.

If you've left out the mirror, you will need to put the opposite side of your glass etching toward the Fresnel than you would with a real overhead projector.
posted by flabdablet at 9:12 PM on May 15, 2007 [1 favorite]

What flabdablet said about being careful about the focal point. You can use a fresnel lens to melt a penny in short order.
posted by plinth at 6:55 AM on May 16, 2007 [1 favorite]

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