Help me build an overhead projector from scratch?
October 17, 2016 11:08 AM   Subscribe

Looking for help with the optics in my from-scratch overhead projector!

I've been tasked with building an overhead projector for a Children's Museum exhibit - one that doesn't look like an old 3M version. So I've got the aesthetics down, but having trouble with the optics.

I'm using a super bright LED module from Cree to replace the MR16 that are usually used in these things: it has equivalent brightness. I sourced a Fresnel lens from an old rear-projection TV but it doesn't look like it's going to work. Any suggestions where I can get a 20" Fresnel lens for less than $200? The old style projectors I cannibalized had much larger concentric ridges

I'm also at a loss as to what kind of lens I need up top to project the image after the Fresnel condenses it. Is the size of my Fresnel related to the size of my projection lens?

Any suggestions would be helpful!
posted by AutoPilot83 to Media & Arts (6 answers total) 2 users marked this as a favorite
For the head unit maybe check out some surplus optics such as this. I assume you'd put this top piece at the focal point of the fresnel, which is likely to be influenced by but technically independent of its size. In your shoes I'd just use the optics out of the head of an actual OHP though.
There are 20" fresnel lenses under $200 on AliExpress, but not all that much under $200 and shipping takes a few weeks.
posted by anonymisc at 12:07 PM on October 17, 2016

I think you can skip the fresnel lens if you're willing to add a diffuser under the glass. You're going to lose a bunch of light with the diffuser, but it sounds like it would be cheaper to add more LEDs than it would be to buy a bigger fresnel lens. You can test the diffuser concept out with a piece of copy paper.

Pretty much any bi-convex lens is going to create an image. I would use one from an old overhead projector since it'll be about the right focal length.
posted by gregr at 12:09 PM on October 17, 2016

Response by poster: Huh. I'll try the whole thing with more light and leave out the Fresnel. The LED rig is about $75 vs. $150 for another large Fresnel.

I just took apart some old OHPs and found some good lenses - seems like they use two meniscus lenses instead of one biconvex lens.

Thanks for the AliExpress lead. Seems like there are some on Ebay for 1/2 of that price, but with some defects - meant for solar cookers!
posted by AutoPilot83 at 12:38 PM on October 17, 2016

It's questions and answers like this that make me hate myself I'm not an engineer. Sounds like a great project!
posted by ouke at 12:47 PM on October 17, 2016

Response by poster: So you can say things like "negative meniscus'?!
posted by AutoPilot83 at 12:54 PM on October 17, 2016

If you're diffusing the light you could just a handful of LED/CFL lightbulbs into your box.
posted by gregr at 1:13 PM on October 17, 2016

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