Projecting big without distance
December 12, 2005 11:53 PM   Subscribe

Lense-filter: I made a few DIY projectors out of overhead projectors and 14" LCD monitors. These works great, and now I want to make it project a large image onto a screen only 1 metre away. I've got a few questions and am looking for any advice [diagram inside]

image here
Grey is the overhead projector, Black is the eyes, Yellow is the light, Green is the screen, and Blue is the LCD

As you can see the eyes are on the other side so it'll be projecting a reversed image onto a transparent sheet.

But with one metre I don't know how to expand the image. Can I put a larger lense infront like in the first diagram? Or should I just scrap the OHP's arm and project straight out like in the right diagram?

And continuing the DIY bent, are there any parts I might salvage to do this or do I have to go commercial?

posted by holloway to Technology (15 answers total)
when we had space constraints in a similar situation, we used a mirror to double the effective throw, although it needs to be perpendicular to the projection source to produce an undistorted image.
posted by gac at 12:19 AM on December 13, 2005

it would be nice to help you, but arnie is putting me off.
posted by a. at 12:31 AM on December 13, 2005

Those drawings are awesome. I want your ray-gun.

You'll have to do something clever, as the focal length of your average overhead projector is rather longer than a meter. Short projections tend to run into the same problem that fisheye lenses do - a curvy kind of distortion that looks cool, but is rather unhelpful.

That said, I don't know much about projector lenses. It does strike me that if you could find a smaller LCD, you could lower the projector head down low enough to get a relatively short throw. But I don't know how short you could get. You will need a smaller LCD, though, as you're essentially trading distance-to-screen for field-of-view on the projector.

Your second image reminds me of the ones that the REALLY cheapo projector kits use. You'd use a fresnel lens to project almost exactly perpendicular to the lens, with little enlargement. I'm not sure whether there are fresnels that would also enlarge in a useful way (there must be, I guess), but you're going to run into image-quality problems.

I think your best bet might be to scavenge another projector, and to extract the lens that the light finally exits from (at the top-right of the boom in your first picture). Hold it in front of the corresponding lens in your existing projector - backwards - and you should both shorten the focal length and decrease the magnification. I think. I'd have to look at a projector to see.

Also, a. - what?
posted by metaculpa at 12:38 AM on December 13, 2005

When viewed in an RSS reader, the diagram is replaced with an obnoxious anti-"bandwidth-stealing" image.
posted by DevilsAdvocate at 12:42 AM on December 13, 2005

a.: Heh... people see Arnie when they're leeching images from me, so I guess you've got a privacy proxy running Instead, get to the diagram by clicking here and then on diy-projector.gif. Clear your browser cache first.
posted by holloway at 12:46 AM on December 13, 2005

Not an answer but a question -- You don't happen to be building a DIY big-screen (projection) TV, do you? If so, you're sort of living a personal dream of mine, so I'll be following the thread closely. I'll try to think of some ideas, too.
posted by SuperNova at 12:53 AM on December 13, 2005

You will need a smaller LCD, though, as you're essentially trading distance-to-screen for field-of-view on the projector
Hmm... so that's the equation eh? Well I have a few 11-14 inch ones, but I guess you mean a 6" or smaller? I'd do some math to figure this out if someone points me to the equation to use.

Might try a 11" in combination with gac's mirror technique.

SuperNova: I guess it could be used as such but it's for a friend's art installation.
posted by holloway at 1:05 AM on December 13, 2005

Come to think of it, you don't need a smaller LCD - you just can't project all of the screen you do have. So if you're okay with losing the edges (it's just art!*) then you might just ratchet down the lens and see what happens. Or you could just render whatever you're rendering at 800*600 in the central part of the screen, with a black border around it. Might work.

posted by metaculpa at 2:28 AM on December 13, 2005

Gac suggested doubling the projection length with a mirror. For what it's worth, some rear projection TVs go a lot more than double, bouncing the image back and forth via a bunch of mirrors to rack up quite a bit of projection distance in a small space. It's useful how light can pass though itself without interferring with the image :)
posted by -harlequin- at 3:27 AM on December 13, 2005

I would search for the shortest FL fressnels (200mm instead of the usual OHP 330mm) and short FL projection lens. You can't leave the projection lens out (second diagram); this would result in a very blurry (though artsy?) picture.

Continuing the DIY bent: replace the halogen bulb in the projector with a Metal-Halide lamp/ballast assembly (250W and 400W are commonly used) if you want more brightness or higher efficiency.

The fressnel lenses can be bought at 3dlens, various FL projection lenses (and varifocal lenses) at surplusshed, or one of the many DIY-projector shops.

Gunawan has made some nice Excel FL calculation sheets.

I think the recommendation to use mirrors is a valid one; you can search the diyaudio forums for some examples (people cramming a projector inside a PC-case) but these mirrors will become very big very fast (if using the short FL optics) and you still won't be able to project a very large image with just 1m throwing distance.

Concerning the size of your LCD: The bigger the panel you start with, the bigger the picture you'll have after 1m using the same optics. But a bigger panel needs bigger fressnels which are harder to find (in short FL variety) and more expensive. You'll also save quite a bit of room behind your screen by using a smaller LCD and optics; room wich can be used to lengthen your throwing distance (using a mirror).

I built one projector myself 3 years ago, a second, smaller one last year and a third one very soon. It's an addictive hobby. Once you have the components you can solve most problems and get answers by trial and error.
posted by Akeem at 3:35 AM on December 13, 2005

Hah... that big scary picture of Arnie in my RSS reader was AWESOME! :)
posted by antifuse at 4:04 AM on December 13, 2005

ahh i see! i'm not going to clear the cache though, i'm keeping him.
posted by a. at 4:43 AM on December 13, 2005

[changed inline image to link to image]
posted by jessamyn (staff) at 4:49 AM on December 13, 2005

Something of a minor threadjack, but please consider posting your instructions over at Instructables. So we can follow in your footsteps, natch.
posted by disillusioned at 5:05 AM on December 13, 2005

I plan on posting instructions one day though I haven't taken enough photos to make a good article. It looks like this one will involve making another from scratch though, so maybe.

I've order a couple of lenses from 3dlens, so when they arrive I'll have a go at this.


Just about the images and arnie,

My images work on blank referrers, and,, etc. However I was getting referrers also from,

I've added these to my whitelist now.
posted by holloway at 1:03 PM on December 13, 2005

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