DIY Video Projector

[SuperDave]

Thanks for the advise Joe...I have started moving my work to a large piece of plywood. My next phase will be quite big but will show me if everything is working properly (like my lamp, lenses, etc....). Once I get a good image I will focus on compressing my design down quite a bit to save a lot of space and be somewhat pleasing to look at.

I do have a question regarding fresnel lenses. The lenses in my OHP are about 10"x10" (just a guess) however my VG150 lcd is 13.5" x 11.5". I need to get a couple fresnels that are larger than my LCD screen (preferably by a couple of inches). Does anybody know where I can get these, either new or used...but preferably cheap....Since they will go infron of the LCD should the cheap plastic ones from an office suppky store work good enough. I really dont want to sacrifice to much quality....but I dont want to make this an extremely expensive project either.

Thanks,
Dave

[contrapasta]

Basically the deal (might call it a scam, I don't know) is that you buy it for Outpost.com for whatever there are charging for (~450 I think) Right now it comes with a $100 rebate. Using your AMEX platinum (or blue if you do it today, this is the last day of Blue's pricematching program) pricematch the 450 price to a cheap internet price, then reap the rebate afterwards.

http://shop2.outpost.com/product/2561551

http://208.158.105.29/P_1195.htm (AFAIK this is a legit site, House of Computers, well legit enought to PM from...)


1)Call up AMEX BVG dept, tell them about it, fax them over printed page of your reciept and printed page of internet price.

2) wait 2 weeks for amex pricematch check.

3a) If anything happens to go wrong (Amex routinely pricematches internet stores, but anything can happen who knows..) Take adavantage of AMEX return policy, AMEX's 90 return policy (anything can be returned, even from stores that don't take returns, like outpost)

3b) Check arrives everything is good, send in rebate and UPC

Well there it is, seems like a lot of work but have fun with it if the VG150 is really what your want! ( I found this at one of those 'deals' sites like fatwallet or anandtech's hotdeals forums)


BTW, interesting read, seems like 15 inch LCDs are actually RISING in price! Yikes



BTW, Link


-Mike

[oftheend]

I was testeing my projector in its final position to get the length of the box i have to build for the front lens, to foucus it correctly. Of course the image was projected where my TV sits, hopefully it will replace it. I was watching a show during my testing, shut off my normal tv during a commercial break, and turned the projector on. When I turned the TV back on, There was a scene in an FBI office (x-files). At first i thought there was someone looking through a gun scope, then i realized that the circular pattern was actually color distortion in the tv. The inner circle was green tinted, the next blue, and the final, the colors were normal. The circles are approximatly where the center of my projected image was. (the tv deguassed it self, so it is fine now.) Does this mean that the projector is emitting some elctromagnetic forces, or could it just be the heat of the light emitted that affected the TV? When the TV was off for hours before my tests, this never happened.

[MuzzMan]

Thanks for the help Undream and gray, invaluable.
It sounds like a converging or plano-convex lens is what is required.
Gray - what you said about not wanting anything other than parallel rays striking the LCD makes sense, but now I am confused as to how the plano-convex lens will function. If parallel rays pass through the regions of the lcd not directly covered by the projection lens, how can they be captured by the converging lens? Unless we are relying on scattering occurring (there will be some), all the light from those regions will miss the lens entirely.

[BodySuitMan]

oftheed, i noticed the same thing when i first tested out my projector. I had set up a screen like 3 inches in front of my TV and projected my image on that screen. I turned on my tv shortly after and the colours where all fuked. It fixed itself, though. Curious phenomenon...

[fender4]

Muzzman,
When a panel is placed on an OHP stage, the rays of light from the fresnel are converging to the head lens, so the rays are indeed not travelling parallel. Several posts back, I mentioned that this could theoretically cause a central hotspot because of the decreased transmission in the periphery (I think this is mostly due to polarizing layers in the LCD matrix). So anything other than 90 degree light rays will be less than optimal. I hope my thinking is correct with this...anyone?!

This is what led me to look at the fresnels themselves. jvisaria confirmed my guess that OHP fresnels are acutally two fresnel--one for making the light from the bulb parallel and one for taking that parallel light and converging it to the head lens. If the LCD could be sandwiched between the two fresnels, this could lead to an increase in transmitted light and/or a more consistent image.

The basic design of an OHP seems to include the very minimum of optics that are necessary for any DIY projector that uses large panels: point-source light, fresnel, LCD, fresnel, projection lens (PCX, DCX, or achromat). Now all we have to do is adapt a more efficient light source (MH or MV, or example) and find a lens with a throw that suits your goal. Combine that with a VG150 like SuperDave and some quiet cooling, and that should be a nice setup . BTW, the PCX lens I purchased from www.surplusshed.com produced (rough estimate) an 85" image from a 9" source at 11 feet.

Hey SuperDave, any luck with the MV bulb?

Good luck everybody!

-f4

[Scot_lad]

Well I just had to reply, since someone stole that nugget of information from me!! I told you all weeks ago about the two-fresnel setup inside an OHP.......

The plan to stick parallel rays of light through an LCD is very clever, but it will not work the way you're going about it. Fresnels (which are by definition approximations to proper lenses) have shite image quality when compared to proper glass lenses. If you look at the surface of the fresnel is it any wonder? It's not a proper lens, and intended really only for light. Why do you think the Big Woody 100" TV project didn't work? Is sticking a fresnel in front of a lit LCD any different to sticking one in front of a big TV? Nope.

Just had to get that out to pre-empt the hundreds of messages saying "I tried this but it didn't work? Why not?" Try looking a few pages back. The forum repeats itself over and over again so you shouldn't have to look too far back to find what you're after.

[fender4]

I apologize, Scot_lad. I honestly didn't see it earlier. It's hard to keep up with everything that is posted, but I try.

Posted by Scot_lad on 03-17-2002 08:29 AM:
....Putting the fresnel in front of the image isn't a good idea, because it is relatively low quality and while that doesn't matter as much for just light, it matters a lot for an image. Notice how in an OHP, there are two fresnels, one which takes the light from the bulb and turns it into a parallel beam, then another fresnel directly stuck to that which takes the parallel beam and converges it through a slide (or an LCD panel) into the high quality glass optics at the top of the OHP arm. Or whatever they're called...

I guess until we can get our hands on those 0.8" high-res LCD's we will be limited to using fresnels. You are right...fresnels are just flat, lower quality versions of a plano-convex lens, but an equivalent PCX lens would be heavy, expensive, and nearly impossile to locate. Aren't most fresnels from OHPs of much higher quality than the "page magnifier" lenses? I guess it depends on the level of quality you expect from a DIY projector. Good point, though. The advantages gained from "sandwiching" probably would be negated by the reduced quality of passing the image through the second fresnel.

-f4

[tech head]

O.K.
My question is, Why not just set the projector up like a regular ohp? Light=>ferensal=>ferensal=>lcd=>lens.

If it works for an ohp, why not for a projector? I mean, that's what the lcd panels were designed for in the forst place.

Joe Morris

[SuperDave]

tech head... That is what most people are doing (at least thats what I'm doing right now).
I will be repeating a lot of previous information here...
The problem is you want the light to go through the LCD screen with prarallel rays. They are designed to work this way. Any of the LCD screens you come across will have a number of polarization filter to assure that light is passed through fairly straight.
Since the LCD screens have these polarization filters on them they will only let the light through that is close to perpendicular(90 degrees - give or take a little). So... there will be a higher amount of light getting through at the center of the screen as opposed to the outer corners of the screen. This is because the center of the screen gets light straight on, wile the futher away from center you get the less and less light gets through because it is hitting the LCD at more of and angle.
I hope this helps.

My question is this...and it is counter inuitive to what I just said...
Since we are already convergring the light with the second fresnel to force it all to go in one direction and it is fairly close to parallel...cant we just remove the polarizing filters from the LCD and let all the light through. I did this with my first test run of my projector and although I had a dim picture...the light was distributed surprisingly easy..... To try and reexplain...The polarization filters on the LCD only allow the light that is coming directly at the LCD to pass through...but since the fresnel is already causing the light to go basically head on with the LCD why can we remove the prolarization filters to allow that small bit of light on the outer corners to pass through???

Ohh and I didnt get a change to work last night...My girlfriend found out she had to travel for work for the rest of the week so I had to help with some things....The good news is I have the remainder of the week to spend alone with my prjector.

Later,
Dave