Hi,
I will build a Psone projector . This is my first project of this kind. I'm really good with electronics (so the wiring part shouldn't be a problem) but i'm a noobs when it come to optic . I have a few question before i start this:


1- I will use a LED array for light source. What i understand from my research is that with a led array, i don't need a fresnel lens between it and the LCD because the light is already projected in a direct line toward the screen. Am i right?

2- I will use a 300mm focal lenght frensel lens between the LCD and the projection lens. So what will be the size of the gap between the 2 lens? (I'm really not shure, but my logic tell me that the gap must be a little shorter then the fresnel lens focal lenght, so the picture will appear as a square at the entrance of the projection lens, am i right?)

3- What should be the distance between the LCD and the fresnel lens? Do they have to be the closer possible?

4- If i get a varifocal projection lens (like this one ebay) what will i be able to adjust? I mean, if i put the projector in a stationnary place and i change the focal lenght from 100 to 200mm, will this change the zoom (size of the projection), or will this change the focus so i can place the projector far/closer from the screen?

5- Does that lens is ok for a DIY projector?? (ebay)

I hope you will be able to help me. I want to be shure that i will be able to get a fonctionnal result with the this material before i spend some money on it, i dont want to be stuck with optic gear that i cant use. Sorry for my bad english if i made some mistake, this is not my native language...


ohh... i forgot to ask, what is the number that come after the f in some lens spec?? I know that lens have a focal lenght (measured in mm) but i dont know what is this other spec.

Hello, and welcome.

1. LED lighting has been attempted, but with limited success. The choice of most builders is Metal Halide lamp due to it's ability to make a brighter projector. Here is a link to someone who tried an LED driven PJ with some success. The LED debate continues... Basically you are correct, an LED light source will not need the rear fresnel, but it is still debateable if the light is collimated well enough to be directed into the triplet. Any light not directed into the projection triplet (projection lens) will not show up on the screen.

2. You can use the focal lenght calculator II in the "Calibration & Tools" area of the forums for determining the LCD to projection lens distance. This will depend on the FL of your projection lens and the throw (distance from projection lens to the screen). Basically you will need a field fresnel that is close to the FL of the projection triplet as this will improve the brightness of the final projected image. If you are going with the standard LL triplet, it has a FL of 320mm, a 300 or 330mm FL field fresnel would be suitable for this lens.

3. You need to maintain a distance of about 15-20mm from the LCD to the field fresnel in order to keep the fresnel rings from appearing on the screen. Having the field fresnel on the triplet side of the LCD allows for keystone correction, allowing more placement options for your PJ, however, having the field fresnel on the lamp side of the LCD ("unsplit" fresnel design) gives the best image quality, but PJ location is limited due to no keystone correction is available.

4 & 5. No, that lens will not work, it is designed for slide projectors, which have a Field of View (FOV) for viewing the size of a 35mm slide. It would not be suitable for a PSone screen as it is too big. Varifocal lenses allow you to change the FL of the lens, allowing more PJ placement options (longer or shorter throw). The LL store carries a varifocal lens for use with small LCD panels like the PSone. You will need to play with the focal lenght calculator as mentioned above to get an idea of what size image you want, triplet and fresnel selection, and PJ placement.

cheers,
gs

First i want to thank you for your fast response.

But i still don't understand why some projection lens can be used in DIY projector (like those from overhead projector) and other can't (slide projector...). If the projection lens have a smaller FOV, why it's not possible to just put the lens a little bit further from the fresnel so the picture is a little bit smaller and more concentrate so it can all fit in the lens? I know i am probably wrong, so can you tell me how can i know if a projection lens could be used in a DIY proj?

Also, since yesterday, i looked at some optic book (it was a photography book, they talked about lens but for camera, not projection, but i think theory is the same) to understand what is the f/ variable of a lens. I finally found that it represent a rate of `focal lenght` on `objectif opening size(where the light can go trought)`. So if the lens have a focal lenght of 200mm and the opening size is 50mm, then f will be f/4. So do this has something to do with the compatibility of a projection lens for a DIY proj?

A projection lens for a slide projector is designed too "see" a 35mm slide, and they also typically have focal length's (FL) that are too small for our application, creating a throw that is much too short. An overhead projector on the other hand is designed to "see" a much larger area, like 11-12" or so. (basically where the transparancies are placed). There are different quality overheads too, some use only a doublet as a projection lens, while some models use a triplet. Opaque projectors can also use triplets, and some members are using those. The FL's of the opaque and overhead projectors are typically longer, and are able to get a useable throw distance for the PJ.

A projection lens must sit a certain distance from the LCD to attain proper focus. (you can use focal lenght calculator to determine this distance, based on your required throw and screen size). The projections lens must have the correct FOV at this focus distance to be usable in our PJ's. The further you move the fresnel away from the LCD towards the triplet, the more impact it will have on the images size, focus, and aberration, with spherical and chromatic abberation being the most detrimental to the image quality. For this reason there is a certain distance you must maintain between the triplet and the field fresnel. I can't tell you the exact allowable distance, but suffice to say, the closer it remains to the LCD, the better the image quality will be.

Yes, 'f' typically denotes focal length while 'f/'# denotes the f-number, or focal ratio of a lens, which is the lenses effective focal length devided by it's aperature. Basically this is a comparative number used to tell us how "fast" the lens is, or basically how much light can be collected. Lower numbers indicate "faster" lenses, or lenses with the ability to capture more light. This is not overly important in choosing a projection lens, as the FL and FOV requirements are much more important.

If you can find a camera lens from a large format camera (large format camera's typically use 4"x5" film), you may find a suitable one for projection lens for a PSone screen, which measures 3x4" (5" diagonal)... of course you'll also need one with a suitable focal length... BUT, in the end you're not likely to find one anywhere near the price you can buy a LL standard triplet, which is only $30.

hope this helps.

cheers,
gs

I finally made my led aray using 10mm leds, maybe a little too big for PSONE screen but that was what i got under my hand. The problem now is when i throw the lgiht trougt the lcd with no lens at all, i get alot of shadow of the image on the screen (i just put a peice of paper in front of the lcd for quick testing and i get the same kind of result no mather the distance between it and the lcd), it looks like each led is creating its own picture. I was able to fit 48 led in the array (8x6), i would try with smaller one like 3mm but i fear that it would end up giving me just more shadow of the picture, but at least the lumens total would be higher. I removed the anti-glare paper surface from the lcd and i dont think now it was a good idea.

Anyway, i know that some of you where succeful using leds for light source, so can someone tell me what i'm doing wrong? Does it need to be a diffuser (the anti-glare) between the led array and the LCD??

the LED tech. for the lumens is not there yet IMO, you would be much better off with a point light source. This is why Commercial projectors do not use LED's cost for one to construct, and not to mention loosing the profit from bulb replacement.

I know commercial projector dont use led. But what i know too is that many people has been succeful using leds for diy projector, i saw alot of picture and alot of post about it. I'm just missing a point somewhere, it's how to get the most of the light from the led to go perpendicular through the lcd and form only ONE picture. I know it will not be as much as bright as if i use a 400w bulb, but my goal is not to make a finish product, it's more to experiment and learn about projector, once ill be able to get something on the screen, ill invest in a good 17" lcd and good optical lens to build a true projector, then maybe ill think about a 400w lamp. Anyway, when my psone projector will be finish, ill probably test it with differents light source (and differents leds and array configuration) to see what give the best result and to see if led are a thinkable alternative. I would be surprise how much light these 4w luxeon leds made by phillips could deliver on the screen, and they never stop perfecting them, with higher and higher lumen output. For now, i need to solve this "shadowy" problem to get going on, so if someone can help me, i would be very happy!!!

It sounds like you have the LEDs too close to the LCD. Move your LED array further from the LCD so that the light from each LED overlaps the light from the others a little to eliminate the dark shadows. When the light hitting the LCD looks more even, try using your projection lens. If you don't have a fresnel to direct the light into the projection lens, the image will be dimmer. If you don't have a projection lens yet, you can use a magnifying glass to project an image, it just won't look great.

Tgreenwood

what about something like this for a smaller projector?

bulb

That would be rubbish not enough light for the amount of heat it would give out espicalluy if u have the 28 in ther

search for prolight 5w led its from hong kong but shipping is cheap.
shmoe

(EDIT)
This is wot i might use for my ps1 pj [url=http://cgi.ebay.com/NEW-1-X-5W-Prolight-H-Power-star-White-LED-W-120lm-F-S_W0QQitemZ7556577822QQihZ017QQcategoryZ66952QQtcZphotoQQcmdZViewItem]
it is 5w and 120 lumens however ther would only be like 20 on the actaul screen so you would need about 10 at least to get a decent picture that is like $200 though.
OR
[url=http://cgi.ebay.com/50X-10MM-JUMBO-White-LED-Lamp-125-000mcd-Free-Ship_W0QQitemZ7579727724QQihZ017QQcategoryZ66952QQtcZphotoQQcmdZViewItem]
These are $20 ultra bright leds