# An idea for 17&quot; LCD Screens

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#### superdaveumo

An idea for 17" LCD Screens

I am still trying to get a grasp on all of this optics (without any optics to play with except a telescope ). I've been doing some calcs and wanted to know if this is even close to right.

So, I wanted to try to get some clarification and share an idea I had. This first picture is a drawing of each fresnel that I've seen available (220mm, 330mm, 550mm, and 790mm). The projection lens assumed here is the 135mm triplet (450mm EFL).

The 4 vertical lines that are 120mm (60mm is halfway) represent the closest distance that the projection lens can be and still get the full light cone from the fresnel lens. The numbers pointing at the vertical lines are the distances from the 2nd fresnel (the one between the LCD and projection lens) to the projection lens.

All angles are half-angles (double them to get the FOV).

This first pic is drawn to scale and is based on a 17", 5:4, LCD that is 13.3" (337.8mm) wide.

An externally hosted image should be here but it was not working when we last tested it.

Two things come to mind with this. First, it would seem that the 790mm fresnel would be a good choice for this setup because it has the same FOV angle as the 135mm triplet lens. But, the focal length would be about 59mm too far. Taking the triplet apart could extend the 450mm FL to the required 509mm FL. But does the image degrade if you take apart the triplet (as some people who have taken the triplet apart have said).

The second is the idea that the triplet's FOV might increase as it moves towards the center of its lens. If this is the case (I don't know if it is), then perhaps you can use only 60mm (instead of 120mm) of the light cone coming from the fresnel to shine into the projection lens (effectively using only a smaller central part of the projection lens).

This second picture is an idea I was thinking might work.

An externally hosted image should be here but it was not working when we last tested it.

Using the standard split setup, simply put a 120mm lens (fresnel 3 in the pic) between the fresnel 2 and the projection lens. Maybe a fresnel or maybe a condenser lens (like the ones used to spread light from the bulb). This 3rd lens would take the light from the 2nd fresnel lens and change the angle the light was taking to a smaller angle (or possible 0 deg angle?) so the 135mm triplet could handle it. It's done with the light source for the LCD, why not for the projection lens too? It would make this a symmectrical setup (the bulb and the projection lens being the end points).

With this 3rd fresnel lens, you could use a shorter focal length fresnel.

#### temp2

Sorry, the links to the pictures seem broken...

#### superdaveumo

Actually, I just checked this page from another machine on another ISP and the thumbnails show up and the pictures load after clicking on them. But thank you for the heads up.

#### davedaxx@yahoo.

I was thinking the same thing I think if you use a third frensnel it would solve all the FOV problems. look at it like a contact lens . You could also adjust the throw to almost any distance, but does the triplet now focues on the third frensnel or is the FL a sum of both lenses? I think I found a source for a negative frensnel. Going to find out if they will sell me one. link http://awi-industries.com/Fresnels.html they have two sizes

If this works it changes everything, throw and FOV fixed, no need for a expenceive long throw lens. 3dlens has a negative frensnel the wide angle PVC its cheap and would be good for testing.

Good luck: davedaxx

#### superdaveumo

I think the idea would be to have a field fresnel that had a shorter focal length. The inserted condenser lens would be at this shorter focal length. Then, the total FL of the field fresnel + the condenser would be the FL of the projection lens.

So, a 450mm projection lens and one 330mm fresnel (field fresnel) and a 120mm condenser. Like I said before, the light wouldn't have to be perpendicular to the projection lens, the light just needs to be condensed enough to lower the angle below the field view of the lens (24 degrees).

I'm not an optics expert. So, if I am wrong, please correct me.

#### redd38

I haven't built my projector yet... so hopefully one of the more knowledgable people can help with this....

It seems to me that we can put condensors and fresnels before the LCD because at that point we are just directing light. But after the LCD we want to reduce the number of lenses because now our light has taken the form of an image and the more lenses we hit the more that image will be distorted. This would be why an unsplit fresnel design produces a better image than a split design. This also means that your 3 fresnel design might work in theory but will produce a lower quality image.

However, it would remain to be seen if the image quality was low enough to justify a \$250 triplet or if a third fresnel was good enough.

So that's my 2 cents... i'm interested to see results from this.

#### superdaveumo

That's a very good point. I didn't think about the added image distortion. Although, people add mirrors after the LCD and that doesn't seem to noticably distort.

This would be very easy for someone to try with a 17" LCD. Just add a condenser lens.

#### davedaxx@yahoo.

A lot of camera lenses have ten or more elements I thing my canon GL has like 12 elements. one element should be no big deal A all glass lens would be best, but for testing a low cost wide angle frensnel would let us know if were on the right path.

#### redd38

But I think there is a difference in the quality of lense between a fresnel and the optics used in your cannon or in your triplet. Those elements are intended for image reproduction, but a fresnel (i think) is just intended for light distribution.

But I could be wrong, just throwing out ideas.

#### Rox

yes, the fresnell does distort the image noticeably. The field fresnell does distor the image but we try to keep it close to the LCD, so the aberrations introduced are minimal, but this negative fresnell is very far from the LCD, the distorntions introduced there are huge. This is a cheap fresnell (the one at 3dlens) so easy to test, but I don´t think this will fix the FOV problem of the 135 triplet.

the negative fresnell will be part of the projection lens, fresnell lenses aren´t very good lenses for image path... so only it is your choice going for that way. Good luck.

#### davedaxx@yahoo.

First It would be nice to find out if it would work at all, if it works but the fresnell makes a lot of distortion then someone could look for a cheep 5 or 6 inch glass or acrylic wide-angle lens. I once saw one sold as a door peep hole I think.
Even if it was \$50 the amount of adjustment that it would give ,mainly realy long throw, would make it worth it. plus I think it might allow the use of short FL triplets?

#### superdaveumo

Yes. I would think a wide angle lens would work. A PCX I think would work too. Those are meant for image reproduction (if I am not mistaken) so you should get minimal image distortion. If this is possible, then you could have an unsplit design but use this new lens for keystone adjustment.

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