Ideal fresnel lens for 15" and 17" LCD

Hello jimhuanq.
The ideal fresnel size for a 17” LCD (4:3 screen ratio) would be 350*285mm.
The focal length should be around 550mm – 650mm.
I think that it would be better to supply two or three different focal lengths instead of different fresnels sizes. The 17” fresnels could be cut down in size to fit the smaller 15” LCD.
The fresnels should be aspherically corrected as well.

DJ

Dazzzla is perfectly right. The main issue with 17" projectors is not that the fresnel is too small but that the focal length is too short to use with the 400mm and up long throw lenses. Fresnels with a longer focal, as Dazzzla said, 550-650mm would be great.

I'm not sure how many different types you were looking to provide but I've also heard many people request 450+mm fresnels in some cases to cut down on size. But I'm not sure what production costs etc. are so 550mm and up would still work.

ideal fresnel

jimhuang,

Your English is better than many of the posts on this forum! I am very happy to read that your company would like to make our ideal fresnel lens. It has been very difficult to design this type of LCD projector because of the difficulty finding a proper field fresnel lens.

If you would like to start with a single fresnel lens that would fit the 15" and 17" LCD projectors, then I suggest a 550 mm focal length lens of 363 mm by 272 mm with aspherical correction.

By my calculations, this would let builders use the most available 450 mm focal length projection lens to get a 100" diagonal screen image from their 17" LCD. This calculation also includes a 20 mm space between the fresnel and the LCD, as well as a size adjustment so the fresnel will completely light the LCD.

Builders using a 15" LCD could also get a 100" diagonal screen image with the fresnel 30 mm from their LCD.

Since this fresnel lens would be perfect for both 15" and 17" LCD projectors, and there are no competitive products, I think you could sell a lot of these. You could also offer this lens in a set with a 220 or 330 mm focal length aspheric fresnel of the same size.

Thank you for asking,
Guy Grotke

This is surprising, I emailed 3Dlens a long time ago and whenever I mentioned fresnels with longer focals the reply I got was 'all you need is 330mm focal length' lol


The ideal fresnel not available for our project would be a 500mm focal length and size it for a 17" panel this way anyone can cut then down to size.


But I'd make them with a .5 groove pitch. No need to split fresnels with the correct focal length lens and the lower pitch will collect more light.



- JCB
www.diybuildergroup.com

Hi jimhuang

Could you please make the fresnel wide enough for the 17.4" wide screen SGI 1600sw monitor?

The SGI 1600sw LCD panel size is 380mm X 245mm (approximately).

Thanks

i would say 360mm*290mm is a very good size, sorry but 17.4" size is not so comon so i don´t think it would be a good choice for you.

about the focal lenght, 600mm is a good choice.
could you tell us the grove picht available?

Correction

Hello jimhuang,

I'm very sorry. I found my mistake: I assumed that a 17" LCD monitor would have a 4:3 shape like lower resolution monitors. That was not correct. I see now that they all have 1280 by 1024 pixels that are 0.264 mm squares. So the true dimensions are 337.92 mm by 270.336 mm, as you wrote in your first posting.

For use as an LCD projector field fresnel, it has to be a bit larger than the LCD because the builder will place it a few centimeters before the LCD. If the fresnel is perfect for the projector lens, then this distance will be very close to 2 cm. If the fresnel focal length and the projector lens do not match as well, then the distance may be as much as 20 cm. Since a 550 mm focal length fresnel would be a very good match for a 450 mm focal length projection lens, I think near a 10 % oversize would work for everyone.

So let me change my recommendation:

550 mm focal length lens of 370 mm by 300 mm with aspherical correction. Also I agree with jcbklyny: Since this fresnel will be used for condensing light, but not for the LCD image, a groove pitch of 0.1 mm is not necessary. If a 0.5 mm pitch will transmit more light, then that would be better.

Sorry for the mistake,
Guy Grotke

ok 370mm * 300mm with 550mm focal is ok i think, what about the corners? will be cut down for manufacturing proceses?

tell us the true square would be inside the 370 * 300 outer square please. Thanks.

Also I agree with jcbklyny: Since this fresnel will be used for condensing light, but not for the LCD image, a groove pitch of 0.1 mm is not necessary. If a 0.5 mm pitch will transmit more light, then that would be better.

Click to expand...

Guy don't forget that some people would like keystone correction. So using the smaller pitch would produce a much better image. It would also make the fresnel more versatile, so that different FL objective lenses could be used. Although a finer pitch fresnel blocks more light, I don’t think that it would be very noticeable. I would think that running the fresnel out of its designed FL would consume more light that the pitch would.

DJ

keystone correction

If I was spec'ing a fresnel to use on the projection lens side of the LCD, then I would make it 0.1 mm pitch. But then I would also make the focal length more like 510 mm, and that would be too short for people not using a split fresnel design!

I was trying to specify one fresnel that would be most useful for people building 15" and 17" LCD projectors. Most of those people will get one of the great 450 mm fl long-throw triplets that are available from a couple of DIY web stores for around $120 US. Since they are building a long-throw projector, it is pretty likely that they can just put it on a counter, a table, or a shelf at the back of their media room. So they don't need to use keystone correction.

I think optical keystone correction is needed by builders of short-throw projectors who have to hang them from the ceiling. But they have no use for a 500+ mm fl field fresnel.

I have to agree with Dazzzla on the keystone issue. For me to raise my projector up to be level with the center of the screen, it would have to be behind my couch. That would put the lens 16 feet from the wall. Using various focal length calculators, my projected area is 165 inch diagonal.(130"W x 100" H) On the floor in front of the couch is an 11 feet distance to wall resulting in a 108 inch diagonal area (85"W x 65"H) which would be ideal from the viewing distance. Hopefully a solution (compromise?) can be found to accommodate the split fresnel and unsplit fresnel designs.

I'm doing an SGI 1600SW projector, and something at least 390mm x 260mm with 550mm FL would be great. Of course, bigger is always fine since we can cut them down some, if necessary.

Mikey p said:

Hopefully a solution (compromise?) can be found to accommodate the split fresnel and unsplit fresnel designs.

Click to expand...

Yes, some smart optics guru giving the DIY community a feature that has existed since the first days of commercial LCD projectors: size adjustment.

I'd like to register my vote for a fresnel large enough for the SGI 1600SW panel.

I know there are many of us who are building a projector with this panel, and we all could use a large fresnel lens.

Thanks.

Roger

if we need just 1 fresnell for every kind of tft size and ratio, then the fresnell should be 400mm * 300mm at least. So 15" lcd users should cut it down to their custom size and 17" widescreen lcd fit as well. Those meassurents are close to 395mm*395mm large fresnell lens sold at 3dlens that most of us use right now, the only difference would be the focal lenght 550mm is ok for most of us.

if every of us go for our perfect fresenll size, it will be caotic. Lets find a universal size of fresnell.

Lets try to make this as easy and as profitable as possible for jimhuang. I suggest we keep to standard sizes that are easy to manufacture. A 15.5"x15.5" (395mmx395mm) is a standard fresnel size and will accomidate any size 17" lcd, and can be cut down to accomidate any 15" lcd.
Now that we have a one size fits all fresnel, we need to agree on a focal length. I have heard 500mm, and 550mm focal lengths. Personally, I am not sure how these will work with 450mm triplets, but since smarter people seem to know it will work, then I say either one will do.

So in the end lets just ask for 395mmX395mm with a focal length of 525mm. The longer we argue, the more frustrated jimhuang will be and the longer this will take to get to market.

Say "I" for all in favor of 395mmX395mm with a focal length of 525mm with 1mm groove pitch.

Good to see this interest from 3dlens.com. I bought my lenses there and they arrived here, at my door, in Canada in about 3 days i seem to remember.

Im using both their 310mmx310mm OHP fesnel set (.5mm) and their higher quality but smaller 260x260mm (.3mm).

Now that wide screen LCDs are coming on the market a big lens for 17" wide screens would be great.

NOT 525!!!

This is one case where the compromise position will not work at all! A 550 mm fl fresnel is perfect for use with the 450 mm fl triplet in a non-split design. Both fresnels would go at least 20 mm before the LCD.

If you want a split design, then the field fresnel has to go at least 20 mm on the other side of the LCD, so the focal length should be (550 - 40) = 510 mm.

A 525 mm field fresnel would have to go right where the LCD is, so nobody would be able to use it! (Fresnels have to be at least 20 mm from the LCD, else you see fresnel rings in the image.)

It is very important that the fresnel focal length is at least 550 mm: It is okay if the focal length is a bit long for your projector design because you can increase the distance between the fresnels and the LCD. But you can't decrease that distance lower than 20 mm if the focal length is too short. All you can do is change to a split design, and that doesn't give you the best screen image. People use 15" - 17" LCDs to get the best image possible. Using a split design violates that goal.