I have a nearly perfect setup for a front screen projection TV in my house. I have a 10' high X 18' wide wall on which to project an image. Directly across from it, about 18 feet away, is my kitchen sink, with a "plant shelf" going across the top about 9 feet off the floor. The ceiling in the whole room is 16 feet high.

Plenty of room to place even a large projector on the shelf and have it project onto the large wall.

I've been wanting to build a DIY projector for months and was well read-up on the pages at DIYProjectorCompany. However, I never could find a lens that would work for me. And I've looked everywhere.

Suddenly this weekend my wife got the itch to get an HDTV and just had to have one. We looked at several and were prepared to buy one until I discovered that apparently the lamp life in a DLP HDTV isn't any better than for a commercial projector and equally expensive. We watch a *lot* of TV, and there's no way I'm going to fork over $250 for a new bulb every 8-10 months just to watch TV.

So....I got her talked into doing the DIY projector. But now I'm back to the same roadblock on not being able to find a lens.

I'm looking to build something that can display at least 720p which means a 15" or larger LCD panel.

I am looking for a target projected image diagonal of between 80" to 100".

Unfortunately, even using the 500mm "pro" lens, it yields a 135" diagonal. Ordinarily, bigger is better, but I have a lot of light contamination in the room. In fact, one wall is nearly solid glass.

Thus, I hate to spread the projected light over such a large area, yielding a darker image that is also having to compete with light in the room. Thus, I want a smaller diagonal that will yield a brighter image.

Does anyone have any suggestions as to where I might be able to find a triplet lens with at least a 650mm focal length? I've looked everywhere, although I haven't looked in a few months.

I'd love any suggestions.

-Michael

Ok. Lets get technical. you say 'about 18 feet away, is my kitchen sink, with a "plant shelf". If you build a "straight shot" projector, the box is about 3 feet long. So how far is the lens going to be from the screen when sitting on the shelf? 15 feet or so?

It's exactly 18 feet wall-to-wall.

I have considered that the projector box could cantilever out off the shelf by a foot or so. Any more than that and it would look considerably odd.

I realize the focal lengths I need necessitate a long projector box. The shelf is about 24 inches deep. I was planning on using a mirror to fold the image around the corner. That way, the box can be as long as is required since it will be running the length of the shelf.

I've also considered that depending on what kind of screen setup I go with, I could move it about 12-inches from the wall, shaving my distance down further.

But even doing all that, I need a lens in the 6XXmm range.

-Michael

Using a 15 inch lcd and a lens to screen distance of 15 feet, with a 22 inch (560mm) opaque projector lens, the screen diagonal is 107 inches. Close enough?

Unfortunately no. Sorry to be so picky, but it just isn't what I want.

I need something that can throw an ~100 inch diagonal around 17-18 feet.

Obviously my other alternative is to find a smaller LCD....no easy task there either, unfortunately.

Any other suggestions?

-Michael

Here's your options as I see them.
1. 12 inch xga lcd, 560mm lens, 17.5 foot throw, 102 inch screen. The problem here is finding a 12 inch xga lcd and controller. Figure on around $700.00
2. 15 inch xga lcd, 700mm lens, 17.5 foot throw, 100 inch screen. The problem here is a 700mm lens. You can probably find a "process lens" on ebay with that focal length. They show up from time to time. Figure on $1000.00 for the lens. Also this setup will need a fresnel in the 800mm range. AWI Industries sells them. They will be pricey.

I got a quote for a 800mm FL fresnel from AWI last year. $130.00 for one lens and a minimum order of $250.00.

Ouch!

I was looking at a total project budget of $500.

Looks like I have a better opportunity of finding a smaller LCD panel than I do finding a lens.

I've noted a few posts from people experimenting with zoom lens arrangements. Any possibility of improving my situation with those? What about using a take-out lens from an existing LCD projector?

I've also considered having my local Opthamologist custom grind some lenses. They've indicated they would be willing to do so at roughly their cost just for the coolness factor. My limitation there seems to be that I'm limited to an lens diameter of roughly an eyeglass blank.....although I've noticed that lenses seem to be getting bigger as a fashion trend lately.

-Michael

Eyeglass blanks run from 65 to 74mm diameter generally.

You can make your own projection duplet, like slide projectors and less expensive overhead projectors have.

For a 700 mm projection lens:

Get two 0.75 diopter (1333mm fl) lenses from your opthamologist and mount them in some kind of a tube so that the lenses are about five inches apart (128mm). Hey presto, a 700 mm focal length projection lens.
Get the largest diameter blanks that you can to improve the field of view and brightness, because I don't know what kind of field of view this setup will give you and if the projection lens will be able to "see" the entire LCD. I "think" the field of view will be about 25 degrees. Not sure.

One nice thing about making your own projection lens, you can mount the lenses in interlocking threaded plastic pipes and adjust the focus to your hearts content.

Before you go shelling out big bucks on lenses at the opthalmologists, you might want to go to your local drugstore and snag a cheap pair of 0.75 diopter reading glasses, pop out the lenses and experiment with them a bit. It'll give you an idea of which way you would like to go.

Tgreenwood

Ooh....*that* is a fantastic suggestion! Thanks.

If that looks like it will work, I can keep an eye out for some surplus 0.75 diopter lens in a larger diameter.

Thanks again.

-Michael

I think this will work for you http://www.surplusshed.com/pages/item/l3517.html it is a 1333 mm fl lens that’s what tgreenwood said you would need but it is only 42mm in diameter.

tgreenwood could you do the same thing to get a 500 mm fl lens to use instead of the pro lens from ll?

Also, could you use the 700mm fl set up with the pro Fresnel from ll?

Believe it or not that one (500mm fl) is a lot harder to make if you just want to use off the shelf available eyeglass lenses.

Three options :

1) Get two 1.25 diopter (800mm fl) lenses and mount them 12.6 inches apart.

2) Get two 1.00 diopter (1000mm fl) lenses and mount them back to back just barely not touching.

3) Get one 2.0 diopter (500mm fl) lens and mount it and use it as a singlet projection lens.

With all three you will end up with a 500mm projection lens, except for number two, the effective focal length will run around 505-510 mm.
This will definitely need experimentation to find out if the field of view is good enough on any of them to "see" all of whatever size LCD you are using.

Remember that the Pro lens is a damn fine triplet lens and these will not give you the same results. Generally speaking, the more lens elements in your projection lens, the better the projection.

For a duplet lens system like 1 or 2 mount the lenses with the bulge of the lenses facing outward. That will help with the spherical aberration (loss of fine focus toward the edges). It may or may not be noticeable, it just depends on how particular you are and what you can live with. Experimentation is the key.

From my understanding, if the setup calls for a 700mm fl projection lens, any style projection lens will do, as long as it is 700mm fl. Is that what it needs?

Tgreenwood

(edit) if I need a positive meniscus lens (eyeglass type lens) I usually get it from Rolyn Optics. Not expensive.

OK...now I'm confused about the fresnels.

Seems like the condenser fresnel (the one closest to the light source) can be whatever focal length is convenient since it sets the distance from the light to the LCD.

The field fresnel (the one closest to the lens) would need to match the focal length of the projection lens.

The LCD needs to be at the focal length of the projection lens so it will be in focus (true?) and the fresnel needs to be at the same focal length so it can narrow down the light rays to converge directly at the projection lens.

Do I have that worked out right, or am I missing something?

So if I needed a 700mm FL projection lens, a 250mm condenser fresnel would work and a 700mm field fresnel would be required. Is this correct?

-Michael

Download the focal length calculator from the forums index and plug in the numbers. You'll get a better understanding of what the requirements are.

I'm guessing I could also mix/match different diopter lenses to get what I want?

In other words, I could use one 0.75 diopter lens and one 0.5 diopter lens about 1-inch apart and get approximately an 800mm doublet?

Seems like with a lens diameter of only 65-75mm using eyeglass blanks, I want the distance between the lenses to be fairly short to open up the field angle?

Could you build a triplet lens the same way? I'm assuming I could get a concave eyeglass lens for the center lens. Would the improvement from a homemade triplet be worth it over a doublet?

Alternately I've run across a 762mm FL 83mm achromatic cemented doublet for $60.00. Would it be any better?

-Michael

1. Yes, but if you use lenses with dissimilar focal lengths you lose out on the spherical aberration correction.
2. Yes, it's all done in the math.
3. Yes
4 & 5.. Technically it's a triplet but it's not optimized to minimize abberations, that's quite involved. So it would actually be worse.
6. An achromatic is one step down from a triplet so it would better than a pair of eyeglass lens. And much better if AR coated too.
May I ask where you found it? Hopefully it's not a telescope eyepiece, I'm not sure if that would be okay.

Before you get too far along with this I should mention something else. The pair of fresnels used magnify the length of the lamp arc.
The formula is: arc length times front/rear fresnel FL.
For a 25mm arc - 25 * 800(front fresnel)/220(rear fresnel) = 90.90 mm which should be a bit smaller than the PJ lens diameter
because the brightest areas of the lamp arc are near the ends.

Ahh...thanks for the explanation on the fresnels. Now I better understand the importance of their focal lenths.

As for the doublet lens, it does appear to be a telescope objective lens, not the eyepiece. I found it on SurplusShed a while back for $60. It's FL is close enough I could make it work. And at $60, it isn't much more expensive that trying to build my own triplet would be.

http://www.surplusshed.com/pages/item/l3547.html

-Michael