i found a huge projection lens at a thrift store. it was in a old projector from the dinosuar days. it had a television inside a box from the fifties or sixties. the projector had no freznels or other lens except for the triplett was about 10-12 inches in front of the screen. i can hold the triplett in front of my small 14" tv and it will project a upside down and backward image on my wall about 10 feet across at a distance of 10 feet from wall to triplett. (have since figured out that anomaly) that said will this lens coupled with an lcd and some IR glass a good relector and a 400W metal halide bulb get me a decent image.
tried to post pics but the computer said the images were to big (i have never posted to a blog before

Pleas post a picture if you can. There is a good chance the lens will work OK. The main problem is it is made for a curved TV screen. If the TV screen is mostly flat you will get good perfomance from it.
If you go with a smaller LCD like a 10.6" you will have better focus on the outside edge.

How large was the CRT? Was the screen flat?

No total attachment can exceed 110KB, so resize your picture and then you can post it.

the crt (tv screen) was 10 inches and curved(in the old box) i held the triplet in front of my own curved tv (14 inch screen) the edges were out of focus and dim. i held in front of a flat screen monitor(17") and the projection was all in focus just dim b/c the monitor is not that bright i was projecting at 10 feet from wall to lens the dimentions of my triplett are lens is 6 1/2 inches across and the height of lens from 1st to 3rd lens is also 6 1/2 inches tall. the lens is the same size circle as a six and a half car audio speaker.

Does it look like this?

Click to view attachment

If it does, the focal length is supposed to be about 320mm


Tgreenwood

You could test the focus by using a transparency with test printed on it and just about any light with a reflector behind it.

With a cheep flood light place the transparency on glass or hang it flat. But the spotlight (it can be low power like 50 watts) about a foot behind the test pattern and set the lens on something and focus the image on the wall.

Block off the extra light with a cloth or something.

There is a good chance that you will not be able to get the whole screen in focus at the same time.

The only sucky thing is finding a cheep transparency. I paid $13 for a pack of them.

that looks exactly like what i have, except the lens tube is inside of a aluminum tube with a flange on one end for bolting to a projector box. i bought the whole proj for $21.50 tax and all at a thrift store in Alabama.

I cut this out of an old PDF I have from when the Beta II lenses were being sold back in the day (1981) in a DIY rear-projection TV Kit called the Poly-vision IV out of Queensland Australia. Looks like you have a rare find there.
Click to view attachment
The focus tube is aluminum, but the lenses and housing are made out of plastic, so don't let it get too close to your projector lamp. One of the members here (I think it was here) tried to use one of the lenses as a pre-condenser and melted the lens nicely.

Tgreenwood

PS. 11.98 inch focal length comes to about 305mm focal length, so my memory wasn't too far off.

Could you be so kind to tell me why this old thing did not have freznells in it, all the proj's i have seen here all have the two extra lenses. would i even need freznells?
thanks for all the knowledge you've given me..

The fresnels are used for what I call "light management", because the only effect they have on projection is the brightness of the image (Well, they also have a small effect on the placement of the triplet projection lens, but not much). They direct the light the way you want it to go.

Since the light coming from a point source lamp hits the LCD in the shape of a cone, the fresnel between the lamp and the LCD makes the light move in a parallel direction so the light goes straight through the LCD. That makes the light more even.

Click to view attachment

The fresnel between the LCD and the triplet takes that straight light and narrows it back down to a point so that if the triplet is at that point all of the light from the LCD will go through the triplet and be projected. Makes the projection a lot brighter than it would be without the fresnel. Without the fresnel funneling the light down to a point, not a lot of the light would hit the triplet and be projected.

The standard LL triplet has a clear aperture of 60mm so the surface of the lens available to catch the light is only 7.4 square inches in area. The LL Pro triplet has a clear aperture of 100mm so the surface of the lens available to catch the light is 12.4 square inches.

The lens you have has a clear aperture of about 6 inches, so the surface of the lens available to catch the light with it is 18.8 square inches.

With such a large lens area the designers probably figured that adding a fresnel between the TV and the triplet wouldn't brighten it up enough to justify the expense, especially since the CRT it was paired with is only 10 inch diagonal itself.

If you want to use an LCD larger than 10 inch diagonal, it would be a good idea to use a fresnel between the LCD and the triplet to funnel more light into the triplet.

You will still need a fresnel between the lamp and the LCD if you are using a point-source light.

Did all of that make sense?


Tgreenwood

Thank you so much i will be ordering the lenses from LL asap!!

The issue as already mentioned with the Beta lens is that it was designed for use with a curved screen CRT/TV tube.
The consequences for our use is that when using with a flat LCD panel the resultant image will not be corner to corner sharp rather the centre will be pin sharp but the further out you go to the corners the less sharpness and the more blur you will see.

The only way to correct that is to find a sweet spot when focusing however it wont be pin sharp.

To address this..one could try ...perhaps at the screen (curving screen corners away from projector till sharp) or perhaps trying to curve a fresnel..or as a last resort perhaps trying to alter spacing in the lens (although I have not heard of anyone trying successfully)

Having said all that I still think it's worth a go experimenting..especially if you have already one to try !

The positive about this lens is that it is the biggest aperture lens I know of and it's why it was used to build DIY CRT Projectors successfully...collecting way more light to the screen than say a Beseler.
Ive considered using mine with a LED light source..maybe one day I will get around to it .

I think you keep getting luckier and luckier.

Remembering that you said that the projection was all in focus with the flat screen monitor made me go back to that Poli-vision PDF again.
In their plans for a DIY rear projector, the projection screen they had used was a curved screen.

It looks like the lens was originally designed for a flat screen source, but since they were using it with a curved screen TV as a source they had to curve the projection screen to compensate to get it all in focus.

You lucky son-of-a gun, it looks like it'll work perfectly with an LCD.

What kills me is that I had one of these lenses about a year and a half ago AND I SOLD IT!!!!!!
AARRRGGGHHHHHH!!

Tgreenwood

I hope he is lucky as well..though I thought the screen was curved because it was Very High Gain in other words to eliminate hot spots the silver screen was curved.

The lens I have Beta II/Polivision IV has focus issues with a flat screen..it will be interesting to see if
'20dollar' results are sharp then perhaps this would indicate more than one model lens was manufactured.

Im dubious though as I'm not aware of any flat screen display back in the 70's/80's

In any case '20dollar' good luck with it it's definitely worth trying !