Could someone help me identify what kind of lens this is and if it's usable in these pj's. Pulled if off of a unit that I don't even know the name of, but thought that lens looked interesting. It's got a concave/convex side; Looking through the lens on a piece of paper with writings on it, I measured the distance at which the writing clearest, it is at 210/220mm or thereabout (this is on the convex side); on the concave side it's clearest at 250mm. The width is 60mm.
Any ideas?

Well, it's hard to get a size reference off of these photos.

1. Check if the lens has a positive focal length, and what that focal length is. To do this, see if it'll fry ants (Or dead leaves) on the sidewalk, and if it will, measure the distance from the lens to the point of light. Since sunlight is perfectly collimated (Or close enough anyway) this will give you reasonably accurate specs for the light.

If it has a positive focal length, then it could most likey be used as a pre-condenser lens for a light engine. It might also work in front of the triplet as a lens to increase the throw of the projection.

Thanks SupraGuy,
I just went outside in the sun and smoked a dry leaf at the pricise dimentions I gave , that's at 220mm. So, going by what you said, it's got a positive focal length, question is where would it work best of the two options you mentioned?
Thanks once again.

Here is a picture showing the size in relation to a CD disk.

That's a lot smaller than first looked. I don't know what type of lens it is, sorry.

assuming it is 60mm diameter and 220mm focal, I would say you can use as condenser lens, but it is very limited in that function. Maybe if you find your trhow too long you can use it to shorten the trhow increasing the image size (supra did small mistake here ).

I expressed it poorly.

What I wrote SHOULD HAVE read:

If it has a positive focal length it can be used as a precondeser otherwise it might be used to increase throw.

But yeah, it's still a mistake.

Greater magnification would only decrease throw (bigger image)....

Thanks guys for your input on this. So Brain, by this statement you're agreeing with what Rox said... [Maybe if you find your trhow too long you can use it to shorten the trhow increasing the image size] ?

yes, the negative lens would make a longer trhow and a smaller image.

It is not the comon case to decrease the trhow on a ll. Maybe if you had a small LCD (8"?) it would be a good idea, but i think precondensor lens is the best option for you.

hello just surfing around the forums and saw yours I think it maybe a fresnel lens a really small fresnel lense:}

hello just surfing around the forums and saw yours I think it maybe a fresnel lens a really small fresnel lense:}

[/quote]

A fresnel lens it is not, but a positive lens it is as these guys have said.
Rox, thanks for clarifying that - is it my correct understanding then that what a precondenser lens does is to increase brightness of the projected image and that it's placed right before the lamp? did I get this right or am I 'equivocado' about this?(I think that's the correct spanish translation of 'mistaken' )