ok so i am undertaking my next projector and i want to make it small... im using a 7" portable dvd player and i want a focal length at about 80mm, but i cant find one in a big enough size or it is made by some specialty company and i have no idea how long it would actually take them to send the lenses to me. So my question is are there any?

i have looked at

3dlens.com
fresneltech.com
jmloptical.com
surplusshed.com
bhlens.com

and couldnt find anything.... if i have to make it bigger than i want i will deal with it but i wanted to make sure i wasnt leaving any stone unturned.

thnx

There are limits to how large a fresnel lens can be made of a given FL. Once the FL gets too short, the edge cuts of the lens have to be so steep that it's no longer practical, or in some cases even possible to have the lens reasonably thin.

80mm is a VERY short FL, I'd be very surprised if a fresnel large enough to cover a 7" LCD exists.

You might want to check out b0rna's Little 10.6 box PLOG.
He used a 10.6 panel with 200mm Fresnels and triplet. He managed to get his box down to about 18".
Building it like this might give you the option to upgrade your panel to something a little larger, when the prices come down.

ah i was not aware of that... learn something new everyday... thnx

lowest FL fres i have ever seen (and big enough for a 7 incher)

http://diy-beer.com/EN/store/comersus_vi...sp?idProduct=69

If you're dead set on having such a short FL you can stack multiple fresnels together. If you stack multiples of the same lens together I think the new focal length can be calculated like this

FL ÷ n = FL2

FL = focal length of each individual lens
n = number of lenses
FL2 = the new focal length of the set

That works for identical lenses, I'm not sure what needs to be done to calculate based on lenses with different focal lengths.


So lets say you can find a few 240 mm fresnels, if you want 80 mm you'd need to stack 3 of them together.

240 ÷ 3 = 80

Very cool info, Hirudin. I didn't know that.

I should have mentioned this before:

Just as a single lens will introduce losses, multiple lenses will introduce more losses. If a single lens only transmits ~92% of the light, 3 lenses are likely to only transmit ~78%, a pretty significant drop.

Also, there's a fair chance shadows/artifacts from the rings of the fresnels will be amplified by multiple lenses. I would imagine the problem would actually be exacerbated by fresnels with the same pitch so therefor could be lessened by using fresnels with different groove pitches.

Also, the shorter the FL of the collimator fresnel's focal length the less "even" the light will appear. There's probably a threshold focal length that is "too short" to be useful.

I don't think I've seen a single build where multiple fresnels were used. Maybe one of the problems above is too great to overcome?

Yep

BTW the effective focal length for lenses with different FLs can be calculated with this equation:

EFL= (F1*F2) / (F1+F2-gap)

DJ