I like Guy Grotke's approach of building a lens to fit the application. He's mentioned using a DIY duplet with something like a 530mm focal length. Now, it's great that we can design a lens with the right properties, but we still have to find the right fresnel. How did you find a fresnel with that long of a focal length, Guy?
long fl fresnel
You don't just need a fresnel that matches the projection lens fl, you need one that matches the LCD to lens distance:
1/fl = 1/LCD to lens + 1/lens to screen
So with a 530 mm fl projection lens and a 14 foot throw distance, you get a LCD to lens distance of 605 mm. Obviously a standard 330 mm fl field fresnel will not work at all. You would have to put it almost 300 mm after the LCD!
I used a LumenLab "790 mm" fl fresnel (which I measured as 770 mm). I put it together with the condensor fresnel (770 - 605) = 165 mm before the LCD. This gives me a converging pyramid of light that focusses the arc image right at the optical center of my projection lens. One issue you have to account for: The pyramid shrinks by the time it gets to the LCD, so you have to cut your fresnels wide enough to light the whole LCD surface.
I would have preferred to use a 625 mm fl field fresnel, but I don't know where you can get one of those off the shelf. There was some talk that LumenLabs was having 650 mm fl fresnels made. If they do start selling them, I might replace my "790" just to make my projector more compact.
You don't just need a fresnel that matches the projection lens fl, you need one that matches the LCD to lens distance:
1/fl = 1/LCD to lens + 1/lens to screen
So with a 530 mm fl projection lens and a 14 foot throw distance, you get a LCD to lens distance of 605 mm. Obviously a standard 330 mm fl field fresnel will not work at all. You would have to put it almost 300 mm after the LCD!
I used a LumenLab "790 mm" fl fresnel (which I measured as 770 mm). I put it together with the condensor fresnel (770 - 605) = 165 mm before the LCD. This gives me a converging pyramid of light that focusses the arc image right at the optical center of my projection lens. One issue you have to account for: The pyramid shrinks by the time it gets to the LCD, so you have to cut your fresnels wide enough to light the whole LCD surface.
I would have preferred to use a 625 mm fl field fresnel, but I don't know where you can get one of those off the shelf. There was some talk that LumenLabs was having 650 mm fl fresnels made. If they do start selling them, I might replace my "790" just to make my projector more compact.
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