projection panel help

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ok i have a projection panel and iv got it all apart my ? is should i use the fresnal peaces that is on the front and back of it or just the monetor part im going to be useing 2 220mm fl fresnals but im wondering if they would help improve my image by useing them also?

front one makes things bigger
back one makes things smaller

guess it would be back one pulls light in and front magnufys the image for ohp

any sugestions would be great

thank you
 
why fresnels

The fresnels are not there to magnify anything. If they do magnify, then you have them too far from the LCD! You can build a projector without fresnels, but then very little light will end up on the screen. The fresnels direct a lot more light into the lens, so your image is watchable.

See the drawing below. Rays spread out uniformly from the lamp arc. Those that hit the condensor fresnel (1) get refracted so they are mostly parallel. In this drawing, the fresnels are split and are placed on either side of the LCD. So the rays go through the LCD in parallel. The field fresnel (2) refracts the rays so they focus an image of the lamp arc right in the middle of the projection lens. That gets the maximum amount of light through the lens and onto the screen.

You can also build a non-split design with both fresnels together before the LCD. Then the light goes through the LCD in a converging cone, and gets focussed in the projection lens.
 

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