Help with lens/lcd/fresnel distances (100WLED,5"lcd)

taez

Member
2013-02-14 5:29 pm
Hello everybody, i recently got back to building my older non-finished projector.
I require some help with the optics though.
Basically it is a 100W 8900LM~9500LM 6300K LED with 44x19mm collimator (placed in a 'collimator mounting base')
next thing is a 5" 800x480 (actual) lcd.
I have a fresnel sandwich from an old OHP - f=220/300 that is cut to be just a bit bigger than the lcd.
Then the lens - 11cm in diameter, doublet, f=should be 150 but here is the thing -

when NOT using fresnels-
the exact distance between front face of the lens and the wall is 2m (205-210cm to be precise).
although the lens' f should be 150, the image can be seen sharp on the wall when the lens' end is 4.5cm away from the lcd screen. The lens inself is 6.5cm wide (from front face to rear face)
and i would be glad if this was a good image, but it's corners are highly distorted (it appears like there is only a center portion of the screen projected on the wall, i can't even find the corners/borders)
When i move the LED away or closer to the lcd practically nothing changes.

With fresnels i have no idea what to do because if i try the advised distances for focal lengths it does not project anything.
My guess is that i have a terrible lens which i need to replace.
I can't find the proper fresnels, just ohp ones, can i use the 220/300 sandwich?
I have seen a couple of projectors which used non-splitted fresnel sandwiches just to light up the whole lcd without measuring the fresnel-to-anything distances.

Sorry for possible mistakes, thanks in advance for your help, i suck at optics stuff. :(