WRITTEN BY SUPRAGUY

What FL you can use for a fresnel is all about inverse square law.

I should really keep this somewhere that I can copy/paste it, 'cause it's a pain to work out.

Inverse square law states that the further away you get from a radiating light source, the intensity (lux) of that light diminishes as the square of the distance increases. This means that if you are 2 times as far away, the light will be 1/4 as bright. 1/2 as bright happens at sqr(2) the distance, or 1.414 times the distance.

1/2 as bright is about as much as we ever want the corners of our projection to be, since more than this is easily visible to the human eye. This much or less, however, we can get away with.

Now say we have a 15" LCD, and we have the collimator fresnel at 220mm from the lamp. The CENTER of the fresnel is 220mm away, but the corners are further. We can use pythagorean theorum to find the distance: 7.5" (1/2 the diagonal size of the LCD) from the center is 190.5mm. The distance therefore is sqr(220*220+190.5*190.5) = 291mm. This is still under the tolerable maximum. (The ratio is 1.32:1)

Say we used a 155mm fresnel, though. The distance from the lamp to the center of the fresnel would be 155mm, but the distance to the corners would be sqr(155*155+190.5*190.5) = 246mm. This is an inacceptable ratio (1.58:1) where the corners absolutely MUST be less than 1/2 the brightness of the center.

But, with that same 155mm fresnel and a 10.6" panel, the corners are only 205mm away, for a ratio of 1.33:1 -- which is again acceptable.

Work it out for a 7" or smaller LCD, and a 110mm fresnel becomes acceptable as a collimator.

Incidentally, this is also how commercial projectors get high brightness from small panels.

Now for the front of the box, this is simply a matter of determining the desired throw of the projector.

A 320mm triplet gives a short throw to a 15" panel, a decent throw to a 10.6" panel, and a very long throw to a 7" or below panel. Thus for small panels, a shorter FL triplet, and matching collector lens becomes possible.