i am already building my projector and just the other day i was testing the box with the lcd and rear fresnel already mounted in. I wanted to see what the image looked like while a movie played so i hooked up all the electronics and plugged it into my PC. I dont have the actual lamp yet and since my box doest have a top on yet, or back or front, i am free to examine everything.

anyways, to watch the dvd playing in my computer i grabbed a 100w standard lightbulb and plugged it in. Its on a hand held socket with a cord so i can move around and shift around the light. I noticed that when i was positoning the 100w bulb at or atleast really near the focal point of the rear frensenl (220mm) i wasnt seeming to get the whole LCD to get even light. So i moved it around a bit and found that the best position seemd to be further back. when i was about 300mm or so from the fresnel i had the whole screen evenly lit. And it didnt really sacrifice overall brightness, center was just as bright as before. Now should we apply this same principle to 400w MH lamps. maybe right at the focal point isnt good enough. does anyone know why past the focal point i was getting better light distribution? in theory, right at the focal point all light rays go parrelel and should evenly light the screen. hmmm.

This has been documented before. Having the arc right at the focal point often gives too much vignetting (bright center, darker edges). Moving the bulb further back up to an inch will give you a more even distribution overall but it WILL sacrifice brightness. There is no way around this unless you can break physics and rewrite the inverse square law the intensity of the light at the fresnel is inversely proportional to the square of the distance from the arc to the fresnel.. so basically at twice the distance, you will see 1/4th the intensity.

Note that the nominal focal length stated in the spec sheet ain't necessarily the true focal length. Also try moving the bulb closer instead of farther away. There will be a sweet spot you'll love.

I just did that actually, moved the bulb forward over 1" closer and it looks AMAZING. Even the corners are brighter.

See?

See diagram below - moving the bulb's arc closer to the fresnel allows the fresnel to encompass the entire arc instead of a small point of the arc - remember that the focal point is 220 mm - this is why I think it's brighter when the bulb is moved closer to the rear fresnel.

Click to view attachment

Yeah, but there's only one, single point that's ever precisely at the focal point. The other light is off-axis light and technically/theoretically junk light. Even with the arc positioned at what you label the bad spot above the whole arc is still "seen" by the fresnel but what comes out the other side of the fresnel is just a qualitatively different mess. At least that's how I think about it. But usually my drummer is playing out-of-rhythm and I still march to him. The take-home point is move that arc fore-and-aft until yer happy.

I know that empirically it works, but in our version of fresnel theory moving the bulb more than inch forward moves the entire arc outside of the focal point - thus all light should now be considered junk - yet it still works? I believe that as long as the arc is enveloping the entire fresnel focal cone, the fresnel 'thinks' it is seeing a smaller light at it's anticipated focal point... maybe?

Yeah, I get what you're saying. And one of my points was that neither of the pro fresnels - I THINK - have EXACTLY the true focal length that they are specified as having. I know for certain the field fresnel has been pretty much shown to not be what's it's sold as being (not that it's a problem at all) but can't remember details for the collimator. I think its true focal length is shorter than specified as well. Folks have to get it set into their heads that adjustments will be necessary and not to set their designs in stone.

Truth.