| cooldrums |
What are the major contributors to hotspotting and ghosting?
The setup i'm using is as follows:
400 Watt M.H. in a alumininum vented box painted black. Using curved piece of aluminum as a reflector. Not optimal reflective properties but good.
The light passes through 2 pieces of standard glass separated about an inch from each other.
Hits a fresnel sandwiched beteeen 2 pieces of regular plexi.
This is sitting right up against the back of the 7" LCD.
Then out to a mirror at 45 degrees, that was taken out of a rear projection TV and cut down to size.
Then out the Fujion Lens that Ace uses.
Focusing is not a problem. Light ouput is as well as minor ghosting.
No condenser lens. |
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| dcarpa |
What bulb arc size are u using. I would think that ghosting can either come from imperfect optics (all optics are imperfect, but how much can be tolerated is the question), and the need to simulate either a light point source or a light plane - a light bulb through the fresnel is an OK way to translate a point source to a plane but the source is never a point so its like having multiple points which end up having multiple images offset from each other - the ghost - so I'm of the strong belief that the biggest factor for a good diy projector is a good light engine, like the commercial types with the small arc sizes - and the short lifes
A reflector is a good way to increase the scalar light output of the bulb but if it doesnt help make the source either a point or a plane it is just going to lead to ghosting or heating - and if you read the threads there is a good deal of argument on what is a good type or reflector - along with some good rivalries on just about any topic (sorry ace and mathias).
I still think that any reflector behind the light to make the source a plane doesnt properly include the source itself so it is always a plane and point added togethor, or a circular reflector that puts the point source at the "center", but that center is the source which can warp the rays path, or an ellipse or parabola that just takes the source and the reflected and makes a combo source, neither is ideal
So - i'll shutup for now - The best way may be to use a high lumens source along with a light trap instead of a reflector so at least we can only focus with the source alone, and not its reflections |
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| cooldrums |
I don't know what bulb arc size is so i'll just give you a link.
https://www.1000bulbs.com/category.php?category=949
I think that my reflector sucks it's basically a piece of aluminum bent into a curve behind the bulb. I tried to put a shine on it with some polish but it lacks. That may be the ghosting?
The light box is aluminum with vents sitting inside one end of my wooden enclosure. See pic
http://www.midcoast.com/~barry/collage.jpg
WHat's not shown is the light channel (for lack of a better word) that is basically 2 pieces of wood for sides that are the width of the bulb and support the glass, plexi, fresnel, and LCD in that order. It starts about an inch away from the light box in the center of the cabinet.
I have a cheap fresnel but it does not show the rings on the screen like the one i was using before. IT is the proper focal length. I bought it from 3Dlens.com
I'll try to put up some more pics of my near finished product. |
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| dcarpa |
That bulb has a pretty big arc (similar to what I'm using because of local availability). I also get some blurring of the whites, but not so much as a full ghost just some leeking of the white in the same direction as the long axis of the bulb - and after some testing I'm pretty sure that its do to he bulb being more of a rectangular size. I've decided so far not to use a reflector because of how hard it is on a big bulb with a big projector to get a good uniform light engine.
I'm pretty happy with my image so far - for watching TV its more than OK since the only issue I have is that the TV sources is the biggest limitation to how it looks, for DVDs its pretty good - definately watchable without complaining but I also have a HDTV and after watching that for awhile its hard to compare it to, but the faults really start showing up when using the computer (games are OK, desktop is where the white leaking and any imperfections become obvious).
Brainchild has some some good pics of some great quality pics using what I think is the same bulb you are - i think if you do a search for "mogul metal halide"it should show them somewhere
Maybe just pull the reflector out and see if the ghosting gets any better - i use some old overhead presentations from work as a test piece for focusing since I can do a lot of moving without risking damage to the LCD and they have crisp edges and its easy to print out test patterns |
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| cooldrums |
| What about the hotspot of light? Will that improve without the reflector? I guess i'll know as soon as i try it. I can get a bright image unles the lights are out even then 60% of the light is centered in the screen in a big ball. |
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| dcarpa |
| The brightness is that different from the center to edge of the fresnel without the reflector? Are you too close to the fresnel maybe? I get a pretty even light distribution with this type of bulb, but my focus is a little out at the corners, thinking the fresnel is a little curved. |
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| cooldrums |
what a headache.
figure out the hotspotting, i needed to use 2 fresnels facing each other, unfortunately my focal length is screwed now.
I took the reflector out for now.
The image is evenly lit accross the screen no but still has ghosting. I'm wondering if the the fresnel focal length has anything to do with that or not?
Mayb because my video cable (unshielded) is running along side to the transformer, would that cause ghosting or just 60 cycle hum lines? |
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| cooldrums |
*******g around with cheap frenels is just not worth it. With the money i have spent on all the cheap ones i could have bought a nice set from Ace by now.
Thank you's to Ace as well for providing endless help! |
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| dcarpa |
I agree on both counts - and then you find that when you first turn the thing on and do small adjustments its great, but after awhile you start to notice every minor detail - and each small defect seems to become more and more obvious - then you swap a part, make it better, and you're happy.. then you see a small defect... and then the smallest defect seems like an elephant on the screen
If I could just find a good quality rollercoaster video file - the kind they used to use at amusement park theaters to try and make kids sick |
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| cooldrums |
See if you can get a free video from like hersey park or some theme park with roller costers. They have some short free ones on the net like at Universal and such. I know disney was giving away a free video.
yeh i went back home today at lunch and put the reflector back in since it wasn't contributing to the ghosting or hotspot.
these cheap fresnels seem to add a yellowish tint to the light when combined. Uuuuggh!
:h_ache: |
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