I was wondering if this idea might make a decent projection TV. I am currently building an LCD projector but was wondering if this is a possibility in theory. The main reason I ask is that 27 to 30 inch CRT HDTV's are getting quite reasonable and this would solve a host of connection and setup problems. Im my case I plan to project from the rear thru a fabric designed for such use, thus hiding what ever type of projector I use. I am quite skilled at a lot of things, however optics is new territory. I would like the opinion of this idea in the illistration from the informed among us.
Thanks
Thanks
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Well heres the thing, Ive worked on rear projection tvs for alot of years (that doesnt make me an optics expert as the only thing that usually goes wront is dirty fluid), and they make a very good picture with three 4-5 nch tubes. The key is the lens is about 2inches from the tube on the other side of the cooling gylcol, so they collect all of the light. I have to believe that a single 30 tube cant put out that much less light than 3 five inch tubes. So the real problem with using tubes for projection is collecting all of the light and not wasting any of it. That is why I figued a large fresnel directly over the tube could focus all of the available light down to a projection lens. I would take a large fresnel lens like say the one behind the outer screen on a rear projection so that you could cover the whole tube face. I may be totally wrong, and this is maybe something that hasnt been tried, I was just wondering if the optics could be made to work like that.
the optics
It would not be brighter with the fresnel. The problem is that each pixel sends light out in a very wide cone. Only a tiny bit of that cone would get to the projection lens, so you would get a very dim image.
When you add the fresnel, a different part of the pixel's light cone would get to the projection lens. But it would not be a bigger part, so it would not be any brighter.
The condensor fresnel in a DIY projector turns the diverging cone of light from the lamp arc point source into a set of parallel rays. Then the field fresnel turns those parallel rays into a cone that converges at the center of the projection lens. This gets a lot of light through the lens and onto the screen.
You CAN use a diffusly backlit (non-parallel rays) LCD in the place of a projection CRT, but then you need a lens wider than the LCD with a very short LCD-to-lens distance. Just like in a CRT projector.
It would not be brighter with the fresnel. The problem is that each pixel sends light out in a very wide cone. Only a tiny bit of that cone would get to the projection lens, so you would get a very dim image.
When you add the fresnel, a different part of the pixel's light cone would get to the projection lens. But it would not be a bigger part, so it would not be any brighter.
The condensor fresnel in a DIY projector turns the diverging cone of light from the lamp arc point source into a set of parallel rays. Then the field fresnel turns those parallel rays into a cone that converges at the center of the projection lens. This gets a lot of light through the lens and onto the screen.
You CAN use a diffusly backlit (non-parallel rays) LCD in the place of a projection CRT, but then you need a lens wider than the LCD with a very short LCD-to-lens distance. Just like in a CRT projector.
This idea won't work.
However, an idea kicking around in the back of my noggin is to see what can be done with rear-projection bigscreen TV. I have to wonder if the lenses could be replaced to throw a 50" TV out to, maybe, 80" without much sacrifice in brightness. Or get rid of the big enclosure entirely and try to project onto a wall...seems to me that the rear-projection material blocks some light, so it might be bright enough.
No idea if it's feasible, and I don't have an RPTV to play with.
However, an idea kicking around in the back of my noggin is to see what can be done with rear-projection bigscreen TV. I have to wonder if the lenses could be replaced to throw a 50" TV out to, maybe, 80" without much sacrifice in brightness. Or get rid of the big enclosure entirely and try to project onto a wall...seems to me that the rear-projection material blocks some light, so it might be bright enough.
No idea if it's feasible, and I don't have an RPTV to play with.
Big screen
Its funny you should mention that, I was thinking the same thing. Most rear projection screens are two pieces, the rear part being a large fresnel. I have a 46" fresnel from a dead rear projection set and was thinking about putting in front of my sony and trying out that very idea. My time to do such playing is limited, however if it works I will post it here. I did take the screen out completely and projected onto a thin sheet and was able to get a pretty good picture at about 77", beyond that the lens wouldn't focus requiring some more space between the lens and the tube. The main problem is that adjusting the tube position and focusing the lens only brings the center of the picture in good, to get the convergence correct on the right and left requires getting into the service menu and putting up a test pattern and recoverging the entire picture. It wasnt worth it when I didnt have any intension of leaving my set butchered up like that.
Its funny you should mention that, I was thinking the same thing. Most rear projection screens are two pieces, the rear part being a large fresnel. I have a 46" fresnel from a dead rear projection set and was thinking about putting in front of my sony and trying out that very idea. My time to do such playing is limited, however if it works I will post it here. I did take the screen out completely and projected onto a thin sheet and was able to get a pretty good picture at about 77", beyond that the lens wouldn't focus requiring some more space between the lens and the tube. The main problem is that adjusting the tube position and focusing the lens only brings the center of the picture in good, to get the convergence correct on the right and left requires getting into the service menu and putting up a test pattern and recoverging the entire picture. It wasnt worth it when I didnt have any intension of leaving my set butchered up like that.
Question for Guy
Guy,
What about 2 say 4" CRT lens positioned in front of an 8" say Hami LCD or 7" Lilliput panel. ?
Could a picture be aligned from such a setup ?
Cheers
Guy,
What about 2 say 4" CRT lens positioned in front of an 8" say Hami LCD or 7" Lilliput panel. ?
Could a picture be aligned from such a setup ?
Cheers
very unlikely
>Could a picture be aligned [from one lens per half the image]?
With really good lenses it might be possible, but I think you would see a lot of blur in the central area of the screen. Lenses generally have their poorest performance at the greatest distances from the center. By using two lenses, each would project its most distorted area in the "good area" near the center of the other lense's image. So the two sets of pixels would not line up well. The result would be "ghosts" in the worst case, or just blur in the best case.
There is a good reason to use 3 LCDs and three lenses: Each can project one color over the whole screen. This is much easier to align, and you can get a lot more light through LCDs that don't have integrated color filters.
>Could a picture be aligned [from one lens per half the image]?
With really good lenses it might be possible, but I think you would see a lot of blur in the central area of the screen. Lenses generally have their poorest performance at the greatest distances from the center. By using two lenses, each would project its most distorted area in the "good area" near the center of the other lense's image. So the two sets of pixels would not line up well. The result would be "ghosts" in the worst case, or just blur in the best case.
There is a good reason to use 3 LCDs and three lenses: Each can project one color over the whole screen. This is much easier to align, and you can get a lot more light through LCDs that don't have integrated color filters.
Guy,
The 3 lens idea sounds like the best solution but what resolution of a picture would you end up with..?
3 x the resolution of a single LCD ?
It's back to finding low cost mono LCD's with good resolution and of small size and cost.
The 3 lens idea sounds like the best solution but what resolution of a picture would you end up with..?
3 x the resolution of a single LCD ?
It's back to finding low cost mono LCD's with good resolution and of small size and cost.
resolution
>3 times the resolution?
Nope, sorry: At best you get the resolution of one of your LCDs.
The benefit is that you can split the light efficiently so most of your lamp's red, green, and blue light goes through one of the LCDs and gets to the screen. (This requires a selective beam splitter that passes one color straight through, sends a second color out the right side, and sends the third color out the left side.)
LCDs with built-in color filters lose at least 66% of the light as the two other colors are absorbed by each filter.
>3 times the resolution?
Nope, sorry: At best you get the resolution of one of your LCDs.
The benefit is that you can split the light efficiently so most of your lamp's red, green, and blue light goes through one of the LCDs and gets to the screen. (This requires a selective beam splitter that passes one color straight through, sends a second color out the right side, and sends the third color out the left side.)
LCDs with built-in color filters lose at least 66% of the light as the two other colors are absorbed by each filter.
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