| rmccoll |
I'm here to help anyone who is interested in building a rear projection, I have succesfully build a rear projection tv use a design I found at www.draperinc.com. I don't know if there are calculation that you can use to figure out how far your projector from the I just used trail and error and it works.
To start out figure how big you want screen, 40" to 50" inches are easy to make but 60 and up are harder, unless you are going to but the projector directly behind you screen. After you figure out how big you want your screen find a mirror that is a couple of inches smaller than your screen or one the size of screen. You need a large mirror if you want to keep it small and keep the image bright.
Now for the projector, first build it so that the image will show correctly on the this mean that it shows up on the wall like a regular tv. Mine is made with the Sony PSone Lcd, a 42 watt(2700 lumen) compact floruescent light bulb and the Figinon rear projection tv lense.
Build a frame for the screen just something to hold it up, my screen was made from a piece of plex-glass and a opaque show curtain. Now here is were the trail and error come in, you have to figure out how to position your mirror and projector to get a image that fits you screen and is focused. Once you get it just right build the rest of your frame around it.
Once you get it build you notice that the screen is %100 viewable from a distance and at eye level, this is because the light rays coming from the projector are not the same length, thats why the center is brighter than the other areas around the screen. The longer the light rays are the darker they are when they reach your eyes. You would need a larger fresnel lense the size of screen to make the light ways equal when they reach your eyes.
I wish could add pictures but I dont how to but more than one image on a single post. |
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| rmccoll |
| This the draper inc design wish my could be this pretty. |
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| 18wheeler |
| could you give more details on the screen you made? Thanks! |
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| rmccoll |
| My screen was made out of a piece of plexi-glass and a opaque shower curtain I got from Wal-mart, iron the wrinkles out of the shower curtain frist then place over the plexi-glass. Another idea is to use sand blasted plexi-glass or the acrylic diamond glass that go over flourescent fixtures. |
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| ace3000_1 |
heya bud, glad to see you have made one of the rear projection tv's! somthing that there isnt much about in here and congrats on your effort and research, im interseted in such a device but i had a slightly different plan, basically the thing was going to be the shape of a crt screen at the back and not using any mirrors, not sure if it would work but i think it could give us run for our money in the optics and trying to get them right, it would be good to set one up so its inbuilt into a wall that way it is like a flat screen just on the wall, either that or cabinet ect, either way there is alot of potentional here for this and i think it would be good if more wer into it, as for your screen it sounds spot on to me, i was going to go with the sandblasted plexi for a front projection screen but here where i am right now its abit hard to get hold of a sandblaster so ill wait till i go back to aust, anyway keep us posted and lets get the ideas in this flowing.
Trev |
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| 18wheeler |
I hung a shower curtain and projected a 40-50in image. The quality was incredibly well, sharp and bright. But the midle is obviously brighter than the edges. I believe it is because the plastics refract lights, and at eye level, more refracted light from the center of the curtain will get to eyes than those from the sides. So I folded the curtain to make a two-layer, and the screen is more evenly lightened. but you have to make two layers closely together, otherwise the image is fuzzy. I would try to strech it on a frame.
rmccoll: is the plaxiglas used for supporting the curtain?
and do you know the price range of draper's rear projection products? I think they use big fresnles to make rear projection screens. |
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| ace3000_1 |
huh interesting, i think what we need ultimatly is a good difuser that wont eat too much light yet yeald a bright image that is even, i have tried plexi painted white on one side for the front projection system but plexi uses light, it was 3mm thick plexi, i think ultimatley a 1mm or 1.5mm thick plexi would do the trick, maybe plexi in the white coulor or a difused plexi, i did try to see what it was like on the other side of the plexi to see if i could see much of a picture on the otherside and yess quite bright actually and clear, i dont know how evenly lit it was as it was only on a small piece of scrap but i think its got potentional, i maybe going to get more plexi this week so ill have a look and see if i can get a few samples to try different types and to see whats best or if this idea is worth persuing, an idea i had was the chistmas spray u put on your windows at christmas for the snow afect, this stuff is quite translucent and difused, abit like the phosfourus in a crt so maybe this will work, the only thing is it will have to be evenly spayed, another idea yeah yeah they keep coming eh? lol is to use the contact u put on school books, use the white that is quite transmisive too and easy to aply, anyway any thoughts keep them coming.
Trev |
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| rmccoll |
My show curtain is taped to the plexi-glass and mounted to the frame.
The price of the RPS depends on the size of your screen, you have fill out a order form and they send the price to you, but i know its probably really high.
The screen I made is not good, you need something that can get the light to your eyes like a large fresnel lens, I believe edmund scientific sells a large fresnel for a solar oven.
Here is another picture, I bought a $20 camera to take the pictures thats why they're so bad. |
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| 18wheeler |
| paint will block too much light. I would sand the plexiglas. to experiment, maybe sand half with coarse sandpaper, another half with fine sandpaper see which side is better. |
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| ace3000_1 |
ive tried the sanding but u see the sanding marks thats the only prob, as for the painting idea the snow paint that i was talking about is the best cos it dont block much light as its translucent, normal paint will depending how thick u put it.
trev |
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| 18wheeler |
coarse sandpaper makes hotspot softer. the result is even better if both sides of the plexiglas(2-3mm thick) were sanded. but the image is not as sharp. so a large 1mm plexiglas may do the trick. it won't be as good as those expensive fresnel type rear projection screen, but it is DIY :)
I way make a 50-60" screen, so the quality of image would be acceptable. besides, my eyes are kind of strained after watching 100" picture from 8-10 feet away. |
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| ace3000_1 |
yeah the 1mm would be the go, a sandblaster is what we realy want, cos the small pits are the difuser and they are like small crystal chips that light up like the glass bead screen for a front projectors as u probally already know, not sure if u know of it or not but comps are getting modded these days and there are things called apogies, u can get the stick on type or the sandblasted type, the sandblasted way out performs the stick on type as it colects the light and re transmits it in the sandblasting, ive done a etching before on this comp of a dragon and i must admit its bright as, also with the sandblasting idea we could experiment with different coarsness of sands to get the optimum picture vs difuser ratio and brightness, im just hoping we can difuse it enough, i think that we can find a way without the need for a big frensel but i think a big frensel would be the optimum way, either way diy rules man and lets keep it cheap.
trev |
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| ace3000_1 |
that diffuser in the lcd's back light can we get a hold of that? that would do the trick just a matter of getting it big enough
trev |
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| mikejz84 |
| Have you thought about digging around getting a trashed rptv and using the screen on it--most bigscreens use a fresnel type lens as the screen--it results in much higher gain then just a rear projection smoked glass style screen. |
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| corwes |
The diffusers behind LCD monitors are very similar to the plastic
film architects draw on ( the non Autocad guys or resistance to change guys.) You do get it in various thicknesses and levels of matt. It is available in rolls or sheets of up to size A zero. Two or three layers might work perfect.
Further there are thousands of different types of plastic film available to the screenprinting industry. You even get translucent silver films ,you name it , it is available .
If we all can work together and each person get some samples or offcuts from a supplier or screenprinter , I am sure we can find a rear projection screen material that match the commercial ones for a fraction of the price. |
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| kcsabresfan |
This rear projection stuff looks really cool... but I want something bigger. I'm using a QA-1650 right now, and even though it's not that great of a screen, I am getting a picture over 10' diagonal and I think it looks great. The screen door is pretty bad, but just a touch of fuzzy focus and it's very bearable. I'm more worried about getting a good clear picture and getting a nice enclosure for it now...
I don't need a 10' picture, but I do want to be at least around 80". If I was going to go with a 50" screen, I'd buy a 50" tv.
I love the way that the whole thing can become an enclosure, could be built into the wall or built into a cabinet, but I just don't want to go to all the trouble for that small of a picture...
How do-able is a larger picture? |
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| ace3000_1 |
heya kc its very doable to make a 200 inch screen lol but do u have the cash and room for that? rear projection tv's use abit more room then a ohp but realy can be much better then a front projection system, and i must say look better too, but they use abit more room, basically the bigger the picture the farther from the wall it will need to be and the prices in optics like a huge frensel starts to get up there, personally i think 60-70inches is all u need for a rear projection tv, most of the new if not all of the front projection projectors are actually designed for 80"screen size even though they go higher, that also includes crt projectors, 80inch is the biggest i think a home owner realy needs, anything bigger then its for a large home cinema unless u want to feel sick, i think a wide screen would be best at 80inch but thats me, but anyway its doable.
Trev |
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| rmccoll |
| How about the plastic that cover flourescent fixtures, they have little diamonds over the whole thing that spreads light evenly. They sell it Lowes, they also have the kind that looks sand blasted but I believe it's not |
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| kcsabresfan |
This rear projection stuff looks really cool... but I want something bigger. I'm using a QA-1650 right now, and even though it's not that great of a screen, I am getting a picture over 10' diagonal and I think it looks great. The screen door is pretty bad, but just a touch of fuzzy focus and it's very bearable. I'm more worried about getting a good clear picture and getting a nice enclosure for it now...
I don't need a 10' picture, but I do want to be at least around 80". If I was going to go with a 50" screen, I'd buy a 50" tv.
I love the way that the whole thing can become an enclosure, could be built into the wall or built into a cabinet, but I just don't want to go to all the trouble for that small of a picture...
How do-able is a larger picture? |
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| Nevermore |
I just found this thread on the internet, and I must say that there are to little people builing an RPTV.
I'm building one myself at the moment, and I hope to finish the damn thing before the end of the year.
I'm using a plexi glass screen, which has one satin surface (Atoglas). For as far I can tell up to now this works great if you can bring enough light to the screen. Satinized plexi has a little smaller grain, but is hard to find. What I did was ordering samples from different producers, before ordering.
For the cabinet itself, I use a two-mirror system. This allows a longer projection distance in a relative small cabinet. I found that the downside is that if the angles of the mirrors some times can give very weird images, but this is just trial and error.
At first I started out with a projection panel, but now I'm switching to a LCD projector. The image is MUCH better, and brighter and it was a real bargain. |
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| nmd_sb |
also in Home Depot (Hardware store) I have seen white translucent plastic board, I dont know what its used for, I am guessing Kabuki theatre :) or strip tease maybe.
I bet that will also work great. |
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| ds21 |
Has anyone used a screen from Harkness Hall, there pretty cheap.
I got lucky at work the other day. I know the A/V guy at work pretty well, and they have an 1997 RCA 60" RP that they've had to fix one to many time, and he gave me first dibs on it. I should have it home next weekend and gut it out, once I get my projector inside and working, I get one of the "Harkness Hall" screens to do a comparison, if it's good I'll construct a bigger unit. |
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| nmd_sb |
hi ds21
if you're not going to use teh RCA lens, can I have it? |
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| ds21 |
| nmd_sb, I'll keep you in mind. |
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| ace3000_1 |
hehehe wow! have a look at that mother of a frensel, i thought i throw this link in here as this place makes custom frensels and custom sizes, not sure of the price though will need to email them, would be good for a screen.
http://www.ntkj.co.jp/products-e.html
Trev |
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| ds21 |
you night want to check this out, could solve your problems with the hotspot, it's a large Fresnel lens 31" x 40" it's the cheapest I've found, and it's light so shipping shouldn't be much.
http://sales.goldmine-elec.com/prodinfo.asp?prodid=9039
if you want bigger! here's one 38" x 50" for $100 plus a lot for shipping:
http://www.alltronics.com/lenses.htm
Havn't found a good place for a Lenticular lens (the grooved one that you feel on the front, but I'm thinking you can get by without it (it the lens that spreads the light out to the sides for viewing of angle) just put the Giant frenel behind what your using now.
I just did a quick test with my gutted 60" RCA and can say this will look incredibe!!! I only live in a small appartment so the limit of 60" is no problem for me. just have to figure out new optical arangment to shorten focal length so I can fit it all inside the TV. |
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| rmccoll |
| Thank you, Thank you And Thank you again, you just saved me alot of time, I was going to use a dremel and a circle guide to cut every little grove into a sheet of plexi-glas to try and make a fresnel lense you savee me alot of time. |
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| rmccoll |
| Now that I have a large fresnel lense I'm going to rebuilt my tv to make it look more professional and brighter. I'm also rebuilding my projector using a gameboy adavnce screen I know people will say that my picture will have a low resolution but I bought a A/V adapter from www.lik-sang.com and was convinced, the picture it produce was better than I thought for a resolution of 160X240 that only equals 38400 pixels, but I'm going with it, I'm just trying to get it built. |
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| rmccoll |
Its been awhile since I have posted but I have made alot of progress. Instead of using a gameboy advance as my screen I'm using a Casio portable TV it's 3'' inches and the resolution is 115,000 which is good enough for me it's way better than 39,800. For my light Iam using a 100 watt halogen bulb it's really small about 3 inches long by 2 inches high. I'm using a condesner lense system I got from a old slide projector because they are 2 in. in diameter and it focus light really well.
I will post picture of my setup tomorrow. I ordered a camera from ebay and it should arrive tomorrow. |
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| freeupyermind |
| If your into the whole rear projection thing then here's something for you to aspire to! |
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| freeupyermind |
| instead of using a big mirror, could u use a big peice of plexiglass covered in mylar to transfer the image from the projector to the screen? |
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| rmccoll |
Yeah I thought about it but it would be easier to buy a plexi-glass mirror.
You can buy them form www.usplastics.com you might want to check this site too they sell everything literally they sell everything, ballast for MID bulbs and the bulbs check them out www.mcmasters.com |
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| dbc105 |
I use to own a rear project tv but some part went bad and it was going to be $600 for parts. I guess my buddy that looked at it still has it in his shop. has anyone used a bad big screen tv and just put a lcd setup in it? the screen and lense are aready there. any thoughts on this would be a help. I am going to try to call him this weekend and see if he still has it.
David
dbc105 |
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| rmccoll |
| I guess it can work. What size is the screen? If going to use a computer LCD you would need to use a lense with a low focal length to get the projected image to fit the screen. |
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| dbc105 |
the screen is 50" to 55". will I have to use a wide screen LCD and if it has a lense inside do you think I could use it? if not how do I figure focal length? I am going to give him a call today to see what he has sitting in his way. if I can make it work it sure would save a lot of work.
David
dbc105 |
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| PeterMitchell |
I know this is a bit off-topic, but I have a Fujinon 4 1/2 lense collecting dust. Just wanted to know if you consider the 4 inch Psone lcd (assuming that is the one you use) decent for projecting purposes (not rear projecting)? Would be nice to build a cheap projector to project a 50-60" image considering the psone screens are so inexpensive.
Seems to me you get a decent picture disregarding the dark sides. Would it be possible to project an image on to a normal screen (not rear projecting) and up a picture of it here? |
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| goldmoth |
| I have a used fresnel from a old big screen. I played around with it a while back using my ohp setup. I suppose the reason rptv manufacturers use a large fresnel instead of a standard grey translucent screen like what is normally used for lcd rear projection is the hot spot problem. Bright in the middle dark on the sides. I have a high quality projection tv, but I can say that I would prefer to use a standard grey screen, with the fresnel you have viewing angle problems and it seems muddier than rear projected lcd images I have seen.. anyone have ideas on how to use a grey screen instead of a big *** fresnel? |
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| dbc105 |
well I got in touch with my buddy and he has several rear projection tv's he is going to throw out. in selecting a case what do I need to look for? type of lenses, fresnels, screens, etc? any and all help is greatly appreciated.
David
dbc105 |
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| mbates14 |
| i have an old 48" fresnel collecting dust around here. its a little scratched, but i guess it works. it just sits and collects dust. |
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| rmccoll |
| Sorry I have not post but I just entered college and just got my internet connection on. The Psone lcd will work I used the sony brand 5 inch lcd for my first one. |
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| dbc105 |
well I picked up 3 bad RP big screens this weekend. I have not had a chance to pull the backs and take a look but plan to do that this weekend and see what I have. I can see in the lower front of one and it has 3 CRTs pointed toward the rear at an upward angle so I am guessing that they are pointing at a mirror. it maybe that I can position the projector part where the center CRT is. rmccoll, do you have any more pics of what you built? Landers(the tv repair guy) said he had several of these things in his way so maybe I can get lenses, fresnels, etc...
David
dbc105 |
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| mbates14 |
yea, thats what you should do. remove teh CRTs, and have the projector lense where the green CRT was.
If it were me, i would attempt to repair the RPTVs before I use them as projects.
I just got a 53" i found of the street, and im currently ordering a vertical deflection IC, which is $4!! thats all thats wrong with it, along with a fusable resistor. and a shorted DC supply diode.
after this, alls well.
ALSO, if one of your 3 sets has a magnovox PTV-375 chassis, I need the convergence waveform board(the one with 40 knobs).
shout at me if you find one. |
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| karmaz |
| quote: | Originally posted by Nevermore
I just found this thread on the internet, and I must say that there are to little people builing an RPTV.
I'm building one myself at the moment, and I hope to finish the damn thing before the end of the year.
I'm using a plexi glass screen, which has one satin surface (Atoglas). For as far I can tell up to now this works great if you can bring enough light to the screen. Satinized plexi has a little smaller grain, but is hard to find. What I did was ordering samples from different producers, before ordering.
For the cabinet itself, I use a two-mirror system. This allows a longer projection distance in a relative small cabinet. I found that the downside is that if the angles of the mirrors some times can give very weird images, but this is just trial and error.
At first I started out with a projection panel, but now I'm switching to a LCD projector. The image is MUCH better, and brighter and it was a real bargain. |
Hi Nevermore,
I'm very interrested by your experience!
Can you give reference (12128001 ?), price, specs, resseler of your screen ?
Results of different sample ?
Any snaphot of the screen ?
Details of your box, type of mirror...
Thanx
The French guy...with the very bad English ;) |
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| dbc105 |
mbates14, not much chance in repairing any of them, the guy I got them from repairs TVs and a lot of parts have been robbed from each one of them. But, they do have screens, lenses, mirrors, etc... That should save me a lot of time and money in the start. Question, will using a LCD monitor screen instead of a panel get the picture quality up to High Definition? I stumbled upon a projector site the other day and the guy was using a LCD monitor and a line doubler on his projector to acheive a very high quality picture. Also, how would you go about getting wide screen?
David
dbc105 |
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| dbc105 |
Well I got the backs off the 3 RPTV I got. 2 of them use a single mirror to reflect the image onto the back of the screen, the third tv uses 2 mirrors, a small one down low and a large one behind the screen. One of these is a Pioneer, it looks to be the best choice. The CRT projection lenses are bolted to a metal plate in a row pointed toward the mirror. I am thinking of making a part to fit in it's place to hold the lense on the top side and the LCD on the bottom side. Using the existing setup the angles should all be ok if I can get the focal points to workout. Can't decide whether to put a PC with a capture card and descaler software or to use a XGA Theater line doubler. Also thinking about using one of the 7" lcd's that ace used in his projector. That is a nice setup. With the RPTV being all enclosed I am not sure what to use for a light source. Should not need as much as a regular projector due to no ambient light. So I am thinking that a 150 watt should be more than enough. Maybe even less. I am going to see about renting a LCD panel for a OHP and do a little testing with light sources before I spend any money. Hang it in the box behing the screen and put the light behing it and hang the back on and look at the screen for an idea of how bright it is. Any thoughts on this anyone. I would like to get a picture like a HDTV has if possible. Bigger is nice but quality is the biggest thing I am after.
David
dbc105http://us.f1f.yahoofs.com/bc/403ce03d/bc/pics/RPTV.jpg?bffBCDABl1oKsH9t |
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| Nevermore |
| quote: | Originally posted by karmaz
Hi Nevermore,
I'm very interrested by your experience!
Can you give reference (12128001 ?), price, specs, resseler of your screen ?
Results of different sample ?
Any snaphot of the screen ?
Details of your box, type of mirror...
Thanx
The French guy...with the very bad English ;) |
I'm a little behind schedule, but I will try to post some pictures and details soon. |
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| dbc105 |
that would be a big help Nevermore.
Thanks |
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| dbc105 |
Ace, you had shown some interest in RPTV. what thoughts and ideas do you have on it? how would you set one up? would you use the 7" lilliput and the mirror setup?
Nevermore, are you still here?
David |
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| ace3000_1 |
heya buddy sure just time is taking my interst in this feild away right now but its doable with a lilliput lcd, i probally would use a mirror, before the mirror there would be the lcd at the bottom with its light engine, then after the lcd i would magnify the image to a mirror so the image is bigger and u have a larger more even throw area, then it should project onto the screen from a right angle, u would need a short focal lens for this and a very bright light, if u wanted to get the best results maybe use a frensel behind the screen the same size as the screen but i gather this may give u a viewing angle problem, but without the frensel behind the screen the image would be very even if you set the optics up right in the light engine.
Trev |
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| rmccoll |
| I know the 7 inch lilliput is a good lcd, but does any know about the 1.8in lilliput lcd? I just ordered off ebay. |
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| rmccoll |
| yes that is the model |
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| acidworkzfx |
I was just wondering, you said use an opaque shower curtain. would this need to be a clear curtain with the foggy effect or a white opaque shower curtain? I noticed also if I hold a piece of paper up to my projector I get a nice pic (coming through the paper) Is this how a shower curtain would work?
Thanks
Andrew:smash: |
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| rmccoll |
| It is a clear shower curtain with a foggy effect you can them at wal-mart for 2.50. When first get it iron the wrinkles out of it, don't iron curtain put a piece of cloth over it then iron. |
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| dbc105 |
ace, thanks for the reply. sounds a lot like the layout of the bad RPTV I removed the back from last week to have a look at the screen. it uses a mirror to reflect the image on the screen. the screen has small vertical ridges on the outside and it is smooth on the inside. you can not see through it though. it is made of a plastic also. I guess that these ridges help with the viewing angle. when you look at new RPTV screens, they are smooth on the outside and seem to have a clear cover in front of the screen. we need a tv repair guy in this thread that has worked on one of these new HD RPTV.
rmccoll, can you post more details or drawings of your design? is the picture dark around the edges or is this just with the photo of the screen? what size is your screen?
thanks,
David |
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| rmccoll |
I have broken this projector but here are the details.The TV is 62 inches tall, 36 inches wide and 30 inches in depth. My screen is only 40" which is not really big. The base where the projector is was the first part I build, then I build the screen which is a piece of plexi-glass followed by the opaque shower curtain put into a frame. My mirror is 18X24 it sits at a 45 degree angle, the top of the mirror is 24 in. away from the screen and 30 in. away at the bottom. After I got that build, I messed around with the projector to get the picture to show right.
At a distance the image shows great but as you come closer to it the area around the image does become darker because I didn't have a fresnel lense to focus the light dircetly at you.
My new projector I'm build will be smaller, brighter and have a slightly larger screen(42in.). |
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| dbc105 |
thanks rmccoll. if you can post a few pics of the one you are building now it would be a big help. the projector setup is of big interest seeing how this is RPTV and not projected on a screen. 40" - 42" is not a bad size in a smaller room. my den is about 20' wall to wall but where the recliners are and where the screen would be out from the wall will onl be about 15'. if I projected a big screen on the wall I would get neck cramps turning my head trying to watch it. I am thinking 50" - 55" wide for 4:3 or maybe 60" wide if 16:9. I am saving some project money to work with so for now I am getting educated. guys on this forum such as ace and yourself are a big help! many thanks. when I get started with my project I will post progress here.
David |
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| rmccoll |
| Here is my new design. The projector will project the image onto a mirror or a white sheet of paper. If I use a sheet of paper the image will show up on the paper and be magnified by a large plano-convex lense or a fresnel. I believe this setup will project the image into the fresnel or lense straighten it out so it project straight at the screen, this will allow me to make my screen without a fresnel to straighten the image out. I don't know if this idea will work but I will post result when I get to it. |
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| rmccoll |
| here is the design |
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| dbc105 |
that is pretty close to what I have in mind but with the projector mounted at an angle right under the screen. I made a drawing of it but how do I insert it into a message? one advantage to your design has is that it allows for a taller screen. will the paper work in place of a mirror?
ace, when I get those seeds I'll send you a few. heck, I'll send everybody a few! LOL! |
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| jrhilton |
To all of you interested to know what Good Rear Projection TV's use as a screen materail, it's simple, it's ground glass, sometimes a thin sheet, sometimes it varies in thickness from edge to center, ground glass has a texture so the viewing angle is wider because when the light hits it it is diffused, and there is less hot spotting, though if you have the F-Lengths correct/optimal with regard to lens hot spotting is not so much of an issue anyway. Ground glass is also used on telecine transfer machines for putting film onto video tape/computer.
Cheaper rear projection tv's have a cheaper Bulk diffuser screen, which uses plastic that has been impregnated with small
particles of material that have a different index of refraction from
the plastic. These particles are dispersed randomly throughout
the plastic, hence the name bulk diffusers. Bulk diffusers have
very high-resolution capabilities, but limited contrast control
relying on tinting to reduce ambient light reflections. This tinting
reduces the projected light through the screen. Bulk diffuser can
have asymmetric viewing angles with the use of a lenticular, but
the addition of a lenticular allows the possibility of moiré
patterns occurring.
Another method used in cheaper rear projection tvs/screens is Beaded Diffusers, Beaded screens use a sheet of transparent beads imbedded into a black plastic substrate. These beads are pushed through the black plastic so that just the tip of the bead is clear through on the viewer side of the screen. By doing this, the ambient light hits the mostly black surface and is absorbed. This allows the beaded screen to display the blackest blacks and hence the best contrast. The back of the bead focuses the projected light through the clear tip of the bead, keeping projected light losses at
a minimum. The beads are small enough for this screen to have high resolution. This screen suffers from the inability to have separate viewing angles in the vertical and horizontal directions, limiting the screen gain or brightness of the display. Because
this is not a lenticular screen, moiré noise is not a big factor, but uneven distribution of the beads can give a mottled look that can be disturbing to the viewer. Speckle is still an issue to be resolved.
The third type often used, again on cheaper tv's/screens are Surface Relief Diffusers (SRD’s). Surface relief diffusers are engineered perturbations in the surface of the plastic that refract the projected light into a diffusive pattern. This type of screen has very fine features allowing for high resolution. The screens can be engineered to have different viewing angles in the vertical and horizontal directions. Since the patterns are random, there is no periodicity to create moiré patterns. Two advantages of SRD’s are high efficiency and a high degree of flexibility in defining viewing angles. Because of their efficiency, SRD’s can have very high gain, however, for good contrast control tinting is sometimes necessary, which negates some of brightness advantage.
Top of the range rear projection tv's use glass-beaded screens, and tinted surface relief diffusers as well as good old fashioned ground glass. I would have to say, IMO the only type of screen that someone can make that will be anywhere near a professional screen is ground glass (sand blasted glass is effectively just the same). Just buy some grinding paste and off you go with some 1500 grade W&D sandpaper, making sure that there is more relief in the centre of the screen if you suffer from very bad hot spotting, if hot spotting is not really a problem, grind the glass evenly all over. If you do use ground glass, you don't need a frsnell, as the ground glass is doing the necessory diffusing.
Hope this helps you all. |
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| dbc105 |
by glass beaded you mean like etched or sand blasted? I have done sand blasting on a car before and did not get one of the windows covered good and the glass got blasted along the edge. looked just like etched glass. if going with the grinding paste, what is it called and where do you get it? this post answers several questions I've had. the screens on the older type RPTV do not look good up close like the new HD screens do, not taking resolution into account. when sitting too close to the older type you see the vertical ridges in the screen. which way is the beaded side of the glass turned? toward the viewer or toward the projector?
Thanks,
David |
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| jrhilton |
******* Adition to origional post*********
I have just though of another alternative if you don't think you are up to making a ground glass screen. Go and buy some rear projection screen material, after all it is designed for this sort of aplication, it used to be used a lot for presentations before the days of video projectors, when 16mm film projectors were the order of the day. You can still get hold of this material easilly, and it is not that expensive. You could be quite clever and get the stuff that is 'sound transparent', this stuff is screen material which has millions of tiny holes in it so sound can travel through it with making the screen material move. Cinemas use sound transparent screens and mount speakers behind the screen. There would be two advantages of doing this:
1) you could mount some speakers behind the screen (just out of the light path) so the sound actually came from the picture, not just below.
2) but better still, it would be great because it would allow great air flow through the whole unit, e.g. hot air from the light source could esceape through the screen! Don't worry, this is not a personal theory, it was one advantage that makers of sound transparent screens used to market their goods on. So you could keep the unit cooler much more easilly.
Again Hope this helps. |
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| jrhilton |
Here is a link showing showing the theory of a rear projection tv, many elements are the same today,
http://www.californiahistoricalradio.com/photos10.html
dbc105, The glass beaded screens have very fine pieces of glass, I mean really fine, e.g. like the size of flour, glued onto a clear substrate, I'm affraid I don't know much about these screens as they are a new idea (well the idea is an old one, but it is only recentley that firms have started to make them) It is the same methods as was used on top of the range screens made back in the 1960's (i'm showing signes of age here!!) Where believe it or not Asbestos dust was epoxy glued onto a sheet, giving a very high quality screen, but I bet you can't get hold of these anywhere now, well, not here in the UK anyway!
As for sand blasting, you need to use really really fine sand, if it is not really really fine you start to get an etched effect. As for grinding glass, I call it grinding paste, as for where you get it, try in an automotive shop , it may be sold as engine cylinder grinding paste, or another alternative would be paint grinding paste which is designed to take the shine off paint. The best bet is to go along to an automotive shop and tell them what you want to do, and see if they have anything that will work. Once you have the paste you mix it into a liquid and using 1500 or 2000 or greater grit wet and dry sand paper you really just sand down the sheet of glass. Look on Google and do a search for people who have made their own 'ground glass focusing screens' for 'sheet film camers'. There are lots of sites out there as I have seen a few myself. If you just want to test to see if you want to go down the glass path just pick up an old pice of glass, any type, just a cheap pice and use usual wet and dry sand paper and just water, the screen wont be as good as glass ground with propper paste but it should give you a good idea of what is possible, though I do warn you, it takes a bit of practice to get right! - I say that from experience!
| quote: | | which way is the beaded side of the glass turned? toward the viewer or toward the projector? |
The ground part would be toward the projector so there is a nice flat pice of glass towards the viewer, the reason for this is it makes it easier to apply a good anti-reflective coating, I beleive it is actually really easy to make these anti-reflective coatings, I know people used to make them out of easilly avalable chemicals - does any one have any ideas here?
Sorry if I keep refering to the past like this, but it has been some years since I did any rear projection work - I used to do a lot with 16mm film projectors - back in the good old days as I like to call them.
| quote: | | when sitting too close to the older type you see the vertical ridges in the screen |
I would guess what you are refering to is actually black plastic strips embeded in some screens, they were called double
lenticular screens with black stripes, the have one problem that they have insufficient resolution for new, higher definition pictures that people want. The black strips were a clever method and simple method I would say of reducing the effects of ambient light bouncing off the screen, they may have been other reasons for these strips bit I'm not sure what they were, if there were any.
I have noticed that people seem to be using shower curtains, I do not think this that this is a very good idea as the screen needs to diffuse the light, plastic shower curtains are very poor difusers, in fact most are not really true difusers at all, they just impair the transmission of the light through it, they don't diffuse it.
******
Another cheap alternative could be artists tracing paper, the good stuff, which almost has a shiny plastic feel to it, you can get this in large lengths and widths, but I have no idea where from. This stuff iinfact acts like a difuser, though not a very efficient one.
Again, hope this helps, keep the questions going, i'm facinated to see how good diy rear projection tvs made by people reading these forums really are, I would guess they shoul be surprisingly good, there is no reason why they should not be as good, in fact they should be much much better than projectors people are making out of old OHP's and Projection panels! As less light is required to project a good image in the short distances involved. |
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| ace3000_1 |
Nice post JR, and yeah age is getting on me too lol though not quite as much as yourself lol, ok for a difuser u can actually buy the stuff, 3m make it and its the same thats use in the backlight of lcd's, i think the best thing to do is to send 3m an emal and ask for a sample pack, from memory they hand these out free, just try to make out your are some big firm willing to buy the product in your email.
Trev |
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| jrhilton |
Trev, I though the days of firms giving things out for free was long gone! It's nice to be wrong sometimes!!
What sort of material is this 3m diffuser, is it a sheet of plastic or is it a pice of film e.g. like an OHP transparency sheet? If it is a thick pice of plactic I would guess it would not work as it will diffuse the light too much, so the projected image may not look right at all. I would guess it may also have a colour in it so that only light of a certain wavelength get's through, as I would guess LCD's would be designed to look best with certain colours of light, what do you think? But if you can get a pice for free go for it, and report the results, as even if it were of no use as a screen it may be of interest to people trying to make an ultra bright backlight style projector. Really you want a very thin material to diffuse the light, so there is not too much light loss, or a thick piece of glass with a thin diffusing layer on one side, e.g. if one side of ground. |
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| ace3000_1 |
its a thin film buddy, 3m makes all different kinds of thin films, difusers, polarisers filters u name it, some are stick on and some are just a thin film, i have a pile of difuser here thats from ripped lcds so i dont need to get any to test but yeah its an option for those who dont have any, your right about the light colour too with lcds, the best colour range is 4000k to 6000k, most backlights in lcd's are actually 6000k now.
Trev |
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| jrhilton |
That 3m diffuser sounds good for people who don't want to bother with ground glass, could you post some pics of some results for people not familiar with diffusers to see so we can compare the quality to that of ground glass?
It would seem that 3m still make everything you can think off, many a time I've heard many storiesof people on the phone to them for hours trying to find out about a a single product as it took so long for people at 3m to search through their product litsts to find the info you want!
It's interesting that you should mention the colour range, I was wondering how people were tackling this problem. It used to be quite easy using film in most cases, all you had to do was photograph a colour sheet and the labs did the rest for you, and you got back a nice 'correct' looking positive print, with all the colour correction done for you. All you then had to make sure was that you were using the correct bulb in the projector.
What is everyone doing when it comes to light temperature, what light sources is everyone using, are people going down the OHP/Projection panel route mounted sideways in a case, projecting onto a screen or using other methods. I seem to remeber that OHP Projection panels only had SVideo and/or just RCA Video inputs, not RGB inputs. Are you all building simple circuits to convert SVIDEO/Video signals into RGB then correcting each colour level then recombining to SVideo again, so as to allow you to easilly compensate for incorrect colour values from OHP style lamps, or change contrast? |
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| dbc105 |
boy I am learning a bunch. that is good. that way you get it right the first time.
David |
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| ace3000_1 |
| quote: | | What is everyone doing when it comes to light temperature, what light sources is everyone using, are people going down the OHP/Projection panel route mounted sideways in a case, projecting onto a screen or using other methods. I seem to remeber that OHP Projection panels only had SVideo and/or just RCA Video inputs, not RGB inputs. Are you all building simple circuits to convert SVIDEO/Video signals into RGB then correcting each colour level then recombining to SVideo again, so as to allow you to easilly compensate for incorrect colour values from OHP style lamps, or change contrast? |
JR most people in here use desktop lcd monitors, they are vga so the colouring of the actual lcd is acurate, the light we are using is metal halide, we are using the colour temp of 4200k - 6000k, im using 4200k now and its white enough for me, another isue i see is the colour of the screen its self, a yellow will yeald better skin tones and add abit more colour while it will also make some colours too dark, a white screen can be too white and make colours too light and tend to wash out the image, so its not just the lcd, light source and filters colour we have to worry about but also the screen, thats where it starts to get technical, i do have some of that diffuser here as im making a front projector right now and not a rear projectiion tv i dont have any shots nor can test in a rear veiw kind of way right now, but ill take a pic of the difuser for you and hold it upto my crt if you'd like.
Trev |
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| MikeCAVE |
Wow glad I tripped into here!!! Awesome feedback... IM into making a 3 wall cave for gaming, most of you have probable seen!
So I need rear screen material but I don't want it to be COSTLY... I did also hear about the OPaque curtain but then somone told me that's not gonna have enough light...
My projectors are NEC VT440 & 2 LT155, 1,100 lumens and 1,500
What and where to buy and how much is what I want to know?
The size I'd like to acheive is 6x8 for each wall... Give or take when everything is set... I have a rear screen 20x30 from Dalite which cost $110 which does look nice.. Im sure getting anthing bigger is gonna be way to much...
Any Ideads Tnx Alot
Mike |
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| jrhilton |
| quote: | | JR most people in here use desktop lcd monitors |
You guys should certainly get awards for ingenuity. Sounds to me like you are getting a good projector/rear projection TV for a bargain price, good work, is all I can say!
As for a light source, metal halide is a good one, they last for ages, but watch out, most pump out a lot of UV, even if there are no marks on the glass of the bulb!
One thing does interest me though, you say that the screens are vga so the colouring of the actual lcd is accurate, I’m not arguing with you there, I’m sitting in front of a laptop screen now and it looks perfect, but when you shine a light source with a spectrum of light output with a relatively high variance around the stated value, e.g. a metal halide bulb, surely you then suffer subtractive filtering so the colour value will change when the light goes through the panel, e.g. basically do you get general colour casts . This is a common problem with Film projection. Or have I got over excited and put a physics lab coat on and have started to read too much into all of this?
As for screens, there are equations for working out what the best colour screen for different colour temperature values are, but I never paid much attention to them, they are probably on the internet somewhere, as when you got an answer, there was usually never any screen that exactly matched what you needed anyway. Though if you are dealing with Rear Projection, you thankfully don't need to worry about this.
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| jrhilton |
Here are some links I though might be usefull to anyone thinking of making a ground glass screen:
http://www.philsan.com/ground_glass.htm
http://rhoadescameras.bizland.com/C...round_glass.htm
http://www.phototechmag.com/previou...okas/dokas.html
These pages are all writen by people making ground glass focusing plates for large format sheet film cameras, but they are exactley the same as for a rear projection tv.
Further to what David asked in an earlier post, aluminum oxide is a great grinding medium or failing that silicon carbide, but this is not as good as aluminum oxide, you should be able to get grinding compounds/pastes containing both of these from an automotive parts shop, or the internet somewhere. |
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| jrhilton |
| quote: | | I have a rear screen 20x30 from Dalite which cost $110 which does look nice.. Im sure getting anthing bigger is gonna be way to much... |
Yeah, you won't get a Dalite one of size 6x8 feet for under $3500 to $4000 USDs, but that is what quality costs, since the 1950's they have always produced great products.
As for alternatives, I would guess a pice of glass of size 6x8 would be very expensive too, plus dangrous. I'll give it some thought as to what you could use as an alternative. |
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| ace3000_1 |
| quote: | | As for a light source, metal halide is a good one, they last for ages, but watch out, most pump out a lot of UV, even if there are no marks on the glass of the bulb! |
yeah they do , they are suposed to be the one bulb that pumps out the most uv, the bulb i have has a uv stop in it wich filters out the uv, not all but atleast 50%, better then none, some use plexi glass as uv filters and some dont use anything, myself i use plexi in my design anyway and a fair amount of it to hold my frensels together, yep and its a optical grade too well it would want to be at 200euro's for half a sheet.
Trev |
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| dbc105 |
How does a euro relate the a US dollar? Who or what is Dalite? I have been in the basement looking at the Pioneer RPTV that I removed the back from. I am going to take some measurements this afternoon and remove the CRT lenses to see if these look like usable parts. I am worried about focal length. Do I have to host an image to post it here or will the forum host it? I guess what I mean to ask is how do I post a pic or drawing? ater I get the measurements I can post a drawing showing what confines I am looking at.
Thanks,
David |
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| rmccoll |
Another screen idea is to use those plastic sheets that cover flourescent light, the kind you see in schools and office buildings they come in alot of textures and they are cheap. You can also use a thin sheet of HDPE plastic, this plastic is used to make those flexiable cutting boards. You can get a sheet from www.usplastics.com
Got a question. Can a 75 watt MR16 light bulb put out 2200 lumens because I found that does and I dont think the specs on it were right. Most 75 watt bulbs put out about 900-1000 lumens and highest I have seen one go was 1200 lumens. |
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| jrhilton |
| You could use 'Low-E' insulated glass or any other insulated glass with an inert gas in the middle of two pains, as insualting glass will stop UV but better still it will also reduce the amount of IR hitting the LCD. Small pieces of Low-E glass, e.g. 10x10 inches, are not that expensive at all. |
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| jrhilton |
| quote: | | Who or what is Dalite |
David, Dalite, or rather 'DA-LITE' are one of the best manafactures of screens for projection applications, IMO the best. They are a German Firm, seem to remember they started back in the 1950's. They make wonderful rear projection screens as well, with lots of options.
Here is their web page:
http://www.da-lite.com
They make every sort of screen, they will also make one of a kind screens for you. But as with all good screens, they cost a lot of money. Most people don't realise that a good screen will cost much more than a good projector. You can easilly spend $10,000 + on a screen.
1 EUR = 1.23840 USD as of 2004.01.31 15:33:13 GMT,
or
1 USD = 0.807495 EUR
David, is there a chance of some pics of the inside of your Pioneer RPTV, as I would be interested to see what they are like inside these days, as i'm sure many other people would be! |
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| dbc105 |
jr, I will have to try to bring a camera home with me next weekend. I don't have one but I can borrow one at work. I have made a drawing with measurements that may help also, if I can get it posted. I removed the bracket that holds the lenses and the CRTs. I then removed the center CRT. There was water? between the CRT and the first lense. The first lense is in a sphere. Above that is an adjustable double lense. Of course there are 3 of these in a row. The size of the CRT screen is about 5" x 4". The screen on the TV is 40" wide by 30" tall. I would like to reuse these lenses if possible. They already have the correct focal length. If I can mount a lcd where the CRT was and have the light come up through it. I don't know about fresnels or the sphere shaped lense. If I can find someone I can borrow a camera from I will get some pics up sooner. Maybe this drawing will give an idea of what I have. I know the mirror angle is wrong and it is not in proportion but it will give you an idea.
Thanks,
David
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| Speedwolf |
Hi all, first post here.
I've been following your posts here for some time.
I've decided to plump for a RPTV as I don't have the space to run a projector.
Rather than using a gutted TFT and bright light, is there anyway to perhaps recycle a CRT monitor? Most people will probably have a spare 15" or 17" CRT knocking around the house that could use a new lease of life, and to be honest turning it into an (almost) HD RPTV is one heck of a way to see in your retirement.
So if someone could see their way to giving it some thought I'm sure a number of us lurkers would be grateful. |
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| jrhilton |
David, great pricure, it seems that RPTVs are much slimmer than they used to be! As for reusing the existing case and lenses ect, well in theory there is no reason why you can't as the focal length of the lens will be correct. However, you will only be able to do this if the lens is designed to project an image from a flat CRT tube, e.g. are the tubes in the RPTV flat or curved? THe liquid you saw was probably coolent. If the CRT's in the TV have a flat serface you will be able to reuse then lens, however if they are curved you will have a problem as a LCD is flat, so the lens only part of the image will be in facus. e.g. the center and towards the edge out of focus. This assumes that the lens is the type that is directley in front of the CRT tubes. I would need an image to work out what is going on if there is an aditional sphere in the light path. I would assume that it would not be too difficualt to put in an lcd if the lens is ok, just put it where the CRT used to be and put a light soure behind, depending on how close the lens is you may not need a fresnel. Also some projection lens are only designed to project an image, e.g. from the CRT that is exactley behind the lens, so you may have limits to the size of the LCD you can use, e.g. up to 5"x4", however the lens may work if you use a fresnall, tests are the way to go ahead to see if it works, bin the thoery books and get some first hand experience/practice is always what I have said!
If you post some pics of the lenses we may be able to help/advise.
Speedwolf, i'm not sure if you could recycle a CRT monitor as there are several problems that you would need to overcome.
1) CRT monitors are much brighter than a standard monitor and are full of cooling liquid to control the heat. Although you can turn the brightness of a CRT monitor up you often sacrafice colour/contrast.
2) Most CRT monitors are curved on the surface, and most lenses are designed to project an image that is flat, in very broad/loose terms. So you may have focusing problems.
3) You may be able to adopt a fresnel+lens approach though brigntness will be an issue, the image will be dim/colour will be poor
4) You may end up with an image that is flipped/upside down unless you have the right lens (though this would depend on you light path.
Here's my advice for an easy to make/good quality RPTV
The best approach I can see for building a rear projection tv, at a good price with out having to do any real work is as follows:
Bits you need:
1) An OHP, must have a triplet lens!
2) A projection panel - with video input, and 2mill cols +, 640x480 mis good as the screen will I guess not be any larger than 60" plus
3) A piece of glass the same size as the screen you want
4) Some mirrors
5)
As it is rear projection and the distance from the projector/bulb will be small a low powered bulb is all that is required, e.g. standard 32v 400w OHP bulb will be more than ample!!!!, don't worry about the light colour, I will come to this.
The OHP will be mounted at the bottom of the case (imagin you are looking at the frot of the RPTV. The ohp will be mounted on it's side so the light source/bottom is at the bottom left of the TV. The mirro/Lens will be in the center. Rotate the OHP so the llight projects out towards the center back of the tv. Now mount a mirror there that reflects the image onto the ground glass screen. If you look at the diagram David kindly posted above, the lens of the OHP will be where the crt tube is so the light is projected towards the morror then onto the screen. Now mount the projection panel onto the ohp so the image is the correct way around on the screen, build an enclosure around it. The advantage of doing this is as follows,:
1) OHP's are cheap to buy used,
2) Projection panels can be bought easilly on Ebay, and have video inputs and remotes ect
3) You don't have to take anything appart!!
4) No real heat problems as Projection panels are designed to be on OHP's for long periods at a time.
5) THe enclosure will reduce the noise of the OHP fan
6) It will work!!, this is basically what I used to do when I was hired to set up a rear projection presentation using 16mm projectors (Oh the good old days!) , and even if you can't be bothered to finish it, you still have an undammaged OHP and projection panel for some other projects!!
From what I remember, projection panels have either video or S-video inputs. Now this means that it is easy to correct any colour problms due to bulb temperatue, here's how.
The process
VIDEO---> Convert to RGB individual signals----->Adjust each colour where neccessory, chrome levels and brightness/contrast------> convert back to S-Video or Video.
Easy, mount the controls for the circuit on the fron of the tv for easy colour ajustment. THere are various circuits on the internet for converting video to RGB and adjusting each level, and best of all they are made from easy to find/cheap conponents and the circuits are easy to build and require no adjusting. If any one wants any more details on these circuits, just post a question, and I will look for some links for you, I just can't remember the site at the moment. Failing that, I'll look through my collection of circuits for one, I know I have one somewhere.
Sorry had to type this post rather quickly and don't have time to grammer check/spell check, so you will have to excuse all spelling/grammer mistakes this time.
I will try and post some diagrams of what I mean, can anyone help me with a good site to host some picures, i'm a bit lost when it comes to hosting/posting pictures to these pages ect. |
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| dbc105 |
I had thought about using an OHP and a high res panel but was not sure about being able to get the OHP mirror far enough away from the rear of the screen. Which OHP have triplet lenses? I will try to remember to take a camera home this weekend to take pictures of the inside of the RPTV. The CRT is flat on the surface facing the lense so that should not be a problem. There is the concern of getting a lcd small enough. The CRT is about 5" x 4" at the face. Hey Ace, what is the width and hight of the lilliput? jr I can host a few pics for you.
David |
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| dbc105 |
oh yea, jr if you run up on the links for that RGB curcuit please post them. Thanks.
David
ps. did anyone see half time of the superbowl game last night? that would have been something on a big screen! LOL! |
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| ds21 |
| WOW, it's been a while since last time I posted on this thread, I've been sidetracked by other fun projects, any way I've got a 60" panasonic and my overhead setup, My biggest problem is that the focal lengh is shorter then my OHP triplett lens (not using the OHP mirror), I think I could lay the projector on it's side and use the OHP mirror, but then it would have to stick out the back of the TV enclosure, I took one of my lenses apart and focused it by hand, but the image ends up being too small (about 40") oh well I've got the origianl tv lenses I've yet to try. |
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| dbc105 |
ds21, do you have any pics or drawings you can post to show us what you have? I have 2 other bad RPTVs in the basement. The one I have been discussing seems to be more suited due to it's design to setting up with a small lcd and using one of the tv lenses. One of the others I believe may be well suited to gut it out and build a shelf to mount an OHP into an let it display to the original screen. All 3 have 40" wide screens so what you are getting may work out real good for me. On the third one the press board bottom of the case seems to have been wet and is sagging badly. I think I will rob the mirror and lenses and screen and trash the rest of it.
David |
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| ds21 |
| dbc105, it's basically configured like the drawing you put up, to take out the gut was something like 3 screws and 2-3 ribbon cables then the whole unit slid out. For the hand focus experiment I just took the top lens of the triplet and faced the concave side toward the screen. The easiest way, I've found, to experement with various lens configurations is to get a measurment from where your lcd sit to a point on the mirror, then a measurment from that point on the mirror to the screen, then add the two measurments, then what I did was just take a candle in a dark room and position it that many inches away from a wall then start holding you lenses inbetween and just move them around and play with them, that way you can see what makes them work. I now know about how far away/apart the triplet lenses need to be I just need to make some kind of mounts for them. |
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| ebswift |
Although I'm new to this forum and to DIY projectors in general I'm really interested in this project. It seems the only practical way to get DIY big screen TV for general use. The projector stuff is nice, but it seems like you've got to get the room darker and use brighter bulbs. This thread should be made permanent because it's the only recent thread I can see focusing on RPTV.
At this stage of the game I'm looking at the materials that can be used for the RP screen. I've skimmed through some commercial stuff, and for the price it seems I could get the whole TV from the local retailer. I've got access to normal projectors so I can test various materials before running off to build the projector itself (based on designs from this forum of course). Basically if screen material isn't readily available I'll forget the idea of RPTV. |
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| dbc105 |
ebswift, going about it the way I am I think should save me a considerable amount of money. I have 3 RPTV's, all bad that I picked up from a buddy that works on TV's. on the ones I dicide not to use I plan to remove the screen, mirror and lenses and trash the rest. This should net me the most hard to find items. check around in your area for someone that does TV repair and see if you can find a bad one you can pick just to get it out of their way. I am also considering the idea of an OHP mounted in the RPTV box focusing on the rear of the screen. Since my screen is only 40" wide there may be enough room to place the OHP behind the screen and only use the mirror on the OHP. This is based of the suggestion that JR made in his post but I am thinking on the smalleer screen I may not need the added focal length in which case I can project right onto the screen. the room light is what got me interested in the RPTV. I will need to be able to use mine all the time. if you have any ideas or thoughts post them. I had not considered the OHP in the tv before JR's post, but now I am thinking it is to simple to pass up.
David |
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| jrhilton |
Here are some links on how to make Video to RGV/S-Video converts and vice versa:
http://www.epanorama.net/links/videocircuits.html
This next one looks great, you could replace the resistors with variable resistors to ajust each colour. It looks cheap too, though remmeber this is designed for PAL, not NTSC, so you would need to adjust it for NTSC ect.
http://www.nexusuk.org/projects/rgb2svid/index.php
I'll post some more links for RGB colour correcting circuits when I can find out what they are again, I did have some myself that I used to use, but I have found that some of the components are not made any more, so they are of no real use anymore - pitty.
As for my idea of using an OHP in a RPTV application, here is a (rather poor) diagram of what I mean, a spectial thanks is due to Davide here for hosting the image for me!
I hope the diagram gives an idea of what I thought of, as for image size, many OHPs these days have very short throw lens, so this should not be a problem, if focal length is a problem, then you could use a zoom projection lens off ebay insted, they are fairly cheap.
A triplet lens is a lens that has 3 elements, the idea is the lens corrects colour distortion, whch effects cheap/OHP single element lens, most good ohps these days would come with 3 or more elements lens I would guess. |
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| faithblinded |
I'd be interested to see what "cheap" zoom projection lensees you are talking about.
All the zoom lenses I've seen "cheap" on eBay have been of a far too small diameter to capture an entire 15" screen.
Furthermore, these small zooms are generally meant to be within an inch of the image source, which was usually about the perfect size for the lens, i.e. 9mm film etc.
Don't let me discourage you though, lets see what lens you're talking about. |
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| jrhilton |
| quote: | | far too small diameter to capture an entire 15" screen |
I'm talking about projection lenses, they don't need to cover the whole screen as they are designed to project a source which is converging/ has aready converged. The are designed to be a certain distance from the focal point of the condensor element of the projector that is before the film/slide ect not the film it's self. As an OHP has a Fresnel Lens the light will be converging to a focal point so you can use a projection lens.
| quote: | | these small zooms are generally meant to be within an inch of the image source |
You could still use these if you have a condensor lens with a long focal length behind the panel, e.g the fresnel lens. Though you want to use a lens that has the widest aperture you can get hold of so the image is as bright as possible, so an 8mm or 16mm film lens would be not be the best bet.
As for ebay,for example I have seen projection lenses for medium format slide projectors (6x6cm slides) go for less than $20 USDs so they are cheap compared to what they cost new. These would be great because many were designed for projecting images from a meter or so for close copy work to 25 meters+ for public viewing. I have even seen a lens that was said to be from a Gaumont 70mm cinema projector, (not that this would be of any use here) but it is stunning what is out on Ebay. Maybe I should have been more clear as to what sort of lens I was talking about in my origional post. I did not mean the type that you will have in a RPTV in front of the crt ect which only project what is just behind them. There certainally are lots of projection lenses out there for 35mm + film sizes, i've seen enough in all my years of working with film, lots must still be out there.
Hope this helps. |
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| Hezz |
JR,
The fact that you are collapsing the light field still will not allow you to use one of those typical projection lenses unless they have the correct focal length and a field of view that works for the LCD's size.
I do not know what the typical focal length of one of those lenses are but it would have to be between 325 - 500 mm to work with a 15 inch LCD. And even if it had the right focal length if it was designed to image a smaller object it will not have adequate field of view for a large LCD.
If the objective lens system will not image the object without any additional aids the lens will not work. All of the other light engine light manipulations are only to increase the light through the system.
Hezz |
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| rmccoll |
Well I got my 1.8inch Lilliput LCD today and to my delight I can use it the controller board is completely out of the way. The resolution on this thing is amazing I see can the smallest words on the screen. The resolution is 116160 which is about the same as the Sony Playstion LCD.
I plan to use a 75 to 100 watt par16 halogen bulb as my light source I may go even higher, the diameter of the lense is 2 inches so I will cut out the condenser lense and put the bulb right behind the lcd, with a piece of ground glass between them. I will use a 80mm fan to cool the whole thing since it's so small. I have not figure what projection lense I'm going to use. When I get my camera back I will post pictures. |
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| ebswift |
Well I got a phone call from "Rosco" yesterday, the price for rear projection screen material in Australia is $48 p/m at 55" wide, and $25 postage. That seems a reasonable price since only about 2m is necessary. There is a choice between the grey and dark grey screen, so I might see if I can get a small sample of each because I don't want to be buying the wrong screen type ;).
Anyway, I'll keep pricing bits and pieces and getting ideas out of this forum for optics etc. I still have no real idea about all the requirements. The guy from Rosco said that distance would be an issue with RP, but I explained to him that I believe it can be done with optics according to what I've read here. Hopefully it won't be too long before I can actually start buying some bits. |
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| dbc105 |
Due to work I have not been able to do anything on my project.
ebswift, it is going to be interesting to see what results you get with the screen material. Is this material smooth on both sides and look like the screens on the newer HDTV or does it have ridges like on the older style RPTV's? What is the material called? That might help in locating it here in the states.
rmccoll, I have seen resolutions numbers like that before but how does that relate to like 800 x 600 on a pc monitor? I am sure it is simple math, I just don't know what it is.
I still like the simplicity of the OHP inside the box directing toward the rear of the screen. I wonder how well the lilliput that ace is using in his projector would do on an OHP? Any ideas?
David |
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| ebswift |
dbc105, Rosco seems to be worldwide, if you go to their home page (http://www.rosco.com) it shows a world map where you select your home country.
I haven't seen the actual material yet, and to be honest I'm not even sure if I'm going to run for this idea at all, I'm still researching. The only thing that would prevent me from going this way is if the resulting image would be inferior to making something that's front projection. I did read somewhere in these forums that the image is inferior, but I really find that hard to believe when you're taking ambient light into consideration. Even in a dark environment, rear projection really should equal or better front projection(?). Maybe someone who has actually experimented with both types can tell us some more. |
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| ace3000_1 |
| quote: | | Anyway, I'll keep pricing bits and pieces and getting ideas out of this forum for optics etc. I still have no real idea about all the requirements. The guy from Rosco said that distance would be an issue with RP, but I explained to him that I believe it can be done with optics according to what I've read here. Hopefully it won't be too long before I can actually start buying some bits. |
heya ebswift, nice find u got going there on the screen, and yep we will and can nut this rear projection out, just a matter of finding a good screen and the right optics, for some parts i can supply a few things like reflectors ect for the light engine, they are hard to find normally and cost a mint to buy, i get them from a wholesaller so i get them cheap, this enables me to sell them cheap and easily aquairable (if thats a word lol) keeping our project to a minimal cost, but yeah i must say u guys in here are starting to move ahead in here, im not doing rear projection just yet, but soon though, and its good to see a good bunch of guys in this feild already onto it, ill be in and out from time to time, good luck.
Trev |
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| dbc105 |
Well I have been tied up for the last week or so. My daughter crashed her car so me and the wife have been having to get her to school and work and also been looking for a replacement car so I have had little time. Has anyone made any progress in the last couple of weeks?
David |
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