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Old 30th January 2004, 04:31 PM   #61
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Default Screen Material

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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Old 30th January 2004, 04:59 PM   #62
dbc105 is offline dbc105  United States
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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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Old 30th January 2004, 05:02 PM   #63
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Default Addition to my origional post above

******* 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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Old 30th January 2004, 05:31 PM   #64
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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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Old 30th January 2004, 06:00 PM   #65
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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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Old 30th January 2004, 06:09 PM   #66
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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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Old 30th January 2004, 06:41 PM   #67
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Default JR

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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Old 30th January 2004, 07:10 PM   #68
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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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Old 30th January 2004, 07:46 PM   #69
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boy I am learning a bunch. that is good. that way you get it right the first time.
David
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Old 30th January 2004, 09:39 PM   #70
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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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