Re: Curved Display, JDome, Immersive 3D Display
Posted: Tue Mar 15, 2011 9:01 pm
Nice thinking. I actually have a Papasan in my living room, but I never thought to use it for something like that. Very slick.
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http://www.diyaudio.com/forums/diy-proj ... post314747" onclick="window.open(this.href);return false;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.
3dvison wrote:What about that foam that comes in a spary can used to make car seats fitted to your shape of insulation foam in a can.
Could you spray out a bunch of that into a tub and then press a balloon into it, let dry then pop balloon ? Or spray around outside of balloon like you were going to shave it with shaving cream, let dry..and POP balloon ?
I am out of town at the moment, I left a few days ago, no time to properly set it up. I promise to take some pictures when I get back, though.mayaman wrote:Palmer no pics? I have to see this.