projection screen
You lose more, at least with a simple white paper sheet screen. You can actually recover part of the light which passes through with an aluminium foil sticked beneath the paper. Probably not worth the trouble, as stacking more papers, or using the thickest you can get from the start, gets you the same result.
In fact projection surfaces are the biggest challenge for the projection principle in general. The screens we can get reflect both the beamer (signal) light and the ambient (noise) light. The brute paleative is using a gross quantity of bulb lumens, but contrast/black level still hurts. Turn the lights off and the picture gets much better, even with a modest lumen bulb.
If we had a screen as black as a CRT or LCD with the lights on, yet equally responsive to signal light, TVs would be pretty much obsolete.
I am wondering whether a plain white paper does not diffuse the light impinging on it equally well, depending on the incidence angle of the beam. Like in the squarer the light hits, the brighter the spot we see. And is the cosine (lambertian) angular distribution the right one to use as a model?
You lose more, at least with a simple white paper sheet screen. You can actually recover part of the light which passes through with an aluminium foil sticked beneath the paper. Probably not worth the trouble, as stacking more papers, or using the thickest you can get from the start, gets you the same result.
In fact projection surfaces are the biggest challenge for the projection principle in general. The screens we can get reflect both the beamer (signal) light and the ambient (noise) light. The brute paleative is using a gross quantity of bulb lumens, but contrast/black level still hurts. Turn the lights off and the picture gets much better, even with a modest lumen bulb.
If we had a screen as black as a CRT or LCD with the lights on, yet equally responsive to signal light, TVs would be pretty much obsolete.
I am wondering whether a plain white paper does not diffuse the light impinging on it equally well, depending on the incidence angle of the beam. Like in the squarer the light hits, the brighter the spot we see. And is the cosine (lambertian) angular distribution the right one to use as a model?
You can get front projection screen material that does not scatter the light in all directions, so you get more light reflected back to the viewing area. (You do need some scattering, else your screen would be a mirror and all you would see is the small bright spot reflecting from the projection lens.)
You can buy or paint your own screen that reflects the light back in a narrow cone (less usable viewing area but brighter) or a wide cone (more usable viewing area but dimmer). I suppose the same factors are present in rear-view screens too. The screen material could be designed to scatter all the light into a specified cone angle. But a simple material like paper would send more light back to the LCD than went out to the viewer! That is very bad.
Another very important factor to consider is that brightness is a product of the square of the screen size: Rear projection screens tend to be smaller than the typical DIY front projector user's screen. Half the diagonal size = four times as bright.
You can buy or paint your own screen that reflects the light back in a narrow cone (less usable viewing area but brighter) or a wide cone (more usable viewing area but dimmer). I suppose the same factors are present in rear-view screens too. The screen material could be designed to scatter all the light into a specified cone angle. But a simple material like paper would send more light back to the LCD than went out to the viewer! That is very bad.
Another very important factor to consider is that brightness is a product of the square of the screen size: Rear projection screens tend to be smaller than the typical DIY front projector user's screen. Half the diagonal size = four times as bright.
The problem is the incoming angle (acceptance), not the outgoing one.
Is there a correlation between the two? As in a narrow output angle screen won't work the same with light coming laterally rather than squarely.
And what paints have those properties? I thought the diy way was to add glass granules, to act as microlenses.
I wonder if it is possible to make a screen respond only to polarised light, while cancelling by destructive interference circular polarised ambient light (eg micro beamsplitter surface with polarisation rotation).
Is there a correlation between the two? As in a narrow output angle screen won't work the same with light coming laterally rather than squarely.
And what paints have those properties? I thought the diy way was to add glass granules, to act as microlenses.
I wonder if it is possible to make a screen respond only to polarised light, while cancelling by destructive interference circular polarised ambient light (eg micro beamsplitter surface with polarisation rotation).
There are a couple of "DIY ways" to make narrow angle screens:
1) Put a thin light scattering layer over a highly-reflective layer. For example, a translucent mix of white and unpigmented base over a plastic mirror.
2) Put a coat of matte-finish clear polyurethane over a uniform coating of "pearlescent" speciality paint. (Behr makes one)
For the narrow-angle rear projection screen, I was thinking of something more like a clear plastic with millions of tiny lenses moulded into the surface. That might be difficult to make.
I don't think polarization can ever get you any gain. But you could easily filter out interfering light, like light from the sky, that is already polarized. Much cheaper to get some good window shades!
1) Put a thin light scattering layer over a highly-reflective layer. For example, a translucent mix of white and unpigmented base over a plastic mirror.
2) Put a coat of matte-finish clear polyurethane over a uniform coating of "pearlescent" speciality paint. (Behr makes one)
For the narrow-angle rear projection screen, I was thinking of something more like a clear plastic with millions of tiny lenses moulded into the surface. That might be difficult to make.
I don't think polarization can ever get you any gain. But you could easily filter out interfering light, like light from the sky, that is already polarized. Much cheaper to get some good window shades!
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