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Black Beam
improve ambient-light contrast by using black surfaces | |
Beamer contrast is a weird thing: It is computed by dividing the luminance of a white pixel by the luminance of a black pixel - this leads to a bad contrast if there is ambient lighting, as anyone can observe with beamers in well-lit rooms. -(1000/0.5 = 2000:1 contrast, same beamer in 300-lit room :
1300/300.5 = 4:1 contrast...)
I propose using a specially coated surface to project on: it only reflects light in one phase, all the other phases are swallowed (as different wavelengths from 300-800nm will have to get through, even with a very thin coating this will mean some leeway in the phases possible, so not perfect black). This means under normal lighting the surface appears black. The beamer would have to be a laser projector tuned to deliver the light in the right phase to the surface (this could be ensured with an interference calibration pattern at the side of the surface). the beamer's light would be reflected, ambient light only to a very small degree, therefore the contrast would be much better
As the laser-beamer will have only 3 or four 4 different, very sharply defined wavelengths, the black can be made even deeper by having the surface totally nonreflective in all other wavelengths.
[link]
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//it only reflects light in a one phase// And this would be
done by......? |
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Also what happens if someone knocks the table on which the
projector is sitting, and it moves by (say) 300 nanometres or
so? |
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////it only reflects light in a one phase// And this would be done by......? // ... in the usual way? You got me there, actually. I just assumed that there was a way to to it, but i could not find it in Google, yet. |
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Knocking the beamer would result in moiré patterns, and strong disapproval by your movie-watching peers. |
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Thanks! I forgot about the category. |
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Actually, I've figured out what you mean, loonQ. YOu
don't mean "phases" you mean "wavelengths". This is
actually clear in your last paragraph, but the earlier
paragraphs only confuse things. |
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So, to translate: create a surface which only reflects at a
handful of very tightly-defined wavelengths. These
wavelengths will only make up a tiny percentage of
background illumination, and hence the screen will appear
dark. However, the projector uses those very same
wavelengths, and hence most of *it's* light is reflected,
giving a bright, high-contrast image. |
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[MaxwellBuchanan]: The last part was interpreted correctly by you, it came to me as an afterthought when i considered that using lasers means using fixed wavelengths. This would already give quite a dark background.
Yet saying 'phase' i mean phase. I still am not sure whether it is possible to create a filter that lets through only light in a specific phase (rotational polarity filters can at least filter for a specific difference in the two wave-components, so my hopes are up) - but if it was possible, this could darken the background still more. |
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Two beams of light may be in or out of phase with each other, but I don't see how light can be at one phase or another with respect to a static object (the screen); there needs to be some time reference. The screen would need to be (somehow) changing its properties at visible light frequencies. Such an active, powered screen would seem to defeat the purpose of using a projector. Still, interesting to think about. |
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Perhaps a resonance could be induced in the screen at a higher, ultraviolet, harmonic of a visible wavelength, using an ultraviolet laser; the visible laser light could then constructively interfere with this. Second harmonic generation may be a good starting point, as the frequencies would already be in phase (I think). I'm just armwaving though. |
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Actually, there is a better solution, of which I have
just though of which. Post, I shall. |
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Actually, there isn't. I was half-way through posting
an idea for a screen that contained UV-absorbing
fluorescent dyes, coupled with a projector that
projected in three different UV wavelengths instead
of in RGB. Then I spotted the flaw. |
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(If it was that the shorter wavelengths will excite the
longer-wavelength-absorbing phosphors, you could put the
phosphors in three layers of decreasing desired excitation
wavelength from projection side to non-projection side,
interlarded with two low-pass layers, a bit like a penetron
but for photons. Should work for both front and back
projection.) |
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