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.
-- loonquawl, Apr 27 2009

//it only reflects light in a one phase// And this would be done by......?

Also what happens if someone knocks the table on which the projector is sitting, and it moves by (say) 300 nanometres or so?
-- MaxwellBuchanan, Apr 27 2009


////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.

Knocking the beamer would result in moiré patterns, and strong disapproval by your movie-watching peers.
-- loonquawl, Apr 28 2009


Thanks! I forgot about the category.
-- loonquawl, Apr 28 2009


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.

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.
-- MaxwellBuchanan, Apr 28 2009


[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.
-- loonquawl, Apr 28 2009


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.

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.
-- spidermother, May 02 2009


Actually, there is a better solution, of which I have just though of which. Post, I shall.
-- MaxwellBuchanan, May 01 2016


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.
-- MaxwellBuchanan, May 01 2016


Which was…?

(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.)
-- notexactly, Jun 09 2019



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