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Negative Light Projector

Paint a surface with a chemical that darkens when exposed to UV; project an image in UV light.
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Chemicals of this type certainly already exist, as they're used in sunglasses that automatically darken outdoors. (The sunglass chemical responds slowly, though.) If we had our hands on a fast-acting variety (which surely exists), then we could project images in UV light that would appear as dark images on the wall.

No more bulky, hot, power-guzzling, noisy projectors, and no need to turn off the lights when giving a presentation!

Dog Ed and egnor: Both of you seem to think that this would require a lot of UV energy. Obviously the point of this idea is to *decouple* the contrast on the display from the power output of the projector. With typical light projectors, the light has to be strong enough that it produces high contrast even when scattered in all directions. The energy that gets to your eyes has to come directly from the projector.

In this case the UV light controls the opacity of the chemical, which should require much less energy. The small control signal comes from the projector; the large energy reaching your eyes comes from the ambient light source.

I'm not a chemist, but there clearly exist lots of chemicals that change colour quickly with relatively little energy input, and lots of reactions that can be triggered by UV. Was it not okay to post this idea because i don't have the chemical formula? If i had it, i'd be busy patenting it instead of posting it here... :)

pingster, Apr 19 2001

Lightbulbs don't produce light, they suck up dark. http://webpages.mar...ksucker_theory.html
[StarChaser, Jun 05 2002, last modified Oct 04 2004]

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The Official Repository - the Woiking Notes are a fascinating mental picture [thumbwax, Jun 05 2002, last modified Oct 04 2004]

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       How do you project the UV image? A hot, power-guzzling, noisy UV slide projector?
Dog Ed, Apr 19 2001
  

       Photosensitive sunglasses require a lot of light -- daylight, basically -- to darken. Assuming this chemical is at all similar, you'd need one big honkin' UV projector, and people sitting near its output would be at danger of sunburn (or worse, if they were close to the lens).   

       Maybe a more sensitive chemical does exist, but I certainly haven't seen anything that reacts quickly to small amounts of light in a narrow spectrum.   

       Can you list some of the chemicals that change color quickly without energy input, and/or reactions that can be triggered by UV? Otherwise, well, WIBNI (though at least it's original). "Maybe 3M has the perfect chemical lying around in a closet already!"
egnor, Apr 19 2001, last modified Apr 20 2001
  

       just paint the screen black and project negatives.
mihali, Apr 19 2001
  

       pingster, you'd have to give the chemicals a "cool down" time between slides.
centauri, Apr 19 2001
  

       UB: If this could actually be done, it would be wonderful. I'm just questioning whether it could be done.   

       (Note also that you could never let sunlight into this room, at least not without heavy UV filters. Also, no color, unless you get really fancy with different bands of UV or something.)
egnor, Apr 20 2001
  

       i like having a dark room in a presentation, it lets one's mind rest if the presentation gets boring. just close your eyes, let hum of the projector's fan soothe you to sleep, and hey presto!! instant nap time. just remember to wake up when the lights come back on, and don't snore too loudly, it's rude.   

       just like my quantum physics class all over again...
mihali, Apr 20 2001
  

       Well from a physic's slant, uv light has more energy than that of normal visable light, so it should be feasable (in theroy at least) to have some kind of passive coating wich shifts the energy states down into the visable spectrum.... I had a simular "halfbaked idea" a few years back, the difference was mine didn't stand a chance 'cause I was speculating about passive infa-red glasses which would (by definition) require additional energy to move infa-red into the visable spectrum, yeah I know.. WRONG...
solomungus, Apr 21 2001
  

       Baked. Well the idea anyway. There is an idea in a book by Alasdair Gray (1982 Janine) for a negative light projector to use with stage lighting. The stage would be white and brightly lit. Sets would be built using projected areas of darkness. It was meant for often changing scenery and low cost theatre productions.   

       I very much doubt this is possible. Gray goes on to imagine some sort of reverse light lazer, so the "lighting" could become 3D. But a nice idea.
mcscotland, Apr 24 2001
  

       solomungus: It is possible to trade off intensity for frequency. Night vision goggles do exist, after all. But you're missing the point; this is not a phosphorescent screen, this is a reflective screen that changes color upon incident UV radiation.   

       I think mcscotland is also missing the point, but I'm not sure because I'm not sure what he or she is talking about.
egnor, Apr 24 2001
  

       You should probably take a look at MIT's photonic crystal work. They are currently working on producing a "band-gap" molecule, or photonic crystal that can be attuned to particular wavelengths of light. With resonant cavity defects they could localize and trap a particular wavelength. I guess the trick here would be to figure out a chemical that will align the traps when exposed to UV.
Reverend D, May 30 2001
  

       Perhaps a better idea would be to use a thinscreen monitor and forget the whole projector!   

       Yes, the monitor would need to be large == $   

       Yes, LCD would have angle-of-view problems, so use VFD (Vacuum Fluorescent Display, I first saw them used in CD Players, reminiscent of TIMEX's Indiglo)   

       Yes, VFD == $$$   

       But, the engineering & production costs of the Negative Light Projecter & Screen would certainly cause a high initial cost, similar to the large VFD   

       AND the VFD uses existing tech, meaning that if enough people want big ones (corporations?), they'll be produced en masse, bringing economies of scale into play: the price drops would probably be similar in both cases.   

       So why invent the negative light projector?
Tegestu, Sep 14 2001
  

       You could probably get the power by using a UV laser and scanning a raster on the wall. I don't think it would be all that power hungry.   

       In this case you need a not too fast response time, about like a CRT phosphor, to get the needed persistence.   

       So ambient light passes through this "paint", reflects off the white wall paint, then back through the paint again to the eye. Sounds reminiscent of liquid crystals. But you need one that is activated by the light rather than electrical charge. Maybe there is such a thing.   

       Hmmmm... a 45 degree twist of the polarization when activated, through twice is 90...
jtgd, Jun 05 2002
  

       Reminds me of the "Kulb" (K for black) which emitted dark. For movie projection in the open in the daytime.
neelandan, Jun 05 2002
  
      
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