Half a croissant, on a plate, with a sign in front of it saying '50c'
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Passive illuminated sign

Concentrates the ambient light
  (+17, -4)(+17, -4)
(+17, -4)
  [vote for,

I just read that some clever MIT people have developed a coating for glass or plastic, which (if I understand correctly) "captures" ambient light and keeps it internally reflected within the sheet of glass, so that most of it comes out of the edges. Their idea is to put photovoltaic cells round the edge of the glass, where the light is concentrated.

Why not make the coated glass into a passive sign ? Just cut shallow grooves into the surface of the glass, and the light will come out there, much brighter than the ambient surroundings (depending on the area of the sign vs. how much groove you cut). You'll probably want to put coating round the edges too.

The photo on the MIT website showed some nice colours - though maybe you can only have one at a time.

Unlike a conventional electrically powered lamp, this sign will always respond to the ambient lighting conditions - bright during the day, and not blinding at night.

It might even be useful for an emergency sign, as an alternative to luminous ones (which fade after some time).

And we'll not need to produce so much electricity to power it, of course :-)

VaquitaTim, Jul 12 2008

MITs report http://web.mit.edu/...cells-faq-0710.html
[VaquitaTim, Jul 12 2008]

The Light of Other Days http://www.scifi.co...ive/shaw/shaw1.html
[bungston, Jul 16 2008]

Light House Light_20House
One of my HB favorites [bungston, Jul 16 2008]


       Interesting technology. Affordable solar panels could be on the market soon.   

       And interesting use for said technology.
Bad Jim, Jul 12 2008

       Just checking... When it's dark, you won't see it at all... but in bright sunlight, the voltage (from the light "captured" at the edge of the pane) goes towards brightening the image to overcome the bright sunlight...?   

       Fabulously Halfbaked, gets my bun [+]
Dub, Jul 12 2008

       No, it's light captured all over the face of the sign that escapes at the edges of the letters.
baconbrain, Jul 13 2008

       Right, baconbrain. Even when it's dark, the little light there is should be amplified, thus the letters should appear relatively bright.   

       < my first buns - thanks everyone :-) >
VaquitaTim, Jul 15 2008

       This would be perfect for roadsigns [+]
Spacecoyote, Jul 15 2008

po, Jul 15 2008

       You may have seen the Energizer battery displays, that have the orange Lucite panel in the top - makes a nice glowing line at the top of the display without any electric power requirements.   

       I *think* I know why there could be problems with this. Tell me how this sounds to you.   

       Any ambient light that strikes your panel at an angle steep enough that it a) gets caught in total internal reflection, and b) is headed toward the edge you want to illuminate - this is the light that will be summed to achieve your edge brightness. So the bigger the sheet, the greater the brightness. (Within reason, of course - internal absorption creates diminishing returns.) But imagine you wish to illuminate a figure such as a letter "B" - the outside edges can be fed light from a huge collecting surface, but the middle bar can't get much. Equalizing the brightness will be a major difficulty. Particularly when you figure that the problem is completely different depending on the angle from which the brightest light is incident on the panel, and the complications of any random shadows.   

       The simpler the lighted figure, the easier it will be. Like, maybe, a glowing line.
lurch, Jul 16 2008

       This idea works. I know because I have a neat little sign with my wife's name carved in it that I had made about 14 years ago.   

       The plastic appears to be doped with a pigment that converts UV to visible (in my case, a harsh neon green/yellow). The edges are cut very smoothly, but where her name is cut, the plastic is quite rough.   

       It would seem that the UV light is stepped down to visible inside the plastic, and internally reflects until it encounters the rough-cut letters which scatter the light such that the letters have a very distinct glow.   

       This seems more along the lines of what those mad scientists are doing with solar - namely causing incoming light to excite a pigment which then emits at an optimal frequency light that remains trapped until absorbed by PV cells along the edges.
TIB, Jul 16 2008

       You have a good point I think lurch. I was thinking that if the grooves are not very deep relative to the thickness of the glass/plastic, then you might get away with it.   

       Thanks for the verification TIB! I read somewhere else on the HB about the conversion of UV into visible light, where it was suggested that most natural solar UV gets filtered out. Are you sure that's how it works ? I have to say I didn't entirely understand the workings of these chaps' pigments, but what you say certainly makes sense - they choose colours at which the PV are most efficient.   

       One other thing is that these things were tried in the 70's (they mention) and maybe this idea would (half) work even with old technology. That might even make it cheap enough to do seriously!   

       my first bones - blech :-(
VaquitaTim, Jul 16 2008

       This is a great ieda, but already well baked per [TIB]. Gotta bone it.
Alpharaul, Jul 16 2008

       The point is to leverage off the newer technology to make large signs. I haven't seen these around much.   

       The concept of using coatings and materials to reduce and increase reflections has been around a long time. We have engineering needs for both. Change the frequency of light to something more visible is baked into everything from reflective road signs to camera lenses. Very low cost, stability, and high efficiency is what MIT has achieved but their process was likely adjusted to optimize the color temperature necessary for solar collectors.   

       Importantly, most humans see a rather broad range of light where an amber color is optimal. This is why amber street lights which have a more expensive investment cost than other colors, are slowly replacing their predecessors because they can be run at lower power and thus use less energy due to human sensitivity to this color. So color conversion to amber means the sign is brighter and can will be brighter looking even during dimmer periods.   

       An issue with large signs, including those internally lit with electric lighting is even lighting. Since in the proposed concept the light is gathered from surface area, the display of light will come from the depletion of the original light medium. The display will naturally become dimmer the further it gets from the original source. The more display, the more loss down steam. Thus a billboard that collects most of the light from its boarder will tend to produce a brighter image at its boarders, progressively getting dimmer toward the center. This will be more noticeable with larger signs and limited source lighting.   

       Cutting the collecting/conducting material is both expensive and difficult to control the image brightness over a large 2d space. It is sufficient to either reduce the capturing coating or perhaps in places to reverse the coating to leak light. At a very crude level, this was popular with lighted glass signs where erasable fluorescent crayons are used to display the day’s menu.   

       Using a coating, this can be done via a grayscale image and something similar to a silk screen process. Ideally, the image can be made to counter the uneven losses over 2d space. It could still keep its original grayscale and also display color, but it would be adjusted to counter losses due to the displayed image configuration.   

       As for storage of light over a long period, well that happens naturally. The speed of light is different in different mediums (hence reflections). The concept of lossless glass with an exceptionally slow light speed is a real problem for fiber communication engineers (where every picosecond counts) but great science fiction fodder when light speed slows to years. See ‘Light of Other Days’, by Bob Shaw published by Analog in 1968.
CwP, Jul 16 2008

       I found that story on the web, cwp. I have linked it. Thank you for pointing it out. What a jewel. I love high sf.
bungston, Jul 16 2008

       This idea, and the story, reminded me of Light House, a whimsical idea involving jello. It caught my imagination, and I have a story roughed out in my head based on the scheme. Read "Light of Other Days" first, then think about the jello.
bungston, Jul 16 2008

       Off topic perhaps but...   

       [bungston] Thank you for linking in the Light House. [lostdog] is a good writer and has captured the gist of what Bob Shaw intended with ‘Light of Other Days’. I gave the Light House a [+] for his idea and writing.   

       Sadly, I can’t keep up with new entries in the HB much less ferret out the older gems. The good news is that people point them out to us on occasion.
CwP, Jul 17 2008


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