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Purely Optical Night Vision

Look Ma, No Batteries!
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A lot of us would like night vision scopes, but they're purty exp--uh, un-cheap. Furthermore, out of us who want them, how many actually NEED to amplify light 25,000 times all the time? Say you're navigating a trail back to camp an hour or two after sunset. Wouldn't it be okay if you could see only, say, 20 times better?

And what about the whole battery thing? Ya gotta put 'em in, wait for 'em to run out, then yank 'em out and put in a fresh batch. Most of us would rather not do that if we didn't have to.

So, how's this? Take some magnifying lenses (optical quality), fold the light path and twist the light around to right-side-up with your choice of mirrors or prisms, the same way binoculars do, but instead of putting lenses at the other end which would magnify the image, put in lenses which just bring it to a focus, so that the full light-gathering capability of the front lenses is exploited. Now, the only thing left to decide is how much light ya wanna let in: the human pupil expands to a diameter about 3/8 of an inch (I'm not sure how exact that is; I just eyeballed it), for a total light-gathering area of 0.11 square inches; with a 3-inch front lens, you get about 7 square inches of area, which lets in about 93 times more light! However, if you actually need to amplify light thousands of times, you might as well go out and buy one of the expensive kind, or else you'll need a whole backpack to stuff yer 12-inch, 2-foot focal length lenses.

So what we've got here, aside from actually working, is something that needs no batteries, something that is lighter than conventional night-vision optics because it has no complex light-amplification systems inside, and something that would cost about as much as a pair of regular binoculars. Sweet.

galukalock, Dec 17 2003

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       How about concave compact disks?   

       "MacGuyver sucks, I learned this on the Halfbakery!"   

       Just imagine the stories I'm gonna tell my kids! <shudder>
Letsbuildafort, Dec 17 2003
  

       Buy a flashlight.
krelnik, Dec 17 2003
  

       This can not be done, a thermodynamics thing. If this worked then we could move energy from a cool source to a warmer target, and then we could make a perpetual motion machine out of it. That would be against the rules in the help file.
Fussass, Dec 17 2003
  

       I don't see any reason that this would have to violate the rules of thermodynamics, since you could trade off uniformity of position for uniformity of angle.   

       Imagine that you had a 10-meter-diameter lens which focused a picture of the moon onto a piece of ground glass 40 meters away which was 10cm in front of your eye. Assume your left eye's pupil is a 1cm disk.   

       Of the moonlight light hitting the lens, about one millionth of it would have hit your left eye's pupil, but nearly all of it will hit the ground glass (and be randomly scattered after doing so). Of that, about 1/500 would hit your pupil (assuming uniform random scattering). If the difuser were properly designed, the fraction could be improved, but even if it weren't there would still be a 2,000-fold improvement in brightness.   

       From what I understand, the fundamental thermodynamic limitation is that an object which is at distance d1 and of brightness b1, if made to appear to be at distance d2, would have its brightness limited to b1*d1/d2. If d2 is much less than d1, b2 could be much greater than b1. Of course, practical limits would get in the way of things (a 10-meter-diameter lens would be a little hard to fit in a pocket) but I don't see any thermodynamic limitations.
supercat, Dec 17 2003
  

       This is sort of how night glasses used to work before the era of fancy electronics - a high aspect ratio (or aomething) to gather in a lot of light at the big end.
DrCurry, Dec 17 2003
  

       Take regular 7X50 binoculars. The lens in front is 50 mm diameter, seven times larger than the maximum pupil of the eye so collects 49 times more light than the unaided eye. All of the light collected by this lens goes into the eye with no loss, as there would be in ground glass. The intensity is also diluted by 49 times because of the magnification. The binoculars cannot be made with lower magnification or bigger lens without also making the exit-pupil larger than that of the eye, in which case the extra light will just spill over. The light presented to the eye must be afocal if it is going to form an image.   

       In other words, the brightness is determined by the F number of the lens as in a camera. The eye comes with certain F number and that is all the light we are going to get in.
Fussass, Dec 17 2003
  

       Your point is understood with regard to purely focal optics. But I don't understand why the optical setup I described couldn't yield a 1,000-fold increase in the apparent brightness of the moon even if the moon's size on the focal plane were such that it occupied the same portion of the viewer's field of view as would the moon seen by the naked eye.
supercat, Dec 17 2003
  

       The principle is the same as using a lupe (magnifying lens) to focus the sun's rays onto a small portion of flammable material to set it on fire. The sun cannot do that on its own from so far away; it needs help. The lupe gathers a lot of light and puts it all in one tiny area, which effectively makes the sun dozens of times brighter (and thus hotter) to an observer within that spot, setting said flammable material (or ant, for some) alight.   

       This principle is also what allows any ordinary human (yes, even you or me) to see Neptune, although our thermodynamically-challenged eyes can't see it alone. Take a telescope with a big ol' front lens, and it takes a whole truckload of light, and slaps it right onto our little retina. All that light goes to one spot, effectively making Neptune (or whatever you point the scope at) brighter for the individual peeping from the other end of the tube.   

       This is no different, except that instead of making objects appear closer, which would dilute the brightness (as supercat said), these glasses leave the perceived distance alone--merely focusing it so you can see it--keeping the image nice and bright.
galukalock, Dec 18 2003
  

       galukalock: I think that for the objects to be made brighter they would have to appear closer; that does not, however, mean they'd have to be bigger or take up more of the observer's field of view. Rather, something like a 10m illuminated object that was 1km away would appear as a 2mm illuminated object that was 20cm away.
supercat, Dec 18 2003
  

       If this were possible, it would have been invented 400 years ago, I'd have thought.
waugsqueke, Dec 18 2003
  

       Your 40 Meter focal length lens [supercat] will form an image of the moon on the ground glass that is .35 Meters in diameter, never smaller. (The moon subtends about a half degree angle.) With the eye at 10 cm away that is a big picture. By the time you move your eye back far enough to make the moon image look normal size those calculations won’t come out so well.   

       In any case the image on the glass will not be brighter than the moon, nor is [galukalok’s] ant-destroying ray hotter than the surface of the sun.
Fussass, Dec 18 2003
  
      
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