Passive night vision goggles?

I was walking my dog the other night and thought about something to see better at night. What are the problems with making something that is:

  1. Passive, by that I mean optical only; no electronic amplification of light.

  2. Gathers 2-10x more light than naked eye.

  3. Does not protrude in front of your face more than … oh, 5cm.

  4. Wide field of view.

  5. Perhaps some distortion would be allowable, depending on how much brighter the center of your vision was.
    I went out and took a look around with some binoculars that have 50mm lenses, and they worked fine. I guess my question could be rephrased ‘How short and light can binoculars be?’

Is a person that designs optical systems called an Optician like the eye-glass makers, or is there a more technical/other term?

Someone who designs optical systems is an optical engineer.

It’s impossible to build passive night vision goggles. Binoculars and telescopes gather more light than the human eye, but it also magnifies the image so the apparent surface brightness cannot be increased. If it were possible it would violate the laws of thermodynamics. When you look at a full moon through a telescope the apparent surface brightness doesn’t change; it just looks larger. So binoculars aren’t much help if you are trying to look at a dim flat surface like a moonlit field. The only exception is when you’re looking at a tiny point of light too small to resolve with your binoculars; in that case the size does not increase perceptively, so the apparent surface brightness does increase.

I did start this reply in quite a comprehensive manner, but on second reading of your question I came to a simpler conclusion and edited it all out.

Sounds like you want opera glasses.

Although I doubt opera glasses have a great field of view, but they are certainly compact.

The NVG (Night Vision Goggles) used by the pilots where I work are amazing things, damned expensive, but boyohboy!

NVG are passive in the sense that they dont transmit any electromagnetic radiation (unlike wildlife camera units who often illuminate their subject with Infra Red lights and film with special IR cameras). NVG do however need battery power to amplify the existing light to a usable level and in that sense are not passive in the sense of part 1 of your question.

What is ther relation between ‘increasing aparant brightness’ and magnification. If some optics could increase mag 10% say, while doubling brightness, that would be ok.

As to opera glasses, that is close, but what I want is to increase brightness, not magnify, to be able to wear it, not hold it, to not have it protrude far from the eyes, and to have a wide field of view.

      • Ehhh, , , -just recalling quickly:
  • Magnification is the inverse of the transmitted light level, so a 2X telescope with the same size objective and eyepiece would cut the light level by 50%.
  • The ratio of the objective lens to the ocular lens is the ratio of light transmittance. So if a telescope is 1X magnification and has an objective lens with twice the area as its eyepiece has, it’s “gathering” twice as much light as another telescope with the same eyepice, and an objective lens the same area as the eyepiece. (this all is assuming perfectly-efficient lenses, so the real losses are somewhat higher)
  • The human pupil is ~6.5mm in people 25 and under, ~5.5 in people 40 and older, and shrinks evenly across the time gap. So for any telescope to increase the apparent light you see, it has to transmit more light to your pupil than your pupil would be able to pick up on its own (taking into account the telescope’s objectiv-to-ocular ratio and magnification).
  • Opera glasses are very compact compared to regular penta-prism and roof-prism binoculars because opera glasses are non-inverting. This optical design doesn’t work well for magnifications above a few “X” however, even with modern lenses–because opera glasses lenses have to be made with very short focal lengths and this causes distortions to occur (chromatic and I forget the other one, the “focus” one…? nuts). The reasons that regular binoculars use inverting prism systems is that this lets them use longer focal-length optics, but folds the optical path up on itself so that the overall instrument is smaller. Generally speaking, longer-focal-length lenses suffer less lensatic distortion.

  • You can use binoculars in some night conditions usefully: 7X50’s would be about 10 times brighter, 10X50’s would be about 6.5 times brighter. As the magnification increases the view gets dimmer though. -And remember that even a cheapo Russian night-vision monocular probably gives a 1200X increase in the light level for a 2-4X telescope. So if you really want to play int eh dark, get some Russian 1X NV goggles with a headband-mount: there’s really no way to accomplish the same thing using only small unassisted lenses.

As I said, the apparent surface brightness remains the same. So the increase in total brightness is the square of the magnification. If you look at the moon through a 2x opera glass, the apparent brightness (light per square degree) remains constant, but its apparent area is now 4x larger, so the total brightness is increased by a factor of 4.

But it’s not OK. If you are trying to read a license plate 20 feet away under moonlight, you can’t use binoculars to make it brighter. It’s the apparent surface brightness that counts, not total light.

I’m sure the bad astronomer could answer this better but I’ll answer it based on my experience with telescopes as a teenager. Actually for alot of astronomy you use a telescope to increase brightness and not magnification.(Well that and using stuff like camera’s and CCD’s.) You don’t really benefit from magnifying individual stars. You use the telescope because you effectively collect light across a circle several inches across and then funnel it down your pupil.(Remember, even a small telescope has an opening of 2" or 50mm. A decent size amateur scope is around 8-12" and gives you a lot of light collecting power.) This means that you can see a much dimmer object than without a telescope. Hell even I’ve had the experience of looking at the moon at low magnification through a telescope. It appeared quite a bit brighter. Of course if you do magnify it does make the image dimmer. However it’s a matter of how big your primary “collector” (mirror or lens) is versus the amount of magnification.

Of course a telescope would be lousy as night vision goggles since they’re so big and cumbersome.

So it’s not possible to make optics that gather light from a large area, and funnel it into your eyes, but that do not magnify the image, so as to make it brighter?

Well you could do something like that with mirrors.(I mean I don’t know of any reason you couldn’t just have a bunch of flat mirrors and just tilt them so they aim their image at the same point.) However I’d think that’d be kind of big and heavy.

Look at it this way, if it was possible to do what u are asking, the military would be using it and not powered NVG as they currently use.

It’s perfectly possible, just not nearly as effective as powered goggles. If the military has one pair of goggles that make it seem a little brighter and another pair that needs batteries and makes it seem like daylight. you can bet they will go for option two.

That’s correct. It’s not just a technical limitation; the laws of thermodynamics say so.

Imagine an ant looking up at the sun. The sun obviously has an extremely high surface brightness (which, by the way, does not depend on the distance from the sun), but the apparent size of the sun is small so there isn’t much light falling on the ant. But then someone focuses sunlight onto the ant with a magnifying glass. The ant looks up and sees that the sun has gotten much larger. The surface brightness of the sun still hasn’t changed, but since it’s larger the total amount of light and heat has increased. If you use a more sophisticated lens/mirror system, you can arrange it so that from the point of view of the ant, the sun fills the entire sky. Surface brightness still hasn’t changed, but that surface brightness mulitplied by the entire area of the sky is a huge amount of light and heat. The ant will receive as much heat as if he were sitting right on the surface of the sun. The corpse of the ant will approach 6000 degrees, the surface temperature of the sun.

Now, imagine you have a 1x magnification optical light intensifier. If you place this in front of the above-mentioned super-magnifying glass, the focused light will get even higher. The ant will be fried to a temperature higher than the temperature of the sun. That means heat is flowing from a cooler object (sun) to a hotter object (dead ant). This clearly violates the laws of thermodynamics. We can conclude, then, that a 1x magnification passive light intensifier is impossible.

I am not convinced that scr’s argument is valid. Given a larger aperture, you collect more light and focus it on the same area. You get more photons from the same source hitting your light receptors.

Consider the case of a 50mm camera lens. A f2.0 lens will collect less light than a f1.4 lens. The faster lens will let you make a shorter exposure. So you can obviously focus more light from a source on the same area with the same magnification.

I’m not sure the thermodynamic argument works either. The specific example probably has more to do with energy density than total energy.

Oops, my mistake. Apparently I confused the fact point sources (IE stars) do get brighter in a telescope and thought extended sources do as well. As SCR4 has said the brightness of an extended object per square unit is at best the same. The apparent size increases so the amount of energy transmitted is greater.(A larger dim object is easier to see than a small dim object.)

Oh well, always good to learn.

I don’t think that scr4’s argument is valid. I can take a telescope of any aperature I want, and give it any magnification I want. So, I could have, for instance, a 6 inch telescope with 100x magnification, and a 20 inch telescope with 100x magnification sitting right next to it, looking at the same object. If I look in both eyepieces, the images will look the same size. But it’s undeniable that the image from the larger scope would have to be brighter: Else what happens to all of the extra photons which are entering the scope?

His thermodynamic argument relies on the claim that a bug under a “brighter” magnifying glass would have to be hotter than the surface of the Sun, but I don’t see why that would have to be the case. Undoubtedly, it would be hotter than under the “regular” magnifying glass, but that’s not solar temperatures, either.

Passive = no transmission of e-mag energy (as mentioned above). Internal amplification is still passive as it is known in the industry, so the way you define passive is not the conventional definition. But that’s OK, I think we all know what you mean.

The reason binoculars are so big is that the light they are focusing has to have enough path length to converge. This path can be folded with mirrors, but that becomes more lossy, heavier, bulkier and more expensive. Plus to make a very compact path you often place mirrors in the light path, so you lose effective gathering area. Heck, most high-power binocs DO fold the light, and they are still somewhat large.

In Waves, volume 3 of the Berkely physics course series, problem 9.32 addresses this somewhat:

With the larger telescope, you can move your eye around a lot and still see whatever it is you’re looking at, while for the smaller one, you’d have to keep your eye in one place.

As you increase the aperture, the exit pupil (diameter of the output beam) will increase. When the exit pupil exceeds the size of the human eye’s pupil, your image has the maximum possible surface brightness (which is the same surface brightness as the raw, unmagnified image). If you increase the aperture even more, the extra light will not get into your eyes.

Why not? If you had an ideal magnifying glass, the flux at the focal point will be the same as the flux at solar surface. It will be thermally equivalent to sitting on the sun.

If you had a 1x magnification optical system that produced a brighter image, you could put as many of them in series as you want. The image will get brighter and brighter. Where is the extra energy coming from?

I am not saying that your argument is not correct, but I think I can address this counter-example. Imagine our hypothetical device has a diameter pointing at what you want to see of 80mm, and the diameter of the ‘light going out towards your eye’ is 8 mm. If you tried to put two of these devices in series, the whole 80 mm diameter of the second device is not getting a brightened image, just the central 8 mm of it is.