That is, does the light-gathering power of say, 80 mm astronomical binoculars result in a brighter field of view? If so, were large-objective optics ever used for night vision before infrared and light-amplificaiion were invented?
I don’t know first-hand about astronomical instruments, but I do know that magnified rifle scopes are a significant aid to night vision, and before the mass-availability of electronic night vision devices, where the best thing a soldier could have on a defensive position at night.
Optics cannot amplify light, it can only magnify the apparent size of an object. Given an object of finite size (say, a distant building or galaxy), a telescope cannot increase the apparent surface brightness (photons per second per square degree). It does increase the apparent size, so the total brightness (photons per second) is increased, but that’s it. Night vision scopes can increase the apparent surface brightness, so a telescope (or binoculars) cannot double as a night vision scope.
Also, night vision scopes are usually sensitive to infrared, and often used with an infrared illumination. Binoculars cannot do that either.
While I’m not sure that scr4 is flat out wrong in the post above, because in fact passive optics do not amplify anything, however what they can do is gather more of the light coming from an object than would normally enter our eyes and direct it them. I know that on the occasions where I have used my 8" telescope to scan the countryside at night that things like distant clumps of trees appear much brighter than when I look at them with the unaided eye. There is a limit of course, if the object is not illuminated at all, it doesn’t matter how large your aperature is, there’s nothing to be gathered.
Yes, but the larger amount of light is also spread over a (apparently) larger image.
It’s not brighter. It may be easier to see because it’s magnified; a large dim object is easier to see than a small dim object. But that’s all.
The exception is a “point source”, i.e. a source of light so small that it cannot be resolved into a finite size even in a telescope. In that case, for all intents and purposese it can be considered to be “amplified” by the telescope.
I do know that scr4 is flat out wrong, if you’ll pardon me saying so. Larger aperatures collect more light, and you can get whatever magnification you want out of that aperature with the right choice of secondary optics (eyepieces and such). So you can actually get a higher surface brightness with purely passive optics. And I have seen binoculars which were built specifically for that: At a star party a few years ago, one fellow had a very nice pair he bought surplus from the Russian army.
No.
One type of passive night vision devices use a special type of photomultiplier vacuum tube that has a photosensitive cathode at one end and a phosphorescent screen at the other. The image is focused on the cathode which releases photo electrons at each point in the image proportional to the amount of light striking that point. These electrons are accelerated by the voltage from photosenstive screen to the photo cathode. When the electrons strike the screen it phosphoresces and you see a synthetic image of the field of view, usually in green.
Another passive type uses an infra-red detector to do the same job except it gives a synthetic image that depends upon temperature differences.
Active infrared devices have an infrared illuminator that illuminates the scene.
Hmmm. This sentence is confusing. I meant to say that the voltage is from screen to cathode. The electrons travel from the cathode to the more positive screen.
No you can’t. There is a lower limit to the magnification; below that limit, the exit beam of the telescope becomes larger than the human pupil, and not all light will go into your eye. In the extreme case, a 100mm telescope with 1x aperture will have a 100mm diameter exit beam. Looking through this telescope, things will look exactly the same as without the telescope: same brightness, same size.
The other reason is that if a purely passive optical system can increase the apparent surface brightness of an image, it violates the laws of thermodynamics. You can use it to increase the energy density of a light beam, which means you can heat something up to a higher temperature than the light source. That means heat flowing from a colder to a hotter object.
Sorry, I meant 100mm aperture and 1x magnification.
Wow. I got about halfway through this thread, and had to check the dates. In this thread, scr4 and Chronos were having pretty much the exact same argument. scr4 was right then, and he’s still right now. Chronos, I think you need to think this through a little bit more.
SCR might be right, but he’s not responsive to the OP. There are binoculars with large objectives known as night glasses and they do improve vision at night. The maximum brightness depends on the size of the exit pupil which is obtained by dividing the objective by the magnification. At night, your pupil opens up to a maximum of 7mm so it can make greater use of the additional light provided by the larger objective. During the day, the pupil closes down to 3.5 mm, so using a larger objective doesn’t buy you anything, and you might as well use lighter binocs.
I’ve seen this effect with even a small monocular – I can distinguish details in questionable light much better with the monocular than without.
I’m sure you’re right, but can you explain how a magnifying glass can burn paper if this is true? You seem to be saying that I can’t use a 10 cm lense to focus light down to a 1 cm radius because it violates the laws of thermodynamics. I can see how what you’re saying could be true for a focused image, but I don’t understand how it is impossible to increase the energy density of a light beam using a lense.
OK, maybe I was being a little too literal. I don’t dispute the fact that large-aperture binoculars are useful for distinguishing dim, distant features. I have a pair of 150mm binoculars myself, and they work wonders for viewing dim objects in the sky (galaxies and nebulas). However, as I explained above, the reason is because they change small dim objects into large dim images.
But night vision scopes can do something binoculars cannot: amplify the brightness without increasing magnification. So you can have a 1x magnification goggles and use it to walk around a darkened city or forest. It’ll be much more difficult to walk around with a pair of 8x binoculars strapped to your head.
(Sorry I have to run - someone else take jawdirk’s question, or I’ll come back later to try to answer it)
There’s a tricky thing called the brightness theorem in optics. The upshot is that, except for a few tricky circumstances (like when the object and image are immersed in indices of refraction), no arrangement of optics, mirrors, and lenses can make the light per unit area per solid angle incident on an image exceed that leaving the object. in fact, because of reflection, scattering, and absorption in your optics you’ll usually be quite a bit lower. This is ultimately a way of saying that your optics can only gather as many photons as the object is emitting – you can’t gather more than there are, so your image of something isn’t going to get brighter than the object is.
Image intensifiers take the incoming photons and amplify them, making their own photons, so they’re not subject to this limitation. But because they do make their own photons (proportionally to those in the image), they’re fundamentally different from passive optical devices, and the two aren’t interchangeable.
The brightness per unit area per unit solid angle on the sun is wayyy more than you need to burn a piece of paper. Even with the losses due to atmospheric scatter and the small collection diameter, there’s more than enough to burn paper.
If this is true, why bother making night binoculars with big lenses? Two reasons:
1.) While you can’t get anything brighter than the original object, you can certainly get it much dimmer. Larger apertures mean more collection area, letting you get closer to that theoretical maximum.
2.) The resolution in your image depends upon the collection aperture. The bigger your lenses, the finer the detail you can see. This is independent of our discussion of the brightness. It’s even independent of the lens quality, up to a point. The concept of diffraction-limited images was discovered in the early 19th century when an amateur astronomer, a British clergyman, found that his poor quality but large diameter lensed-telescope let him resolve two close-together stars that his great quality but small diameter telescope couldn’t. The bottom line is that the maximum resolvable optical frequency is proportional to the size of your lens, so the bigger the lens, the smaller the detail you can see.
CalMeacham explained it much better than I could.
The way I think about it: from the standpoint of the ant being fried by the magnifying glass, the image of the sun isn’t amplified. It’s simply enlarged. But if you kept the sun’s apparent surface brightness (amount of light per square degree) and increased its size, the ant receives more than enough light to burn it to a crisp.
With the most powerful magnifying glass you can make, the ant would see the sun expand to fill the sky. At that point the ant is heated up to the temperature of the sun (ignoring absorption of light by the atmosphere, etc). But never hotter.
Now if you could get any arbitrary magnification and aperture, you could arrange the optics so that the ant would see something brighter than the sun fill up the sky, and get heated to a temperature hotter than the sun. This is clearly impossible (because it violates the laws of thermodynamics).
Hm. I still need to think about this some more, but I think that scr4 might be right after all: The thermodynamic arguments are pretty convincing (and in this house, we obey the laws of thermodynamics!). That said, though, from my experiences with looking through large-aperature binoculars, I seem to recall that I had better color discrimination than with my naked eyes, and color discrimination (of a large, uniformly-colored scene like a grassy hillside) should be a function of surface brightness. Then again, perhaps I’m just misremembering the colors; I’ll have to pay more attention next time I have an opportunity to look through such optics. In the meanwhile, I’ll concede the point until and unless I can come up with a counterargument.
Having now thought about this some more, let me now unambiguously admit error. My previous error was that the configuration I was thinking about required parallel light rays coming in. But parallel light rays, of course, require either a point source, as scr4 already mentioned, or a lossy filter of some sort, which would at least negate any benefits from the lens system. So, I sit corrected.