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    Re: Night Vision Scopes
    From: George Huxtable
    Date: 2005 Jun 29, 22:24 +0100

    Brooke Clarke wrote-
    >The devices commonly called "Night Vision" are light amplifiers, not IR
    >based scopes like were used in W.W. II.  They take the existing light
    >and make it brighter.  Although I have not done it, I expect that in the
    >middle of an ocean on a moonless night you could in fact see the horizon
    >with a night vision scope.
    >Astronomers call the optical type of scope you describe a "rich field"
    >scope.  That means that the exit pupil diameter is about 7 mm, which is
    >the diameter of a night adapted eye.  For example a 7x 50 binocular has
    >an objective diameter of 50 mm which when divided by the 7 power
    >magnification yields about 7 mm exit pupil.  Any scope whose objective
    >diameter divided by it's magnification that yields about 7 mm is good
    >for viewing with a dark adapted eye.  For daytime use where the eye's
    >pupil is only a few mm diameter you can use a scope with a smaller exit
    >pupil diameter.
    What Brooke says about such an optical telescope (and it applies just the
    same to "night binoculars") is quite correct.
    But there's an additional point to be made about such optical devices,
    which is often not appreciated, bur was touched on in earlier discussion of
    this topic on Nav-l. It's this-
    No night-glass or telescope or any other such device can do anything to
    enhance the brightness of a night-scene at the retina, to be any greater
    that what the naked-eye itself sees.
    A "night-glass", as Brooke explains, has a big enough objective to collect
    all the light that will go into the enlarged pupil of a dark-adapted eye,
    given a certain magnification. In that respect, it's better than a
    "day-glass", which has a much smaller objective for the same magnification,
    but is still quite big enough to collect all the light that can go into the
    much-smaller eye-pupil in daylight (only about 2mm dia. as opposed to 7mm).
    In daylight, both these oculars will perform exactly the same. Only at
    night will the night-glass do better. But even then, what you see in a
    night-glass is no brighter than what you can see without it. In fact, it's
    somewhat less bright, because of the light-loss inherent in passage through
    the glass surfaces.
    To take Brooke's example, a x7 night-glass with a 49 mm. objective can
    collect all the incident light falling on it and compress it into a narrow
    pencil 7mm. dia, just big enough to fill the pupil of a dark-adapted eye.
    If the objective was bigger than 49mm, then that outgoing pencil would be
    wider than 7mm.,  and light would be wasted in striking the iris rather
    than in passing through the hole. The ratio between the diameters of the
    incoming pencil of parallel light (defined by the size of the objective)
    and the outgoing beam exiting the eyepiece is exactly the same as the
    magnification of the ocular, 7x in that example. Indeed, that's a valid and
    simple way to measure the magnification. It's universally true, and doesn't
    depend in any way on the details of the optical design.
    If we neglect any light loss in transit through the glass or in crossing
    its surfaces, then the night-glass collects 49 times as much light-energy
    to pass into the pupil, compared with the light-energy that would enter the
    pupil without the night-glass, simply because of the 49x increase of
    area.  That light now forms an image in the retina. Because of the
    magnification of x7, every object, focussed on the retina, occupies 49x the
    retinal area than it did without the glass. So the light-energy per unit
    area on the retina, which is the definition of brightness, is no greater
    with the glass than without it.
    This conclusion seems to contradict common experience. I agree that when
    you approach a dark harbour, searching for unlit moored craft, a
    night-glass certainly SEEMS to help. In fact, it helps by making the images
    bigger, rather than brighter. Surprising, but true. That conclusion
    surprised me when the question arose, when last discussed on this list.
    The only way to increase the surface brightness of an image, then, is with
    a device that can actually feed additional energy, such as the night-vision
    scopes that Brooke refers to.
    >Earlier, Alexandre Eremenko wrote:
    >>I have never used a night vision device,
    >>but on a pure theoretical ground I predict
    >>that it will NOT help to see the sea horizon:-)
    >>(And for the stars and the Moon you don't need
    >>any night vision anyway).
    >>Can anyone verify this theoretical prediction?
    >>P.S. I mean the common modern night vision devices based on infrared
    >>radiation. Another tipe of "night vision scope" was invented
    >>in XVIII century, and this was simply a Galileo scope
    >>with small magnification and with large
    >>object lens diameter. (Approximately of the same type as
    >>the standard straight non prismatic scopes of modern sextants).
    >>These scopes indeed help with horizon or any other object at
    >>night simply because they collect more light.
     From the argument presented above, I suggest that Alex is wrong about
    the  Galilean "night vision scope" he describes; at least, in comparison
    with the light-collection of the naked-eye. It would certainly be better,
    at night, than a "day-glass" with a smaller objective, however.
    It also seems to me unlikely that his prediction on a "pure theoretical
    ground" of the inefficacy of an amplifying "night vision device" will hold
    water. It seems unreasonable for Alex to ask readers to verify such a
    questonable proposition, before he has presented any arguments in its support.
    Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222,
    or from within UK 01865 820222.
    Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
    5HX, UK.

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