Re: optics data

On Tue, 2 Dec 2003 23:34:30 -0000, "Dirk Bruere at Neopax"
<[EMAIL PROTECTED]> wrote:

>
>
>"Tony Williams" <[EMAIL PROTECTED]> wrote in message
>news:[EMAIL PROTECTED]
>> "Dirk Bruere at Neopax" <[EMAIL PROTECTED]> wrote in message
>news:<[EMAIL PROTECTED]>...
>> >
>> > Which reminds me of something I should know, but I never liked doing
>optics
>> > on the physics course...
>> > Is it possible to build a light intensifier of arbitrary power simply by
>> > expanding the dia of the objective and keeping the magnification low?
>>
>> Sorry, but no. What you will do is increase the size of the exit
>> pupil, but as your eye's pupil will be no bigger than 7mm it can't use
>> any more light than that. That is why the standard 'night glasses'
>> measure 7x50. That gives you a 7.1mm exit pupil, and a 7x100 would be
>> no brighter.
>
>Still not clear.
>Why cannot the light coming into a (say) 100mm objective *all* be
>transferred to the eye?

Tony is incorrect in this case.  If you wanted to make a 7x100 lens
then it should be twice as long (and twice the diameter) of a 7x50
lens, unless you alter the curve of the lens.  Obviously this can
become extremely bulky and heavy.

At the extreme end, the physical dimensions of the lenses come into
play.  As you decrease the f/# further and further the convex curve of
the lens must get fatter and fatter until the focal point is inside
the lens itself.  

Also, with extremely convex lenses you introduce errors such as
spherical aberration and coma into the system.  Basically only the
chief ray that travels straight down the middle is free of error.  The
marginal rays that traverse the edges do not focus to the same point.

There are ways to deal with spherical aberration, principly through
aspheric lenses -- i.e. introduce a conic curve to a spherical lens.
I know they are used now in aircraft HUDs but they are probably too
expensive still for normal soldiers.  There are cheap molded aspheres
available but they probably aren't up to military standards.

--
Robb McLeod ([EMAIL PROTECTED])
A bus station is where a bus stops.  A train station is where a train stops.
On my desk I have a work station...