MadSci Network: Physics

Re: How does the dot in red-dot gunsight appear visible relative to the target?

Date: Wed Jan 12 21:03:26 2000
Posted By: Matthew B. Weyerich, Technical Coordinator,ES&R Dept., CPI Corp.
Area of science: Physics
ID: 946968178.Ph

Hello, Mark!

I love this question because I've enjoyed target shooting ever since I got 
my first BB Gun years ago. I've seen the "red dot" sights you 
mention…wasn't lucky enough to have one on my gun…and I've always wondered 
how they worked. Time to find out!

My first clue was the word "reflex". I work for photography company, so I 
went about asking people what the "R" in "SLR" (Single Lens Reflex) meant. 
That helped me think about prisms and things - which I'd suspected from the 
start- but provided no solid answers.

I also looked for "reflex sight" on the web, hoping to find an explanation. 
No such luck.

Then I thought, "Hey, somebody invented this thing, and there's probably a 
patent. Patents usually have drawings which tell you how the thing works." 
BINGO! (There are several. Some are listed below.)

I'd like to answer your questions before I just dump the links on you, 
though, so, here's what I can tell you:

1) The dot always appears visible in relation to the target because you are 
looking at a COLLIMATED image.
2) The dot appears at infinity because you are looking at a COLLIMATED 
image. (By the way: Another term for "reflex sight" is "infinity 
sight"…your question is "right on target"!)

A collimated image (actually you are seeing TWO images) can be thought of 
as one in which the light rays striking your eye are kept parallel. (You're 
right, by the way. Astronomers have to be concerned with collimation. It's 
something you have to account for if you are looking at stars through a 
telescope, as stars are "point sources of light", assumed to be 
"infinitely" far away.) Collimation simulates infinity, as far as optics 
are concerned.

[Fun Aside: About 30 feet from my desk is an "autocollimator". The device 
is basically a pipe with a lens at one end, and a perpendicular, adjustable 
viewing lens about 1/3 of the way from the other end.  Not much to look at, 
but "magic" happens inside! (It should: the eyepiece alone costs more than 
a car!) We look through camera lenses at a mirror with this thing and it 
tells us exactly where the focal point of each lens / camera combination 
is…parallel rays in, they meet at the mirror, parallel rays out. If not, we 
can tweak the device's focus and find out how far off the lens focus is. I 
think of this device as our "infinity simulator"….kind of a cross between 
Marvin the Martian's "Illodium Q-32 Space Mod-u-la-tor" and the irritating 
way  I go back in time when I put instant coffee in the microwave!  Back to 
the topic at hand, though…]

Basically, if you line up the dot and the target you are looking at two 
superimposed images, the dot-portion of which is always going to be "at 
infinity", because it's rays are kept parallel with that of your target. If 
you shift your position / viewing angle, you see different rays, but 
they're still kept parallel, so the dot "stays with" the target as long as 
you can see the target in the sight ring. (Kind of spooky to look at, but 
it really works!)

The "magic" inside your son's scope is math and optics. There are different 
ways to get the job done (prisms are one way), but, the principle is the 
same: reflect one image into another. If the focal lengths of the optical 
systems involved are precisely controlled and kept equal, the rays from 
both images are kept parallel. Move your eye, see a different set of 
parallel rays. Presto! Nifty gun sight!

I'll rely on the pictures in the links below to show you how this works. 
(It's pretty simple if you follow the lines in the patent drawings.) If 
that doesn't explain it, or, if you want more information on this topic, 
please feel free to e-mail me at, or, 

I've really enjoyed researching this question, and I'd be happy to help if 
you need to know more!

Your MadSci,


At this site use the search engine with the text "reflex 

Why you should never, EVER, point your BB Gun at a helicopter: (See the 
XM-60 reflex sight pics, then visit the homepage. Interesting visual 
history & audio files here.)

One of many manufacturers (These guys make the reflex sights the army 
uses.) :

Some cool sights you could buy (The "Russian Reflex" looks like something I 
might buy… go back to the homepage for more BB Gun / paintball gizmos.) :

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