The scope is keyed in to order to ensure that it is installed right side up. How many door scopes can one install upside down? This is the first optical device that I have seen that has a top!
If one holds the viewer and identifies the key, then rotates the unit 90 degrees, the image on the ground glass rotates 180 degrees. I do not understand how this is possible as there are no moving parts that align to gravity or anything internal to rotate independently of the entire assembly.
The box has a drawing of the internals and it seems to be a set of prisms, some lenses and the ground glass. This is strictly a curiosity for me, not a technical requirement. I feel a bit out of place among the highly technical dissertations already posted. If there is a more appropriate newsgroup, please let me know."
(When I first saw this posting, I thought that there must have been something he was missing. Then, I remembered the dove prism.)
The optical system includes a dove prism or an equivalent optical system made from other prisms or mirrors. The dove prism is crudely shown below:
__________ _
In ----> / \ ----> Out / \
/______________\ /_____\
Dove Prism Right Angle Prism
(Top is right angle)
You can see a similar effect using a normal right angle prism (the type in
binoculars) by sighting through it as though it were a short dove prism.
A portion of your view will exhibit this phenomenon.Light rays are deflected through refraction at the entrance and exit surfaces to the bottom of the prism which acts as a mirror by total internal reflection.
It is the mirror that is critical to get the 2X rotation effect. This is similar to looking at a scene reflected in a mirror. If you rotate the mirror X degrees, the reflection will rotate 2X degrees. What is unique about this configuration is that the refraction at the entrance and exit of the prism enable the input and output to be coaxial.
I would guess that this 2X rotation is just a byproduct of the door-scope needing to create a non-inverted image on a ground glass. The lenses would normally invert the image, so the dove prism or its equivalent reinverts it.
Then again, maybe this is a California company that has a really good product planning process and is ready with a device for correcting tilt after the BIG one: "Don't rotate the house, just adjust our viewer". :-)
Here is an experiment you may be able to try with simple mirrors:
Get yourself 3 small mirrors, say two that are 2" x 2" square and one that is 2" x 6" or so.
Arrange them as follows:
\ /
In ---> a\ /a --> Out
The angle, a, should be about 30 degrees (Internet graphics stinks!).If you sight through this, it will give you a fair approximation of the dove prism. Since there are 2 additional reflections rather than refractions, the details of the optics are slightly different, but does basically the same thing.
Another not quite as effective approach is to hold a regular mirror at arm's length at a position where you can see something in front of you. Then, rotate this around you while maintaining that object in view. It will appear to rotate through twice the angle of your arm movement.
And, the answer to the other question on your mind: "Can you put two dove prisms in series to get 4X rotation, etc.?" is NO - you end up with a No-0p!
(From: Mark W. Lund (lundm@plasma.byu.edu)).
I will try to get through this, but a physicist without a chalkboard is like an Italian with his hands tied. The trick of understanding a dove prism is to realize that the "beam" is refracted down towards the hypotenuse , bounces off it, then is refracted again at the second glass surface. The reason that the prism is not the same for every angle of rotation is that the image bounces off the hypotenuse differently as it is rotated. My favorite way to look at prisms is with a pencil. You hold the pencil, point up, and imagine the pencil traveling through the prism. If the hypotenuse is down then as the pencil travels through the prism the eraser hits the reflective surface first, causing the pencil to tilt. By the time the tip hits the mirror, the eraser is already bounces off, so it is evident that the pencil has flipped over, and the image through the prism will be upside down. Now if the hypotenuse is rotated 90 degrees to the pencil, then the pencil will go through the prism without rotating end for end, but only side to side. To complete the mystery, rotate the prism another 90 degrees and the pencil is going end for end again.
So we have rotated the prism 180 degrees but the image is rotated 360 degrees. These things are really interesting, many an hour I have spent with my own dove prism, rotating. I don't know what the point is with your door viewer, but it probably uses a lens to project the outside image onto a frosted screen to prevent the inside image from getting out. The dove prism would be used to make the upside down image from the lens right side up again. It seems a little expensive for a simple door viewer, but I am in favor of clever consumer optics, it is really hard to find any these days.
(From: Bob Knowlden (knowlden@ll.mit.edu)).
If you have access to optics books, check out the dove prism or the k-mirror. (The dove prism is mentioned briefly in "Principles of Optics", by Born & Wolf.) These are both straight-through devices with an odd number of reflections (with folds in the same plane). Any such device will produce an image that has flopped parity (like your image in a mirror) but that rotates at twice the rate you rotate the prism (rotate the prism 45 degrees, the image rotates 90, etc.). I presume that your "door scope" does the same. If you want to see something funkier, find a reference on the Pechan prism., which is used as a compact beam rotator.
None of the above is necessarily obvious. (If you want to be nasty, ask someone who knows a lot about optics why a mirror gives a left/right reversal. It's a bit like asking a mathematician to define a number.)