# Right angle prisms

I use right angle prisms to make an image alignment.

Let's suppose 1st and 2nd image are the same. The 1st image should be aligned with the 2nd image.
My problem is that right angle prism deviates input ray about 8°. ( Prisms are made of bk7 material which has refractive index = 1.5168 )
Can anyone let me know where and how I must re-arrange prisms( where to add that 8° gap) so that it will eliminate that deviation and the rays of picture 1 and ray of picture 2 will enter the camera in a correct way?
Thank you

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sophiecentaur
Gold Member
Hi and welcome.
Can you tell me why you reckon the blue ray from the mirror will not go back out of the top face - obeying the reflection laws at the 45 degree face?
The sort of trick you are trying to achieve is normally done with half silvered mirrors, I think.

Lok
A few questions:
So you use 2 separate prisms arranged by hand in the way shown above?
The 8 deg deviation comes from the inequality of the two input/output faces of the prism? (If this is the case there is the possibility that the 2 images cannot be aligned or their intensity will be very different)
Are you sure your 2 images are parallel?

Other than these how about getting a Beam splitter?
http://en.wikipedia.org/wiki/Beam_splitter

Thank you both of you for the replies.
@Lok
How should l use of the beam splitter? (One ray enters and two rays go out in 90°).
But I need two images ( one up, one down i.e. 180° between them) to be seen together .

I also found this solution

The black lines are the mirrors. The red and green are the light sources.
But not sure what kind of the mirror is that lower right mirror.
Does anyone know??
In this configuration the mirrors are set as 45 degrees.

sophiecentaur
Gold Member
You need a half silvered mirror, also known as a stereoscopic mirror. Obtainable from this link that I found. And others,I'm sure. It seems that a 2" X 2" mirror would be \$12. Not cheap but these claim to be high quality.

Do you mean that the lower right mirror is stereoscopic mirror and all others are common mirrors?

sophiecentaur
Gold Member
You only need a stereo mirror when you want two paths for the light. In your diagram, all but one mirror are fully reflecting. Have you Googled anywhere else? It would be a good idea to read as much as you can.

Lok
How should l use of the beam splitter? (One ray enters and two rays go out in 90°).
But I need two images ( one up, one down i.e. 180° between them) to be seen together .
I was trying to get more info. It seems you can use other mirror setups but the input images are fixed to their 180 angle. So a Beam splitter is not ideal.
I also found this solution...
The solution will work just fine,just remember that the bottom image exits mirrored while the top one does not, if that does not present an issue all you need is the above mentioned half-mirror.

@Lok: Can you please explain a little more what you mean by:the bottom image exits mirrored while the top one does not
Thanks

sophiecentaur
Gold Member
@Lok: Can you please explain a little more what you mean by:the bottom image exits mirrored while the top one does not
Thanks
One image undergoes one reflection whilst the other image undergoes two reflections. One is laterally inverted and the other is laterally inverted twice. This can be taken sorted out for in the way the two images are presented to the system in the first place. Not an insoluble problem.

@sophiecentaur: Thank you for your explanation.
In your first reply from Aug7-14, 10:06 PM, you pointed out I forgot the blue ray from the mirror will go back out of the top face.
Do you think that will distort the picture( makes it less significant)?
Thanks

sophiecentaur
Gold Member
@sophiecentaur: Thank you for your explanation.
In your first reply from Aug7-14, 10:06 PM, you pointed out I forgot the blue ray from the mirror will go back out of the top face.
Do you think that will distort the picture( makes it less significant)?
Thanks
Light hitting the 45° face of the top prism (blue ray) should be totally internally reflected (same as the yellow ray is shown, reflecting of the face of the other prism). I do not understand why you are drawing the ray going straight through it. 45° prisms are used for the very reason that you get 100% reflection this way. The only way to avoid 100% reflection is to have a substance with higher refractive index than air behind the face (changing the critical angle). Otherwise, you can expect zero(blue ray) light to emerge the way you have drawn it.

If you don't know about total internal reflection then you should google it. This is very relevant to your invention. It's why the other prism works with the yellow ray!

You say:
to avoid 100% reflection is to have a substance with higher refractive index than air behind the face.
By FACE do you mean the place where the blue ray enters the prism( here, in the picture, it is the top) ?

sophiecentaur
Gold Member
A triangular prism has three relevant faces. I am referring to the face where the reflection occurs. That's where there is TIR (Total!) and it's where you would need to suppress TIR. Have you read any other sources of information, to find how / why the prisms are used in this way? The web is chock full of them and it would help to learn as much as you can, rather than just to work on the answers you are getting here.
If you just want a result then it would be best to buy something off the shelf and that way you should get what you need but it will cost you a bit, I expect.

You say:to avoid 100% reflection is to have a substance with higher refractive index than air behind the face (changing the critical angle).
But there is the other glass prism behind the face and glass has higher refractive index than air.
I also tried to glue prisms together( using Liquid optically-clear adhesive) and it stopped working as I expected( the blue ray did not go the way I wanted.) But if there is air between those two prisms, the blue ray seems to go the ( similar) way as drawn.
My problem, however, is the deviation that those prisms cause and
1st image ( yellow ray) can not be aligned with the 2nd image( blue ray)
Can anyone explain that deviation?

sophiecentaur