# Optical aid adventure



## petcnc (Nov 3, 2014)

The inspiration came from "Model engineers Workshop 1"




A pair of cheap binoculars my kids turned into bits and pieces revealed some interesting glass prisms!

I found out they are called "porro prisms"


After some consideration instead of throwing them prisms in the dust bin, I thought I could use it for the project.




As you can see They are VERY CLEAR and you can use it to view from a 90 degree angle.

All I need was a metal bar to support it and to scratch a cross on the horizontal surface so I can "aim" directly on the target.

*Step 1. making the bar.*

Using a scrap round cast iron bar, I created a 45 deg angle at one end.




I thought I should create two different diameter rounds at the other end (16 and 12 mm) so it can go in two different collet sizes.




The finished bar ready to receive the prism.




Testing the prism fitting




Looks ok for the purpose...




Some strong glue, a bit of paint and 24 hours later all set!




*Step 2 the final touch
*
Now the presision work to "engrave" the aiming aid on the horizontal surface of the prism.
I thought I could use a cheap Diamond Dresser Pen I bought from e-bay.




I centered the dresser pen using a 6mm collet and secured it on the mill vise pointing upwards.

Then I put the bar with the prism in a 16 mm collet and touching the diamond point I scribed a cross using only the Y axis of the mill (scribe line 1 then rotate prism 90 degrees and scribe vertical line.




Then I thought "why not making two circles as well?"
I went for it. Circles made but the diamond decided to leave this world and join it's ancestors...

I murmured a few words about China and my incompetence to handle precious stones!!!!
But even with a broken diamond (I managed to break a diamond!!!) the circles were created.




*Step 3 The tool in use
*
I put an iron bar with a 4mm hole in the mill vise and tried to center the hole using my new tool!!




This is what I see through the prism!!!




*The moral of the story:*
When something breaks don't try to fix it! Make a tool out of it instead.

Thank you for reading this.

Petros


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## chuckorlando (Nov 3, 2014)

Pretty slick there. Nice


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## echesak (Nov 5, 2014)

Hey...  That's an interesting idea.  I can't tell you how many prisms and other pieces of optics I have from my Holography days.  I may have to give something like that a try.

Thanks for sharing, 

Eric


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## ariscats (Nov 6, 2014)

Try to rotate the spindle 90o degrees left and right and check if it keeps center.If not,the 45o degrees are not 45o.Also make small touch with
a center drill on a scrap piece,replace the drill with the optical aid without moving anything and check again for centering.I am not trying to
"shorten" your idea,is a very clever one.I'm just trying to point somethings from my experience.A good idea also is to fill the target scratches
with something black and ,more,you could add a shadow piece so only light through the prism will show to improve contrast.
If "improvements" are made i will appreciate much any new photos.
Thanks for the idea.
Ariscats


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## chips&more (Nov 6, 2014)

Over here in the States, we would call that “yankee ingenuity”, great job!


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## petcnc (Nov 7, 2014)

chips&more said:


> Over here in the States, we would call that “yankee ingenuity”, great job!


Whaoo! shall I feel a little bit like a yankee now?

@ ariscats:

Thanks for the kind words pal, I have already tried the 90 rotation and there was no movement. 
Great idea with the black filling. I will borrow some black nail polish from my wife's stuff and see how it works.
I'm not sure I understood the shadow part though... Do you mean to fix something on the eye side like old cameras had?


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## Ulma Doctor (Nov 7, 2014)

excellent work,
 it never dawned on me to use a prism for anything else than chilling out with some pink floyd :jester:


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## ariscats (Nov 8, 2014)

petcnc said:


> Whaoo! shall I feel a little bit like a yankee now?
> 
> @ ariscats:
> 
> ...



Something like a black carton tube (hood?) about 10 cm to cut reflections of ambient lighting


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## Ebel440 (Nov 12, 2014)

Great idea I may try it out. I just got an email from a company called surplus shed they have Porro prisms on sale this week for 7 $ if anyone's looking for one I have no connection to the company but have gotten stuff before from them and was happy with my order


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## petcnc (Nov 12, 2014)

*Optical aid mystery*

Trying the tool I had some hard to explain results that I wish to share with you all.

*MYSTERY 1.* 

When the prism is very close to the surface you wish to center (a couple of mm) you see the surface very accurately (i.e. at the same position) no matter the angle your eye is in relation to the vertical side of the prism. 
In other words it doesn't matter if you are in position "A" or "B" or "C" you see the hole to the same place (see photo)



I (wrongly) thought that the situation will be the same when you move the prism at a height from the surface you want to center.
In that case I found out that happens exactly what happens when you look through a mirror.
The Image seems to relate to the angle you are looking it! Different angles show the image at a different height (see next drawing).




To make things worst different horizontal angle shows the image in different horizontal position.

Solution: none!!!!


*MYSTERY 2. *

I had another porro prism and decided to make another tool. This time instead of applying glue to the sides of the prism I covered all the long side (hypotenuse) with contact cement to make it "As strong as possible".




To my huge surprise I COULD NOT SEE A THING through the prism! Total darkness.
To find out what happened I asked Mr Google to enlight my ignorance.
Mr. Google pointed at:
http://www.astro.caltech.edu/~lah/ay105/pdf/Fundamental-Optics.pdf

There at chapter "Prisms" I found out that:

_TOTAL INTERNAL REFLECTION_
_Rays incident upon a glass/air boundary (i.e., an internal reflection) at angles_
_that exceed the critical angle are reflected with 100-percent efficiency_
_regardless of their initial polarization state. The critical angle is given by_


_and depends on the refractive index, which is a function of wavelength. If,_
_at some wavelength, the refractive index should fall to less than √2=1.414,_
_the critical angle will exceed 45 degrees, and total internal reflection (TIR)_
_will fail for a collimated beam internally incident at 45 degrees on the_
_hypotenuse face of a right-angle prism. Reflectance decreases rapidly at_
_angles of incidence smaller than the critical angle.
_
In pure english it means that for a prism or a fiber optic *surrounded by air* if the 
angle is 45 deg or more we have *TOTAL INTERNAL DEFLECTION* 



Ehm!! in my case the long side (reflective side) was covered by contact cement and not air!!!
That explained the absense of an image as I expected according to (a) at above drawing as the light instead of reflecting was going through the cement and God knows where through it.

Solution: leave the long side clean of glue as I did in the first tool I made

*NEW DISCOVERY*

So I have solved the mystery of no reflection and, if the prism touches air and no glue at the long side I also discovered that you can light the surface below  from the front of the prism!! (see below)




CONCLUSION
It was a learning journey for me! A real adventure! I enjoyed every moment of the construction and, within its limitations, the tool is very accurate and easy to use.

Thank you for reading this


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## hman (Nov 12, 2014)

*Re: Optical aid mystery*



petcnc said:


> When the prism is very close to the surface you wish to center (a couple of mm) you see the surface very accurately (i.e. at the same position) no matter the angle your eye is in relation to the vertical side of the prism.
> In other words it doesn't matter if you are in position "A" or "B" or "C" you see the hole to the same place
> ....
> when you move the prism at a height from the surface you want to center ... I found out that happens exactly what happens when you look through a mirror.


Indeed, once you get “outside the glass,” a Porro prism is nothing but a mirror.



petcnc said:


> In pure english it means that for a prism or a fiber optic *surrounded by air* if the angle is 45 deg or more we have *TOTAL INTERNAL DEFLECTION*





petcnc said:


> ....
> Solution: leave the long side clean of glue as I did in the first tool I made


I was thinking of this very thing when I read your first post.  I’ve actually seen similar prisms (for astronomical use) with pieces of black paper against the reflecting face.  I think this is to be sure that stray light isn’t transmitted into the prism.  And if nothing else, it will help protect the back side of the prism from any stray glue.


petcnc said:


> *NEW DISCOVERY*
> 
> So I have solved the mystery of no reflection and, if the prism touches air and no glue at the long side I also discovered that you can light the surface below  from the front of the prism!! (see below)
> 
> View attachment 87665


You might have some fun using my “beamsplitter” illuminators between your eye and the Porro prism. (See also post #2125 in Project of the Day.)






petcnc said:


> CONCLUSION
> It was a learning journey for me! A real adventure! I enjoyed every moment of the construction and, within its limitations, the tool is very accurate and easy to use.
> 
> Thank you for reading this


 Thank YOU for an inspiring idea!


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## petcnc (Nov 13, 2014)

*Re: Optical aid mystery*



hman said:


> Indeed, once you get “outside the glass,” a Porro prism is nothing but a mirror.
> 
> I was thinking of this very thing when I read your first post.  I’ve actually seen similar prisms (for astronomical use) with pieces of black paper against the reflecting face.  I think this is to be sure that stray light isn’t transmitted into the prism.  And if nothing else, it will help protect the back side of the prism from any stray glue.
> 
> ...



Thanks for giving me new ideas to test on the "optical aid tool v.2"
Is it collaborative thinking or what?

Petros


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