How does reflective photoelasticity work on transparent glass?

[BirefringentStress]

TL;DR Summary

Glass Photonics {subsidiary of stress photonics} is one example of a company that uses reflective photoelasticity for stress analysis systems, however how does reflective photoelasticity work on transparent materials such as glass, if light transmits through it and does not bounce off it?

https://glassphotonics.com/all-products/grey-field-polariscopes/gfp2600-product-line/

In this link you can see a photoelasticity instrument that is all-in-one, and compact. Traditional photoelasticity (left hand side, (a), in this picture) is not compact and has an awkward apparatus setup where there are polarising filters in front of the subject specimen , and polarising filters behind the subject specimen. This makes sense for glass, as light transmits through glass and therefore you would need an analyzer and camera/sensor to detect the polarized light that passed through the transparent specimen. But this product is not a traditional photoelasticity apparatus, its like a standalone camera with an integrated polariscope that you can point at glass and suddenly get stress analysis information!

The reason the website says this photoelasticity apparatus is so compact and works is because it uses kaleidoscope optics. (as stated on the product description in the glassphotonics website: " kaleidoscopic optics")

But I don't understand what that is!! And I've tried a literature review and gotten nowhere. Look at fig 1 of this paper if you want a schematic of a kaleidoscope integrated with a polariscope

Is it like reflective photoelasticity? (right hand side (b), in the previous picture). Even then, how are they getting light to reflect off glass, glass is transparent so why would light be reflected?

And if light is transmitted through and there is nothing to receive the transmitted light through the other side of the transparent glass, how are they receiving any polarized light that's been through the glass??

There is this paper published by stress photonics talking about how they used reflective photoelasticity to analyse stress in a car windshield, but they applied a reflective coating/substance in the inner surface of the glass to allow reflection of light. My question is how does the advertised product (the GFP-2600) work, that definitely does not use reflective coatings or require it, because you can see they use the apparatus on float glass freshly manufactured and being transported on rolling pins in the factory fresh from annealing. It is used as in-situ real time stress monitoring. So there is no shot of a reflective coating applied to the float glass there, because it's freshly produced right and why would they ruin the glass with an opaque reflective coating that is constantly spraying float glass?

I'm so confused, I may be very wrong as I am just a layman. Please someone help me out! Or point me in the right direction, I have tried contacting the company themselves but they have ignored me.

 

[Tom.G]

Google is your friend (well, mostly).

Critical angle of reflection.
https://www.bbc.co.uk/bitesize/guides/zv4pd6f/revision/2

above found with:
https://www.google.com/search?hl=en&q=critical+angle+of+glass

Cheers,
Tom

 

[BirefringentStress]

Hi Tom,

For fig 1 of this article, (polariscope with kaleidoscope optics, like the GFP-2600) https://link.springer.com/article/10.1007/BF02428172, how does it receive the reflected light if light in mirrors is reflected at such a large angle? It seems like you would need to have your sensors/receivers a lot farther to the right of the light source to be able to catch the reflecting light.

But in thw GFP-2600, it os just like a video camera apparatus and only has one integrated unit with all the working stuff inside. And no analyser filter needed to be outside of the camera system.

 

[BirefringentStress]

I think I just need to wrap my head around this to get it, so in the paper it says that the objective lens was "produced by sectioning a conventional convex lens into quadrants and separating the quadrants by a small amount". And the objective lens seems to act like a beam splitter? Because two light beams emerge after passing through it somehow (I don't understand that).

I also don't get the part I highlighted with the red arrows, light is shone onto the specimen and there are two light beams reflected off it separated by roughly 30 degrees? If there is one light source how did 2 light beams get reflected off the specimen and in two different directions?

 

[Tom.G]

I also don't get the part I highlighted with the red arrows, light is shone onto the specimen and there are two light beams reflected off it separated by roughly 30 degrees? If there is one light source how did 2 light beams get reflected off the specimen and in two different directions?

Those dashed lines show the boundary of the field-of-view (light cone) at the various parts of the light path. They meet at the focal point of the instrument where the item being looked at is located.

Understanding ray tracing may help understanding somewhat:

(found with:
https://www.google.com/search?hl=en&gbv=2&q=Ray+tracing+Optics)

A further search found this, somewhat closer to the image you supplied.
http://opticampus.opti.vision/cecourse.php?url=ray_tracing/
(found with:
https://www.google.com/search?&hl=en&gbv=2&q=Ray+tracing+Optics+of+positive+lens)

Try this site for another description of the theory (bit of a heavy read though).
http://gemologyproject.com/wiki/index.php?title=Polariscope
(found with:
https://www.google.com/search?hl=en&q=polariscope+wikipedia

I suppose you could pay the USD$40 for the original research report, but it is so old (20 years -- and patents last 17yrs) that the information is almost certainly freely available somewhere; for instance if the device is being manufactured, there is likely an explanation on the manufacturers web site.

Have Fun!
Tom