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First draft.
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Reflection photoelasticity method
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Reflection photoelasticity
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==========================
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## 1. Reflective polariscope
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## 1. Building a reflective polariscope
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Reflective polariscope can probe the photoelastic fringes with light source and camera on same side of the photoelastic specimen. As shown in the figure below, the reflective polariscope contains 5 basic elements:
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* The light source.
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* The *Polarizer*, which is usually a circular polarizer (plotted as a combination of a linear polarizer and quarter-wave plate below) between the light source and photoelastic specimen.
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* The mirror to reflect light.
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* The *Analyzer*, which is usually a circular polarizer (plotted as a combination of a linear polarizer and quarter-wave plate below) between the camera and the photoelastic specimen.
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* The camera.
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Reflective polariscopes allow for the visualization of the photoelastic fringes when the light source and the camera are located on the same side of the photoelastic specimen. As shown in the figure below, they consist of 5 basic elements:
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* A light source
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* A single *polarizer*, which is ideally a circular polarizer located between the light source and photoelastic specimen. This is shown with both of its parts: a linear polarizer plus a quarter-wave plate.
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* A reflective surface, that can be either a mirror below the sample or a coating on the particles.
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* An *analyzer*, which is a circular polarizer located between the photoelastic specimen and the camera. For a darkfield polariscope, this matches the incident light (and can even be the same optical element); for a brightfield polariscope, this is complementary.
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* A camera.
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Similar to the [transmissive polariscope](https://git-xen.lmgc.univ-montp2.fr/PhotoElasticity/Main/wikis/transmission-photoelasticity), both the *Polarizer* and the *Analyzer* are usually circular polarizers. Thus the principle axis of the quarter-wave plate in figure below must form $`45^{\circ}`$ angle with the direction of polarization of the linear polarizer. It is important to point out that a dark field reflective polariscope uses circular polarizers with same chirality for both the *Polarizer* and the *Analyzer* (see section 2. for mathematical proof), whereas the transmissive polariscope uses circular polarizers with different chirality.
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