When a material is observed through the polariscope, the presence or lack of stress areas should not be construed to be conclusively indicative of improper or proper heat treating or sealing. The polariscope has not been established as a conclusive analytical test instrument in these areas. Ideas for utilizing test instruments may occur more easily if the basic elements of the theory of operation is understood. Therefore, a brief explanation of polariscope theory follows. Figure 2 has been drawn to help clarify the narrative.
Any light source emits a very large number of photons of light oriented at random.
These photons are a light wave mixture linearly polarized in all possible transverse directions. When two polarizers are mounted in the crossed position, i.e., with their transmission directions at right angles to each other, no light is transmitted through the combination. If a material that refracts light is inserted between the crossed polarizers, the linearized light passing through the first polarizer will have its direction altered by the refracting material and some light will pass through the second polarizer to the viewer. The areas of the doubly refracting material that refracting light from the first polarizer will be seen as patches of color when viewed through the second polarizer. The Bethlehem model P5 polariscope is being utilized by some districts as an experimental inspection tool.