The Polariscope is an essential tool for determining strain patterns developed during fabrication and manufacturing. It permits immediate determination of strains in most transparent materials. It is valuable in production or as a laboratory tool wherever glass is fabricated, welded or bent. Check liquid in glass thermometers for stress and strains.
As polarized light travels through strained glass or plastic, it undergoes a retardation proportional to the amount of stress. A Polariscope is an instrument which can be used to qualitatively view this retardation.
Tech Note: When a photoelastic material is subjected to a load and illuminated with polarized light from the measurement instrumentation (called a reflection polariscope), patterns of color appear which are directly proportional to the stresses and strains within the material. The sequence of colors observed as stress increases is: black (zero stress), yellow, red, blue-green, yellow, red, blue-green, yellow, red, etc. The transition lines seen between the red and green bands are known as "fringes." The stresses in the material increase proportionally as the number of fringes increases. Closely spaced fringes means a steeper stress gradient, and uniform color represents a uniformly stressed area. Hence, the overall stress distribution can easily be studied by observing the numerical order and spacing of the fringes.
The photo above shows the strains in a clear plastic tape dispenser. The Polariscope shows the true strain patterns in transparent materials.