Measurement of low retardation using reflective optical system based on the photoelastic principle

A new measuring technique that can measure retardation and can output magnitude and direction of plane stress in each glass of a panel composed of double transparent pieces of glass has been developed using reflective confocal optics. The linear polarized probe beam is incident to the glass and we can detect a reflected beam converted to orthogonal polarization caused by the photoelastic phenomenon. Using the high extinction ratio (10⁶) beam-displacing prism as a polarization discriminator, we can measure the photoelasticity by rotating the polarization of the probe beam from 0 to π rad without disturbing the optical axis. This system has the ability to measure retardation. The lowest one is estimated as nearly 0.066 nm for 700 μm thickness glass which corresponds to 0.03 MPa stress from our calibration line.