1. Multiple Wavelengths Photoelastic Constant Determination of Glass by Tardy Method and a Hyperspectral Camera

In most birefringent materials, photoelastic constants (or stress-optic coefficient) of various wavelengths are commonly required for application in modern optical measurement and/or electronic products. Nevertheless, the birefringence in glass is weak. In this paper, a four-point bending beam test was performed using a circular polariscope with Tardy method. A pair of images with analyzer at angles γ and γ + 90° was acquired by a hyperspectral camera with wavelength range from 400 ~ 1000 nm, separated by 3 nm. By using image subtraction, the quarter-order fringe plus γ/π for each wavelength can be determined. Then the photoelastic constant can be calculated through the stress-optic law. Test results show that the photoelastic constants can be determined in the wavelength range from 450 to 780 nm using the hyperspectral camera. Comparing to the result by using single wavelength, the difference between them is less than 3%. The combination of four-point bending test, digital image analysis, and hyperspectral camera makes the calibration of photoelastic constant much faster and efficient.