Patent ID: 7236241

Claim:
A method of making a standard tool for calibrating devices that analyze stress in photoelastic material, the method comprising the sequential steps of: partially annealing a starting piece of tempered glass, the starting piece having a first thickness and a first surface with a first dimension that is perpendicular to the first thickness; removing a peripheral portion of the partially annealed starting piece to leave a working piece having the first thickness and a second dimension less than the first dimension; cutting a rectangular parallelepiped from the working piece; polishing two opposing faces of the rectangular parallelepiped, the two opposing faces being spaced apart by a measurement distance through which light passes during stress analysis in a stress analysis device, the two opposing faces being perpendicular to the first surface; and determining a birefringence of the rectangular parallelepiped across the measurement distance using a calibrated stress analysis device, the rectangular parallelepiped being the standard tool that has the determined birefringence for the measurement distance.