A method of making a standard tool for calibrating polarimeters that
analyze stress in photoelastic material, includes the steps of partially
annealing a starting piece of tempered glass so that the starting piece
has a retardance below 250 nm/cm, removing a peripheral portion of the
starting piece (preferably, at least about 20% of its radial dimension)
to leave a working piece, cutting a rectangular parallelepiped from the
working piece, polishing two opposing faces of the rectangular
parallelepiped where the two opposing faces are spaced apart by a
measurement distance through which light passes during stress analysis in
a polarimeter, and determining a birefringence of the rectangular
parallelepiped across the measurement distance using a calibrated
polarimeter. The standard tool is the rectangular parallelepiped having
the determined birefringence for the measurement distance. The method can
be used to make a set of the standard tools with different measurement
distances.