7.3 Stress Birefringence

Depending on the annealing condition, optical glass retains slight residual strain in most cases. This can be observed as optical birefringence. It is measured by optical path differences and specified in nm/cm.

Stress birefringence of a rectangular plate is measured at the middle of the long side where maximum values occur at a point 5% of the width from the edge. A disc is measured at 4 points located 5% of the diameter from edge. The maximum value of the 4 points is shown as the birefringence value.

We guarantee the strain according to the grade of anneal as follows:

On request, we shall supply birefringence data for precision annealed blanks in a "BMC" (Birefringence Measurement Chart) certification.