Centripetal forces in a rotating glass disk generate mechanical strains that are visible in polarized light. Dynamic stress birefringence in rotating disks has a practical effect on several classes of optical instruments, including flying-height testers for the data storage industry. We provide a model for stress-induced polarization effects, and describe an interferometric technique for mapping birefringence in a 100-nm diameter disk spinning at 12,000 rpm. The polarization interferometer employs a laser diode and a homodyne phase receiver to detect polarization- dependent phase shifts as small as 1 mrad at a data rate of 250 kHz.