A sensitivity of 0.03-mµ birefringence is achieved by the introduction of a plastic disk (~6-mµ birefringence) rotated at 30 Hz to modulate the birefringence at 60 Hz and a suitable detector. Two components are found in the detector signal: the one (120 Hz) that depends on the modulator birefringence is rejected; the other (60 Hz) which depends linearly on the compensator birefringence and changes phase at extinction is utilized. An A.E.I. BTH compact mercury are (with interference filter for 546 mµ) operated on dc is the light source; a photomultiplier 1P21 is the detector. The signal is first fed into a parallel T notch filter which reduces the 120-Hz component five-hundred-fold and then into a three-stage narrow-band (5-Hz) active filter (utilizing three integrated-circuit operational amplifiers) that provides a. possible gain of 151 dB for the 60-Hz component. To obviate tedious manual setting of the compensator for the required traverse of the specimen, a 60-Hz servo motor is linked to the tangent screw of the analyzer and operated by a power amplifier for the active filter-output signal. Potentiometers on the specimen-translation screw and on the analyzer tangent screw permit the data to be plotted on an x-y recorder. For vitreous silica specimens 1 cm deep, 3 mm wide, irradiated with electrons of 1-mm range (0.6 MeV), the effect of a radiation-induced dilatation as small as 3 × 10-8 can be observed, equivalent to inserting an atomic sheet into a 1-cm long specimen. Alternatively, 1% dilatation of a surface layer 30 Å thick can be detected.
W. Primak and R. Kampwirth, "Automatic Photoelastimeter for Determining Very Small Dilatations," Appl. Opt. 10, 327-332 (1971)