We describe an instrument for measuring linear retardance in transparent optical components using two photoelastic modulators. The instrument contains a He-Ne laser (632.8 nm), a polarizer, two low-birefringence photoelastic modulators at different frequencies, an analyzer, and a silicon photodiode detector. A sample is placed between the two modulators. The detector signals corresponding to linear retardance in a sample are analyzed using lock-in amplifiers. A computer program calculates and displays both the retardation magnitude and the angle of the fast axis. The reported instrument is essentially a polarimeter specifically designed for measuring low-level linear retardance in high-quality optical components. It provides a retardation sensitivity of better than 0.005 nm (0.003 deg or 5×10~(-5) rad with a He-Ne laser at 632.8 nm).
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