The Hinds Instruments Pem-90 Photoelastic Modulator Systems are an instrument used for modulating or varying (at a fixed frequency) the polarization of a beam of light. Hinds photoelastic modulators are used for measurement of circular and linear dichroism, birefringence, optical rotation, and for ellipsometry, polarimetry, reflection difference spectroscopy, and FTIR double modulation.

The PEM-90 principle of operation is based on the photoelastic effect, in which a mechanically stressed sample exhibits birefringence proportional to the resulting strain. Photoelastic modulators are resonant devices, each producing oscillating birefringence at a fixed frequency in the low frequency ultrasound range (20 kHz to 100 kHz). These factors result in a number of very useful advantages which are unique to the PEM including wide acceptance angle, large aperture, and high modulation purity.

In its simplest form the PEM-90 consists of a rectangular bar of a suitable transparent material (fused silica, for example) attached to a piezoelectric transducer. The bar vibrates along its long dimension at a frequency determined by the length of the bar and the speed of a longitudinal sound wave in the optical element material. The transducer is tuned to the same frequency and is driven by an electronic circuit which controls the amplitude of vibration. The oscillating birefringence effect is at its maximum at the center of the fused silica bar.