The Photoelastic modulators (PEM) are modulation devices that can change the polarization state of light, causing a dynamic phase retardation of transmitted light. The photoelastic effect is used to change the birefringence of the optical element in the photoelastic modulator. Badoz in the 1960s and originally called a. Hinds Instruments is the world's leading developer of technologies based on the principles of polarization modulation.