Imaging with photoelastic modulators

Photoelastic modulators (PEMs) are among the most robust and precise polarization modulation devices, but the high frequency free-running nature of PEMs challenges their incorporation into relatively slow CCD and CMOS imaging systems. Current methods to make PEMs compatible with imaging suffer from low light throughput or use high cost intensified CCDs. They are not ideal for some analyses (microscopy, reflectivity, fluorescence, etc.), and likely cannot be extended to polarimeters with more than two PEMs. We propose to modulate the light source with a square wave derived from particular linear combinations of the elementary PEM frequencies and phases. The real-time synthesis of the square waves can be achieved using a field programmable gate array (FPGA). Here we describe the operating principle.