The photoelastic modulator (PEM) has been applied to a variety of polarimetric measurements. However, nearly all such applications use point-measurements where each point (spot) on the sample is measured one at a time. The main challenge for employing the PEM in a camera-based imaging instrument is that the PEM modulates too fast for typical cameras. The PEM modulates at tens of KHz. To capture the specific polarization information that is carried on the modulation frequency of the PEM, the camera needs to be at least ten times faster. However, the typical frame rates of common cameras are only in the tens or hundreds frames per second. In this paper, we report a PEM-camera birefringence imaging microscope. We use the so-called stroboscopic illumination method to overcome the incompatibility of the high frequency of the PEM to the relatively slow frame rate of a camera. We trigger the LED light source using a field-programmable gate array (FPGA) in synchrony with the modulation of the PEM. We show the measurement results of several standard birefringent samples as a part of the instrument calibration. Furthermore, we show results observed in two birefringent biological specimens, a human skin tissue that contains collagen and a slice of mouse brain that contains bundles of myelinated axonal fibers. Novel applications of this PEM-based birefringence imaging microscope to both research communities and industrial applications are being tested.
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.