# Principle and Implementation of Stokes Vector Polarization Imaging Technology
## 3 Implementation of Stokes Vector Polarization Imaging Technology
### Non-Simultaneous Imaging
#### 3.1.2 Electronically Controlled Polarization Imaging  
Electrically controlled polarization devices are generally electrically controlled phase delay devices, including Nematic Liquid Crystal (NLC), Ferroelectric Liquid Crystals (FLC) and Photoelastic Modulator (PEM) [17; 18; 19]. They can be combined with a fixed Angle polarizer to form a polarizer and a polarizer. Liquid crystal spatial light Modulator (LC-SLM) is usually used as the variable delay wave plate. Different voltages are applied to IC-SLM at different times to collect different polarization images. As shown in Figure 4, the schematic diagram of fixed variable delay waveplates in [PERSON] et al. 2010 research paper [20] is cited. However, the strong attenuation of the liquid crystal to the light leads to the detection range is extremely limited, whereas the inevitable electrical noise, heat and other problems of the electronic control have a serious impact on the detection accuracy, and the non-simultaneous working mode makes it impossible to carry out real-time detection.  
The linear liquid crystal NLC and ferroelectric liquid crystal FLC are linear phase retarders, but the phase delay is fixed, and the voltage changes in the fast axis direction of FLC, and this direction is bistable, that is, there are only two fast axis directions of FLC that can be realized and the difference is 45\({}^{\circ}\); FLC uses direct current drive, different polarity corresponds to different fast axis Angle; The switching time of FLC is extremely fast, usually less than 100 m, which makes high-speed measurements of polarized images possible. Similar to NLC, the FLC-based polarizer also contains a fixed polarizer and two  
Figure 4: Schematic diagram of fixed variable delay wave plate [20].  
Figure 3: High-speed non-simultaneous polarization imaging system [16].  
FLCS. By changing the polarity of the voltage, the fast axis angles of the two FLCS change alternately to produce four different polarization states. Similar to NLC, the photoelastic modulated crystal PEM has a fixed fast axis orientation and phase delay varying with voltage. However, compared with NLC, PEM has been used by more and more researchers in recent years due to its high driving voltage, polarization properties that are not easily affected by temperature and faster phase delay switching speed [17; 18; 19].However, the phase delay of PEM cannot be controlled at will, and the modulation speed is too fast for imaging system, so the modulation process of PEM is quite different from that of rotating polarization device and NLC.  
The acoustic-optictunable Filter (AOTFM) and Liquid Crystal Variable Retarders (LCVRs, Meadowlark Optics, Inc., Frederick, CO, USA) developed by [PERSON] et al., us Army Research Laboratory, the system principle diagram of the imaging spectral polarization detection device of LCVR is shown in Figure 5[21]. In addition, [PERSON] et al., university of Arizona, proposed a VRFTSP (Variable-retardance, Fourier-transform imaging spectropolarimeter) in 1999. The VRFTSP uses a four-mirror Sagnac beam splitter as the angular shear interferometer of the system. The channel polarization modulation module consists of two electronically controlled variable phase retarders and a linear polarizer placed in the collimating light path in front of the interferometer. However, the existence of slits behind the imaging objective limits the optical flux and spatial resolution of the system. VRFTSP adopts push-sweep mode and changes the phase delay of electronically controlled variable phase retarder four times to obtain four groups of polarization-modulated interference fringe image sequences.