# Review on Complete Mueller Matrix Optical Scanning Microscopy Imaging
## 2 Mueller Matrix Optical Scanning Architecture
### Temporal Domain Encoding/Decoding  
In the 1970s/1980s, the very first development in polarization-resolved microscopy was of interest in the research community for its advantage of extracting the label-free contrasts. Due to the low technological requirement, the temporal control of the polarized light was the first approach for such instrumentation based on the use of rotating optical features or electro-optics modulators. In a single-point measurement, the technique was based on the differential polarized (linear or circular) intensity collection, leading to the determination of only few Mueller matrix elements [32; 33; 34; 35; 36; 37]. Thus, the method has been deported into an SLM configuration for imaging high-ordered macromolecules and biopolymers, such as chromatin or blood cells [38; 39; 40; 41; 42; 43]. The main motivation for such implementation was to substitute the fluorescence techniques by imaging and tracking very localized molecules based on the single optical response of the sample without sample preparation. Later, the first full MM polarimeter imaging associated to an SLM configuration have been proposed for ophthalmology, based on OCT imaging in 1999 [44]. It allows the imaging of the full MM in reflection configuration using LCVRs. Even if there is no need of any moving part, the technique consists in measuring 16 successive double-pass images with an exposure time of 4 s, resulting in a full acquisition in one minute. Considering that this approach required a low power source for not damaging the retinal tissues, the method is coupled with an important post-data treatment for reducing the noise and increasing the Signal-toIn the confocal mode, an aperture slightly smaller in diameter than the Airy disc image is positioned in the image plane in front of the detector [46]. The ability of this method to reduce out-of-focus blur signal, and thus permit accurate non-invasive optical sectioning that makes confocal scanning microscopy so well suited for the imaging and three-dimensional tomography of biological specimens. It has several additional advantages over conventional optical microscopy, including the possibility of a significant improvement in lateral resolution and the capability for direct non-invasive serial optical sectioning of intact specimens. In 2005, the first confocal Mueller reflection microscope was developed for the retinal diagnosis [27]. The scheme suggested to increase as much as possible the polarization encoding/decoding speed rate allowing the highest number of measurements for improving optically the SNR. For this reason, the generation of the polarization states is performed thanks to two Pockels Cells (PC) allowing the encoding of the polarization in few MHz. In an effort to speed up the decoding time, the method was coupled with a combination of multiple polarization resolved detectors based on the Division of Amplitude (DoA) method [47]. Thus, the time for getting the full MM is around 50 ms and reach a lateral optical resolution of 30 \(\upmu\)m. However, the implementation of such a technique into multimodal system is difficult considering the physical space required. Furthermore, using multiple polarization optics leads to errors in aligning the optical axis taken into account from the ECM. Nowadays, the multiple reflection optics used for this method, such as beamsplitters, fresnel rhombs or wollaston prism, could be reduced in very miniature modules of few centimeters designing from metasurfaces [48]. In 2008, an already existing commercial transmission SLM was modified for MM reflection imaging of glaucoma [49]. The system was completely automatized based on the use of Liquid Crystal Variable Retarder (LCVR) and required 72 successive images  
Figure 2: (**a**) Block diagram of a typical MM SLM configuration based on the sequential encoding/decoding of the polarization states from Reference [45]. (**b**) Schematic presenting the extraction of the MM images from the measured intensities images in the sequential approach. PSG: polarization states generator.