Significance Traditionally, light intensity was utilized in optical imaging, resulting in multi-dimensional physical quantities such as spectrum, polarization and phase, and the light field information are lost, which lead to the poor performance or even failure of the traditional method in harsh conditions. However, polarization imaging technology utilizes the polarization property of light, which is insensitive to background illumination, ambient temperature and contrast. Meanwhile, polarization characteristic of light can be reserved more probably in low than that of the light intensity, and so it is more applicable to achieve effective detection of targets in special environments. Based on the unique advantages of polarization imaging, the technology is widely used in the fields of communication, imaging and detection.
Progress Firstly, four types of traditional polarization imaging systems of time-sharing (TS), division-of-amplitude (DOA), division-of-aperture (DoAp) and division-of-focal-plane (DoFP) are introduced. Except for the TS polarizaition imaging method, the other three methods all performed well in real-time imaging. The TS polarization imaging system is simple in structure and is commonly used in polarization differential imaging and 3D imaging. The DOA polarization imaging system is relatively complex and difficult to calibrate, resulting in its poor practicality. Structure of DoAp polarization imaging system is relatively compact, but the image alignment is relatively complicated. The DoFP polarization imaging system became a focus in recent researches, owing to its advantages of low energy loss, compact structure and fast imaging. For this technology, low extinction ratio of the micro-polarization array produced during the fabrication process was significantly improved with the enormous progress in processing technology and this approach is most likely to be predominant in future polarization imaging.Based on the traditional polarization imaging system, polarization 2D/3D imaging technology has been studied and made great progress. Based on polarization difference and image fusion, the polarization 2D imaging technique that has achieved good imaging results in underwater and haze environments is illustrated in detail. 2D imaging through strong scattering media and separation of high and low polarization targets are still challenging at present. For polarization 3D imaging technology, this paper provides a detailed description of the methods to solve the azimuth and zenith angle multivalence problems in the imaging process. Although high-precision 3D reconstruction of a single object in the natural environment is currently possible, the relative height of the target rather than the absolute height is recovered (Fig.5). In addition, with the existing polarization 3D imaging technology, it is unable to achieve 3D shape recovery for discontinuous and dynamic targets, and further research is still indispensable to solve these problems.With the development of micro-nano processing and integration technology, smaller and more integrated metasurface structures have been studied and applied to polarization detection. At present, full polarization detection, and polarization imaging has been realized by using polarization devices based metasurface (Fig.6).
Conclusions and Prospects Polarization imaging technology is elaborated in two aspects of polarization detection and imaging. For polarization detection, four traditional polarization imaging systems were introduced respectively. Therein, the DoFP polarization imaging system has drawn more attention due to its unique advantages of fast imaging speed and good integration. Depending on the spatial dimension, polarization 2D/3D imaging based on the traditional polarization imaging system have good prospects for detection and imaging in different fields. In order to achieve polarization detection and imaging more efficiently and conveniently, polarization devices based metasurface are fabricated and applied. Scientists at home and abroad are dedicated to continuously optimize the imaging process from five aspects of generation, transmission, modulation, acquisition and processing of polarized light, and various advanced processes and methods are effectively combined to achieve on-line polarization imaging with high stability.
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