# Mueller Matrix Decomposition and Image for Non-Destructive Testing of UAVs Skin
## 1 Introduction  
Mueller matrix polarimetry (MMP) is an important optical measurement method due to its non-destructive, non-contact, high precision, etc. [1]. The Mueller matrix polarimetry provides the means to determine all 16 elements of the Mueller matrix of an optical sample. So far, Mueller matrix polarimetry has been widely used in biomedicine tissue [2, 3, 4, 5], remote sensing [6], optical communication [7], etc. To extract the important properties hidden in measured Mueller matrices, several decomposition methods have been proposed. In the sum decomposition, the Mueller matrix is decomposed into up to four non-depolarizing parts based on the coherency matrices [8, 9]. In the Brosseau method [10], the depolarizing Mueller matrix is decomposed into the polarized and depolarizing parts, respectively. Lu-Chipman decomposition represents an arbitrary Mueller matrix as a sequence of elementary polarization components which are retarders, attenuators, and depolarizers [11]. Based on Lu-Chipman decomposition, the depolarizing Mueller matrix can be factored into five terms representing elementary polarization devices: a diagonal depolarizer stacked between two retarder andattenuator pairs [12]. In the differential method, 16 differential matrices were derived corresponding to the characteristics of depolarizing anisotropic media [13]. The paths correlation matrix (PCM) of the rough surface sample is decomposed into single scattering and depolarization related to multiple scattering [14]. Among these methods, the sum decomposition deal with both physical realizability and matrix filtering of Mueller matrices. Even though these methods have been used in several aspects, they are still challenging for complex multiple scattering materials such as camouflage targets, composite materials, seawater surfaces, etc. The multiple scattering randomizes the intensity, phase, and orientation of the incident light. The contrast and resolution are greatly reduced, and the sharpness of the intensity image becomes bad due to the multiple scattering [15]. Furthermore, polarization information is inevitably lost because of rough surface scattering, random noises, and system errors [16; 17]. Therefore, it is usually difficult to directly use polarization information due to the lack of explicit associations with specific microstructures. Some traditional methods encounter difficulties in dealing with the Mueller matrices of multiple scattering mediums. The unmanned aerial vehicles (UAVs) industry is one of the world's fastest-growing sectors. In recent years, UAVs have been widely used in military and civilian fields around the world, such as reconnaissance, surveillance, attack fighting, aerial photography, oceanography, meteorology, agriculture, etc. Most skins of UAVs are made of composite material, which has advantages in structural, aerodynamic, stealth performance, and a low radar cross-sectional area (RCS) [18]. Defects and damages in manufacturing usually seriously affect key parameters such as the stiffness, strength, and service life of UAVs [19]. The NDT techniques are widely used in the manufacturing, power, petrochemical, aircraft industries, and play an important role in certifying the integrity of manufactured components and for routine maintenance inspections of UAVs. The conventional NDT techniques include magnetic particle inspection, liquid penetrants, X-ray micro tomography (XRMT), ultrasonic, infrared thermography, microscopic method visual, optical interference technique, etc. [20]. Nevertheless, some traditional thermographic systems are inconvenient and bulky for many applications [19]. Some NDT methods such as X-ray inspection, ultrasound, and eddy current methods cannot meet the challenges posed by new composite structures [21]. To the best of our knowledge, few researchers reported that MMP has been successfully used in the NDT of UAVs skin. To address these issues, it is necessary to find some innovative MMP methods to carry out the NDT of UAVs skin with complex multiple scattering.