# Mueller Matrix Decomposition and Image for Non-Destructive Testing of UAVs Skin
## 3 Experiments
### The Results of UMMMD  
Figure 3 shows the images of intensity of the UAV skin. As shown in Figure 3a, the average light intensity image is obtained from the 36 coded images. As shown in Figure 3b, the distribution of the scattering intensity on the skin surface is uneven. A lattice structure image of the skin is obtained. The lattice structure is generated by reflection from a number of yarn nodes. It is found that the incident polarized light is scattered multiple times due to the complex layered structure, including resin materials, the rough surfaces of yams, honeycomb structures, gaps, bends, etc. Due to the randomness of multiple scattering, the outgoing intensity is weak and difficult to be detected.  
The Mueller images need to be normalized since the detected light signal is as weak as the order of \(10^{-5}\). The Mueller matrix measures the reflection signals from the UAV skin, and the measured Mueller elements are between \(10^{-5}\) and \(10^{-7}\) orders of magnitude. It is because the resin surface of the UAVs skin is rough that the internal scattering caused by the 3D structure is serious, and its reflection is weak. The Mueller matrices of all pixels are normalized by dividing their first Mueller matrix elements. Moreover, the eigenvalues for each pixel need to be normalized, since the value of \(\lambda_{1}\) is greater than 1, and its corresponding gray level is greater than 255. To solve this problem, the eigenvalues are divided by the sum of the eigenvalues to ensure that the eigenvalues lie in the range of [0, 1], and the corresponding grayscale values vary from 0 to 255. This operation ensures an accurate mapping of the image grayscale and intensity, which is convenient for the subsequent calculation and image display.  
Table 1 is the decomposition results, where the Mueller matrix is obtained with 36 coded images. The measured \(M\) reflects the total polarization response from the UAV skin. \(\lambda_{i}(i=1,2,3,4)\) and the entropy are calculated with the coherency matrix. In Table 1, the entropy shows that the multiple scattering is significant and the scattering type is not unique.  
Figure 3: The intensity image. (**a**) The image of average light intensity; (**b**) 3D image of light intensity.  
Figure 2: The microscopic images of the sample. (**a**) The reflection microscope image; (**b**) the transmission microscope image.  
In the case of isotropic depolarization, only the first element in depolarization matrices carries the information of depolarization. However, when the depolarization is anisotropic caused by non-uniform scattering, these methods cannot provide information for different polarized states. The non-uniform scattering of the UAV skin results in different polarization responses under different polarization states. It is noted that both the Cloude method and UMMMD have anisotropic depolarization matrices. Similar to the Cloude method, the key polarization parameters in UMMMD are simplified. Nevertheless, the depolarization processes of the two methods are essentially different due to different depolarization spaces. In UMMMD, the first element of the depolarization matrix represents the depolarization degree and depolarization coefficients [14,24], and the other three elements represent the relative depolarization degree. In the Cloude method, the first element in the depolarization matrix represents the total noise rather than the depolarization degree of the system, and the other three elements represent the magnitude of noises under different polarized states. In general, \(\lambda_{i}(i=2,3,4)\) of the coherence matrix of the UAV skin are small, but their relative deviations are large. Consequently, \(\eta_{i}(i=2,3,4)\) are different from each other, and UMMMD could provide a larger dynamic range of depolarization.  
Therefore, UMMMD provides more polarization information related to non-uniform multiple scattering than some conventional methods. In particular, this method can describe differences and changes in depolarization for different polarization states.