## 2 Materials and Methods
### Experimental Setup  
A homemade transmission MMIP system based on the dual-rotating-retarder (DRR) method is used. The experimental setup and schematic diagram are shown in Figure 1.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Normal** & **BE** & **MM** & **Total** & **Mean Age** \\ \hline Patients & 21 & 20 & 23 & 64 & \\ Cases & 853 & 851 & 847 & 2551 & 68 \(\pm\) 12 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The number of patients and detection cases of unstained skin sections detected by MMIP.  
The light from a broadband LED (MWWHLP1, Thorlabs, Newton, NJ, USA, 400-700 nm) is modulated by a polarization state generator (PSG). The PSG consists of a polarizer (P1, LPVISE100-A, Thorlabs, USA, 400-700 nm) and a rotatable achromatic quarter-wave plate (R1, AQWP10M-580, Thorlabs, USA, 350-850 nm). The fast axis azimuth of R1 is successively rotated to 15\({}^{\circ}\), -15\({}^{\circ}\), 52\({}^{\circ}\), -52\({}^{\circ}\) directions by an electric turntable (KPRM1E/M, Thorlabs, USA) to generate four independent polarized lights. The modulated polarized light is then focused on the sample by the condenser (C, Nikon MBL11300), and the outgoing light carried the sample polarization information. After interacting with the sample, the forward scattered light is collected by an apochromatic objective lens (MRL00202, Nikon, Tokyo, Japan, 20\(\times\)/0.40). Then each polarization state of the forward scattered light is analyzed by a polarization state analyzer (PSA), which is composed of a rotatable achromatic quarter-wave plate (R2, AQWP10M-580, Thorlabs, USA, 350-850 nm) and a polarizer (P2, LPVISE100-A, Thorlabs, USA, 400-700 nm). The fast axis azimuth of R2 is successively rotated to 15\({}^{\circ}\), -15\({}^{\circ}\), 52\({}^{\circ}\), and -52\({}^{\circ}\) using an electric turntable (KPRM1E/M, Thorlabs, USA) to achieve four linearly independent analyses. Hence, a set of sixteen intensity images can be collected in one experiment, and the corresponding Mueller matrix can be derived by the linear algebra operations.  
To improve the accuracy of the measurement, a calibration is performed using the eigenvalue calibration method (ECM) with standard samples, including air, a linear polarizer, and a quarter-wave plate [36]. The mean values of each Mueller matrix element of the standard samples after calibration are given in Equations (1)-(4). \(M_{\text{air}}\), \(M_{P-0}\), \(M_{P-90}\), and \(M_{r-30}\) represent the normalized Mueller matrix of air, the linear polarizer oriented at 0\({}^{\circ}\) and 90\({}^{\circ}\), and the quarter-wave plate oriented at 30\({}^{\circ}\), respectively. The measured Mueller matrices are constructed with an error below 1%. The detailed calibration process can be found in our previous publications [46; 63; 64; 65].  
\[M_{\text{air}}=\begin{bmatrix}1.0000&0.0030&0.0012&0.0003\\ 0.0008&0.9944&-0.0106&0.0025\\ 0.0001&0.0003&1.0060&-0.0055\\ -0.0008&-0.0111&0.0056&1.0037\\ \end{bmatrix} \tag{1}\]  
\[M_{P-0}=\begin{bmatrix}1.0000&0.9982&0.0023&0.0053\\ 0.9928&0.9946&0.0033&0.0034\\ -0.0001&-0.0001&0.0048&0.0003\\ -0.0021&-0.0028&-0.0010&-0.0010\\ \end{bmatrix} \tag{2}\]  
Figure 1: The experimental setup and schematic diagram.  
\[M_{P-90}=\begin{bmatrix}1.0000&-0.9907&-0.0028&0.0004\\ -0.9977&0.9909&0.0027&0.0000\\ -0.0019&0.0016&0.0003&-0.0006\\ -0.0040&0.0038&0.0006&-0.0003\end{bmatrix} \tag{3}\]  
\[M_{r-30}=\begin{bmatrix}1.0000&0.0026&-0.0023&-0.0015\\ 0.0000&0.2519&0.3936&-0.7747\\ 0.0011&0.3988&0.7503&0.4936\\ 0.0019&0.7724&-0.4927&0.0061\end{bmatrix} \tag{4}\]