# Mueller Matrix-Based Approach for the Ex Vivo Detection of Riboflavin-Treated Transparent Biotissue
## 3 Results  
### Polarimetry of Porcine Cornea in Transmission and Reflection Mode  
In a first step, we recorded the MM of unmodified porcine cornea ex vivo by using our polarimetry system in transmission mode [20]. For this purpose, the cornea was carefully placed in a 3D printed ring holder. The result of a MM measurement with illumination at a wavelength of 532 nm is shown in Figure 2.  
A clear non-zero signal can be seen in some diagonal and off-diagonal entries in the MM, with \(M(2,2)\), \(M(2,3)\), \(M(3,2)\), \(M(3,3)\) and \(M(4,4)\) having the largest mean values. These values indicate the polarization-changing properties of the sample. The signals are not homogeneously distributed over the whole surface area of the cornea. This may be because the cornea cannot be held with tension in the current holder. Therefore, the surface remains curved. The birefringent properties of the cornea are already known from the literature [41]. Spatially resolved features and the anisotropy of the human cornea were shown using an imaging polarimeter, demonstrating negative biaxial material features [42; 43]. It was clearly observed here that our polarimetry setup was capable of resolving spatial features on such samples. In order to improve similar measurements in the future, the special cornea sample holders consisting of spherically shaped quartz glass elements could be used.  
The measurements, discussed in the remainder of this work, were carried out in reflection mode as shown in Figure 3[38]. The effect of non-optimal sample fixation is smaller in this mode. Another reason is that the measurements in reflection mode are the only option for the needed in vivo measurements.  
A photograph of the experimental setup used for this study is shown in Figure 3. The cornea is clamped in a specific holder that prevents it drying out. To do this, an artificial anterior chamber behind the cornea was filled with Dextran 500 soluble salt. Figure 4 shows a measurement image obtained with the polarization state HH of the illuminating source and detector, as ensured by the PSG and PSA elements.  
An overexposed area can be seen in the center of the cornea. Since the cornea is not planar, there is always a part of the surface for which the angle of incidence of the exposure source is equal to the angle of reflection to the detector. From this area, a part of the light is directly reflected from the surface of the sample and causes the overexposure of the CCD camera sensor. A circular, dark area located on the left side of the cornea image stems from the opening to the rear chamber through which fluid exchange can be realized.  
From the setup it is clear that the hemispheric shape of the cornea needs to be considered. The effects of polarized light incident to a curved surface can be a source of measurement error. For the following measurements, we tried to reduce the influence of the hemispheric shape by maintaining the same relative orientation for each sample. We assumed that the radius of the curvature of the cornea was approximately constant for the different samples.