Mueller Matrix Measurement System for Skin Polarimetry as Additional Module for Non-Contact Dermatoscopy

The skin is the biggest human organ. It is also easily accessible and visible from the outside. Diagnosis of skin diseases such as melanoma skin cancer is often based on optical parameters, e.g. the widely known ABCDE rule of dermatoscopy. Not only cancer, but also inflammatory skin diseases are rated according to criteria which are at least partially based on optical criteria (see PASI- or EASI-scores) and often include subjective criteria. The standard instrument for an optical examination of the skin is the so-called dermatoscope. It is an epiluminescence microscope which nowadays often provides digital images of the suspected lesion. Due to the design as a contact-based device, most dermatoscopes face problems like suppression of perfusion, among others. That is why we developed a compact non-contact dermatoscope. To obtain further information about, for example, the tissue composition, an additional Mueller matrix (MM) measurement system was developed which can acquire the 4×4 MM spatially resolved for a 2D-image in a comparably short time. The MM (M _m ) contains all information about the polarization changing properties of a sample. It allows to calculate the Stokes vector of the outcoming light (S ^→ o) for every state of incoming light (S ^→ i) by applying S ^→ o = Mm ·s ^→ i.. The concept of measuring the MM is well known in the literature [1,2]. In our approach, we investigate the potential of a MM measuring system as an extension to a non-contact dermatoscope. The ultimate goal is to realize a diagnostic camera, which can take overview and zoomed images under comparable conditions for each examination and provides additional diagnostic information by measuring the MM of the area of interest. This information is particularly interesting for inflammatory skin diseases like Morphea where contribution of the chiral molecule collagen is important. To validate the potential of the MM measurement system, samples with known chirality like sucrose where studied. For this, cuvettes of poly(methyl methacrylate) (PMMA) and polystyrene (PS) filled with different concentrations of sucrose solved in pure water were utilized. The MM reliably differentiates between different concentrations of sucrose in the water. Also, as Fig. 1 shows, the system was able to quantify the slight changes in the polarization probably induced by stress birefringence which is material dependent.