Optical systems have shown their potential in non-invasive medical diagnostics over the last years. While most imaging systems use information on wavelength or phase, e.g. OCT, in our approach we focus on the polarization properties of biotissue. We designed a Mueller matrix (MM) measurement setup for in vivo investigations on skin tissue. The MM describes the polarization-changing properties of a sample.. Thus, it is possible to calculate the MM from images taken with different polarization states of the illuminating and the observed light. For medical application, an important requirement is that the process is fast to enable in vivo measurement, avoid motion artifacts, and reduce stress for patients. In our setup, we use a combination of two polarizers and four liquid crystal retarders to quickly change between polarization states. The system is able to measure the location dependent MM of a target for different wavelengths. It is designed for measurement in reflection mode, however, upon simple modifications, it can be used in transmission mode as well. One interesting field of application is diagnostics for inflammatory skin diseases. Here, for example, changes in the structure and concentration of collagen could provide diagnostically valuable information. We evaluated our system on different skin phantoms to investigate the diagnostic advantages compared to standard approaches. In the future, our system could be part of a non-contact dermatoscopic device and provide extra information for the physician.