This paper describes the development of a new optical tool to detect in-situ wetting in membrane distillation. The principle of the detection tool is based on the phenomenon of light transmission. A dedicated experimental setup was developed. The proof of concept of this optical tool is validated using in parallel the Detection of Dissolved Tracer Intrusion (DDTI) method that was previously developed, and which is based on the detection of remaining salt traces in wetted pores by scanning electron microscopy and x-ray dispersion spectroscopy. The in-situ tool was used in standard operating conditions (feed temperature 40 degrees C, laminar flow and vacuum pressure 6 kPa) for vacuum membrane distillation with deionized water, a synthetic saline solution, and seawaters. Controlled wetting was then induced by adding a surfactant in the feed after 30 min of operation and wetting dynamics could be effectively visualized. Additionally, the scalability (macro and mesoscales) of this tool was verified and wetting was visualized at different locations on the membrane surface and compared with the wetting observations made at a global scale. A definition of pore wetting is also proposed.