This paper presents a new approach to safety and transparency in physical human-robot collaboration by using conventional projector and camera equipment. Our system is able to establish an arbitrarily shaped light barrier around the workspace of a robot. The barrier can be made dynamic, i.e. it can adapt to the actual movement of the robot. This is a highly desirable property in scenarios requiring human robot collaboration. We will discuss a method for visually detecting violations of the safety barrier including compensation of ambient light. Furthermore, we argue that this method complies with several requirements regarding reliability in safety critical systems.