In a method or device for function-monitoring an actuator in a process engineering plant, said actuator comprising a valve and an actuating drive for actuating said valve. A portion of the illumination beam is passed through, or reflected off of, a subject to create a subject distribution, and a portion of the reference beam is passed through a reference object, or reflected off a reference object reflector, to produce a reference distribution. The release valve may also include a check valve for inhibiting passage of gas from the outlet to the inlet.