Ultrasonic sensors are a useful alternative where optical sensors come up against their physical limitations.  They are ideal in applications in which objects have uneven surfaces or are under difficult ambient conditions.  They also work well in detecting highly transparent media and moving, highly reflective liquid surfaces.  Checking the presence of highly transparent foils and measuring filling levels in liquid containers are typical uses of ultrasonic sensors.  A major advantage of ultrasonic sensors is the reliable background suppression resulting from their sound time-of-flight measurement principle.