US Patent 9233471 Debris monitoring

A debris monitoring system includes a receptacle, a first and a second emitter, and a first receiver. The receptacle defines an opening to receive debris into the receptacle. The first and second emitter are each arranged to emit a signal across at least a portion of the opening. The first receiver is proximate to the first emitter to receive reflections of the signal emitted by the first emitter, and the first receiver is disposed toward the opening to receive an unreflected portion of the signal emitted by the second emitter across at least a portion of the opening.