An apparatus including at least one emitter configured to emit energy; at least one receiver configured to receive the emitted energy, where the at least one emitter is mounted on at least one of: a robot arm, an end effector of the robot arm, a substrate on the robot arm, or a substrate process module, where the at least one receiver is mounted on at least one of: the robot arm, the end effector of the robot arm, the substrate on the robot arm, or the substrate process module.