PS-ML4-US2D-PHOTO SENSOR M18 RETRO 4.2M PP 2M 20.The PS-ML4-US2D-PHOTO SENSOR M18 RETRO 4.2M PP 2M 20 is a durable photo sensor developed for industrial accuracy. With a 4.2-meter range, a tough polypropylene housing, and precise optics, it performs consistently in any environment.