The VTF-18-4N1240 part from Sick is an 18 mm diameter through-beam photoelectric sensor with a range of 4 m and a dark-on, NPN output. It features a built-in LED indicator that shows the status of detected objects, and is designed for use in challenging environments. The VTF-18-4N1240 is rated up to IP67 and can be used in temperatures up to 80°C, making it resistant to dirt, dust, water, and other liquids, and vibrational and shock applications.