Ultrasonic Distance & Proximity Sensors
Ultrasonic sensors use time of flight of sound in air to sense objects and to measure distance. Compared to other measuring principles ultrasound is very robust. It works well in dirty environments and sound waves are reflected by almost all surfaces, rendering this technique insensitive to material, colour and surface structure of the detection target.
Click here for a video demonstration of how these sensors perform in dirty. dusty conditions and with transparent materials