Using an infrared source and detector, visually opaque materials such as silicon wafers or black plastic bottles can be measured. Based on the reflected light from the two (or more) surfaces, the interference pattern can be used to determine thickness. If there are multiple layers, those individual thicknesses can also be determined, as well as the total thickness. Since the optical probe contains no electronics or moving parts, it is very robust for inline applications, even in wet, acidic or dirty environments.
