n The thickness of many calendered, extruded, and cast products must be measured while they are produced in order to control the thickness within specified tolerance limits. The simplest methods called contact gauging, use tools such as calipers, micrometers, and rolls that physically touch the product being measured. Today these are mostly used for checking and backup while non-contact methods make on-line measurements and transmit signals to computers, which in turn order process adjustments to be made. Non-contact gauging devices employ nuclear radiation (see  Beta-Ray Gauge), infrared radiation, ultrasound, air nozzles with means for measuring back pressure which varies with product thickness, electrical-capacitance sensors, and optical devices employing beams of light.