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The first rule of noncontact temperature measurement is that it is almost always a surface measurement. The sensor is pointed at a spot on the surface of the product and measures the IR radiation from this spot. The instantaneous size of the target spot depends on the sensor's collecting optics and the working distance to the target. The measurement is made in radiance and converted to an equivalent temperature value, if needed, by inserting a correction for the measured or estimated target surface emissivity. Typically, the decision to control a process is not made until a simple IR sensor is deployed and data are gathered over a period of operating time, under a variety of expected operating conditions, and at different points in the process. The condition (quality, consistency, etc.) of the product is correlated to the temperature variations experienced at the various monitored locations. The point (or points) in the process where temperature behavior is most closely related to product condition is selected as the permanent monitoring/control site (or sites). Variations in signal at this site are used to control the process. This can be done manually, with an observer maintaining the temperature reading within allowable limits by manually turning the process on and off, or by adjusting the process driving control (gas valve, heater current, speed adjustment, etc.). The trend, however, is to close the loop by means of a computer/data processor for automatic control of the process. The three control types described above—monitor, open-loop control, and closed-loop control—are illustrated in Fig. 10.1.
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