1. A diagnostic field device for detecting a condition of a process conduit which carries a process fluid in an industrial process, comprising: an infrared detector comprising a plurality of pixels configured to receive infrared radiation from the process conduit and responsively provide a plurality of pixel outputs, wherein a first pixel of the plurality of pixels is configured to receive infrared radiation from a first location on the process conduit, a second pixel of the plurality of pixels is configured to receive infrared radiation from a second location on the process conduit and a third pixel configured to receive infrared radiation from a third location on the process conduit and responsively provide an output; a memory containing thermal profile information related to outputs from the first, second and third pixels during normal operation of the industrial process; a microprocessor configured to normalize the outputs from the first, second and third pixels using the thermal profile information contained in the memory and further configured to identify a process anomaly related to the condition of the process conduit based upon a nonlinear relationship between normalized outputs from the first, second and third pixels; and output circuitry configured to provide a diagnostic output indicative of the identified process anomaly.