This document is an attempt to establish and define the parameters of importance for valid calibrated infrared signature data and to lay down guidelines to be adapted to each individual system in order that exchanged data between laboratories could be made useful and comparable. It is assumed that the data gathering instrument is a thermal imager. The initialization of the calibration parameters characterizing the system is discussed, followed by a description of the field calibration procedures. Then the basic equations for image analysis and conversion to radiometric data are presented for a general and ideal case. Three special cases are considered: the effect of ignoring the atmospheric corrections, the effect of having reference blackbodies at a range different from the target, and the effect of using a system with built-in references.