In some applications, it is necessary to extract spatial information from infrared images, i.e., the world position associated with each pixel must be known. This issue has been widely dealt with in the image processing field applied to traditional cameras [82,83]. In IRT, spatial calibration is not common; however, there are specific calibration procedures, both for infrared line-scanners and for infrared cameras. In [84], a procedure is proposed for the calibration of infrared line-scanners, and the uncertainty is analyzed. In this case, the calibration is based on a specific model of the infrared line-scanner used. In [85], a procedure is proposed for the calibration of infrared cameras. It is based on a grid of burning lamps that is used for the geometric calibration. This device is used to create correspondences between logical coordinates (sensor) and physical coordinates (world). The authors apply the proposed procedure to two different infrared cameras and achieve an accuracy lower than 1 mm. In [86], a procedure is proposed for the calibration of infrared cameras using a different approach. The work explores the use of small lamps that warm up when switched on, but concludes that this approach cannot be used to obtain high accuracy. Instead, the authors propose a new calibration device based on the reflectance of metals from the cold temperatures of outer space. The results provide an accuracy lower than 0.25 mm for common infrared cameras. In general, these works use calibration devices to build a model that can be used to determine the relation between camera units (pixels) and real world units (millimeters). This gives a benefit to the temperature measurements, as spatial information can be added to these measurements.
The SNR metric is used to quantitatively assess the signal-to-noise ratio of the acquired and processed data [15]. The quantification of a defect is based on the definition of two areas: the defective area and the reference area. The defective area encloses the pixels where the defect appears in the infrared image. This area is considered the signal. The reference area, also called the sound area, is a defect-free area used to calculate the thermal contrast. This area is considered the noise. The reference area is selected independently for each defect and must be located close to it. Thus, the reference area is known to have received the same excitation energy as the defect, which minimizes errors in SNR calculations due to non-uniform heating.
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