Table 2.
Preprocessing techniques
| Preprocessing image techniques | |
|---|---|
| FPN | FPN is the result of differences in responsivity of the detectors to incoming irradiance. It is a common problem when working with FPA. FPN for a particular configuration can be recovered from a blackbody image for later subtraction from the thermogram sequence. |
| Badpixels | A badpixel can be defined as an anomalous pixel behaving differently from the rest of the array. For instance, a dead pixel remains unlit (black), while a hot pixel is permanently lit (white). In any case, badpixels do not provide any useful information and only contribute to deteriorate the image contrast. A map of badpixels is generally known from the FPA manufacturer or they can be detected manually or automatically; the value at badpixel locations is then replaced by the average value of neighboring pixels. |
| Vignetting | Vignetting is another source of noise on thermograms that cause a darkening of the image corners with respect to the image center due to limited exposure. It depends on both pixel location and temperature difference with respect to the ambient. A correction procedure has been proposed.3 |
| Temperature calibration | A transformation function is used to convert the grayscale values g provided by the infrared camera into a linearly incrementing physical quantity, eg, temperature. The procedure3 is used to position the IR camera in front of a reference temperature source (such as a blackbody and a thick copper plate) brought to various known temperatures. As the reference temperature source is varied, the IR images are recorded. Average of the central pixels in the field of view allows getting the calibration curve through a polynomial fit |
| Noise smoothing | One of the most useful preprocessing (and postprocessing) techniques is noise smoothing. For instance, neighbor processing can be performed bypassing a mask or kernel through the image. More elaborate noise removal techniques are available. |
Note: Data adapted from Ibarra-Castanedo et al.17
Abbreviations: FPA, focal plane array; FPN, fixed pattern noise; IR, infrared.