Infrared image processing
After the pseudo-static sequence is obtained, principal component thermography (PCT) is applied in the reconstructed sequence. PCT, originally proposed by Rajic in Ref. [16], extracts the image features and reduces undesirable signals. It relies on singular value decomposition (SVD), which is a tool to extract spatial and temporal data from a matrix in a compact manner by projecting original data onto a system of orthogonal components known as empirical orthogonal functions (EOF). The first EOF will represent the most important characteristic variability of the data; the second EOF will contain the second most important variability, and so on. Usually, original data can be adequately represented with only a few EOFs. Typically, an infrared sequence of 1000 images can be replaced by 10 or less EOFs.
shows an example of a line region (from the sample shown in Fig. 2a) inspected by using the flying laser spot approach. The reconstructed line obtained from the flying laser spot inspection was, at early times, approximately 3 mm wide which is the same diameter as the focused laser beam that heated the sample's surface. The length of the line was 100 mm, which is the same dimension of the sample. Fig. 5a shows the region that was inspected on the surface of the sample, Fig. 5b shows the first EOF image, normalized between 0 and 1, obtained from the PCT application on the pseudo-static reconstructed sequence and Fig. 5c shows the binary image obtained from Fig. 5b. The binary image was obtained based on an automatic calculated threshold. The adopted binarization approach is described in Ref. [17]. Both the first EOF image and the respective binary images are used in the following steps of the approach proposed in order to assess the fiber orientation.