## 4 Results and Discussion
The sector and focused scan images were obtained by sweeping the ultrasonic beam in the SUT in a field of view of \(\pm 22^{\circ}\) with a step size of \(0.5^{\circ}\) and focused on a focal length of 60 mm from the transducer surface, approximately corresponding to the backwall distance. The obtained sector and focused scan images from the PuC-based system and the standard pulse-echo system are presented in Figure 5. It can be observed in the images that the PuC method demonstrates a higher sensitivity than pulse-echo for the deeper defects and a clearer visualization of the backwall. This could be better noticed by looking at the surface images of the deeper defects, i.e., defects no. 3 and no. 4, as reported in Figure 6. The sensitivity of the pulse-echo image rapidly decreases with the distance from the surface, as expected [40]. On the other hand, this effect is significantly less prominent for PuC. We believe this is mainly due to this reason: in the proximity of the transducers, the PuC system suffers a reduction of sensitivity as the transmitting and receiving beams do not overlap completely (in elevation direction). As long as the distance from the transducer increases, this effect decreases, which then improves the gain of the SNR provided by PuC-PA method. We note also that the defect shape is better reconstructed in the PuC images, even if a smaller number of elements were used. Figure 4: Examples of the signals involved in the pulse compression ultrasonic measurement: the input chirp signal (**top-left**) applied to the transmitting element; the echo signal acquired through the reception element of the array transducer (**top-right**); the impulse response of the sample under test (SUT) retrieved by convolving the acquired signal with the matched filter (**bottom**). Additionally, the FMC data were acquired by using both systems. For PuC measurements, in addition to the linear array transducers used for the sector scan, a pair of 16-element linear array transducers was used as well, with a central frequency of 4 MHz, and the pitch and length of the element equal to 2 mm and 16 mm, respectively. The FMC data consisted of 16 \(\times\) 16 and 43 \(\times\) 43 A-scan signals for the PuC and the pulse-echo data. The TFM images were constructed by considering a square spatial grid with a mesh size of 0.1 mm in both directions. The delay-and-sum (DAS) method was utilized in the frequency domain for the TFM image reconstruction [15, 16]. The A-scan signals were first converted to the frequency domain through the fast Fourier transform (FFT), then phase-shifted with the required time delays, summed up at the corresponding spatial location, and converted back to the time domain. However, the summation of all the possible A-scan signals contributes to the appearance of the artifacts in the images, especially near to the surface of the transducer. This was filtered out by considering a generated/received field with an angular aperture between \(+60^{\circ}\) and \(-60^{\circ}\) for each element. Figure 7 reports the TFM images obtained from the PuC procedure by using the 2.25 MHz pair of transducers and chirp signal with a central frequency of 1.5 MHz and a bandwidth of 180% of the central frequency, while the TFM image obtained with the pulse-echo system is presented in Figure 8.  
Figure 5: Sector scan images of the CFRP sample obtained by using the full aperture of the UT array transducers; (**a**) image obtained with the pulse-compression-based system and (**b**) image obtained using with the pulse-echo system. Figure 6: Surface view of a zoom of the sector scan images of Figure 5, around the location of the defects no. 3 and no. 4. Results of the measurements with (**a**) pulse-compression and (**b**) pulse-echo systems. The images are represented in three different ways: the amplitude of the full wave field and its energy in both linear and dB scales have been visualized. It can be observed that the two measurement setups exhibit a very similar sensitivity for detecting the defects, i.e., defect no. 1 to no. 4.