## 4 Results and Discussion
### Data Contrast Enhancement in Temperature, Amplitude, and Phase Image  
Figure 9 shows the temperature, amplitude, and phase images of LIT without compensation techniques applied. Figure 10 shows LIT's temperature, amplitude, and phase images applying the ATC compensation technique. Figure 11 shows LIT's temperature, amplitude, and phase images applying the TRC compensation technique. As shown in Figures 9-11, it can be seen that the ATC technique qualitatively improves the contrast compared to the results without the compensation technique applied. As contrast improved in both temperature, amplitude, and phase images, more defects were detected in the images. In particular, in the case of phase images, row C defects (defect depth of 6 mm) and row D defects (defect depth of 7 mm) were detected, qualitatively improving the detection possibility. Unlike ATC, which compensates for the temperature difference, TRC is a method that compensates for the temperature difference as a ratio, so it is thought that the compensation can be applied by expanding or reducing the ratio when the temperature difference between the reference sample and the test sample is large.  
Figure 8: Sample surface temperature distribution according to lamp type: (**a**) horizontal direction, (**b**) vertical direction.  
Figure 7: IR Images at 50 s of modulation frequency 0.01 Hz along each lamp type: (**a**) parabolic type, (**b**) HA type.  
Table 2 shows the contrast values applied to each compensation technique in raw, phase, and amplitude images according to the lamp type. To analyze the improvement effect compared to contrast, data were acquired along the vertical and horizontal profiles in the sound area close to the specimen center, as shown in Figure 7. Standard deviation can compare the degree of data scattering around the average value and can be expressed in the same units as the raw data.  
It can be seen that the temperature standard deviation is significantly lower when the equivalent array lamp type is applied. When applying ATC, the standard deviation was found to decrease in all cases, and in the case of TRC, it decreased in most cases, but the improvement effect was insignificant. By applying the compensation sheet using the reference sample, the contrast of the thermal image was confirmed to be improved to a maximum of 54%, close to 1/2, and the effect of ATC was overall better than that of  
Figure 11: Images along the data type at modulation frequency 0.01 Hz with the TRC compensation method applied: (**a**) raw (temperature), (**b**) amplitude, and (**c**) phase.  
Figure 9: Images along the data type at modulation frequency 0.01 Hz: (**a**) raw (temperature), (**b**) amplitude, and (**c**) phase.  
irregular TRC. TRC is a method of compensating for the temperature difference by ratio. When the temperature difference between the reference sample and the test sample is large, compensation is expanded by the ratio. In addition, as the contrast of raw data is improved by the compensation technique, the overall contrast of the phase and amplitude is also improved, and it can be seen that the improvement rate in the horizontal direction is better than the vertical direction. This is believed to be because the resolution of the IR camera is 640 \(\times\) 480 pixels, a rectangle that is longer in the horizontal direction than in the vertical direction.