Figure 6 Detection of sub-surface bubbles in sample B using 4 μm OCT with XCT verification. (a,b) OCT and (c,d) XCT surface- and subsurface en face projections of the scan area, respectivly. (e,g,i) OCT and (f,h,j) XCT cross-sections of bubbles (1)-(5). Note that the XCT scale is a physical scale, while the OCT scale is optical path distance (OPD) (i.e. multiplied by n).
3.3 | Multilayer coatings and crack detection
Scanning sample C near the SPIFT impact region revealed no visible damage below the surface. However, it revealed how a difference in material properties can lead to a difference in the OCT image contrast. The images in the previous section were obtained from sample B, which has a single ~3.5 mm coating layer, causing the OCT signal to slowly decay with increasing depth. However, sample C has multiple thin coating layers, as shown in the microscope image in Figure 7(c), which provides a different OCT image contrast. From the corresponding B-scan in Figure 7(d), the top coat is clearly delineated as a bright band in the image, while the second putty/filler layer show only clear signal contrast from the large filler particles. The transition from topcoat to filler is clearly seen in the corresponding line scan intensity plot in Figure 7(e). The peak at 0 μm OPD represents the air-coating interface, while the slowly decaying signal represents the back-scattered signal from inside the coating. The change in slope at the 223 μm mark indicates that the filler material has a significantly higher scattering coefficient, and therefore the reflected signal quickly reaches the noise level of the OCT system. The top coat of sample C also appears to have a slightly higher scattering coefficient compared to sample B, and comparing the measured thicknesses also result in a higher refractive index of n = 233 μm/133 μm = 1.68.
It is clear that the penetration depth of OCT depends both on the system parameters, such as laser wavelength and sensitivity, as well as material properties, such as absorption and scattering coefficients. Most materials, with the exception of water, has low molecular vibrational absorption in the 3.5-6 μm window, and since scattering in general decreases with increasing wavelength, the penetration depth could be improved by increasing the laser wavelength. However, since the axial resolution scales with λ2/Δλ, increasing the wavelength also reduces the optical resolution unless a much broader spectrum is used [27]. While SC lasers covering the entire 2-10 μm wavelength band has been demonstrated [28], it becomes increasingly challenging to find a suitable detection system that can operate in this region without lowering the detection speed or system sensitivity.