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.