Figure
5 OCT imaging of sample B using both 1.3 μm and 4 μm OCT. (a) Close-up
of the impact area taken with the OCT onboard camera. (b,c) 1.3 μm and 4
μm OCT surface en face projection of the impact area, respectively. (d)
XCT verification of subsurface voids. (e,f) 1.3 μm and 4 μm OCT
subsurface en face projection of the impact area, respectively.
Horizontal dashed lines indicate the scan positions in (g) and (h),
which are offset due to different scan orientations. (g,h) Superposition
of 10 B-scans using 1.3 μm and 4 μm OCT, respectively.
3.2 | Subsurface bubble detection using 4 μm OCT and X-ray CT
In the previous section, a subsurface void was detected directly below
the impact area and the observation was verified by XCT. In this
section, the OCT imaging contrast and penetration depth is further
compared with images obtained using XCT. Figure 6(a,c) show OCT and XCT
en face images, respectively, of the surface of sample B just next to
the impact. The area shows signs of strain from the impact in the
top-left region, including an exposed surface bubble and a crack. The
lower regions appear seemingly unaffected by the impact. Figure 6(b,d)
show the corresponding subsurface region, revealing several bubbles of
varying diameter. For ease of comparison the observed bubbles are marked
(1)-(5). Since the bubbles are different sizes and found at different
depths, the OCT projection in Fig. 6(b) could not exclusively capture
the dark air region of all bubbles simultaneously. Therefore, bubbles
(2) and (4) appear as bright white spots, which marks the strong
reflection from the air-coating interface at the top of the bubble. The
bubbles are also clearly seen in the cross-sectional images in Figs.
6(e-j). However, compared to XCT it is clear that the penetration depth
of 4 μm OCT is still very limited. To measure the penetration depth in
physical units using OCT requires knowledge of the refractive index.
However, using the known 9 μm voxel size of the XCT images a physical
scale bar was generated, and from that the penetration depth could be
evaluated. For example, the top and bottom of bubble (4) is located
about 247 μm and 425 μm from the surface, respectively. The deepest
point observed with OCT is the bottom of bubble (1), which is located
650 μm below the surface, and this was only possible because of the
reduced scattering inside the hollow cavity combined with a strong
reflection at the air-coating interface. Still, it presents an advantage
compared to ultrasonic techniques that cannot image through air. Using
the known depth of bubbles (1)-(4), the refractive index of the coating
was calculated to be n=1.59±0.03.
Although the XCT images provide a much clearer identification of
bubbles, there are other features where OCT provides a better contrast.
Because the contrast of OCT is determined by reflectivity and
scattering, it is sensitive to small changes in the refractive index and
orientation of particles. For this reason, the OCT surface topography
shown in Fig. 6(a) is much more detailed than the corresponding XCT
image in Fig. 6(c). Similarly, the horizontal cracks seen in the left
side of Fig. 6(i) at a similar depth to bubble (5), is not visible in
the corresponding XCT image in Fig. 6(j). Only the larger crack located
much deeper is seen. OCT could therefore have an advantage in detecting
small cracks that have not yet opened sufficiently to be visible by XCT.