Figure
3 (a) Experimental setup for the 4 μm OCT system. (b) OCT imaging depth
for different spectrometer integration time. The scale bar corresponds
to 200 μm assuming a refractive index of n=1.
3 | RESULTS AND DISCUSSION
3.1 | Comparing OCT at 1.3 μm and 4 μm
Several commercial OCT systems for imaging scattering media operate near
the 1.3 μm wavelength region. However, at 1.3 μm the penetration depth
in coatings with pigments and additives is very limited due to strong
scattering from these particles [23]. Therefore, a transparent
coating (sample A) was tested first using the 1.3 μm OCT to establish a
benchmark for the subsequent measurements in more realistic coatings.
Figure 4(a) show a close up en face image (X/Y) of the impact
area captured with the OCT onboard camera. The corresponding surface
topography imaged using OCT is shown in Figure 4(b). It is clear from
the topography of the impact site that the dark regions visible in
Figure 4(a) are primarily located below the surface and therefore only
visible because the coating is transparent. This becomes apparent when
visualizing the OCT signal retrieved from below the surface, here
visualized as an en face volume projection in Figure 4(c). The
projection was made by superimposing individual en face images from just
below the surface down to where the signal completely disappears. In the
cross-section, additional information can be obtained about the
morphology and length of the cracks. Figure 4(d) show a single B-scan,
whose position is indicated by a vertical dashed line in Figure 4(b).
The complex structure of the cracks can be further visualized using
volume projections, as shown in Figure 4(e), which is constructed from
the maximum intensity of 200 B-scans. The region of the projected volume
is indicated by the horizontal dashed arrows in Figure 4(b). It is clear
from the OCT images that the cracks propagate at an angle with respect
to the surface, with some reaching several hundred microns in depth
(divided by the refractive index of the coating). From the volume
projection, there is some indication that the cracks propagate around
certain areas, leaving dark voids in between. These could be formed from
microscopic defects that form a cavity when exposed to the force of the
impact.