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.