SD-OCT Base Unit
Chapter 5: Imaging Artifacts
Page 40
MTN012389-D02
5.3. Autocorrelation
The fundamental principle of SD-OCT is a frequency analysis of an interference signal entering the
spectrometer. In the usual case, this interference signal is created by photons returned from the sample
interfering with photons returned from the reference arm.
In case a sample has at least one highly reflecting surface, the reflections off this surface can interfere with
other photons returned from the sample. The OCT engine cannot distinguish between interference created in
respect to the reference arm and interference created within the sample. Figure 50 shows the B-scan of a
sample (laminated foils).
Figure 50 Autocorrelation from a Surface Reflective Sample in a B-Scan
When blocking the reference arm – by turning the reference arm intensity knob counter-clockwise - the signal
caused by autocorrelation remains in the B-scan, while the primary interference signal disappears. This is
shown in Figure 51.
Figure 51 Autocorrelation with the Reference Arm of the Imaging Scanner Blocked
For thickness measurement of foils or related features, the autocorrelation can be a wanted feature. For avoiding
this effect, proper index matching (see Figure 48) or tilting of the sample is suggested.
