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SD-OCT Base Unit Chapter 5: Imaging Artifacts
Rev E, November 7, 2018 Page 49
As an example, a material with wedge is analyzed:
Figure 62 Complex Structure in Image
The block shows “standard” behavior on the right side where the surface is perpendicular to the incoming beam.
In the chamfered area there is diffraction and the beam travels under an angle through the block. The real
angled physical path is enlarged a little (
a
d
and
b
d
), and
s
d
is not in-line with
a
d
.
These paths are displayed strictly vertical in the OCT image showing the expected optical path lengths
i gi
d n
.
The beam is displayed without rotation.
This difference between the real diffracted optical path and the displayed OCT image makes it difficult to perform
an inverse ray tracing because all the diffractive interfaces need to be determined in 3D.
Even this determination needs to be undertaken step after step since the first interface affects the OCT image
of the second interface and so on.
The most challenging part is the light grey area in the sample marking the imaged field. In the left edge of the
chamfered block there is an area which is not reached by OCT light and therefore cannot be visualized at all.
On the other hand there are structures that are measured twice because of the two different optical paths leading
to these structures.
In very complex structures these effects become more and more difficult to handle – Just assume spherical or
curved interfaces, bubbles, inhomogeneous materials, possible imaging aberrations in the sample etc.