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SD-OCT Base Unit Chapter 5: Imaging Artifacts
Rev E, November 7, 2018 Page 47
Measurement Depth in OCT Systems
The spectral resolution of a frequency domain OCT system defines its possible measurement depth. This depth
is the maximum detectable optical path length difference limited by the Nyquist criteria. In real materials the
measurement depth of OCT systems as well as the axial resolution is reduced. The reduction of the resolution
depends on the material properties between the two measured interface signals used. The reduction of the
imaging depth is a result of the materials in the sample image as visualized in the graphic below:
Figure 60 Measurement Depth With Refractive Media
In the image, the incoming beam from above is scanned over a structure made of two different materials named
one (d
1
, n
g1
) and two (d
2
, n
g2
) stacked on a flat surface (red line). The imaging range is displayed as a light
grey area. Vertical structures are barely visible.
The materials are displayed in the OCT image with an axial dimension corresponding to the optical path lengths.
In most cases, the sample is not well known. The measurement depth in air (vacuum) is known and the optical
path lengths of the materials are obtained. The material properties (e.g. group refractive indices) must also be
known in order to determine the real physical thickness of the materials.
The loss of imaging depth depends on the thickness and the group refractive indices of the materials displayed
within the image. It is calculated as follows:

 
i
i gi
d n loss ) 1 (