SD-OCT Base Unit
Chapter 4: System Operation
Rev C, June 25, 2021
Page 33
Advanced Adjustments
Focus and Choice of Objective Lens
In OCT, an objective lens is used to focus the light beam on and into the sample, as well as to collect the
backscattered light. The focused nature of the beam means that the lateral extension of the beam is different
along the depth axis. Since OCT records signals along this depth axis, the collected signal corresponds to
different lateral extensions of the beam and therefore the resolution is different along the depth axis. The best
resolution can be obtained from the axial focus plane; the further away from the focal plane the light is
backscattered from, the more blurred out the signal will appear.
The length of the axial focus depends on the optics of the OCT system and especially on the choice of the
objective lens. For samples with a small axial focus length, an objective lens with a high numerical aperture is
appropriate. This will grant a small lateral and axial focus
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, hence allowing a more in-depth analysis of the
sample but also restricting the total axial depth that can be investigated. For samples with a deep region of
interest, it is more helpful to choose an objective with a low numerical aperture, thus enabling the system to
collect signals from a larger focus band. However, low numerical aperture objective lenses will decrease the
lateral resolution of the image.
Figure 36 OCT B-Scans of Scattering Particles taken with LSM02, LSM03, and LSM04
Thorlabs offers three scan lens kits for different purposes. Figure 36 shows the difference in lateral resolution
and depth of focus for the OCT-LK2 (high resolution imaging), OCT-LK3 (general purpose), and OCT-LK4 (long
depth of focus) objectives. Note the change in signal strength and lateral resolution over depth caused by an
increasing focus band.
Additional lenses with larger scan range and depth of focus as well as higher lateral resolution are available on
request. Please contact Thorlabs’ OCT support (see Chapter 13) for more information on how to incorporate
different objective lenses into your OCT setup.
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Please note that the axial resolution is given by the specifications of the OCT base unit and thus independent from the selected
objective lens.
