Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

METHODS
Slit lamp
Scheimpflug Camera
Image plane
Lens plane
CCD
CCD
Object plane
Fig. 2. 12 The Scheimpflug camera.
However, the Scheimpflug camera introduces a geometrical distortion (Fig. 2. 13, top
row), because the magnification is not constant over the image. Additionally, because
of the refraction from the different ocular surfaces, the Scheimpflug camera also
introduces an optical distortion (Fig. 2. 13, bottom row), due to the fact that each of
the ocular surfaces is seen through the previous one (i.e., the anterior lens is seen
through the posterior and anterior cornea).
Geometrical
distortion
Distortion
Corrected
Optical
distortion
Fig. 2. 13. Image distortion in Scheimpflug imaging.
A validation of the correction of the images is needed before it can be reliably used to
obtain posterior cornea surface shapes (Dubbelman et al., 2002, Rosales and Marcos,
2009).
2.1.3.3. The Pentacam Scheimpflug imaging system
We used a Pentacam (Oculus GmbH, Wetzlar, Germany) anterior segment imaging
system based on the Scheimpflug principle (Fig. 2. 14). The slit rotates around the
optical axis of the instrument, allowing the capture of 25 anterior segment diametral
sections at different angles, and therefore 3-D elevation maps of the anterior and
posterior corneal surfaces are obtained. According to the manufacturer, this device
provides quantitative information of the posterior cornea, as it automatically corrects
for geometrical and optical distortions. The commercial software also provides
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