Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

CHAPTER 2
A complete description of the accuracy and sources of error of the technique can be
found in Sergio Barbero’s thesis (Barbero, 2004). Many software tools have been
developed in the Visual Optics and Biophotonics lab during the last years for data
export, and analysis using custom algorithms. One of the most important applications
developed in previous works (Barbero et al., 2002b, Barbero et al., 2002c) is the
estimation of corneal aberrations from the Atlas Placido disk videokeratoscopy
(briefly described in Section 2.3.1).
2.1.3. Corneal topography with Scheimpflug Imaging
2.1.3.1. The Scheimpflug principle
Fig. 2. 11 (a) shows a conventional optical system. The object plane, the lens, and the
image plane are parallel. In imaging systems following the Scheimpflug principle (Fig.
2. 11 (b)), the object plane, the lens plane and the image plane are no longer parallel,
and they cross a line called the Scheimpflug intersection. The major advantage of the
Scheimpflug geometry is that a wide depth of focus is achieved.
a) b)
Object Lens Image Lens
Object
Image
Fig. 2. 11. The Scheimpflug principle.
2.1.3.2. The Scheimpflug camera
In Fig. 2. 12 the basic principle of slit lamp photography and of Scheimpflug imaging
is illustrated. Slit lamp photography (Fig. 2. 12 (a)) has not been used in this thesis,
but it is a well established technique in clinical practice and the concept behind it
serves as an introduction to Scheimpflug imaging. A narrow slit beam of very bright
light produced by a lamp is focused on the eye which is projected with a 10x to 50x
magnification microscope on a imaging sensor (a CCD chip in this example). The
width, length and orientation of the slit is generally variable. A scheme of the
Scheimpflug camera, for use in opthalmology is illustrated in Fig. 2. 12 (b). The
Scheimpflug camera improves slit lamp imaging by using the Scheimpflug principle.
The image plane (the CCD) and the lens plane intersects in a line. Therefore, the
images correspond to sections of the eye, in which all the points are in focus. Sections
of the entire anterior segment of the eye can be obtained.
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