Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

INTRODUCTION
1.2. THE OPTICAL SYSTEM OF THE EYE
1.2.1. Cornea
The eye is, from an optical point of view, an image forming device than achieves its
high optical power (60 diopters) with two refractive elements, the cornea and the
cristalline lens (Atchison and Smith, 2000). The images of the outside world are
projected onto the retina (the eye fundus) directly connected to the brain. Figure 1.1
shows the main optical elements, with dimensions and refractive indices.
The cornea is an inhomogeneus cellular and fibrilar structure, composed of
different layers: Epithelium, Bowman’s Membrane, Stroma, Decemet’s membrane and
Endothelium (Barbero, 2006). The cornea is alive, and protected from environmental
elements by the tear film. The Stroma is the thickest layer (90% of the corneal
thickness, around 500 m).
Fig. 1.1. The eye can be considered an optical system (Atchison and Smith, 2000).
The dimensions and refractive indexes of the human eye are shown.
The anterior cornea is the front surface of the eye and has a regular and stable
shape. The tear film has the important role of providing the anterior cornea with a
smooth and polished optical surface. In optical terms, the anterior cornea is the most
important optical surface, providing 48 diopter of optical power, and it is extremely
transparent. The average cornea is not spherical. The radius of curvature increases
from the apex or, in other words, the cornea flattens in the periphery. If we consider
the equation of a conicoid:
(X 2 + Y 2 ) 2 +(1+Q) Z 2 -2ZR = 0 , (1.1)
(where Z is the coordinate along the optical axis, R the apical radius and Q the
asphericity), and fit it to real corneas, the average R obtained will be around 7.7 mm
and the average Q will be -0.25 (see table 2.3 in Ref. (Atchison and Smith, 2000)).
This shape corresponds to an ellipsoid.
Some optical inhomogeneities inside the cornea contribute to the scattering of
light, what allows the imaging of optical sections of the cornea with different
instruments and techniques as slit lamp imaging, Scheimpflug imaging and Optical
Coherence Tomography.
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