Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

CHAPTER 2
Several routines for processing corneal surfaces, available in the laboratory from those
works, was used during this thesis.
For the purposes of this thesis, we will refer to “first surface aberrations” and
“corneal aberrations” synonymously. This term will refer to the anterior corneal
aberrations for naked eyes, aberrations of the artificial cornea in artificial eye
measurements or the aberrations of the anterior surface of the contact lens in eyes with
fitted contact lenses. We therefore intentionally exclude from the “corneal
aberrations” the effect of the posterior surface of the cornea, the effect of the anterior
cornea of the eye behind a contact lens or the effect of a tear lens between a contact
lens and the cornea (that will be considered part of the “internal aberrations” of the
eye).
2.3.2. Ocular aberrations: Laser Ray Tracing
The experimental measurements of ocular aberrations in this thesis were performed
using Laser Ray Tracing (LRT) technique. This technique is well suited for the
measurement of eyes post-ablation (due to its high dynamic range that allows
successful measurements of highly aberrated eyes) and eyes with contact lenses
(especially for soft multifocal contact lenses as, besides the extended dynamic range, it
has flexibility in the configuration of the sampling pattern).
In brief, a set of parallel laser pencils samples the eye’s pupil, by means of a
scanning system (using different sampling patterns). Aerial images of the retinal spots
corresponding to different entry pupils are collected sequentially onto a cooled CCD
camera. The centroids of the set of aerial images are computed. The deviations of the
centroids from the principal ray are proportional to the derivatives of the wave
aberrations. The wave aberration is obtained by fitting the derivatives to a Zernike
polynomial expansion (up to the 7 th order) using a least-mean-squares procedure. All
measurements were made foveally (the subject was asked to fixate a stimulus). A
CCD camera centered at the optical axis of the instrument was used to monitor and
center the pupil and also to monitor possible shifts of the contact lens, were present.
Subjects were stabilized by means of headrest and dental impression attached to a
three-dimensional positioning system. A near infrarred laser diode (786 nm) was
normally used as a light source. Light exposure was at least two orders of magnitude
below safety limits.
2.3.2.1. LRT basic concepts
The LRT technique was first proposed in 1993 (Penney et al., 1993) and it was applied
to measure ocular aberrations in human eyes in 1997 (Navarro and Losada, 1997,
Molebny et al., 1997). A deeper description of the method can be found at Moreno-
Barriuso’s thesis (Moreno-Barriuso, 2000).
This is a double pass technique, since light is delivered into the eye and the
reflection from the retina is captured on a CCD camera (Fig. 2. 19). In the first pass
the pupil of the eye is sequentially sampled with laser pencils parallel to the optical
axis. Each ray is deflected by a specific angle depending on the slope of the
wavefront at that particular point on the pupil plane (defined by the optical
characteristics of the surfaces it goes through), and therefore will impact the retina at a
specific point. In an aberration-free system, all rays superimpose on the same retinal
location. However, when optical aberrations are present the rays hit the retina at
different positions. In the second pass the light is reflected off the retina, exiting the
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