Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

INTRODUCTION
a point focus: the different rays will intersect the image plane (retina) at different
points, and the wavefronts will no longer be spherical.
Thus, the optical aberration can be described in terms of wave aberration maps:
the distance that each point of the wavefront departs from the ideal sphere at the exit
pupil (Charman, 1991). An example of wave aberration map is shown in Fig. 1.5,
where the colour indicates the distance between the wavefront and the reference
sphere. Besides in terms of wave aberrations, optical aberrations can also be
represented as: transverse aberration (departure of a ray from its ideal position at the
image surface), or longitudinal aberration (departure of intersection, where this occurs,
of a ray with a reference axis, from its ideal intersection) (Atchison and Smith, 2000).
Fig. 1.5. Schematic representation of the wave aberration. Wave aberration values
(distances between the distorted aberrated wanefront and the spherical reference) can
be represented as z coordinate referred to the pupil plane (three-dimensional
representation) or can be represented as a colour gradation (Aberration map).
From the wave aberration maps, two classic descriptors of the optical quality of an
optical system can be directly derived, after simple mathematical operations: the
Modulation Transfer Function (MTF) and the Point Spread Function (PSF). These two
are alternative ways to describe the optical quality of a system. The MTF quantifies
the loss in contrast associated to each spatial frequency. The higher the MTF, the
better the image provided by the system. The PSF is the impulse response of the
system: the degraded image of an ideal point, as imaged by the system. Finally, the
retinal image associated with any observed image image can be simulated. A common
procedure (Barbero S and Marcos, 2008) consist of convolving the ideal image with
the PSF of the system.
Figure 1.6 shows two examples, corresponding to the same eye (right eye of the
author of this thesis), under two different conditions: naked eye (top row) and
correction of aberrations (botton row) with adaptive optics (explained in Section 1.5.3)
(Marcos et al., 2008). When the eye is aberrated (normal condition, top row) the
aberration map has considerable deviations from zero (the reference), the MTF drop is
sharper (from the initial point: unity at the center), the PSF suffers a larger spread at
the retina and the retinal images are blurred. When the aberrations of the eye are
corrected (botton row), the residual wavefront map is closed to zero at all points, the
MTF fall is gradual, the PSF is a point and the retinal image sharp.
25