Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

INTRODUCTION
which image was shadowed on the subject’s retina when viewing a distant point light
source through the “aberroscope”. Aberrations were estimated from the distortions of
the grid. In 1900 Hartmann used Scheiner’s idea to measure aberrations in mirrors and
lenses, using an opaque screen perforated with numerous holes, which is commonly
referred to as wavefront sensor. Smirnov (Smirnov, 1961) modified the Scheiner’s
disk in order to sample the whole pupil, and Howland (Howland, 1968) modified the
Tscherning’s aberroscope with a crossed cylinders lens to study aberrations in camera
lenses. It was not until eighteen years later that it was applied to measure aberrations
of the human eye subjectively (Howland and Howland, 1976), with the subject
drawing the perceived distorted grid. In 1984, Walsh et al. (Walsh et al., 1984) turned
Howland’s aberroscope into an objective method by photographing the image of the
grid on the retina.
1.4.5.3. The Hartmann-Shack (HS) aberrometer
In 1971 Shack and Platt (Shack and Platt, 1971) improved Hartmann’s screen by using
an array of microlenses (or lenslets) instead of the perforations to analyse the
wavefront coming out of the optical system to study. The array of microlenses is
called a Hartmann-Shack (HS) wavefront sensor, and it is composed of a number of
microlenses with the same focal length, arranged in a known geometry.
Figure 1.10. Schematic diagram of the working principle of Hartmann-Shack (SH)
sensor. A point source emits from the retina, and the spherical wavefronts are
distorted by the eye aberrations. Each lenslet will sample a portion of the wavefront
on a different phase, and will form a point image away from its focal point (reference)
a distance proportional to the phase distortion. Modified from original by S. Marcos.
A diagram of the working principle of a HS sensor for measuring the aberrations
of the eye is shown in Fig. 1.10. In this technique (Liang et al., 1994), a point source is
created on the retina by illuminating the eye with a narrow beam (generally from a
superluminescence diode). In an aberration-free optical system the wavefront
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