Alfredo Dubra's PhD thesis - Imperial College London

1. Introduction
in the measurements and noticed that the aberrations change with the accommodating
distance. In the same work, Smirnov suggested the use of contact lenses for correcting
high order aberrations. Many of Smirnov’s ideas and observations are the basis of
modern wavefront sensing techniques, both subjective and objective and some of his
ideas, such as the compensation of the aberrations have been put into practice.
Following Smirnov’s work, at least two more subjective or psychophysical wavefront
sensing methods (i.e., those in which the aberrations of the eye are estimated from
responses provided by the subject being examined) have been developed, based on
wavefront slope evaluation in the pupil plane. Firstly, the cross-cylinder aberroscope
proposed by H. and B. Howland [5], in which a grid is sandwiched between two crossed
cylindrical lenses and projected onto the retina. The retinal image of the grid is
distorted by the aberrations of the eye. The subject is asked to draw this grid so
that the aberrations can be estimated the drawings. This technique did not become
popular given the strong reliance on the subject’s ability to replicate the image seen.
Later, in 1992 Webb et al.[6] and then He et al.[7] built more automated versions
of Smirnov’s experiment, where the wavefront slope at different positions within the
pupil is measured by feedback provided by the vernier acuity of the subject.
In 1969 Berny [8] used a completely different approach by performing a Foucault
knife-edge test experiment to measure the wavefront aberration itself, rather than the
wavefront slope.
With the development of more sensitive films and cameras, a number of objective
techniques for evaluating the aberrations of the eye were developed in recent years.
In 1984 Walsh et al. [9, 10, 11] transformed the subjective Howland and Howland
cross-cylinder aberroscope into an objective method by recording the image of a grid
projected onto the retina with a camera.
In 1997, Navarro et al.[12, 13, 14] developed an instrument to evaluate the slope of the
wavefront, sampling the pupil at different locations with a narrow collimated incoming
beam and recording its displacement on the retinal surface with respect to a reference
spot.
Recently, in 1996 Ragazzoni [15] proposed a new slope wavefront sensor, in which the
beam to analyze is moved in a circular pattern over a glass pyramid producing four
images that are combined to estimate local wavefront slopes. This wavefront sensor
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