
The objective evaluation often quality of the optical system of the
human eye has been carried out generally in terms of the optical
transfer function from measurements of aerial images of lines,
edges, or gratings tests formed by a double pass through the optical
media of the eye.17-In this way, only one-dimensional information
is obtained in each deter- mention. Because of the asymmetries of
the wave aberrant- ton of the eye, simultaneous dimensional
information should be obtained for a more complete evaluation. An
objective method based on the photographic recording of the retinal
image of a grid and that provides dimensional in- formation has been
presented. 8 The method allows one to estimate the coefficients of
the wave-aberration polynomial through measurements of the grid
deformation. However, owing to the irregularity of the retina, 9 it
seems that a point test would be more adequate and straightforward
for evaluate- ton purposes. In spite of that, the point image has not
been used, mainly because of the difficulties involved in its recording or measurement. As far as we know, only in Ref. 5 was a
point test used, but the aerial image was scanned with.
Radial gratings, so the system behaved as if the test were a grating,
and only information in one direction were obtained. It must be
pointed out that, besides the normal difficulties in the study of the
living human eye, that is to say, energy.
In what follows, the experimental method for recording the aerial
image of the point source is first described in detail. Afterward the
PSF and the OTF are obtained from the digitized aerial images.
Results and conclusions are presented at the end.
The experimental system for recording the aerial image of a point is
a conventional double-pass setup and a modification of the ones
described in previous works.9"10 Its main Chirac- touristic are as
follows: a spatial-filter pinhole is used as the accommodation and
fixation test object, a laser beam is the light source, and a micro
channel light intensifier is used as the registration step. As was
mentioned before, this method permitted us previously to follow, in
real time, the dynamic behavior of the eye optical system; however,
the intensifier that was used destroyed the linearity of the recording
pro- cuss. The setup used in this work keeps the same main
characteristics, but the image intensifier has been eliminate- end
from the system to preserve linearity. This can be done because, if
only instantaneous or short-term images are ob.- tainted, the retinalirradiance limitations can be consider- by relaxed. In this way, the
resulting aerial images can be made bright enough to be recorded by
conventional means.
The experimental setup is depicted in Fig. 1. A He-Ne laser beam (8
mW) is expanded by a 20X microscope object- tive (M) and filtered
by a 20-, tm pinhole (0) that also acts as the test object. The beam,
collimated by a lens L, (f' = 120 mm), enters the eye after reflection
in a pellicle beam split- tar (BS) and forms an image on the retina
(0'). The corneal irradiance in the observer is of the order of 0.3
mW/cm 2, more than 1 order below U.S. security standards." 1
The angle of incidence (a = 520) has been chosen to obtain the
maximum polarization degree (0.95) caused by reflection in the
beam splitter. The light reflected in the retina leaves the eye, and,
after transmission through the beam splitter and an optional polarizer
(A), the lens (L 2) forms an aerial image (0") on the
photocathode of a calibrated TV camera has been computed. In this
expression I (x,
y) is the average aerial image and I (x, y) the
image computed by autoconvolu- tion of the PSF. It turns out that,
to obtain a normalized error smaller than 0.01 chosen as a criterion,
at least 64 Images should be added. However, to avoid excessive
dis- comfort to the observer, and to shorten the duration of the
experiment, only 32 images are added, and posterior low- pass
filtering is performed on the resulting image.
The characteristics and the location of reflection in the retina are
matters on which authors have not generally agreed, 5’14-'6 and
further research is required. It is the pur- pose of this paper not to
study reflection in the retina but to
Present a methodology for the evaluation of the quality of the optical
system of the eye; however, both subjects are closely related,
especially when the double-pass method is used.
In this sense the
speckle structure that appears in all instantaneous images leads us to
think that reflection takes place not at the membrane but at a
posterior surface, at the receptor surface, or more probably at all
planes after the interior membrane. The results of Rohler et al. 5 and
O'Leary and Millodot, 17 in which reflected-light-maintaining
polarization takes place at the receptor surfaces and corre- lates with
absorbed light, will be assumed in what follows.
1. M. F. Flamant, "Etude de la repartition de lumiere dans l'image r6tinienne dune
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1046-1050 (1962).
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(London) 186, 558-578 (1966).
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des lebenden menschlichen Auges im reflektierten
Licht," Vision. Res. 9, 407-428 (1969).
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Vision. Res. 21, 445-454 (1981).
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8. G. Walsh, W. N. Charmin, and H. C. Howland, "Objective technique for the
determination of monochromatic aberrations of the human eye," J. Opt. Soc. Am. A
1,987-992 (1984).
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A hybrid optical-digital method for the determination of the PSF and the dimensional OTF for
individualized human eyes is presented in this paper.
The method is based on recording the aerial
image of a point source. The experi- mental system is a double-pass ophthalmoscopy setup in which a
spatial-filter pinhole is used as the object test and a laser beam is the light source. Short-term aerial
images are recorded with a calibrated TV camera and fed into the com- puter, where they are
preprocessed and averaged. By de- convolution, the PSF and the dimensional OTF are ob- tained. The
method is implemented in such a way that recording and computation can be carried out on a routine
basis with minimum discomfort for the observer.
The results obtained with this procedure can be used not only for individualized optical evaluation
purposes but also as physical input data for the study of further steps in the.