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Corneal Optical Aberrations and Retinal Image Quality in Patients in Whom Monofocal Intraocular Lenses Were Implanted
Antonio Guirao, PhD;
Manuel Redondo, PhD;
Edward Geraghty;
Patricia Piers;
Sverker Norrby, PhD;
Pablo Artal, PhD
Arch Ophthalmol. 2002;120:1143-1151.
ABSTRACT
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Objectives To compare retinal image quality and optical corneal aberrations in
patients in whom monofocal polymethyl methacrylate intraocular lenses (IOLs)
were implanted with those in healthy subjects of a similar older age (60-70
years old) and to use the results to suggest improved optical designs of IOLs
to maximize retinal image quality.
Methods A double-pass apparatus was used to measure retinal image quality for
3-, 4-, and 6-mm pupil diameters. Corneal aberrations for a 4-mm pupil were
calculated by a ray-tracing technique from the elevations provided by corneal
topography. Two groups of 20 subjects of a similar older age were studied:
in one group, polymethyl methacrylate monofocal IOLs were implanted; and in
a second group, healthy subjects were used as a reference.
Results The average retinal image quality was similar in older healthy patients
and in patients in whom IOLs were implanted, with both groups having a significantly
worse image quality than healthy younger subjects (aged 20-30 years). Both
groups were more tolerant to defocus than younger subjects.
Conclusions The average retinal image quality of patients in whom IOLs were implanted
was worse than that of healthy younger subjects despite the good optical quality
of isolated IOLs. This apparent paradox can be understood by the nature of
the aberration coupling in the eyes that undergo implantation. The ideal substitute
for the natural lens is not an IOL with the best-isolated optical performance,
but rather one designed to compensate for the aberrations of the cornea—a
design somehow inspired by the crystalline lens of younger subjects.
INTRODUCTION
THE IMPLANTATION of intraocular lenses (IOLs) is common practice in
cataract surgery and is a solution to cataracts and aphakia. In general, it
is a rather successful procedure; however, there are still some important
questions requiring additional study. For instance, a major concern is the
retinal image quality and visual performance of patients in whom IOLs have
been implanted relative to those of healthy subjects of a similar age. In
addition, it is important to explore improved IOLs designed to provide additional
capabilities, such as some range of pseudoaccommodation without a considerable
loss of performance, or new aberration profiles in the IOLs to improve retinal
image quality after surgery.
Many of the answers to the previous questions are related to the type
of optical aberrations present in the IOLs and, more important, to how these
combine with the eye's aberrations to produce the final retinal image. Today,
IOLs are manufactured to meet standard specifications1-2
of high optical quality when tested on an optical bench.3-4
However, the final retinal image quality of patients in whom IOLs were implanted
was not better than that of healthy subjects,5-6
despite the fact that typical monofocal IOLs have better optical quality than
the healthy crystalline lens.7-9
This apparent disagreement between measurements in vivo and in vitro can be
partially explained by considering the possible tilts and/or decentrations
of the implanted IOLs, which would reduce the final optical performance of
the eye. However, typical values for these effects are too small10
to fully explain the observed optical deterioration. A better explanation
arises from an understanding of the aberration coupling of optical systems.
An aberration profile that is appropriate for an isolated IOL, or even for
a lens considered in a theoretical eye or an International Standards Organization
model eye, may be inadequate to compensate for the optics of the corneal surface
in real eyes. Deteriorated optical quality of the cornea after surgery would
also limit the performance of the eye in which an IOL was implanted, although
the enormous improvements in surgical techniques cause this possibility to
be of minor relevance.11
In this context, this study examines the retinal image quality and corneal
aberrations of 2 groups of 20 subjects of a similar older age: in one group
(aged 56-80 years), polymethyl methacrylate monofocal IOLs were implanted;
and in a second group (aged 60-70 years), healthy subjects were used as a
reference. This will allow us to further understand the optical performance
of eyes in which IOLs have been implanted.
SUBJECTS AND METHODS
The optical performance of the living eye can be measured by different,
and in most cases, complementary procedures. By direct recording of the double-pass
retinal image,12-14
an overall estimate of the eye optics is obtained, usually expressed through
the point-spread function or the modulation transfer function (MTF). By using
aberrometric techniques,15-18
the optical aberrations of the whole eye are obtained and the retinal image
or the MTF is calculated. Furthermore, by using computer ray-tracing techniques,
the aberrations produced by the anterior surface of the cornea alone can be
determined from the corneal shape.19 Finally,
by comparing the corneal aberrations with the overall retinal image quality,
it is possible to establish the relative contribution to aberrations of the
different ocular elements.8
RECORDING DOUBLE-PASS RETINAL IMAGES: APPARATUS AND PROCEDURE
A double-pass apparatus adapted to record retinal images in a clinical
environment was used for this study. Figure
1 is a schematic representation of the apparatus and the procedure
used to measure the MTF. The basic principles, operation, and computational
analysis of this method have been reported extensively elsewhere.12-13,20-21 In
brief, the image of a green (543-nm) helium-neon laser–generated point
source is formed on the retina. The light reflected from the retina formed
the double-pass image that was recorded by a slow scan–cooled charge-coupled
device camera. The double-pass image was sent to a personal computer for processing.
Two apertures of equal size conjugated with the eye pupil plane acted as the
artificial entrance and exit pupils, when the natural pupil of the eye was
dilated. A second camera was used to control the pupil centration with respect
to the measuring beam (not shown in the Figure
1). The typical light exposure in the apparatus was more than 3
orders of magnitude below safety standards.22
The subject's head was placed on a chin rest mounted on 2-dimensional positioners,
allowing the centering of the natural pupil with respect to the artificial
pupil.
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Figure 1. The double-pass apparatus and
procedure. Double-pass images of a point source are recorded by a charge-coupled
device (CCD) camera after double pass of the light through the ocular media.
From the retinal image, the modulation transfer function (MTF) of the eye
is computed.
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The eye's pupil was dilated and the accommodation paralyzed by instilling
approximately 40 µL of 1% cyclopentolate hydrochloride. In some subjects,
an additional 20 µL of 2.5% phenylephrine hydrochloride was instilled
to dilate the pupil up to 6 mm in diameter. The refractive error in the subject
was corrected by moving the relative positions of a Badal system (L2 and L3),
and a cylindrical trial lens corrected astigmatism if required.
Double-pass images were recorded under the best refraction for 3 pupil
diameters: 3, 4, and 6 mm. For 4-mm pupils, additional images were recorded
for defocus of ±0.5 diopters (D). For each condition of pupil size
and focus, 3 double-pass retinal images and 1 background image were recorded.
The duration of each exposure was 4 seconds. The final double-pass image was
the result of averaging the 3 retinal images and subtracting the background
image, and the ocular MTF was calculated as the square root of the modulus
of the Fourier transformation of the double-pass image. From the 2-dimensional
MTFs, 1-dimensional MTFs were computed by averaging across all directions.
The ocular MTF was used as a description of retinal image quality. This function
describes the reduction in contrast from the object to the image produced
by the optics of the eye for each spatial frequency and can be related to
the contrast sensitivity function. To characterize the overall optical performance
of the eye, we use a single variable, the Strehl ratio, which is defined as
the quotient of the area under the actual aberrant MTF curve and the area
under the diffraction-limited MTF curve, corresponding to a perfect system.
ABERRATIONS AND OPTICAL QUALITY OF THE CORNEA
The corneal optical aberrations (for a 4-mm-diameter pupil) produced
by the anterior surface of the cornea from the elevation data provided by
a corneal topography system (MasterVue System; Humphrey Instruments, San Leandro,
Calif) have been estimated.19 Figure 2A shows a diagram of the procedure. From the elevation at
each point in the pupil, we calculated the corneal wave-front aberration, W, as the difference in optical path length between the
principal ray that passes through the center of the pupil and a marginal ray:
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Figure 2. A, The ray-tracing procedure used
to estimate the corneal aberrations (W), which are computed as
differences in optical path length between marginal and principal rays. X,
Y, and Z represent coordinate axes; r (radius) and 
(angle), radial coordinates of an arbitrary point at the exit pupil of the
eye; n and n', refractive
indexes; and z, d', and s', distances. B, From W, the point-spread
function (PSF) and the modulation transfer function (MTF) for the cornea are
estimated.
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where n and n'
are refractive indexes and z, d', and s' are distances, as represented in Figure 2.
The corneal wave-front aberration was represented (up to the fourth
order) as a weighted sum of the first 15 Zernike polynomials, Zmn (m and n are natural numbers representing the order of the Zernike polynomials):
where r represents the radial coordinate over
the pupil (Figure 2).
Each Zernike coefficient, cmn, represents
an individual aberration. The lower orders correspond to the known Seidel
aberrations. For example, c±22 represents astigmatism; c±13, coma; and c04, spherical
aberration. We calculated the astigmatism as follows:
where r0 is the pupil radius in
millimeters, and the Seidel spherical aberration and the Seidel coma are as
follows:
and
The spherical aberration obtained in this way is positive if marginal
rays focus in front of the paraxial focus. The remaining higher-order aberrations
measured are lumped into a single variable:
microns, with i excluding astigmatism, coma,
and spherical aberration.
From the corneal wave-front aberration, the MTF and the Strehl ratio
corresponding to the anterior surface of the cornea were also calculated (Figure 2B is a schematic representation of
the procedure). We tested the accuracy of the complete procedure using reference
surfaces for a pupil of up to 6 mm in diameter. The error in the estimation
of the aberrations is lower than 0.05 µm for a 4-mm pupil, demonstrating
that this method is sufficiently accurate for this study. Additional details
of the procedure and its accuracy are described by Guirao and Artal.19
SUBJECTS
Forty subjects distributed into 2 groups participated in the study.
The first group consisted of 20 patients (10 women and 10 men) in whom monofocal
rigid polymethyl methacrylate IOLs (BUV-95, Storz, St Louis, Mo) of 13-mm
diameter were implanted, using a conventional wide-incision technique. The
surgery, which used extracapsular cataract extraction, was completed during
1997 in all patients. A 6-mm incision was made in the superior conjunctiva
at the base of the limbus. An anterior capsulotomy was performed before removal
of the cataractous lens, implantation of the posterior chamber IOL, and extraction
of the viscoelastic gel injected for aiding the surgery. Finally, the wound
was closed using a nylon 10-0 suture. Patient ages ranged from 56 to 80 years
(mean ± SD, 67 ± 3 years). The second group contained 20 healthy
older subjects (9 women and 11 men), with ages ranging from 60 to 70 years
(mean ± SD, 63 ± 3 years). This group of healthy subjects was
used as a reference for comparison with the group in which IOLs were implanted.
Subjects and patients were selected after passing an ophthalmologic
examination with the following exclusion criteria: a refractive spherical
or cylindrical error of more than 2 D, keratometric astigmatism of more than
1.5 D, a corrected visual acuity lower than 20/25, any previous surgery on
the eye to be tested in healthy subjects, amblyopia, any known ocular or retinal
pathological features, and IOL decentered more than 1 mm in patients in whom
an IOL was implanted. The study followed the tenets of the Declaration of
Helsinki, and signed informed consent was obtained from every subject after
the nature and all possible consequences of the study had been explained.
Data are given as mean ± SD unless otherwise indicated.
RESULTS
RETINAL IMAGE QUALITY
Figure 3 shows the average
MTFs for the group of patients with implanted IOLs and for the reference group
of healthy older subjects, for each pupil diameter at best focus. The MTFs
are similar in both groups for the 3 pupil diameters. The slight differences
between groups are not significant (90% confidence level), although the average
MTF for the group with implanted IOLs is systematically slightly worse for
each of the 3 pupil diameters. The MTF for healthy younger subjects (aged
20-30 years) is significantly better than for older subjects for every pupil
diameter.13-14 As an example, Figure 4 shows the MTFs for healthy older
subjects and subjects in whom IOLs were implanted, together with the curve
for healthy younger subjects,13 for a pupil
diameter of 4 mm. The Strehl ratio for every eye as a function of age is plotted
in Figure 5 for the 4-mm pupil.
The average Strehl ratios are as follows: 3-mm pupils, 0.19 ± 0.07
(reference, 0.22 ± 0.06); 4-mm pupils, 0.12 ± 0.03 (reference,
0.14 ± 0.03); and 6-mm pupils, 0.07 ± 0.02 (reference, 0.08
± 0.02). Figure 6 shows the
comparison between the average MTFs at best focus and at 2 small defocus values
(±0.5 D), for the 4-mm pupil. The results for the positive and negative
defocus were similar, indicating that the position of best focus was correctly
determined. The relative decay of the MTF with defocus is similar for the
patients in whom IOLs were implanted and the older subject reference group.
This indicates that the tolerance to defocus is similar in patients with IOLs
and in healthy older subjects (and larger than in healthy younger eyes13).
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Figure 3. Ocular modulation transfer functions
(MTFs) averaged across all subjects in each group, healthy older subjects
(aged 60-70 years) and patients in whom intraocular lenses (IOLs) were implanted
(aged 56-80 years), as a function of the spatial frequency. A, Pupils with
3-mm diameters. B, Pupils with 4-mm diameters. C, Pupils with 6-mm diameters.
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Figure 4. Ocular modulation transfer functions
(MTFs) averaged across all subjects for a 4-mm pupil for healthy older subjects
(aged 60-70 years), patients in whom intraocular lenses (IOLs) were implanted
(aged 56-80 years), and healthy younger subjects (aged 20-30 years). The curve
for the younger subjects is from Guirao et al.13
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Figure 5. Optical quality of the eye via
the Strehl ratio, calculated from the ocular modulation transfer function
of each healthy older subject (aged 60-70 years) and each patient in whom
an intraocular lens (IOL) was implanted (aged 56-80 years), as a function
of age (4-mm pupil diameter).
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Figure 6. Average ocular modulation transfer
functions (MTFs) for the 4-mm pupil at best focus and at a defocus of 0.5
diopters (D). A, Patients in whom an intraocular lens was implanted. B, Healthy
older subjects (aged 60-70 years).
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CORNEAL ABERRATIONS
Figure 7 shows the values
of different corneal aberrations for the patients with IOLs and the healthy
older subjects, represented as a function of subject age. Figure 7A shows the corneal astigmatism, which is similar for both
groups. On average, the astigmatism is slightly larger in the patients in
whom IOLs were implanted (-0.9 ± 0.5 D) than in the reference
group (-0.7 ± 0.5 D), although the difference is not significant.
Values of the corneal spherical aberration are similar in the 2 groups, as
shown in Figure 7B, and the mean
± SD is the same for both groups (0.7 ± 0.2 µm). The astigmatism
and spherical aberration results suggest that the base shape of the cornea
(main curvature and asphericity) is similar in healthy subjects and in patients
in whom IOLs were implanted. However, other corneal aberrations are higher
in patients with IOLs. Figure 7C
shows the values of coma. There is a tendency for coma to increase slightly
with age, as previously reported.23 In addition,
some patients in whom IOLs were implanted have a higher value for this aberration.
The value of coma in the group in which an IOL was implanted (0.91 ±
0.58 µm) is significantly higher than in the reference group of healthy
older subjects (0.57 ± 0.27 µm). A similar situation is seen
for the rest of the higher-order aberrations (Figure 7D); the value in the patients in whom IOLs were implanted
(0.08 ± 0.07 µm) is higher than in the healthy older subjects
(0.05 ± 0.02 µm). Some patients in whom IOLs were implanted have
more corneal aberrations. This is probably because of the large incisions
(6 mm) required for the implantation of the rigid IOLs used.
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Figure 7. Corneal aberrations for each healthy
older subject (aged 60-70 years) and each patient in whom an intraocular lens
(IOL) was implanted (aged 56-80 years), for the 4-mm pupil, as a function
of age. A, Astigmatism. B, Spherical aberration from equation 4. C, Coma from
equation 4. D, Rest of the higher-order aberrations up to the fourth order,
from equation 5. The equations are given in the "Aberrations and Optical Quality
of the Cornea" subsection of the "Subjects and Methods" section.
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The MTFs associated with only the anterior surface of the cornea were
calculated from the previously mentioned corneal aberrations. The comparison
of the average corneal MTFs in both groups is shown in Figure 8 for a pupil with a 4-mm diameter. The optical performance
of the cornea is slightly worse on average in patients with IOLs. The relative
reduction of the MTF from the reference group to the group in which an IOL
was implanted is similar for the cornea (Figure 8) and for the complete eye (Figure 3). Figure 9 plots
the Strehl ratio obtained from the corneal MTFs for each subject and patient.
Values for the corneal Strehl ratio are as follows: 0.21 ± 0.09 for
the patients in whom IOLs were implanted and 0.26 ± 0.10 for the reference
group.
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Figure 8. Average corneal modulation transfer
functions (MTFs) for the 4-mm pupil for the healthy older subjects (aged 60-70
years) and the patients in whom intraocular lenses (IOLs) were implanted (aged
56-80 years).
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Figure 9. Optical quality of the cornea
via the Strehl ratio calculated from the corneal modulation transfer function
of each healthy older subject (aged 60-70 years) and each patient in whom
an intraocular lens (IOL) was implanted (aged 56-80), as a function of age.
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COMPARISON OF CORNEAL WITH OCULAR PERFORMANCE
Figure 10 shows the Strehl
ratio for the complete eye vs the cornea in patients in whom IOLs were implanted.
There is a correlation trend (r = 0.43, P = .04), statistically significant, implying that the eyes with poorer
image quality are the eyes with poorer optical corneal quality, as expected
for eyes with IOL implants. The dispersion indicates individual differences
in IOL implantation (tilts and decentration). The same type of IOL perfectly
implanted in every eye would yield a perfect correlation (r = 1.0) (Figure 10). Figure 11 shows the same comparison between
ocular and corneal optics in the group of healthy older subjects. In this
case, there is no correlation, which indicates that the natural crystalline
lens plays a different role in every eye.
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Figure 10. Correlation between the optical
quality (Strehl ratio) of the complete eye and that of the cornea for patients
in whom intraocular lenses were implanted (linear regression, r
= 0.43, P = .04).
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Figure 11. Correlation between the optical
quality (Strehl ratio) of the complete eye and that of the cornea for healthy
younger subjects (aged 20-30 years) (to be compared with Figure 10).
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COMMENT
The average retinal image quality of patients in whom IOLs were implanted
was similar or slightly worse than that of healthy subjects of a similar age,
and clearly worse than that of healthy younger subjects. These results indicate
that, despite the good optical quality of IOLs, the retinal image quality
of eyes with IOL implants is generally worse than that of eyes with natural
lenses. However, as an advantage, the tolerance to defocus is higher in older
subjects with IOL implants and in healthy older subjects than in healthy younger
subjects. This is a beneficial effect appearing in eyes with more aberrations.
At best focus, the retinal image quality is worse than that for a system with
less aberrations, but this retinal image quality remains similar for moderate
amounts of defocus. As a consequence, eyes in which IOLs have been implanted
and healthy older eyes are more tolerant to small refractive errors than healthy
younger eyes.
Corneal aberrations were similar or slightly larger in the patients
in whom IOLs were implanted than in the reference group of healthy older subjects.
The IOLs were rigid, requiring surgical procedures with large incisions (6
mm), which is likely the cause of the increased aberrations shown by a few
patients (Figure 7). Small-incision
foldable IOLs would be expected to yield no differences in presurgery and
postsurgery corneal aberrations. Corneal aberrations in the 2 groups studied
herein were larger than those in healthy younger subjects.23
However, the change in corneal aberration found with age was relatively small23; other researchers24
found nearly no changes. What is more important is that the small increase
in corneal aberrations alone, due to either incisions or aging, cannot account
for the limited retinal image quality in the patients in whom IOLs have been
implanted.
These results show an apparent paradox: while IOLs yield an extremely
good image quality when measured on an optical bench, the final retinal image
performance in eyes with implanted IOLs is only similar to that of healthy
older eyes and is clearly inferior to that of younger eyes. Because an increase
in corneal aberrations cannot fully explain this observation, there are 2
plausible explanations. First, the ocular performance after IOL implantation
may be limited due to inaccurate placement of the IOL. Tilts and/or decentrations
of the IOL will produce aberrations that may be comparable to the aberrations
of natural crystalline lenses. The dispersion found in Figure 10 shows that these aberrations of the IOL are present after
implantation, because a constant IOL for all of the patients would have produced
a perfect correlation between the corneal and the ocular optical quality.
Nevertheless, this reason alone cannot explain why patients in whom IOLs were
implanted did not show better image quality than healthy older subjects. According
to Guirao et al,13 the values for the ocular
Strehl ratio of a group of 20 healthy younger subjects between the ages of
20 and 30 years (4-mm pupil) ranged between 0.19 and 0.33 (mean ± SD,
0.26 ± 0.04). For patients in whom IOLs have been implanted, the Strehl
ratio (Figure 5) ranges from 0.07
to 0.17 (mean ± SD, 0.12 ± 0.03). Therefore, all of the patients
had a lower image quality than even the worst of the healthy younger subjects.
Despite the potential misplacement of the implanted IOLs, in a sample of 20
patients, one would expect to have at least a few with no or little decentration
and then have optical performances as high as those in healthy younger subjects.
An additional reason can fully explain our results and the apparent
paradox. The ideal substitute for the natural lens is not an IOL with the
best-isolated optical performance, but rather one designed to compensate for
the aberrations of the cornea (Figure 12 shows this explanation). Thus, an improved design for an IOL would
have an aberration profile that compensates for the corneal aberrations, to
maximize the quality of the retinal image. However, an aberration-free IOL
(diffraction limited) or an IOL with an aberration profile with the same sign
as the corneal aberrations will produce larger total ocular aberrations. The
ideal solution to this problem would be a customized IOL for any individual
cornea.
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Figure 12. Representation of the coupling
between the cornea and the intraocular lens (IOL). An IOL without aberrations
will produce an eye with the aberrations of the cornea and relatively poor
retinal images. However, an IOL with aberrations approximately contrary to
those of the cornea will produce an eye nearly free of aberrations. WA indicates
wave aberration.
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A concern associated with IOLs with customized aberration profiles is
that if the position of the IOL cannot be precisely controlled in the surgery,
then the final aberrations could be similar or even larger than those of conventional
IOLs.25 A useful approach would be the use
of a customized IOL with aberrations adapted to compensate only for spherical
aberration. The correct balance of the corneal spherical aberration of the
patient will still be achieved even in cases in which the IOLs are slightly
decentered or tilted.26 However, the improvements
in surgical techniques will probably allow precision in IOL positioning,11 enough at least for partial aberration compensation,
and will make worthwhile the correction of more complex patterns, including
several aberrations. This kind of customized design would mimic the behavior
of the lens in younger subjects, in whom the corneal aberrations are partially
compensated by the natural lens.8 This is not
generally the case in older eyes, in which the changes in the aberrations
of the lens with age reduce the compensation of aberration present in younger
eyes, resulting in a reduced quality of the retinal image.27
Thus, older subjects undergoing cataract surgery could benefit from customized
IOL designs to reduce the ocular aberrations.28
In summary, we measured the retinal image and the corneal aberrations
in healthy older subjects and in patients after the implantation of monofocal
polymethyl methacrylate IOLs. This method is useful for testing the clinical
success of IOL implantation and for exploring the possibility of producing
more efficient IOL designs. These results clearly suggest that an IOL with
good optical quality (aberration free) is not the best design choice. A better
IOL design would be the one that corrects for the corneal aberrations, producing
lower total ocular aberrations and, hence, a higher-quality retinal image
and an improved visual performance.
AUTHOR INFORMATION
Submitted for publication November 6, 2001; final revision received
May 1, 2002; accepted May 16, 2002.
This study was supported in part by Pharmacia, Groningen, the Netherlands;
and by grant PB97-1056 from the Dirección General de Enseñanza
Superior, Madrid, Spain (Dr Artal).
We thank all participants in this study for their cooperation.
Corresponding author and reprints: Pablo Artal, PhD, Laboratorio
de Óptica, Departamento de Física, Universidad de Murcia, Campus
de Espinardo (Edificio C), 30071 Murcia, Spain (e-mail: pablo{at}um.es).
From the Laboratorio de Óptica, Universidad de Murcia, Murcia,
Spain (Drs Guirao, Redondo, and Artal); and Pharmacia, Groningen, the Netherlands
(Mr Geraghty, Ms Piers, and Dr Norrby).
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18. Iglesias I, Berrio E, Artal P. Estimates of the ocular wave aberration from pairs of double-pass retinal
images. J Opt Soc Am A Opt Image Sci Vis. 1998;15:2466-2476.
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19. Guirao A, Artal P. Corneal wave aberrations from videokeratography: accuracy and limitations
of the procedure. J Opt Soc Am A Opt Image Sci Vis. 2000;17:955-965.
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20. Artal P, Marcos S, Navarro R, Williams DR. Odd aberrations and double-pass measurements of retinal image quality. J Opt Soc Am A. 1995;12:195-201.
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21. Artal P, Iglesias I, López-Gil N, Green DG. Double-pass measurements of the retinal image quality with unequal
entrance and exit pupil sizes and the reversibility of the eye's optical system. J Opt Soc Am A. 1995;12:2358-2366.
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22. ANSI Z136.1, Safe Use of Lasers. Orlando, Fla: Laser Institute of America; 1993.
23. Guirao A, Redondo M, Artal P. Optical aberrations of the human cornea as a function of age. J Opt Soc Am A Opt Image Sci Vis. 2000;17:1697-1702.
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