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Generation of Higher Order Aberrations with Soft Contact Lens by Excimer
Laser Ablation
Geunyoung Yoon*, Keiu Tokumura, Takahisa Jitsuno, and Masahiro Nakatsuka
INTRODUCTION
METHOD
Although the human eye has a relatively simple
optical system, it is not perfect. It has been well known
that the human eye suffers from the wavefront aberration
that includes both lower (defocus and astigmatism) and
higher order aberrations. The lower order aberration has
been effectively corrected with conventional ophthalmic
lenses such as spectacles and contact lenses. However, it
is more complicated to correct for the higher order
aberrations. Ocular wavefront sensing technique [1]
allows us to precisely understand optical quality of the
eye by measuring the wave aberrations. It also makes it
possible to correct the higher order aberration using
advanced methods such as adaptive optics [2], laser
refractive surgery [3] and customized optics [4]. Optical
and psychophysical tests have demonstrated that
correcting these aberrations significantly improves visual
performance even in normal eyes. The visual benefit is
even more substantial when correcting the aberration in
eyes having abnormal corneal conditions such as
keratoconus (abnormal cone shape cornea) and
penetrating keratoplasty (corneal transplant).
Adaptive optics is a powerful and noninvasive tool
to achieve higher order correction in real time. However,
it is an impractical method simply because of the size of
the entire system. Although refractive surgery has been
proven to be practical and effective to correct the
aberration, it is a non-reversible surgical method and its
availability is restricted by factors such as corneal
thickness and the amount of the aberration. The interest
in using customized optics to correct the aberration has
been increased. This special optics includes phase plates,
customized contact lenses and intraocular lenses (IOLs).
Phase plate (spectacles capable of correcting the higher
order aberration) has been proven to be an effective way
to compensate for the static aberrations in the eye. This
special optical component has an arbitrary surface profile
created to correct the eye’s aberrations on a plate made
of optical plastic. The successful correction of the higher
order aberration resulted in significant visual
improvement in both normal and abnormal eyes.
However, we also found that decentration of the eye’s
pupil due to changes in a gaze angle results in poor
correction performance causing reduction of visual
benefit. To overcome this limitation, customized soft
contact lenses which follow most of the eyes’
movements was proposed as an alternative way to
correct the aberration.
This report presents the feasibility of generating the
higher order aberration with soft contact lenses of which
surface was ablated by an excimer laser.
A laser ablation algorithm was developed to
calculate the laser pulse distribution to create an arbitrary
pattern of the higher order aberration. Convolution
theory was used to design effective overlapping of laser
spots. The algorithm allowed for minimizing both the
residual RMS error and surface roughness by
determining laser ablation parameters such as beam size,
ablation rate and transition zone. With this algorithm, an
ArF2 pulse laser (Xantos S-200 ArF, Coherent) was used
to ablate the surface of soft contact lenses. The laser spot
diameter was 500 µm and the laser pulse fluence was 0.2
mJ that produced an estimated ablation depth per pulse
of 0.05 µm on hydrated soft contact lens material. Figure
1 shows a schematic diagram of the apparatus for the
laser ablation with the excimer laser. A telescope with
convex and concave lenses was used to demagnify the
original laser beam. A two-axes steering mirror was used
to scan laser spots onto targeted locations on the surface.
Pulse repetition rate was 40 Hz limited by scanning
speed of the steering mirror. After the ablation, the
higher order aberrations of the soft contact lenses
immersed in saline solution were measured with a
Shack-Hartmann wavefront sensor to evaluate the
accuracy of the laser ablation [5].
Fig. 1. Optical layout to ablate a surface of soft contact
lens with an ArF excimer laser.
RESULT
Theoretical calculation with the laser ablation
algorithm indicated that a proper transition zone
(extrapolated from the ablation profile of the optical
zone) of at least half of the laser spot diameter can
significantly reduce residual ablation error along the
edge of the optical zone. The calculations for different
amounts of overlapping spots also showed the tradeoffs
between the amplitudes of the residual error and surface
roughness. Both the residual error and surface roughness
were decreased with the increased amount of spot
overlapping. However, with too much overlapping, the
residual RMS error was increased again. This is because
of the fact that an ablation rate per pulse can not be
infinitely small. Therefore, it is important to precisely
determine the ablation parameters that minimize the error
and roughness. In our experiment of ablating the surface
of soft contact lenses, we created different individual
Zernike’s higher order aberrations, coma (3rd order),
trefoil (3rd order) and spherical aberration (4th order) with
1-µm RMS error. These aberrations were chosen since
they have been measured as the most dominant higher
order aberrations in eyes with abnormal conditions. The
measured RMS values of the ablated soft contact lenses
for coma, trefoil and spherical aberration were 1.02 µm,
0.85 µm and 0.97 µm, respectively. Although those
targeted aberrations were accurately generated with the
laser ablation, small but, significant amounts of other
undesirable higher order aberrations were also observed.
The residual higher order RMS wavefront errors of these
samples were 0.26 µm, 0.31 µm, and 0.24 µm,
respectively.
differences in wavefront shapes are caused by the
induced other higher order aberrations.
CONCLUSION
We successfully demonstrated the feasibility of
using an ArF excimer laser to produce the higher order
aberration on the surface of soft contact lenses. The same
methodology can be used to ablate the back surface of
soft contact lenses. Back surface customized soft contact
lenses can have the potential to significantly reduce the
ocular higher order aberration and to improve the
stability of lens position after blinking, which provides
stable correction of vision especially in eyes with
abnormal corneal conditions.
ACKNOWLEGEMENT(S)
This research was supported by NIH Grant No.
5R01EY14999 and a RPB Grant.
REFERENCE(S)
[1] Liang J, Grimm B, Goelz S, Bille J, “Objective
measurement of wave aberrations of the human eye with
the use of a Hartmann-Shack wave-front sensor”, J. Opt.
Soc. Am. A 11, 1949-1957 (1994).
[2] Yoon G, Williams DR “Visual performance after
correcting the monochromatic and chromatic aberrations
of the eye”, J. Opt. Soc. Am. A 19, 266-275 (2002).
[3] Yoon G, Jeong TM, Cox IG, Williams DR. “Vision
improvement by correcting higher-order aberrations with
phase plates in normal eyes” J. Refract. Surg.; 20 (5),
S523-527 (2004).
[4] MacRae SM, Applegate RA, Krueger RR “The future
of customization”, Customized Corneal Ablation; The
Quest for Supervision, Slack Inc., Chapter 30, 339-346
(2001).
[5] Jeong TM, Menon M, Yoon G. “Measurement of
wave-front aberration in soft contact lenses by use of a
Shack-Hartmann wave-front sensor” Appl. Opt.; 44 (21)
4523-4527 (2005).
*Department of Ophthalmology and Institute of Optics,
University of Rochester
Fig. 2. Wavefront maps for target and ablated aberrations,
(a) spherical aberration, (b) trefoil, and (c) vertical coma.
Wavefront maps for targeted and ablated aberration
were also shown in Fig. 2. There was a good agreement
in both amplitudes and shape between the maps. Slight