Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Objective type
Reexamination Certificate
2000-10-20
2003-04-22
Lateef, Marvin M. (Department: 3737)
Optics: eye examining, vision testing and correcting
Eye examining or testing instrument
Objective type
Reexamination Certificate
active
06550917
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to measurement of the refractive error in the eye, more particularly to methods and techniques for compiling a topographic mapping of these refractive errors.
2. Description of Related Art
Measurements of aberrations in an eye are important for diagnosis of visual defects and assessment of acuity. These measurements and their accuracy become increasingly important in light of the growing number of ways, both surgical and non-surgical, that aberrations can be corrected. These corrections rely on accurate, precise measurements of the entire ocular system, allowing successful screening, treatment and follow-up. Enhancements in the accuracy of ocular measurements may aid in improving the identification of patients in need of correction and the performance of the correction itself.
There are a number of current methods used to measure performance of the ocular system. The most widely used and well established are psycho-physical methods, i.e., methods relying on subjective patient feedback. The oldest of the psycho-physical methods is the foreopter or trial lens method, which relies on trial and error to determine the required correction. There are psycho-physical methods for measuring visual acuity, ocular modulation transfer function, contrast sensitivity and other parameters of interest.
In addition to these subjective methods, there are also objective methods for assessing the performance of the ocular system. Such objective methods include corneal topography, wavefront aberrometry, corneal interferometry, and auto-refraction. Many of these methods only measure the contribution of specific elements to the total refractive error. For example, much work has been directed to measuring the topography of the cornea and characterizing the corneal layer. However, the corneal shape only contributes about 30-40% of the total refractive error in most cases. In order to measure the bulk of the refractive error and to provide a complete mapping for diagnosis and correction, additional information and measurements are needed.
Another method for determining the refraction of the eye is auto-refraction, which uses a variety of techniques to automatically determine the required corrective prescription. These automated techniques include projecting one or more spots or patterns onto the retina, automatically adjusting optical elements in the auto-refractor until the desired response is achieved, and determining the required correction from this adjustment. However, auto-refractors are not considered especially reliable. Further, auto-refractors measure only lower order components of the aberrations, e.g., focus and astigmatic errors.
Recently, the eye has started being considered as an optical system, leading to the application of methods previously used for other optical systems to the measurement of the eye. These methods include interferometry and Shack-Hartmann wavefront sensing. These techniques are of particular interest because they measure the complete aberrations of the eye. This additional information allows measurement of non-uniform, asymmetric errors that may be affecting vision. Further, this information may be linked with any of the various corrective techniques to provide improved vision. For example, U.S. Pat. No. 5,777,719 to Williams describes the application of Shack-Hartmann wavefront sensing and adaptive optics for correcting ocular aberrations to make a super-resolution retina-scope. U.S. Pat. No. 5,949,521 to Williams et al. describes using this information to make better contacts, intra-ocular lenses and other optical elements.
Wavefront aberrometry measures the full, end-to-end aberrations through the entire optics of the eye. In these measurements, a spot is projected onto the retina, and the resulting returned light is measured with an optical system, thus obtaining a full, integrated, line-of-sight measurement of the eye's aberrations. A key limitation of the instruments used in these measurements is the total resolution, which is ultimately limited by the lenslet array of the instrument. However, selection of the lenslet array is itself limited by several factors, most importantly the size of the spot projected onto the retina.
A schematic illustration of the basic elements of a two dimensional embodiment of a Shack-Hartmann wavefront sensor is shown in
FIG. 2. A
portion of an incoming wavefront
110
from the retina is incident on a two-dimensional lenslet array
112
. The lenslet array
112
dissects the incoming wavefront
110
into a number of small samples. The smaller the lenslet, the higher the spatial resolution of the sensor. However, the spot size from a small lenslet, due to diffraction effects, limits the focal length that may be used, which in turn leads to lower sensitivity. Thus, these two parameters must be balanced in accordance with desired measurement performance.
Mathematically, the image on the detector plane
114
consists of a pattern of focal spots
116
with regular spacing d created with lenslets
112
of focal length f, as shown in FIG.
3
. These spots must be distinct and separate, i.e., they must be readily identifiable. Thus, the spot size &rgr; cannot exceed ½ of the separation of the spots. The spot separation parameter N
FR
can be used to characterize the lenslet array
12
and is given by:
N
FR
=
d
ρ
(
1
)
The relationship between the size of a lens and the focal spot it creates, where &lgr; is the wavelength of the light, is given by:
ρ
=
1.22
f
λ
d
(
2
)
for a round lens or
ρ
=
f
λ
d
(
3
)
for a square lens. Thus, for a square lens, the separation parameter can be given by:
N
FR
=
d
2
f
λ
(
4
)
This is also known as the Fresnel number of the lenslet. To avoid overlapping focal spots, N
FR
>2. In practice, the Fresnel number must be somewhat greater than two to allow for a certain dynamic range of the instrument. The dynamic range of a Shack-Hartmann wavefront sensor can be defined as the limiting travel of the focal spot such that the edge of the spot just touches the projected lenslet boundary, given by:
θ
max
=
d
2
-
ρ
f
or
(
5
)
θ
max
=
d
2
f
-
λ
d
=
[
N
FR
2
-
1
]
λ
d
(
6
)
Thus, the dynamic range is directly proportional to the separation parameter and the lenslet size.
A particularly useful arrangement for a Shack-Hartmann wavefront sensor ocular measuring system places the lenslet array in an image relay optical system at a plane conjugate to the pupil or corneal surface. In this configuration, the spot size on the detector of the wavefront sensor is given by:
ρ
2
=
1
M
f
L
f
e
ρ
1
(
7
)
where M is the magnification of the imaging optics, f
L
is the focal length of the lenslet array, f
e
is the focal length of the eye and &rgr;
1
is the spot size on the retina.
Comparing Equations (5) and (7), it is evident that the dynamic range of the wavefront sensor is limited by the size of the spot &rgr;
1
projected on the retina. For a practical system, the dynamic range must be able to resolve errors in the optical systems. Thus, the dynamic range is a key limited parameter of the entire system design. In previous implementations of the Shack-Hartmann wavefront sensor used for ocular measurement, the dynamic range has been increased by increasing the size of each lenslet. However, the eye itself can have significant aberrations. Thus, any beam projected into the eye will become aberrated, spreading the focal spot and increasing the spot size &rgr;
1
on the retina.
Various techniques have been implemented to address this problem. A small diameter beam has been used so that the total wavefront error is minimized across the injected beam. Another proposed solution projects the light into the eye at the focal point of a long focal length lens, operating as a field lens so that the size of the focal spot is not affected by the eye aberrations. In practice, fo
Armstrong Darrell J.
Copland Richard J.
Neal Daniel R.
Topa Daniel M.
Lateef Marvin M.
Sanders John R.
Volentine & Francos, PLLC
WaveFront Sciences, Inc.
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