Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Objective type
Reexamination Certificate
2001-10-04
2003-01-28
Manuel, George (Department: 3737)
Optics: eye examining, vision testing and correcting
Eye examining or testing instrument
Objective type
C351S221000
Reexamination Certificate
active
06511180
ABSTRACT:
DESCRIPTION OF RELATED ART
Various objective techniques (retinoscopy, autorefraction, photorefraction) can be used to measure the spherical and cylindrical refractive errors of the human eye. They are fast and constitute an attractive alternative to performing a subjective refraction. Objective refraction is not only useful but often essential, for example, when examining young children and patients with mental or language difficulties. However, one major concern is the ability to properly determine objectively the refraction of the observer. Since all those objective methods are based on the light reflected from the retina and emerging from the eye, the ocular aberrations reduce the accuracy of the measurement. The eye suffers from many higher-order aberrations beyond defocus and astigmatism, which introduce defects on the pattern of light detected. Thus, photorefractive methods are based on paraxial optical analysis, and it has been shown that there can be a significant degree of measurement uncertainty when the spherical aberration of the normal human eye is considered. Aberrations also influence the retinoscopic measure. Although autorefractors provide reliable measurements of the refractive state of the eye, their limitations in accuracy and repeatability are well known. For example, there are discrepancies between autorefractive and subjective measurements, especially with astigmatism, or when the degree of ametropia is large. Such discrepancies are described in M. Elliott et al, “Repeatability and accuracy of automated refraction: A comparison of the Nikon NRK-8000, the Nidek AR-1000, and subjective refraction,”
Optom. Vis. Sci
. 74,434-438, 1997; J. J. Walline et al, “Repeatability and validity of astigmatism measurements,”
J. Refract. Surgery
15, 23-31, 1999; and A. M. Thompson et al “Accuracy and precision of the Tomey ViVA infrared photorefractor,”
Optom. Vis. Sci
. 73, 644-652, 1996. Also, retinoscopy and autorefraction usually disagree to some extent, as described in E. M. Harvey et al, “Measurement of refractive error in Native American preschoolers: Validity and reproducibility of autorefraction,”
Optom. Vis Sci.
77, 140-149, 2000.
Patients' preference with regard to autorefraction is described in M. A. Bullimore et al, “Patient Acceptance of Auto-Refractor and Clinician Prescriptions: A Randomized Clinical Trial,”
Visual Science and its Applications
, 1996 Technical Digest Series, Vol. 1, Optical Society of America, Washington, D.C., pp. 194-197, and in M. A. Bullimore et al, “The Repeatability of Automated and Clinician Refraction,”
Optometry and Vision Science, Vol
75, No. 8, August, 1998, pp. 617-622. Those articles show that patients prefer the clinician's refraction; for example, the former article states that the autorefractor has a rejection rate around 11% higher than the clinician. The difference in rejection rates suggests that autorefraction is less accurate than the clinician's prescription. Thus, the state of the art in autorefraction provides room for improvement.
In that context, the development of an objective method that makes use of the higher-order aberrations of the eye and provides accurate estimates of the subjective refraction is an important challenge. Such a method would be extremely useful, for example, to refine refractive surgery. As more patients inquire about refractive surgical procedures, the accurate measurement of refractive errors prior to surgery becomes more important in assessing refractive outcome. Another important issue regards the aging of the eye. A difference has been found between subjective refraction and autorefraction for different age groups. That difference probably comes from the fact that the ocular aberrations increase with age. The age dependency is described in L. Joubert et al, “Excess of autorefraction over subjective refraction: Dependence on age,”
Optom. Vis. Sci
. 74,439-444, 1997, and in A. Guirao et al, “Average optical performance of the human eye as a function of age in a normal population,”
Ophth. Vis. Sci
. 40, 203-213, 1999. An objective method that considers the particular aberration pattern of the subject would provide reliable estimates for all the age groups. Such a method could also automatically give a value of refraction customized for every pupil size and light level, since both aberrations increase with pupil size and visual acuity depends on luminance conditions.
The higher-order aberrations of the eye can be now measured quickly, accurately and repetitively, for instance, with a Shack-Hartmann sensor. A method and apparatus for doing so are taught in U.S. Pat. No. 5,777,719 to Williams et al. It seems then almost mandatory to optimize the use of this new information in a more accurate instrument.
The simplest case of vision correction is shown in
FIG. 1A
, in which the only error is of focus. That is, the rays R passing through the edge of the pupil
103
of the eye
102
are focused on a paraxial plane
110
which is spatially separated from the plane
112
of the retina. Accordingly, the only correction required is to shift the plane of focus from the paraxial plane
110
to the retinal plane
112
. The images before, at and after the paraxial plane
110
are shown as
124
,
126
and
128
, respectively.
The diagram in
FIG. 1B
shows an example of the image formation by a myopic eye
102
′ with negative spherical aberration. Without spherical aberration, all the rays R′ would focus on the paraxial plane
110
, and then the refraction of the eye would be calculated from the spherical negative lens required to displace the focus plane to the plane
112
lying on the retina. However, due to spherical aberration, the rays R passing through the edge of the pupil
103
converge at a plane
104
closer to the eye (marginal plane). That simple example shows how the distribution of rays in different planes produces images
114
-
122
with different quality. The refraction of that eye should be the one required for displacing a plane of “best image” to the retina.
A similar phenomenon occurs when astigmatism must be corrected. Depending on the higher-order aberrations of the eye, to maximize the image quality, the amount of astigmatism to correct could be different, beyond the paraxial zone, from that corresponding to the Sturm's interval (distance between the two focal planes determined by astigmatism).
Such a situation is shown in FIG.
1
C. The rays R″ and R′″ from different locations in the pupil
103
of the eye
102
″ have focal points which are not coincident or even coaxial. Thus, the images taken at various locations are shown as
130
,
132
and
134
. In cases in which not all aberrations can be corrected, or in which all aberrations can be only partially corrected, it is necessary to determine which of the images
130
,
132
and
134
is the best image.
The best image is not, or at least not necessarily, achieved by correcting the defocus and astigmatism corresponding to the paraxial approximation, which does not consider the effect of higher order aberrations. The question then is what is such a “best image.” By a geometrical ray tracing the answer is that the best image would correspond to a plane where the size of the spot is minimum. That plane, shown in
FIG. 1B
as
106
, is called the plane of least confusion (LC) and, for example, for a system aberrated with spherical aberration lies at ¾ of the distance between the paraxial and marginal planes. Another candidate is the plane
108
where the root-mean-square (RMS) radius of the spot is minimum. In the example of
FIG. 1B
, that plane lies midway between the marginal and paraxial planes. However, the spots determined geometrically do not accurately reflect the point spread function (PSF), which is the computed retinal image based on the results of the wavefront sensor and which should be calculated based on the diffraction of the light at the exit pupil. The distribution of light in a real image is usually very different from the image predicted
Guirao Antonio
Williams David R.
Blank Rome Comisky & McCauley LLP
Manuel George
University of Rochester
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