Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Objective type
Reexamination Certificate
2000-06-27
2001-07-10
Manuel, George (Department: 3737)
Optics: eye examining, vision testing and correcting
Eye examining or testing instrument
Objective type
C600S452000
Reexamination Certificate
active
06257723
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to devices and methods for mapping the topography of an eye, such as the human eye, for purposes such as fitting a contact lens to the eye, pre- and post-operative evaluation of the eye, and diagnosis of abnormalities of the eye, such as astigmatism, keratoconus, and corneal warpage induced by contact lenses. More specifically, this invention relates to devices and methods that utilize elevation measurements of the eye to improve slope measurements of the eye, and that utilize the improved slope measurements in combination with the elevation measurements to provide enhanced mapping of the topography of the eye.
BACKGROUND
One conventional device for mapping the topography of an eye is referred to as a “Placido.” As shown in respective front and side views in
FIGS. 1A and 1B
, a Placido
10
typically includes a series of illuminated concentric rings
12
. In order to map the topography of the eye
14
, the Placido
10
is positioned in alignment with the eye
14
so the rings
12
reflect off the tear film on the cornea
16
and travel through an aperture
18
in the Placido
10
to a camera
20
that records images of the reflected rings
12
(to clarify the illustration,
FIG. 1B
depicts the reflection of only some of the rings
12
off the cornea
16
). Analysis of these recorded images, including analysis of the shape and position of the reflected rings
12
, provides an approximation of the slope of the eye
14
at the points on the eye
14
where the rings
12
were reflected. A surface suitable for display can then be mathematically “fit” to the approximate slopes at these points using various techniques well-known to those of skill in the art.
As shown in more detail in a top view in
FIG. 2
, analysis of the recorded image of a point P on one of the rings on the Placido
10
reflecting off the cornea
16
of the eye
14
, passing through the aperture
18
in the Placido
10
, and striking a Charge Coupled Device (CCD)
30
in the camera
20
at point I proceeds as hereinafter described. A central portion
32
of the cornea
16
enclosed by the innermost ring of the Placido
10
reflecting off the cornea
16
is approximated by fitting the portion
32
with a partial sphere having a radius of curvature R
0
. Also, the apex point E
0
of the cornea
16
is assumed to have a Normal
36
(i.e., an orthogonal vector) that is coincident with the optical axis
38
of the camera
20
. The point E
1
on the cornea
16
where point P reflects off the cornea
16
is then approximated by assuming a constant curvature between ring edges on the cornea
16
.
Using this “constant curvature” technique, a radius of curvature R
1
and coordinates (x
1
, z
1
) are determined iteratively for point E
1
such that a Normal
40
at point E
1
has equivalent angles of incidence a and reflection &THgr;. The surface of the cornea
16
between points E
0
and E
1
is then assumed to be a partial sphere having radius of curvature R
1
. This process is repeated until (x,z) coordinates and a Normal are approximated for all points of reflection of the rings of the Placido
10
off the cornea
16
. Knowledge of the Normal of each of these points then permits the calculation of a slope at each point and, in turn, the fitting of a surface to the points as previously described. More information regarding the general operation of Placidos may be found in U.S. Pat. No. 3,797,921 to Kilmer et al.
Because the described Placido utilizes certain assumptions about the eye being measured that are not necessarily true, namely, that the curvature of the cornea between successive Placido rings is constant, and that the surface normal at the apex of the cornea is coincident with the focal axis of the camera, the Placido is not as accurate as is desirable. Consequently, other techniques have been devised for more accurately mapping the topography of an eye.
One such technique, referred to as “ORBSCAN™,” was introduced by the Assignee of the present invention, Orbtek, Inc. of Salt Lake City, Utah, and is disclosed and claimed in U.S. Pat. Nos. 5,512,965 and 5,512,966 to Snook. As shown in a top view in
FIG. 3
herein, in this technique, a first slit beam
50
of light is stepped from right to left across an eye
52
that is to be mapped, and a second slit beam of light (not shown) then steps from left to right across the eye
52
. When the slit beam
50
reaches the anterior surface
54
of the cornea
56
of the eye
52
, it splits into two components: a specular reflection
58
from the anterior surface
54
of the cornea
56
, and a refracted beam
60
that penetrates the cornea
56
and is refracted (i.e., bent), in accordance with Snell's Law, by the index of refraction between air and the cornea
56
. The specular reflection
58
serves no purpose in this technique.
The refracted beam
60
is scattered within the cornea
56
by a mechanism known as diffuse scattering. Reflections
62
from the intersection point C
ant
between the diffusely scattered refracted beam
60
and the anterior surface
54
of the cornea
56
, and reflections
64
from the intersection point C
post
between the diffusely scattered refracted beam
60
and the posterior surface
66
of the cornea
56
, then travel through the focal point of a lens
68
to impinge on a CCD
70
of a camera
72
at respective points L
ant
and L
post
. Because the relative positions of the light source (not shown) for the slit beam
50
, the eye
52
, the lens
68
, and the CCD
70
are known, the reflections
62
impinging on the CCD
70
at known point L
ant
allow calculation of the space coordinates (x
ant
,y
ant
, z
ant
) of the point C
ant
. Also, the reflections
64
impinging on the CCD
70
at known point L
post
, as well as knowledge of the index of refraction between air and the cornea
56
, allow calculation of the space coordinates (x
post
, y
post
, z
post
) of the point C
post
. A similar diffuse reflection
74
from the lens
76
of the eye
52
, and from the iris
78
of the eye
52
(diffuse reflection not shown from the iris
78
), along with knowledge of the index of refraction between the cornea
56
and the anterior chamber
83
of the eye
52
, allow calculation of the space coordinates (x, y, z) of points along the respective anterior surfaces
80
and
82
of the lens
76
and the iris
78
. Of course, the second slit beam works in the same manner to measure space coordinates (x, y, z) as the first slit beam
50
.
By stepping a pair of slit beams across the eye
56
from left to right and from right to left, this technique allows the direct measurement of space coordinates (x, y, z) for thousands of points on the anterior
54
and posterior
66
surfaces of the cornea
56
, and on the anterior surfaces
80
and
82
of the lens
76
and the iris
78
. Surfaces suitable for viewing can then be mathematically fit to these known points as previously described. Since no assumptions are made regarding the shape of the cornea
56
, lens
76
, or iris
78
, the technique more accurately portrays the surfaces of these parts of the eye
52
.
Unfortunately, inaccuracies exist in this technique as well. In particular, limitations in the density of pixels on the CCD
70
, and errors in the relative positions of the slit beam
50
, the eye
52
, the lens
68
, and the camera
72
, limit the accuracy of the measurements using this technique typically to about ±2 &mgr;m (micrometers or “microns”).
Therefore, there is a need in the art for an improved device and method for mapping the topology of an eye.
DISCLOSURE OF THE INVENTION
In a preferred method of the present invention for mapping the topography of an eye, elevation measurements of the eye are collected using a slit beam diffuse reflection system, such as the ORBSCAN™ device previously described. An approximating b-spline surface is then fitted to the elevation measurements. Slope measurements of the eye are collected using a Placido as previously described, but the slope measurements are referenced to points on the b-spline
Broadus Charles R.
Sarver Edwin J.
Bausch & Lomb Surgical, Inc.
Manuel George
Trask Britt Law firm
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