Optics: measuring and testing – By polarized light examination – With polariscopes
Reexamination Certificate
2002-05-31
2004-03-09
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
By polarized light examination
With polariscopes
C351S215000
Reexamination Certificate
active
06704106
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to polarimeter systems for measuring polarization properties of light and more particularly to an ophthalmological system for measuring the birefringence of structural elements in the eye.
2. Description of the Related Art
The polarimeter is well-known in the optical arts and is reviewed here briefly to establish some of the terminology required for this disclosure. A single-beam polarimeter measurement usually consists of an optical signal in a single state of polarization. Some form of “analyzer” within the polarimeter removes all but a single state of polarization from the incoming light, which is then measured and recorded with a suitable detector as a charge-coupled device (CCD). A series of measurements is usually made with a different state of polarization being recorded for each. These measurements allow both the degree and orientation of the optical signal polarization to be estimated and recorded. The single rotatable analyzer passes only light polarized parallel to a specified axis so the analyzer must be rotated about the optical beam axis to measure light polarized in different directions. A single fixed analyzer passes light polarized parallel to its axis and cannot be rotated but a polarization rotator such as, for example, a half-wave plate, may be placed in the optical beam axis to rotate the plane of polarization of the incoming optical signal before it reaches the fixed analyzer. Light polarized in different directions can thus be measured by rotating the half-wave plate.
A half-wave plate has a preferred (“fast”) axis. Light polarized parallel to this axis passes through the half-wave plate unchanged. Light polarized perpendicular to the fast axis (parallel to the “slow” axis) is retarded by half a wavelength. The net effect of this is to rotate the plane of polarization of the light so that the axis of the half-wave plate bisects the angle between the planes of polarization in the incoming and outgoing light. Using similar reasoning, it may be shown that the net effect of a precise one-quarter wavelength retardance is to bias the linear polarization components of the entering light into equivalent circular polarization components, as is well-known in the art.
The single beam polarimeter is exemplified by the polarimeter
20
shown functionally in FIG.
1
. The optical signal
22
arrives along the optical beam axis
24
and the half-wave plate
26
rotates the original polarization angle
22
to a new angle
28
by means of the position of its fast axis
30
. The fixed analyzer
32
then blocks all of optical signal
28
except for the particular component
32
parallel to the analyzer axis
36
, which is then received by the detector
38
. Detector
38
may then generate an electrical signal
40
representative of the intensity of the optical signal
34
.
FIG. 2
shows a reference direction
42
aligned with analyzer axis
36
of fixed analyzer
32
within an arbitrary focal plane at detector
38
. The orientation of rotating half-wave plate
26
is specified by the difference angle
44
between reference direction
42
and half-wave plate axis
30
. The combination of fixed analyzer
32
and rotating half-wave plate
26
can be thought of as equivalent to a single rotating analyzer that rotates twice as fast as half-wave plate
26
. As shown in
FIG. 2
, the anti-clockwise angle
44
from reference direction
42
to half-wave plate axis
30
is doubled to give the effective analyzer position
46
. Thus, by rotating half-wave plate
26
over a 180-degree range, the effective analyzer position
46
is rotated over a complete 360-degree cycle.
References to birefringence herein refer to intrinsic birefringence or form birefringence, a property of a material that causes a change in the polarization of light which passes through it. Birefringence has two components; orientation (or axis) and magnitude. Form birefringence is found in materials consisting of a substantially parallel array of many small cylindrical structures that are small with respect to the wavelength of the light passing through it. Such form birefringence is a measurable property of the retinal nerve fiber layer (RNFL) that is useful for determining RNFL thickness. Form birefringence is also a measurable property of the Henle fiber layer that is similarly usefull for determining Henle fiber layer thickness.
Knowing the thickness of a patient's RNFL can be crucial in diagnosing glaucoma and other optic nerve diseases. The RNFL birefringence introduces retardance into any polarized beam of light passing through the RNFL when the beam polarization axis is neither parallel nor perpendicular to the nerve fiber bundles making up the RNFL. Birefringence is an optical property associated with the anisotropy of a medium through which polarized light propagates, and is manifested by the retardance of some components of the light resulting from variation of light velocity in the medium with propagation direction and polarization axis. When light propagates perpendicularly to the optic axis of an anisotropic material, the two orthogonally-polarized (S and P) components of the light, one with polarization parallel to the fast axis and the other with polarization perpendicular to the fast axis (parallel with the “slow” axis), travel through the material at different velocities, introducing a phase shift between the two components. This phase shift is known in the art as retardation or retardance and is herein denominated “retardance.”
A beam of light entering a patient's eye encounters the retina and scatters back from it. The polarization state of the emerging directly-backscattered light changes based on the amount of retardance between the two S and P components. A retardance map can be generated based on the backscattered light that represents the thickness of the RNFL and, hence, that is useful for diagnosing maladies of the eye.
Accordingly, the commonly-assigned U.S. Pat. Nos. 5,303,709, 5,787,890, 6,112,114, and 6,137,585, entirely incorporated herein by reference, disclose laser diagnostic devices that measure the thickness of the RNFL by measuring the amount of retardance of laser light in the RNFL layer, with the amount of retardance then being correlated to RNFL thickness in accordance with principles known in the art. Likewise, the so-called Henle fiber layer, which includes photoreceptor axons and which has radially distributed slow axes centered about the fovea in the macula of the eye, is also form birefringent and consequently, its thickness also can be measured for diagnostic purposes using laser light.
However, portions of the eye (hereinafter collectively denominated “anterior segments”) that are anterior to the retinal nerve and Henle fiber layers may also be birefringent. For example, both the cornea and lens are birefringent. Moreover, the axial orientation and magnitude of birefrigence of the anterior segments may vary significantly from person to person. Because a diagnostic beam must pass through these anterior segments, the laser beam retardance caused thereby must be accounted for, to isolate the retardance of posterior segments such as the retinal nerve fiber and Henle fiber layers. When measuring RNFL birefringence from the front of the eye, a compensating device is needed to remove the retardance contribution of the anterior segments from the birefringence measurement.
The above-mentioned U.S. Pat. No. 5,303,709 disclosed a corneal compensator for neutralizing the effects of the birefringence of anterior segments of the eye on a diagnostic beam meant to measure the thickness of the RNFL. The compensating structure of the '709 patent includes a polarization-sensitive confocal system attached to a scanning laser retinal polarimeter. The detector of this apparatus includes a pinhole aperture set to be conjugate with the laser source and the posterior surface of the crystalline lens so that only reflected light from the posterior surfaces of the crystalline lens is captured a
Anderson Michael
Papworth William
Zhou Qienyuan
Jester Michael H.
Laser Diagnostic Technologies, Inc.
Pham Hoa Q.
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