Apparatus for visualizing the eye's tear film

Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Objective type

Reexamination Certificate

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Reexamination Certificate

active

06447119

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a device for evaluating the curvature or shape of the cornea of the eye, and more particularly, to a corneal measurement device that assists with pre-operative or post-operative measurements of the cornea, with contact lens fitting and with the diagnosis of diseases of the cornea. Additionally, the present invention relates to an ophthalmic instrument for visualizing disruptions in the eye's tear film which covers the cornea.
BACKGROUND OF THE INVENTION
The cornea, being the front surface of the eye, provides its major refracting surface and is important to quality vision. Recently, a number of corneal surgical techniques have been developed for correcting visual deficiencies, such as near-sightedness, far-sightedness and astigmatism. In order to assist with such surgical techniques, a number of devices have been proposed or developed to evaluate the topography, i.e., the shape or curvature, of the cornea. In addition, such corneal topography techniques are useful for fitting contact lenses and for the diagnosis and management of corneal pathologic conditions, such as keratoconus and other ectasias. For example, prior to performing a corneal surgical technique to correct a refractive error, the patient is preferably screened using a corneal topography device to rule out the possibility of subclinical keratoconus.
Corneal topography is typically measured using a series of concentric lighted rings, known as a keratoscope pattern
5
, shown in FIG.
1
. In one typical embodiment, shown in
FIG. 2
, keratoscope pattern
5
is created by a keratoscope target
10
, consisting of illuminated concentric rings which emit light rays which are projected onto the cornea of a patient's eye
15
. Light rays
12
,
20
are reflected off patient's cornea
15
, and a portion of light ray
20
is captured by an objective lens
25
and focused onto an imaging system
30
, such as a video camera. A computer
35
is utilized to compare the image captured on imaging system
30
with a stored reference pattern, or other known information, to identify any distortions in the captured image and thus calculate any deformations in the patient's cornea.
While conventional corneal topography devices have achieved significant success, such devices suffer from a number of limitations, which, if overcome, could significantly enhance their accuracy and utility. In particular, earlier designs for topography devices have incorporated large keratoscope targets, causing the overall size of the prior art devices to be quite large. In an operating room or a doctor's office, however, where space is at a premium, it is desirable to minimize the overall size of the topography device.
In addition, commercially available topography devices, such as the design illustrated in
FIG. 2
, typically measure the topography of only a relatively small area of the cornea. For example, in the design shown in
FIG. 2
, the light beam is emitted from a large, flat, backlit keratoscope target
10
and is then reflected off cornea
15
. Thereafter, a portion of light
20
reflected off cornea
15
is focused by small objective lens
25
at the center of keratoscope target
10
onto imaging system
30
, such as a CCD chip. Additional light rays
12
reflected from the peripheral portions of cornea
15
, however, are not captured by objective lens
25
and are therefore not imaged onto imaging system
30
. Therefore, such prior art devices are unable to measure the peripheral cornea.
To overcome this problem, prior art devices have attempted to capture the light rays reflected from the peripheral portions of cornea
15
by designing a keratoscope target
10
′ in the shape of a cylinder or cone, as shown in
FIG. 3
, encompassing the peripheral cornea. In this manner, light rays emitted by cylindrical or conical keratoscope target
10
′ will form a pattern
5
of illuminated rings which will be reflected off cornea
15
. The reflected light rays, including light rays reflected off the peripheral portions of cornea
15
, will be captured by objective lens
25
and imaged onto imaging system
30
. To be effective, however, cylindrical or conical keratoscope target
10
′ must be positioned very close to the eye, and thereby tends to impinge on the patient's brow and nose. In addition to being potentially uncomfortable and potentially contributing to the spread of disease, the close approach of keratoscope target
10
′ makes the design very error-prone, as a slight error in alignment or focusing causes a large percentage change in the position of the keratoscope rings relative to the eye.
In addition, current systems tend to provide poor pupil detection and do not accurately measure non-rotationally symmetric corneas, such as those with astigmatism. The location of the pupil is particularly important in planning surgical procedures for correcting visual deficiencies. In current systems, pupils are typically detected by deciphering the border of the pupil from the image of the keratoscope rings. This is particularly difficult with conventional designs, however, as the intensity transition from the black pupil to a dark iris is minimal compared to the intensity transition from a bright keratoscope ring image to a dark interring spacing. As a result, the pupil detection algorithms in current systems often fail.
Furthermore, current systems have difficulty detecting the edges of the keratoscope rings and difficulty separating ring images from background iris detail. Conventional corneal topography systems image the iris along with the keratoscope rings, as know as “mires”. Particularly in patients having light-colored irises, however, the bright reflection from iris detail obscures the rings, thereby making detection of ring edges difficult. Finally, conventional devices utilize high intensity visible light to illuminate the keratoscope target and therefore cause discomfort to the patient. The high intensity light is required because relatively little light is actually reflected from the cornea and captured by the measuring devices.
As is apparent from the above discussion, a need exists for a more compact corneal topography device. Another need exists for a topography system that allows a large area of corneal coverage without the focusing problems and invasive approach of previous designs. A further need exists for a system incorporating improved pupil detection by using an image that does not include the keratoscope rings. Yet another need exists for a topography device providing improved separation of the corneal reflection of the keratoscope pattern from the iris detail. A further need exists for a topography system utilizing light levels that are not unpleasant for the subject undergoing measurement. An additional need exists for a topography device that permits accurate measurement of non-rotationally symmetric corneas, such as those with astigmatism.
SUMMARY OF THE INVENTION
Generally, according to aspects of the present invention, a method and apparatus for measuring the topography of the cornea are provided. The method and apparatus utilize a virtual image of a keratoscope pattern or other diagnostic pattern, which is projected at a desired distance in front of the patient's eye. Since the topography is evaluated with a virtual image, there is no nose or brow shadow, allowing better coverage of the cornea and providing a design which is relatively insensitive to focusing errors.
In certain embodiments, however, it has been found preferable to position the image of the keratoscope pattern at some other location. For example, a virtual object of the keratoscope pattern may be formed just behind the cornea such that after being reflected from the surface of the cornea is re-imaged just in front thereof. Likewise, however, distortions in the cornea are observed in the reflected real image of the keratoscope pattern.
The disclosed topography system includes a structured light source, preferably consisting of an illumination sou

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