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, the 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 the patient's eye 15. Light rays 12, 20 are reflected off the patient's cornea 15, and a portion of the light 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 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 the cornea 15. Thereafter, a portion of the light 20 reflected off the cornea 15 is focused by a small objective lens 25 at the center of the keratoscope target 10 onto the imaging system 30, such as a CCD chip. Additional light rays 12 reflected from the peripheral portions of the cornea 15, however, are not captured by the objective lens 25 and are therefore not imaged onto the imaging system 30. Therefore, such prior art devices are unable to measure the peripheral cornea.
To overcome this problem, prior devices have attempted to capture the light rays reflected from the peripheral portions of the 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 the cylindrical or conical keratoscope target 10' will form a pattern 5 of illuminated rings which will be reflected off the cornea 15. The reflected light rays, including light rays reflected off the peripheral portions of the cornea 15, will be captured by the objective lens 25 and imaged onto the imaging system 30. To be effective, however, the 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 the 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. 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.