The most commonly used methods in the prior art for gaze tracking an eye (also referred to as measuring or monitoring the fixation direction of the eye) entail: (a) illuminating the eye using one of two methods and (b) acquiring and processing a video image of the illuminated pupil. In the first method of illuminating the eye, known as the "bright-pupil method," a light source is positioned so that it is nearly coaxial with respect to a line between a video camera and the eye. As a result, the pupil appears in the video image to be brightly lit. In the second method of illuminating the eye, known as the "dark-pupil method," the light source is positioned so that it is substantially off-axis with respect to the line between the video camera and the eye. As a result, the pupil appears in the video image to be dark when compared with the iris and other surrounding features. When using either of these two methods of illuminating the eye, another source of light, a point source, is located at a distance that is large when compared with the radius of curvature of the cornea. An image of the point source of light (referred to in the art as a corneal reflex) appears in the video image as a bright spot in the region of the pupil.
As is well known in the art, if the eye rotates, both the corneal reflex and the pupil move, but at different rates, whereas, if the eye merely translates, both the corneal reflex and the pupil move at the same rate. Thus, the relative distance between the corneal reflex and the center of the pupil is a measure of rotation angle or fixation angle of the eye. As a consequence of this, the most commonly used methods in the prior art for gaze tracking of the eye (measuring or monitoring the fixation direction of the eye) entail finding the center of the pupil. Typically, this is done by locating several points on the pupil/iris boundary and using straightforward geometry to determine the center of the pupil.
Since the corneal reflex is extremely bright and small, it is a straightforward matter to locate it. However, problems are frequently encountered in determining the center of the pupil. In the bright-pupil method, difficulties arise if the pupil is small. In such a case, the illuminated pupil will be rather dim and the contrast between the pupil and the iris will be low because the brightness of the pupil varies inversely as the square of the pupil diameter. Further difficulties arise in the bright-pupil method if the eye requires substantial refractive correction. In such a case, the brightness distribution in the pupil will be nonuniform, which nonuniformity creates problems in locating the pupil edges. In the dark-pupil method, difficulties arise if the contrast is low. Such low contrast occurs, for example, if the cornea or the lens act as diffuse scatters due to natural aging of tissues or the presence of cataracts. Further, depending on the wavelength of the illumination and natural pigment variations, the brightness of the iris may be naturally low. In either case, the pupil/iris contrast may be too low to determine the center of the pupil accurately.
A less frequently used method of gaze tracking the eye (monitoring or measuring the fixation direction of the eye) requires imaging features on the fundus of the eye, for example, the papilla (nerve head) or a distinctive pattern of blood vessels. Difficulties arise in this method because of cloudiness of the ocular media.
All of the above-described prior art methods rely on feature discrimination which arises from differential brightness of reflected light. However, these prior art methods make no distinction between reflected light originating from a structure of interest and reflected light originating from all scattering centers anterior to the structure of interest. This ambiguity results in loss of information. Furthermore, information about axial location of scatterers, i.e., depth of scatterers in the eye, is either nonexistent, or at best (using stereo optics) is difficult to extract.
In light of the above, there is a need for a method and apparatus for gaze tracking the eye which overcome the above-described problems in the prior art.