Image processing for improved eye-tracking accuracy

Video cameras provide a simple, noninvasive method for monitoring a subject's eye movements. An important concept is that of the resolution of the system, which is the smallest eye movement that can be reliably detected. While hardware systems are available that estimate direction of gaze in real time from a video image of the pupil, such systems must limit image processing to attain real-time perfor­ mance and are limited to a resolution of about 10arc minutes. Two ways to improve resolution are dis­ cussed. The first is to improve the image processing algorithms that are used to derive an estimate. Off­ line analysis of the data can improve resolution by at least one order of magnitude for images of the pupil. A second avenue by which to improve resolution is to increase the optical gain of the imaging setup (i.e., the amount of image motion produced by a given eye rotation). Ophthalmoscopic imaging of retinal blood vessels provides increased optical gain and improved immunity to small head move­ ments but requires a highly sensitive camera. The large number of images involved in a typical exper­ iment imposes great demands on the storage, handling, and processing of data. Amajor bottleneck had been the real-time digitization and storage oflarge amounts of video imagery, but recent developments in video compression hardware have made this problem tractable at a reasonable cost. Images of both the retina and the pupil can be analyzed successfully using a basic toolbox of image-processing rou­ tines (filtering, correlation, thresholding, etc.), which are, for the most part, well suited to implemen­ tation on vectorizing supercomputers.

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