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while users of search coil systems must worry about slippage of the coil relative to the
eye. Retinal imaging, on the other hand, provides a direct measure of what is on the
retina. In the scanning laser ophthalmoscope (SLO) [8], a laser beam provides
illumination for imaging retinal structures, while simultaneously allowing modulation
of the beam intensity to deliver patterned stimulation. Recent SLO designs have in-
corporated adaptive optics to correct the eye’s aberrations [9], enabling the acquisition
of diffraction-limited images of retinal structures, and images of the foveal cone mosaic.
Using an adaptive-optics-enhanced SLO (AOSLO), it has been shown that the
functional foveal “center” (defining the line-of-sight) has a slightly different position
for steady fixation target versus smooth pursuit of a moving target [10].
Today, the majority of eye monitoring systems in general use are based on digital
images of the front of the eye, captured with a remote video camera and coupled with
image processing and machine vision hardware and software. Such systems are called
passive eye monitors, or video-based eye-trackers. When these systems first appeared,
they generally required special-purpose image-processing hardware to allow real-time
measurement of gaze. Today, thanks to the steady increase of microprocessor power, it
is possible to do a decent job entirely with software.
Passive eye monitoring technology became popular by being completely remote
and non-intrusive, while offering reasonable accuracy at an affordable cost. It can be
slower (sampling at about 30 to 60Hz in most of the times) and less accurate than
invasive eye monitoring techniques, but provides a more natural experience for the
subjects. In many applications, the available accuracy may be adequate to answer the
questions of interest. This technology, for example, allows a disabled person to
compose electronic messages, browse and navigate through web pages, turn on-off
monitors, or call an assistant. Installed in a motor vehicle, a passive eye monitor can
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