Humphrey Field Analyzer II - i
General Information
Confidential and Proprietary
52235C1105
1 - 11
Field Service Guide
1.7.2
Gaze Tracking
The
HFA II - i
uses two systems for measuring patient fixation: the standard Heijl-Krakau blind-
spot monitoring and the IR Gaze Tracking System. Both methods can be used, either together
or alone, or they can both be turned off, as required. This description covers the IR Gaze
Tracking System.
The direction of a patient's gaze is determined in two steps: first, a reflex marker is established on
the corneal surface; and second, the location of the pupil center is determined.
Gaze tracking is initialized in the following manner when a selected test is first started:
The patient is asked to fixate on the central illumination LED. Gaze tracking turns on the reflex
gaze IR LED located just under the diamond fixation pattern and turns off eye illumination
briefly . Light from the LED is reflected off the cornea, and back to the IR sensitive camera
(Figure 1-1). The majority of the cornea appears black except for the reflected spot. This image
is digitized and stored in memory. The reflected spot is referred to as the reflex marker
(Figure 1-2). Because the corneal surface is rounded, the reflex marker will move very little even
if the patient's eye rotates, and thus the marker becomes a (relatively) stationary reference point.
Next, the system locates the pupil center by illuminating the entire eye with the two IR LEDs
located either in the bottom of the bowl, or in the trial lens holder (when in the raised position).
The iris appears bright with a dark pupil (Figure 1-3). This image is also digitized and stored in
memory. It is the relationship between the location of the reflex marker on the cornea and the
location of the pupil center that determines fixation (Figure 1-4).
Note - When gaze tracking is being initialized, it appears as repetitive "strobing" when
viewed by the operator via the video insert on the HFA II - i monitor.
During a test, each time a spot is projected into the bowl, the locations of the reflex marker and
the center of the pupil are compared to the initial images stored in memory. If the patient is
fixating correctly, the positional relationship between the reflex marker and the pupil center will
be the same as that of the stored images (Figure 1-4). If the patient is off fixation, the positional
relationship between the reflex marker and the pupil center will be different, as in Figure 1-5.
The greater the misalignment, the higher the mark on the Gaze Graph (Figure 1-6).
(continued)
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