Subject Seating

Any stationary chair can be used, but to the extent that the type of chair minimizes subject motion, eye-tracking accuracy will be maximized. Align the subject so a level gaze looks at about the center of the display screen. A chair with adjustable seat height, such as a secretary’s chair, may help in making this adjustment.

For testing the system, you can use the model eye target bar (a black plate with a white 4 mm spot and ball bearing). Place the model eye so the white spot is about where a subject’s eye will be and focus the camera on the white spot. The model eye is useful for practicing set up and discrimination of the pupil and corneal reflection (CR) without requiring a cooperative subject.

Use the velcro band to place the MHT sensor on the subject’s head, with the sensor over the eye (usually the right) that will be tracked (Figure 3-1). It is possible to place the sensor anywhere on the subject’s head that is deemed convenient, although usually over the forehead is the simplest.

6.2 Obtaining an Eye Image

Check the “Illuminator” check box on the Eye Tracker Interface main screen to turn on the illuminator, and raise the illumination level by moving the slider labeled “Illuminator” to the right about half way. Be sure the “Pan-tilt Tracking” radio button, near the bottom left of the Interface screen, is set to manual and that the switch on the control unit rear panel is in the “P/T” position.

Pull down the “Pan-tilt” menu and select “Setup” (Figure 5-6). The “Zoom”, “Gain”, and “Iris” slide switches should be all the way to Max and the “Shutter” slide switch should be at Long. These are the default positions for these slide switches. It is best to have them set to the default settings when first attempting to acquire a subject’s eye image. Changes in these settings take effect immediately and remain set until they are changed. If the program is closed and re-started, the last settings, not the default settings, will be in effect.

6.2.1 Aiming the Camera

With the Illumination on, use either the infra red remote control or the arrows on the Pan-tilt/ Setup window to aim the pan-tilt camera at the eye (or model eye). (See section 3.3.2 on the use of the camera wide angle lens in aiming the camera.) The inner ring of arrows move the module a short distance for each click, while the outer arrows move the module a greater distance for each click. If the “Reverse left/right” and “Reverse up/down” boxes near the bottom right of the Pan-tilt/ Setup window are not checked, clicking an arrow moves the eye monitor

image in the direction of the arrow. If the “Reverse” boxes are checked then the pan-tilt camera will move in the direction of the arrow and the image (in the eye monitor) will appear to move in the opposite direction (as thought the “window” through which the image is being viewed were moving in the direction of the arrow). Camera movements may also be reversed using the hand-held control. The user should experiment to find out which setting of the “reverse” camera directions will be the least confusing to use during calibration of the subject.

Once the subject’s face is centered in the wide angle view, use the hand-held control to make small movements of the camera until the subject’s eye is visible in the Eye Monitor. The image of the eye may be blurry at this point. As the eye comes into view in the Eye Monitor, tapping the arrow keys may be necessary to center the image of the eye.

6.2.2 Focusing the Camera

The camera auto-focus is “on” when the camera is first powered up. Switch the auto- focus to “off” using the hand-held control. There are two rocker switches on the hand-held control, one for fast movement of the field of focus, the other for slow movement (Figure 6-1). Pressing the top of the switch moves the image closer to the camera (T - Telephoto). Pressing the bottom button moves the image away from the camera (W - Wide angle). Slow movements are much easier to use. Using the rocker switches, it should be possible to bring the image of the subject’s eye clearly into focus in the Eye Monitor.

If the camera is unable to focus, move the “Zoom” slide in the Pan-tilt/ Setup window to

the left, toward Min, until focus is restored. The zoom slide will not need to be moved far; it is probably best to move it by clicking the left arrow once or twice.

You should now see a clearly-focused image of the subject’s eye in the Eye Monitor. Once the pan-tilt module is aimed at the eye or (or the model eye), pull down the Pan-tilt menu and select “Set Home.” This will cause the program to memorize the current position of the pan-tilt module. Subsequently selecting “Home” from the Pan-tilt menu will cause the module to

return to this memorized position. The Home position can be helpful for reacquiring the subject’s eye when the system is not configured with the MHT option. The “home” button on the Pan-tilt remote control does not correspond to the Home settings in the Eye Tracker Interface menu and is not a substitute for it. Users with the MHT option can skip this step.

If the MHT option is not being used, skip to Section 6.4.

6.2.3 Enabling the MHT System and Setting Sensor to Eye Offset

This procedure does not require that valid Pupil and CR recognition be established, but the eye image must be clearly focused and centered in the Eye Monitor.

Once the image of the eye is in focus in the Eye Monitor, completing the calibration described here allows the system to compute a vector connecting the center of the MHT sensor with the center of the eye. This allows the system to calibrate for differences in the shape of a subject’s head, the location of the eye relative to the forehead, and the sensor position with respect to the eye. This procedure must be done for each subject.

It is assumed the System Settings dialog window, under the Configure menu, is set to “Pan-tilt Optics with MHT option” (Section 4.2.4 and Figure 4-3), and the MHT Transmitter and the Sensor-to-Pan-Tilt calibration procedures have been completed (Section 5.2).

The mounting surface of the sensor and the head band worn by the subject are equipped with mating Velcro. The sensor should be mounted just above the eye being monitored (Figure 3-1).

Enable the magnetic head tracker, using the MHT pull-down menu (Section 6.2.3 and Figure 4-4). Once the MHT system has been Enabled, the Sensor-to-Eye vector shortcut button turns black (Figure 6-2).

With the subject’s eye image centered and focused in the Eye Monitor, click on the Sensor-to-Eye vector shortcut button. If the MHT system has not been Enabled, this button will be greyed out and clicking will have no effect. The eye image should be centered in the Eye Monitor when the shortcut button is clicked, not when “OK” is clicked in response to the “Sensor Calibration Complete” message.

The Sensor-to Eye vector button represents the X, Y, and Z axes of the MHT system. Clicking on the button causes the Eye Tracker program to calculate a vector from the MHT sensor to the center of the subject’s eye. Once the vector has been established, the program uses the location of the sensor to “find” the subject’s eye, should it be moved out of the camera’s field of view. The Sensor-to-Eye Offset can also be set using the Pan-Tilt/ Setup window “Set” button (Figure 5-6), but this is a much less convenient method.

A pop-up box will display “MHT Sensor Calibration Complete” and Pan-Tilt Tracking at the bottom of the Eye Tracker main screen will automatically switch to “Auto.” Click “OK.”

The camera should now be able to reacquire the subject’s eye automatically following a large head movement.

If the system does not have a magnetic head tracker, leave Pan-tilt tracking set to manual until proper pupil discrimination is achieved as described in section 6.3.