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7.9.4
Measuring and Using the Eye-to-Camera Distance
Before putting the eye tracker in position, measure the distance between the camera and the
head coil mirror as well as the mirror to the eye. The sum of these distances is the eye-tocamera distance (ETCD) and will be used as discussed in Section 7.7 to:
1. Select the lens to use
The ETCD is used to select whether to use a 35 mm, 50 mm or 75 mm lens. These
operate at viewing distances of 50-90 cm, 90-120 cm and 100-150 cm respectively (see
Table 2). Affix the appropriate lens to the Camera Head by turning the threaded lens
into the Camera Head.
2. Set the lateral position of the Camera Head and Illuminator on the Mounting Bar. Use
the ETCD to set the separation between the Camera Head and Illuminator, by aligning
the edge of the Camera Head and Illuminator clips with the ETCD value marked on the
Mounting Bar scales. For more details see item number 1) in Section 7.7.2.
3. Focus the Illuminator by setting the edge of the Illuminator cover to the ETCD as
marked on the scale on the Illuminator’s top. For more details see item number 4) in
Section 7.7.2.
4. Align the Iluminator to maximally light where the Camera Head is pointed as detailed in
Section 7.7.3.
While measuring distances, also be sure to measure the eye to screen top and bottom, as well
as the width and height of the projected image. This time the measurement is in millimeters.
Use these values to follow “Section 8.4 Customizing Your PHYSICAL.INI Settings” and to modify
the PHYSICAL.INI file settings on your Host PC.
7.9.5
Align the Camera Head and Illuminator
Follow the instructions from Section 7.7.3 to align the Camera Head and the Illuminator so
that no matter where the camera is pointing it is well lit. Having performed the alignment
outside of the bore, put the eye tracker into position.
For an alternative approach to aligning the Camera Head and Illuminator with the mount in
place, see the following section.
7.9.6
Put a Volunteer/Confederate in the Scanner and Find the Eye(s)
To aid in the fine positioning of the camera and illuminator, have the Display PC projecting the
global image of the camera as you finely adjust the camera and illuminator position by moving
the Mounting Bar. Be sure it is the global image that is being projected as this will show the
entire range of the camera and make it easier to get the camera’s field of view to include the
eye.
At this point, it will be easiest to have a volunteer in the bore with the head coil and head coil
mirror in place, as though ready to be scanned. The volunteer should be able to report line of
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Long Range Mount Installation
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