KINEMATICS - Biomechanics and Motor Control of Human Movement - page 62

called OPTOTRAK. Such a system consists of three cameras mounted in
line on a rigid frame, as shown in Figure 3.14. The left and right lenses are
mounted to face slightly inward and their linear diode arrays are mounted
horizontally. Thus, their scan of the pulsed light will define the location of a
marker in a vertical plane. The middle lens is mounted with its diode array
mounted vertically, and its scan will define a horizontal plane. Figure 3.15
depicts this arrangement. The left and right detectors each define the location
of all markers in a vertical plane; the intersection of these two vertical planes
is a vertical line. Thus, any markers on this vertical line will record the same
signal on the left and right cameras. The middle camera has its lens facing
directly ahead with its diode array mounted vertically. Thus, this camera will
define all markers in a horizontal plane. The intersection of this horizontal
plane with the vertical line defined by the other two cameras is a unique 3D
point in space. Thus, as each infrared diode (IRED) pulses, its x , y , z coor-
dinates in the global reference system (GRS) are recorded. The pulsed light
from a second light source yields a different vertical line and horizontal plane
and, thus, a different set of x , y , z coordinates. There are some unique advan-
tages to such an active system. There is no specialized software required (as
in television) to identify which marker is which. Thus, in laboratories where
Figure 3.14 An OPTOTRAK system with three lenses, each with a linear diode array.
The outside two lenses face slightly inward and each defines a vertical plane, while the
middle lens defines a horizontal plane. See Figure 3.15 to see how these three diode
arrays define a marker in 3D space. (Courtesy of the Gait and Posture Laboratory,
Department of Kinesiology, University of Waterloo, Waterloo, Ont., Canada.)