KINEMATICS - Biomechanics and Motor Control of Human Movement - page 63

Figure 3.15 OPTOTRAK system with two outside lenses facing slightly inward with
their diode arrays, each defining markers in a vertical plane. Any marker on the inter-
section of these two planes will define all markers on this vertical line. The middle
lens array defines all markers in a horizontal plane. Thus, the intersection of a marker
on this horizontal plane with the vertical line will define the unique coordinates of a
marker in 3D space.

the number and location of markers is changing from day to day, there are
no problems with marker labeling, which makes this system flexible for the
changing research requirements. Also, because of the precision of the IRED
array, the precision of the x , y , z coordinates is better than that of TV systems,
which are constrained by the distance between the scan lines. The precision
for an OPTOTRAK camera mounted as shown in Figure 3.15 at a distance of
4 m from the subject is 0.03 mm with noise 0.015 mm (Gage et al., 2004).

Possible disadvantages are the number of IREDs that can be mounted and
the potential encumbrance of the cables connecting the power source to the
active IREDs.

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3.3.6 Advantages and Disadvantages of Optical Systems

Advantages

1. All data are presented in an absolute spatial reference system, in a plane
normal to the optical axis of the camera.
2. Most systems (cine, TV) are not limited as to the number of markers
used.
3. Encumbrance to movement is minimal for most systems that use
lightweight reflective markers (cine, TV), and the time to apply the
markers is minimal.
4. TV cameras and VCRs are reasonably inexpensive.
5. Cine and TV systems can be replayed for teaching purposes or for
qualitative analysis of the total body movement.