KINEMATICS - Biomechanics and Motor Control of Human Movement - page 58

The human error involved in this digitizing has been found to be random
and quite small. For a camera 4 m from a subject, the root-mean-square (rms)
“noise” present in the converted data has been measured at 1 - 1.5 mm.
3.3.4 Television ∗
The major difference between television and cinematography is the fact that
television has a fixed frame rate. The name given to each television image
is a field. In North America, there are 60 fields per second; in Europe, the
standard is 50 fields per second. Thus, television has a high enough field rate
for most movements, but it is probably too low for a quantitative analysis
of rapid athletic events. The f -stop, focus, and lighting for television can be
adjusted by watching the television monitor as the controls are varied. Many
television cameras have electronic as well as optical controls that influence
brightness and contrast, and some have built-in strobe lighting. Also, focus
can be adjusted electronically as well as optically. The major advantage of
television is the capability for instant replay, which serves both as a quality
control check and as an initial qualitative assessment. Second, the television
signal can be digitally converted by a “frame grabber” for immediate analysis.
3.3.4.1 Television Imaging Cameras. Some technical problems can result
from the use of standard vidicon television cameras. The strong signal from
a reflective marker produces a distinct circular image when the marker is
not moving. However, when there is a rapid marker movement, the circular
image blurs and produces a trailing edge. Thus, the triggering threshold for
conversion into two levels (black and white) must be carefully set to ensure
a circular digitized image. A more reliable way to get rid of the blur is to
use a strobe system, which results in the exposure of the TV imaging tube
for a millisecond or less. The strobe, in effect, acts as an electronic shutter.
Strobe systems also eliminate a second problem associated with a continu-
ously exposed imaging tube: skewing of the marker coordinates because of
the time delay in the scanning from the top of the image to the bottom of the
image. It takes about 15 ms to scan one TV field; thus, a head marker could
be scanned 10 ms before a foot marker. The strobe system freezes all marker
images at the same point in time in the same way that a movie camera does.
Newer charge coupled diode (CCD) cameras have mechanical or electronic
shutter controls that eliminate both blurring and skewing. A further devel-
opment is the infrared camera, which does not use visible light and is not
influenced by reflections from light sources other than those sources required
to get the desired circular reflection from the markers. Figure 3.12 shows a
typical infrared camera mounted permanently from the ceiling in a clinical
gait laboratory. The active infrared lights form a “donut” shape about the
camera lens and are pulsed at 120 Hz for a period of less than a millisecond.
∗ Representative paper: Winter et al., 1972.