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3.3 Environmental preparation
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Space, walkway, light
control |
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Control frame: optimal
recommendation for the purpose of the lab |
Space of Room
Gait analysis requires space for the subject to walk in and to maintain a
reasonable camera-to-subject distance. Of course, larger space without any
obstruction would be better.
There are three factors to be considered, length of walkway, calibrated
volume and distance from calibrated volume to camera location.
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The length of walkway depends on the population and speed of gait. For
normal gait of adults, about 10 m of walkway is needed for the sampling of gait
with constant velocity excluding accelerating or decelerating phase of gait. For
children or pathologic gait patients shorter walkway can be used without any
problem.
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The calibrated volume is the volume of sampling for analysis. The
calibrated volume should encompass the range of movement of all markers during
the motion we are to analyze. Usually, at least 1.5 gait cycle (about 2m along
the walkway for normal adults and less for children or patients with pathologic
gait) should be included. About 1m or more would be good for the width of the
volume. For the height of the volume, it should be at least 2m if whole body
(including head) is interested and at least 1.2 m if for lower body only.
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There should be some distance between the calibrated volume and the
locations of cameras. This distance depends on the characteristics of cameras
used.
For example, a 4.8 mm lens on a 1/2 camera
provides a 69 degree angle on your lens. This calculates out to a 1 meter field
of view for every .73 meters of distance of the subject to the camera. Hence the following can be used for a 4.8 mm lens
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Field of View
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Distance to camera
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1 meter
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.73 meter
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2 meters
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1.46 meters
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3 meters
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2.18 meters
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A 6.0 mm lens provides an angle of 58 degrees or a D= .902 Field of View 8.0 mm
lens provides an angle of 44 degrees or a D= 1.25 field of view (from Dan India
posting to CGA-L).
But because cameras are located a little diagonally to the walkway rather
just perpendicular, more calibrated volume is sampled with less distance. And APAS/Gait
based on real video, which enables manual correction of merging
effect, can encompass more volume of calibration with shorter distance than
other infrared based systems.
Light control
APAS/Gait does not depend so much on light control as usual infrared based
system. It will be enough to shield the direct daylight from the windows. There
is no need to control any lights.
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