determine the relationship between the phase structures (time lag) of light
intensity time series records measured at specified locations within the field of
view of the camera. It is assumed that the physical relationship between the
visual signature of the wave in image space and the true wave in object space is
spatially constant.
Equipment Installation and Experiment
Preparation
The data acquisition component of the CIIS comprises several commercially
available video components. Synchronized SonyTM SSC-54A color video
charge-coupled device cameras are mounted at vantage points above the physical
model on support beams. Synchronization of the cameras is required for
simultaneous data acquisition from each camera view. Manual iris NavitarTM
lenses with focal lengths of 8 and 6mm provide the desired degree of spatial
resolution. Figures 20 and 21 show pixel resolutions for a typical imaged area in
the physical model for camera stations located 3.8 m (12.5 ft) above the free
surface for the 8 and 6mm lenses, respectively. For example purposes, the
cameras are located arbitrarily in a rectilinear coordinate system. The 8mm lens-
camera combination is oriented with a tilt angle of 55 deg off nadir and a roll
angle of 0 deg, and it has a horizontal field of view of 40 deg. This camera
orientation results in < 2-cm pixel resolution in the direction perpendicular to the
camera azimuth over the entire imaged area. Pixel footprints in the direction of
the camera azimuth are highly resolved in both the near and far fields (< 4 cm).
The 6mm lens-camera combination has a tilt angle of 48 deg, a roll angle of
0 deg, and a horizontal field of view of 54.6 deg. The shorter focal length
provides a wider field of view and slightly lower resolution. Cross-azimuth pixel
resolution is < 2 cm over the imaged area, and along-azimuth resolution is
< 4 cm.
Camera-lens combinations are calibrated at the Field Research Facility,
Duck, NC, Coastal and Hydraulics Laboratory, U.S. Army Engineer Research
and Development Center, to determine lens distortion coefficients and optical
characteristics of a given camera-lens combination prior to installation. Cameras
identified as 1 and 2 in this report have 6mm focal length lenses, and cameras
identified as 3 through 6 have 8mm lenses. Each camera is connected to a
PanasonicTM AG-1980P videocassette recorder (VCR) via BeldenTM 8281F
composite video cable. Super Video Home SystemTM (SVHS) cable is desired
for increased videotape resolution; however, long cable lengths between cameras
and VCRs (>30 m) dictated the use of composite video cable to prevent loss of
signal strength. The video signal from each camera passes through a
recorded by the VCR at a frame rate of 30 Hz. VITC time code is recorded on
each video frame to allow identification of a specific frame of interest during
VCR playback and image digitization.
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Chapter 5 Video-Based Wave Direction Measurement
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