Number of Pixel Sampling Elements and CIIS Arrays

Structures 3 & 4 Pixel Array Element Locations:Camera 1

Figure 33. Example 10-sec-duration timestack recorded in the diffraction region in the diffraction region during the Structure 4 monochromatic wave experiment

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Table 5

Number of Pixel Sampling Elements and CIIS Arrays

Collocated CIIS

Sampling

Experimental

Arrays

Camera View

Arrays

Elements

Arrangement

no effort was made to remove information contained in the original analog

signals. During the playback process, the image processor collects time series

records of pixel intensity at locations corresponding to measurement array

elements. The software reads VITC on each video frame to locate the image

records of interest.

Pixel intensity time series records are called timestacks. Pixel intensities are

recorded as unsigned numerical representatives of gray values ranging from 0

(dark) to 255 (bright). Figure 33 shows an example timestack collected at a

sample rate of 30 Hz for duration of 10 sec during a random wave test. Bright

regions of the timestack are associated with the wave crest and wave slope facing

the illumination source. Dark regions of the timestack are associated with the

wave trough and slope of the wave facing away from the illumination in the

shadow of the wave.

Time series analysis

As previously mentioned, the premise for use of CIIS as a wave direction

measurement system is that, for waves propagating past a fixed point on the free

surface, the intensity of directional specular reflection from the surface of the

wave slope varies from light to dark. It is assumed that, outside of the wave-

breaking region, the phase structure of the timestacks is a proxy for the phase

structure of the true propagating wave signal. To exemplify this assumption,

Figure 34 shows synchronized time series records of reflected light intensity and

wave height from a collocated pixel sampling element and capacitive wave rod.

These measurements were obtained during a Structure 4 monochromatic wave

test in the diffraction region, and temporal variations from peak to trough are

strongly correlated.

Chapter 5 Video-Based Wave Direction Measurement

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