Argus image types and conventions
Three types of images are gathered from the Argus cameras. Every hour, a snapshot, a time exposure and a variance image are collected for each of the stations cameras. A snapshot serves as simple documentation of conditions, but offers little quantitative information. The time exposure images provide us with much more information. The ten minute time exposures of the nearshore wave field average out natural modulations in wave breaking to reveal a smooth band of white which has been shown to be an excellent proxy for the underlying, submerged sand bar topography (Lippman and Holman, 1989 and Van Enckevort and Ruessink, 2001, Argus literature). The third type, the variance image, displays the variance of the light intensity signal during the same ten minutes of time exposure. Variance images help identify regions which are changing in time, from those, which may be bright in the time exposure, but are unchanging in time.
Besides time-averaged video data, data sampling schemes can be designed to collect time series of pixel intensities, typically at 2 Hz, with which wave and flow characteristics can be investigated. Pixel intensity time series show the fluctuations in time of the intensity in a pixel. A time stack image is an image in which the intensity of an array of pixels is plotted against time. To illustrate, the figure below shows the strong correlation which is commonly observed between a time series of pixel intensities and the wave height signal obtained from a wave gauge at the same location (Lippmann and Holman, 1991, Argus literature). The small phase lag between the two indicates that maximum intensities correspond to the white, foam-covered face of the breaking wave, which precedes the passage of the actual wave top. Comparisons for non-breaking waves also show strong coherence, but often a larger, still fixed, phase difference.