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180 CHAPTER 5. SMALL-SCALE AIR-SEA INTERACTION<br />
5.1.7 Combined slope/height measurements of short wind waves : ISHG<br />
Participating scientists Roland Rocholz, Martin Schmidt<br />
Abstract A new technique for the simultaneous measurement of wave height and wave slope of<br />
water surface waves has been developed. The Imaging Slope/Height Gauge (ISHG) is an imaging<br />
system that is designed to measure waves at different length scales. Using a high speed camera, the<br />
high spatiotemporal resolution provides an insight into hydrodynamic processes such as microscale<br />
wave breaking and small scale wave-wave interaction.<br />
f 2<br />
high speed camera<br />
(lens set to infinity)<br />
lens (L 2 ) for telecentric<br />
imaging (f 2 = 50 cm)<br />
water body<br />
lens (L 1 ) for telecentric<br />
illumination (f 1 = 25 cm)<br />
diffusing screen<br />
rays are only<br />
for illustration<br />
(entrance pupil of the<br />
camera lies at the focal<br />
point of lens L 2 )<br />
f 2 (mean water height is at<br />
focal distance to L 2 )<br />
water height<br />
(6 to 16 cm)<br />
f 1 (diffusing screen is at<br />
focal distance to L 1 )<br />
programmable LED-array<br />
(320x1W, λ = 880nm)<br />
Figure 5.7: Sketch of the wave imaging system<br />
for the simultaneous measurement of waves height<br />
and slope.<br />
Background Since water surface waves<br />
strongly influence transfer velocities due to the<br />
induced turbulence, such wave measurements are<br />
used for monitoring the conditions in gas exchange<br />
experiments. Unfortunately, the slope calibration<br />
of typical imaging wave measurement techniques<br />
like the color imaging slope gauge used at the<br />
Heidelberg Aeolotron so far is depending on the<br />
wave height of larger waves. To overcome this<br />
source of systematic errors, a new technique is<br />
under development with the following aims:<br />
• The slope of the waves should be measured<br />
without biases due to the wave height<br />
• The image size should not change with wave<br />
height<br />
• The wave slope and height should be measured<br />
simultaneously<br />
• Simple set-up with a single camera<br />
Funding DFG, Ja395/13: ”Mechanismen des<br />
Gasaustausches zwischen Atmosphäre und Ozean:<br />
Laborversuche und Modellierung”<br />
Methods and results Like the color imaging<br />
slope gauge the slope measurement of the new<br />
Imaging Slope/Height Gauge (ISHG) is based on<br />
light refraction (shape from refraction) and makes<br />
use of an area-extended light source with defined<br />
spatial variation of radiance, produced by a programmable<br />
LED array. This Imaging Slope Gauge<br />
gives a 2-D slope map of the water surface. The<br />
Imaging Height Gauge gives the 2-D height map.<br />
This is also used to correct the slope map for<br />
height dependencies. The method is based on<br />
light absorption and works with light in the near<br />
infrared range. The optical setup provides a telecentric<br />
imaging. The camera is virtually located<br />
at infinity so that height variations of the water<br />
surface do not affect the image scaling.<br />
Outlook/Future work The ISHG system at<br />
our small wind wave flume is still under construction.<br />
The calibration and first wave measurements<br />
are expected to take place at the end of year 2005.<br />
It is planned to assemble the same system at the<br />
wind-wave tank of the University of Hamburg and<br />
at the wind-wave facility AEOLOTRON, University<br />
of Heidelberg. Combining the ISHG with infrared<br />
imaging will be the next step of extending<br />
the system.<br />
Main publication Jähne et al. [2005c]