22 December 1992 Focusing of sunlight by sea-surface waves: new measurement results from the Black Sea
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Proceedings Volume 1749, Optics of the Air-Sea Interface: Theory and Measurement; (1992); doi: 10.1117/12.138865
Event: San Diego '92, 1992, San Diego, CA, United States
Abstract
Under clear skies, strong fluctuations in the downwelling irradiance, Ed, prevail in shallow water as a result of the focusing and defocusing of sunlight by surface waves. Such temporal fluctuations were measured in the Black Sea, usually at a depth of 1 m, from a fixed platform located 600 m off the coastline. A method of thresholding analysis was applied to 109 time-series records of Ed (525 nm), each of which lasted 10 min. The frequency of occurrence of flashes of intense foci (intensity exceeding the time-averaged irradiance, Ed, by > 50%) decreased exponentially with increasing flash intensity. The frequency and intensity of flashes, hence the slope of the exponential relationship, all varied with wind-wave conditions and atmospheric lighting conditions. The best conditions for wave focusing were characterized by light winds of 2 to 5 m s-1, solar elevation > 40 degree(s), and diffuseness of surface irradiance < 40%. Then, at a depth of 1 m, the flashes > 1.5 Ed occurred at rates as high as 6 Hz. The most intense flashes exceeded Ed 5-fold at rates of 10-3 Hz. These results, which are consistent with our previous observations, substantially improve the database on still poorly documented wave focusing effects.
© (1992) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Jerzy Dera, Slawomir Sagan, Dariusz Stramski, "Focusing of sunlight by sea-surface waves: new measurement results from the Black Sea", Proc. SPIE 1749, Optics of the Air-Sea Interface: Theory and Measurement, (22 December 1992); doi: 10.1117/12.138865; https://doi.org/10.1117/12.138865
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Light sources and illumination

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