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Spotlight on Optics


  • March 2013

Optics InfoBase > Spotlight on Optics > Robust approach to directly measuring water-leaving radiance in the field

Robust approach to directly measuring water-leaving radiance in the field

Published in Applied Optics, Vol. 52 Issue 8, pp.1693-1701 (2013)
by ZhongPing Lee, Nima Pahlevan, Yu-Hwan Ahn, Steven Greb, and David O’Donnell

Source article Abstract | Full Text: XHTML | Full Text: PDF

Spotlight summary: In this paper, Lee et al., have solved a long standing problem associated with measuring in-situ water leaving radiance, namely the need to model processes associated with the air-sea interface. Obtaining water leaving radiance is critical to vicariously calibrate and validate (cal/val) remotely sensed radiance measured by Earth Observing satellites such as MODIS and VIIRS. The latter need to be vicariously calibrated because sensor characterization prior to satellite launch is insufficient to obtain the desired accuracy.

Currently, the methods of choice to obtain water-leaving radiance are variations on two approaches. One obtains the water-leaving radiance below the interface, extrapolates it to above the surface using measured or modeled attenuation and then corrects for interface effects (e.g. Snell’s law). Alternatively, measurements are obtained above the water. The above-water measurements, however, suffer from contributions of sun and skylight radiances that are specularly reflected from the wave modulated air-sea interface into the above water detector, and hence need to be removed. A combination of judicious measurement angle relative to the sun, combined with radiative transfer calculations (assuming idealized sky conditions), is employed to remove those contributions. In any case, both approaches, introduce significant uncertainties to the water-leaving radiance.

The method proposed and tested by Lee et al. solves these problems using commercial instrumentation already used by many labs, namely radiometers mounted on a buoy. The crux of the approach is to position the upwelling radiance meter outside the water and to surround it by a cone (similar to a dog collar) reaching down into the water, such that sun and skylight do not contribute directly to the measurement (e.g. only the component that has interacted with the water is measured). This approach, which was first described by co-author Y. H. Ahn in an internal report in 1999, is implemented and tested in the current manuscript.

This novel method is associated with two sources of uncertainty, namely (a) under some conditions spectra collected are unacceptable (wave induced motions cause cone to pop out of the water or submerge the radiometer), and (b) self-shading by the system decrease the measured radiance compared to the ‘true’ water leaving radiance. While Lee et al., provide effective means to deal with (a) and with (b) to first-order, further research is needed to bound the uncertainty associated with self shading (a problem shared by all existing systems).

This work is a significant step forward in improving in-situ measurements of water-living-radiance, necessary for Ocean Color cal/val activities. I have no doubt it will stimulate novel and exciting research to further constrain the uncertainties involved. I have little doubt many in the Ocean Color community will adopt this method or, in the least, test it to compare against the methods they have used to date.

--Emmanuel Boss

Technical Division: Information Acquisition, Processing, and Display
ToC Category: Atmospheric and Oceanic Optics
OCIS Codes: (010.0010) Atmospheric and oceanic optics : Atmospheric and oceanic optics
(010.4450) Atmospheric and oceanic optics : Oceanic optics
(120.0120) Instrumentation, measurement, and metrology : Instrumentation, measurement, and metrology
(120.0280) Instrumentation, measurement, and metrology : Remote sensing and sensors

Posted on March 22, 2013

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