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Introducing a mini-catamaran to perform reflectance measurements above and below the water surface

Jean-Marie Froidefond and Sylvain Ouillon

Open Access Open Access

Abstract

An innovative platform is tested to perform reflectance measurements at sea. This platform is a mini-catamaran with two hulls 1m long and set 0.7m apart, fitted with optical sensors. It can be used far away from an oceanographic ship to avoid its hull influencing the measurement. Reflectance measurements were performed with a TriOS radiance sensor placed +2cm or -2cm from the water surface and a TriOS irradiance sensor. Tests were carried out in calm seas and with cloud cover. The processing to derive marine radiances from raw measurements is detailed. When the radiance sensor is above the interface, it limits the sky reflections on the targeted surface and the radiance is identical to that deduced from measurements below the surface. When the sensor is placed at +3cm above-water or higher, glint affects the measurements. The mini-catamaran shows a good ability to measure marine reflectance with an adapted measurement protocol. Except for very turbid waters, it seems preferable to perform upwelling radiance measurements below the surface.

©2005 Optical Society of America

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Figures (9)
Fig. 1.
Fig. 1. Calibration radiance spectra measured using the FieldCal device.

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Fig. 2.
Fig. 2. Measurement platform fitted with TriOS irradiance and radiance sensors.

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Fig. 3.
Fig. 3. Testing device comprised of an aquarium placed on a white reflectance Spectralon® plate and in the shade to avoid major reflections on the glass tank.

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Fig. 4.
Fig. 4. Spectral radiances obtained following the schema of Fig. 3. The below-water spectrum L(λ,-1) corresponds to a non-corrected radiance.

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Fig. 5.
Fig. 5. Irradiance measured at Aiguillon under a covered sky on 17 February 2004 between 10:44 and 10:46 am and between 10:50 and 10:52 am.

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Fig. 6.
Fig. 6. Upwelling radiance measured at +2cm±1cm (17 red curves) and at -2cm±1cm (13 blue curves, uncorrected radiance) simultaneously with an irradiance measurement presented in Fig. 5. Aiguillon, 17 February 2004.

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Fig. 7.
Fig. 7. Radiance measured above-water at +3cm (7 red curves) and below the sea surface at -2cm (33 blue curves, uncorrected measurements), Teychan, 18 February 2004.

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Fig. 8.
Fig. 8. Remote-sensing reflectance calculated from irradiance and radiance shown in Fig. 5 and 6 at Aiguillon on 17 February 2004: Rrs=Lw/Ed with Lw computed from Lu(-2) in blue (measurements taken between 10:44 am and 10:46 am) and Rrs=L(+2)/Ed in red, measurements taken between 10:50 am and 10:52 am).

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Fig. 9.
Fig. 9. Remote-sensing reflectance and corresponding concentrations in Total Suspended Matter (S) and in chlorophyll-a (chl) for 7 stations in the Bay of Arcachon, 17 and 18 February 2004.

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Equations (13)

Equations on this page are rendered with MathJax. Learn more.

R ( λ , z ) = Eu ( λ , z ) ⁄ Ed ( λ , z )
Eu ( λ , 0 − ) = Eu ( λ , z ) ⁄ e − [ Ku ( λ ) . z ]
R ( λ ) = f . [ b b ( λ ) ⁄ ( a ( λ ) + b b ( λ ) ) ]
a ( λ ) = a w ( λ ) + a ch ( λ ) + a s ( λ ) + a y ( λ )
b b ( λ ) = b b w ( λ ) + b b ch ( λ ) + b b s ( λ )
Rrs ( λ , θ , ϕ ) = Lw ( λ , θ , ϕ , 0 + ) ⁄ Ed ( λ , 0 + )
Lw ( λ ) = t ⁄ n 2 * Lu ( λ , 0 − )
L t ( λ , 0 + ) = L w ( λ ) + ρ L sky ( λ ) + Δ L ship ( λ )
θ water = arcsin [ ( 1 ⁄ n ) * sin θ air ]
Ω IFOV = 2 π ( 1 − cos θ )
Lw ( λ ) = 0.98 * L ( λ , − 2 )
Rrs ( λ ) = 0.98 * L ( λ , − 2 ) ⁄ Ed ( λ , 0 + )
Ed ( λ , z ) = Ed ( λ , 0 − ) . e − [ Kd ( λ ) . z ]
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