4SM illustrations normalization

Optical calibration, bathymetry, water column correction and bottom typing of shallow marine areas, using passive remote sensing imageries

Normalization of BOA reflectances

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Reflectances in %

Reflectances are dimensionless ratios.
Through the visible spectrum:

Reflectance of bright coral sand
- increases steadily from 0.28 to 0.65
Reflectance of dark red algae Sargassum
- is stable at 0.02 from 400 to 500 nm
- then peaks at 0.05 at 600 nm
- then decreases to 0.02 at 670 nm
- then rises sharply to 0.25 at 750 nm

Reflectance of terrigeneous sand at Mahares is ~1/3 that of coral sand

Reflectance of deep water exceeds that of dark seagrasses and algae below ~525 nm:

the negative bottom contrast case

Normalized reflectances in 4SM

dots: raw reflectances r in %
plain: normalized reflectances

At all wavelengths in 4SM:
- at the base of the atmosphere,
- L=(DN-La)*200/(DN-La)_{bright beach}

Color composite: through the normalization process,
we typically want to enhance
the remote sensing image
so that beach sands display as
shades of grey in any color composite.

Soil Line: then for all pairs of bandsthe slope of the Soil Line is 1.

Distortion: Because the reflectance of the reference-sand increases steadily through the visible spectrum, this normalization distorts ("morphs") the spectrum of spectrally contrasted bottom signatures. This might have consequences on the water column correction process in 4SM.