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Imaging Fluorescent Dye Concentrations on Soil Surfaces

Uncertainty of Concentration Estimates

Jan Vanderborght^*,a, Paul Gähwiller^,b, Hannes Wydler^b, Ute Schultze^b and Hannes Flühler^b

^a Laboratory of Soil and Water, Katholieke Universiteit Leuven, Vital Decosterstraat 102, B-3000 Leuven, Belgium
^b Soil Physics, Inst. of Terrestrial Ecology, Swiss Federal Institute of Technology, ETHZ, Grabenstrasse 11a, CH-8952 Schlieren, Switzerland

View larger version (24K): [in a new window]   Fig. 1. Schematic of the image acquisition setup.

View larger version (53K): [in a new window]   Fig. 2. Outline of the image processing steps to derive dye concentrations from the fluorescence image. Left-hand side panels are images of the fluorescence signal of brilliant sulfaflavine (BF) on horizontal cross sections of four soil cores from the Unterehrendingen soil (upper and lower cores in the panel are taken from the topsoil and subsoil, respectively): A(x) is the raw fluorescence signal; Aff(x) is the flat field corrected fluorescence signal; Affr(x) is the background subtracted fluorescence signal corrected for variations in soil surface reflection; and Ct(x) is the total concentration pattern. Right-hand side panels represent correction factors <F(x)>/F(x) for inhomogeneous illumination (flat field correction); correction factors R0/Rff(x) for variations in excitation light reflection at the soil surface; and calibration relations between corrected fluorescence and total dye concentration, Ct.

View larger version (26K): [in a new window]   Fig. 3. Calibration relations for different soil layers and the two dyes, brilliant sulfaflavine and sulforhodamine B: total dye concentration in the calibration sample, Ct, is plotted vs. the noncorrected fluorescence signal Aff, and vs. the fluorescence signal corrected for variations in light reflection at the soil surface by Eq. [5] Affr, and by Eq. [7] Affr.

View larger version (61K): [in a new window]   Fig. 4. Corrections of the brilliant sulfaflavine fluorescence signal for variations in light reflection at the soil surface (soil core from the 0.5- to 0.6-m layer of the Unterehrendingen soil): a) noncorrected signal, Aff(x); b) corrected signal without subtracting the background fluorescence before the correction, Affr(x); c) corrected signal with background fluorescence subtracted before correction, Affr(x); d) corrected signal with background fluorescence subtracted and fluorescence and reflection images smoothed by a 9 by 9 median filter before correction, ffr(x). Right-hand side panels are pixel values along a transect (marked in Panel b).

View larger version (17K): [in a new window]   Fig. 5. Illustration of the background fluorescence signal, detection limit, and typical dye fluorescence signal.

View larger version (12K): [in a new window]   Fig. 6. Range of correction factors for variations in excitation light reflection at the surface of soil cores from the 0- to 0.1-m, 0.2- to 0.3-m, and the 0.5- to 0.6-m layer of the Unterehrendingen soil for the excitation light wavelengths of brilliant sulfaflavine (BF) and sulforhodamine B (SB). Horizontal bars mark the interval that is defined in Eq. [10] and that contains 90% of the correction factors.

View larger version (15K): [in a new window]   Fig. 7. Summary of uncertainty factors of the estimated brilliant sulfaflavine (BF) and sulforhodamine B (SB) concentrations and of correction factors normalized to one with which the flat field corrected fluorescence signal, Aff(x), is multiplied to obtain the concentration estimates on soil surfaces in cores from the Unterehrendingen soil.