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of 
In the field the water was filtered over Whatman GF/C 
filters, followed within 1 day by a filtration over 
.2 yin membrane filters. Organic compounds passing 
this filter are here defined as aquatic humus. 
Absorption spectra of humus were measured with a 
Perkin-Elmer 551S double-beam spectrophotometer, 
using cuvettes of .01 or .1 m. Reflection spectra, 
the compound system humus and water, were measured 
with a multispectral scanner, built from a 
grid-monochromator and a photodiode array. Signals 
from both instruments were handled by a 
Hewlett-Packard 86B computer. 
An aquarium was used to measure reflection spectra. 
A parallel beam from a slide-projector at an angle of 
8° with the vertical was used as a light source. This 
beam passed the layer of water of .345 m and was 
diffusely reflected by a white bottom plate. Radiance 
emitted from the aquarium was measured. 
In underwater optics usually the attenuation 
coefficient for a particular wavelength is used, 
implying both light absorption and scattering. For 
the present samples light scattering was 
insignificant. In the spectrophotometric measurement 
this could be checked using an integrating sphere; in 
the reflection geometry the measured radiance was 
dominated by the light reflected by the white plate, 
given the small depth of the water column. So in this 
work we use the spectroscopic definition of 
extinction coefficient, as applied to clear 
solutions. 
The wavelength range considered here is 300-800 nm. 
The short-wavelength cut-off is chosen such as to 
avoid contributions to the absorption from inorganic 
compounds; the upper limit followed from instrumental 
restrictions. 
3 RESULTS 
The absorption spectrum of freshwater humus deviated 
significantly from the exponential funtion with 
d = -.014 nm 1 . The extinction, on a logarithmic scale, 
of one sample is given in figure 1. In the_same 
figure the model result, with d = -.014 nm 1 and 
Xo = 440 nm, is given. 
In the spectrum shown in figure 1 the difference 
between the measured extinction and the exponential 
function is about 50% at 600 nm. This difference is 
of the same size as the water extinction at that 
wavelength, and is therefore not negligible. 
Another illustration of the error introduced by 
application of the exponential function (1) is the 
wavelength ( nm) 
Figure 1. Difference between measured absorption 
spectrum and exponential model. The extinction scale 
is logarithmic. The dashed line is the exponential, 
with d = -.014 nm 1 and X 0 = 440 nm, normalized to the 
measured extinction at 440 nm. 
Figure 2. Illustration of the variability of the 
absorption spectrum, on a logarithmic scale, of 
freshwater humus from different locations in The 
Netherlands. The spectra were normalized at 440 nm. 
The dashed line is the exponential model, with d = 
-.014 nm 1 and Ao =440 nm. 
following, simplified, example of a depth measurement 
in the aquarium from remotely sensed data. Reflection 
spectra from a dilution series of humus with 
demi-water, in the aquarium, were measured at a water 
depth of .345 m. It was found that the reflected flux 
decreased exponentially with the sum of extinctions 
by water and the variable amount of humus. This 
indicates that in this case the scattering of the 
water sample is insignificant. 
From this exponential dependence the bottom depth 
can be calculated, using the reflection at two 
arbitrary wavelengths, 500 nm and 630 nm. Other 
inputs for the calculation were: a constant and known 
bottom reflection, water absorption at 500 nm and 630 
nm according to Smith and Baker (1981), and the ratio 
of the humus extinction at these two wavelengths. Two 
ratios were used: the ratio of the measured 
extinction of the humus sample shown in figure 1 and 
the ratio based on the exponential model, with 
d = -.014 nm 1 . 
Table 1 contains the result of the depth 
calculation for the dilution serie. As a parameter 
for the humus dilution the humus extinction at 500 nm 
was used. The calculated depths, using the measured 
humus extinction and the exponential function, and 
the relative error, when the exponential function was 
used, are given. 
Table 1. Error introduced by the exponential humus 
absorption spectrum in an experimental depth 
measurement. Depth was calculated from reflection 
data, using directly measured extinction values and 
values derived from the exponential function, for the 
humus dilution serie. The dilution of the humus is 
represented by a(500), the extinction at 500 nm. 
Relative errors are shown in the last column. 
a(500) 
depth 
rei.error 
measured 
model 
ratio 
ratio 
m 1 
m 
m 
% 
3.55 
.353 
.545 
58 
2.67 
.344 
.492 
43 
1.32 
.354 
.435 
26 
0.62 
.340 
.382 
11 
0.10 
.348 
.362 
5 
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4 DISCUSSU 
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classifie 
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26:671—6f 
Smith, R.C. 
of the' c] 
Applied ( 
Zepp, R.G. 
photochen 
in water: 
substance