68 
Everybody using the fluorescence intensity thus has the 
problem to find a good approximation for the baseline. The 
usual approximation is the most simple one, that is a straight 
line. In this case the problem is reduced to the problem of 
finding two suitable points which define this line. For the 
evaluation presented here, the straight-line-approximation 
was made, and the fixpoints of the baseline were choosen as 
reflectance values at 663.5 nm and 737.5 nm. At these wav 
elengths, the reflectance spectra have minima, and these 
minima do not change their position when chlorophyll con 
centration changes. Figure 4 illustrates the construction of 
the baseline and gives the definition for the terms line height 
(LH) and fluorescence line height (FIJI). 
4.2 Fluorescence Intensity 
Since the (correct) baseline represents the reflectance of a 
non-fluorescing medium, the line height FI I is the fluores 
cence intensity. It is proportional to chlorophyll concen 
tration N (a = factor of proportionality), if no shadowing 
effects occur. However, for fluorescing particles in the water, 
two effects lead to a decrease of the detected signal: First, the 
illuminating light is absorbed with the probality p t before it 
reaches the particles. Second, the fluorescence light emitted 
by the particles is absorbed with the probability p 2 before it 
reaches the detector. Thus, 
Z,//=atf(l-/>,)(!-ft), 
where /?, and p 2 depend on N. If the absorption of the excit 
ing light is neglected (p t — 0) and the reabsorption of the 
emitted light is assumed to be proportional to N (II = factor 
of proportionality), the equation reads as follows: 
LH{N) = a/V(l - IfN). (1) 
Equation (1) describes the relation between LI I and N at 
fixed detection wavelength X. Since a as well as II depend on 
X, the curve FII(N) depends on the detection wavelength 
chooscn. Figure 5 gives two examples for the function 
FII(N). 
LH X = 6 8 5.3 nm 
Reflectance (%) 
Figure 4: Construction of the baseline and definition of LH and 
FLH. The baseline is a straight line between the values of 
reflectance at X u = 663.5 nm and X a = 737.5 nm. The differ 
ence between the reflectance and the baseline in the interval 
). u to X 0 is called line height LH, the maximal value of LH is 
called fluorescence line height FLH. 
LH X = 710.3 nm 
Figure 5: LH as a function of chlorophyll concentration at two detection wavelengths X (tank 2). The solid line is a fit 
according to equation (1).