Forward and Backward Fluorescence

In this section the ideal case of vanishing reabsorption, Ke = 0, is discussed, where Fb + Ff= Ftot- A large area of the sample should be irradiated close to /to = 2/3, what is a very convenient geometry in most spectrometers, or diffusely via an integrating sphere, what is less convenient but guarantees homogeneous density of irradiation. Under these conditions Eqs. (8.27) and (8.28) are sufficiently accurate for quantitative evaluation. 

In layers with very small optical pathlengths (Sd 1) the forward and backward fluorescences are equally intense like in nonscattering media. However, with increasinglayerthickness, Ff grows slowerthan Fb, passes through a maximum, and decreases 

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Figure 8.18. Intensities of forward and backward fluorescence of a scattering layer as a function of the layer thickness (ff = 1 cm-1, 5° = S = 100 cm-1). The corresponding curves of diffuse reflectance and diffuse transmittance at Xa are also given.

Figure 8.19. Normalized intensities of forward and backward fluorescence, diffuse reflectance and transmittance of a scattering layer (S = 50 cm"1, d = 1 cm) as a function of the absorption coefficient of the fluorophore.

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