Total Internal Reflection Fluorescence (TIRF) is a powerful technique for visualizing focal and close contacts between the cell and the surface. Practical application of TIRF has been hampered by the lack of straightforward methods to calculate separation distances. The characteristic matrix theory of thin dielectric films was used to develop simple exponential approximations for the fluorescence excited in the cell-substratum contact region during a TIRF experiment. Two types of fluorescence were examined: fluorescently labeled cell membranes, and a fluorescent water-soluble dye. By neglecting the refractive index of the cell membrane, the fluorescence excited in the cell membrane was modelled by a single exponential function while the fluorescence in the membrane/substratum water gap followed a weighted sum of two exponentials. The error associated with neglecting the cell membrane for an incident angle of 70 degrees never exceeded 2.5%, regardless of the cell-substratum separation distance. Comparisons of approximated fluorescence intensities to more exact solutions of the fluorescence integrals for the three-phase model indicated that the approximations are accurate to about 1% for membrane/substratum gap thicknesses of less than 50 nm if the cytoplasmic and water gap refractive indices are known. The intrinsic error of this model in the determination of membrane/substratum separations was 10% as long as the uncertainties in the water gap and cytoplasmic refractive indices were less than 1%.

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