Simulated Images. Based on Eq. 6 and a custom IDL program, the expected intensity patterns P’ and S’ for p-pol and s-pol excitation, respectively, are shown for a diI-labeled spherical granule fusing with (and truncated by) a diI-labeled planar plasma membrane. In this particular simulation, most of the sphere is still intact; only the lower one fourth of its radius is truncated off. The schematic line drawing (white) shows a side view of this configuration at the same scale as the simulated images. The effects of finite evanescent depth, optical resolution limit, and nonparallelism of the diI dipole with the membrane are all included in generating these P’ and S’ patterns, thereby simulating what appears at the CCD array image plane. The pixelation of the CCD array is superimposed along with an outline showing which pixels are actually used to integrate the total intensities P and S. The ratio P’/S’ and the sum P’+2S’ are also shown. The corresponding pixel by pixel ratios and sums on experimental data (as pixelated by the camera) are used to determine lateral positions of diI/membrane morphology features at the time of exocytosis. However, extended temporal tracking of the p-pol and s-pol ratios and sums uses the spatially integrated values P and S (without the primes) before forming the P/S and P+2S combinations. The predictions of the simulations are sensitive to the assumed parameters, which are set close to the actual or expected experimental values: granule radius = 150 nm; Airy disk half-width (out to first minimum) = 211 nm; evanescent field depth = 110 nm; side length of CCD array pixel (as projected onto the image) = 73 nm; angle β between membrane normal and diI dipole = 69°. The P’ and S’ images are shown with the same grayscale; the P’/S’ and P’+2S’ each have their own gray scales.