We have completed a model to predict the three-dimensional position of a sp
herical particle from the image formed by a high numerical aperture microsc
ope. Although this information may be exploited in a number of different wa
ys, we are particularly interested in determining the three components of v
elocity for seed particles following a flow. The basis of the model is to u
se Mie theory to determine the complete electromagnetic field scattered by
a spherical particle and then to propagate the scattered field across the i
nterface between sample and air and then through a high aperture microscope
and ultimately to a detector.