1. A method for modeling S-distortion in an image intensifier, said method comprising: identifying a reference coordinate on an input screen of the image intensifier, wherein the z axis intersects the reference coordinate and is perpendicular to the input screen at the location of the reference coordinate, and wherein the x axis intersects the reference coordinate and is perpendicular to the z axis, and wherein the y axis intersects the reference coordinate and is perpendicular to the x axis; computing a set of charged particle velocity vectors, said charged particle velocity vectors including a first component for the velocity of a charged particle along the z-axis and a second component for the velocity of a charged particle in an x-y plane that is along the x-axis and y-axis; computing a set of magnetic field vectors, said magnetic field vectors including a first component for the magnetic field within the image intensifier along the z-axis, a second component for the magnetic field within the image intensifier along the x-axis, and a third component for the magnetic field within the image intensifier along the y-axis; and, computing the force exerted on said charged particle in said image intensifier along said x-y plane using at least said set of charged particle velocity vectors and said set of magnetic field vectors.