1. A method for simulating an aerial image projected from an optical system, said optical system including a pupil and a mask plane, the method comprising the steps of: providing a mask to the mask plane; obtaining parameters for said optical system; calculating a kernel based on an orthogonal pupil projection of said parameters of said optical system onto a basis set; obtaining parameters of said mask; calculating a vector based on an orthogonal mask projection of said parameters of said mask onto a basis set; calculating a field intensity distribution using said kernel and said vector, and obtaining aerial image data from said field intensity distribution.