1. A method for optimizing the illumination conditions of a lithographic apparatus by computer simulation using full resist calculation, the lithographic apparatus comprising an illuminator, a projection system, and a mask having a pattern to be printed in a layer of photoresist material formed on a substrate, the method comprising: defining a lithographic pattern to be printed on a wafer; choosing a resist model of a resist process to be used to print the pattern in the layer of photoresist material; selecting a grid of source points in a pupil plane of the illuminator; calculating separate responses for individual source points, each of the responses representing a result of a single or series of simulations using the resist model; and adjusting an illumination arrangement based on analysis of accumulated results of the separate calculations.