1. A method for determining an image of a patterned object formed by a polychromatic lithographic projection system, the projection system having a radiation source of a finite spectral bandwidth for illuminating the patterned object and a lens for imaging the patterned object to a planar image space including an image plane within a resist layer, the method comprising: providing patterns for the object; providing a spectrum of the radiation source to be used in the lithographic projection system; providing an intensity and polarization distribution of the radiation source; providing a lens impulse response in the spatial domain or in the spatial frequency domain of the image; forming a polychromatic 4D bilinear vector kernel comprising a partially coherent polychromatic joint response between pairs of points in the spatial domain or in the spatial frequency domain; determining the dominant polychromatic 2D kernels of the polychromatic 4D bilinear vector kernel; and determining the image of the patterned object from convolutions of the object patterns with the dominant polychromatic 2D kernels.