In this work, we model the effects of excimer laser bandwidth on optical proximity effects in high-NA ArF dry and immersion lithography. We quantify the errors introduced by using common approximation methods for the laser spectrum, such as the modified Lorentzian and Gaussian forms. Although these approximations are simple to use, and their symmetry properties can lead to reduced simulation run-times, they typically induce significant CD error when compared to the use of measured spectral profiles, which are obtained from high-resolution spectrophotometry. In this paper we establish some accuracy benchmarks and demonstrate the need for inclusion of information about the spectral profile-for the laser type of interest-in order to achieve sub-nanometer image calculation accuracy required for optical proximity correction. We further assess the speed-accuracy tradeoffs in terms of data truncation and sampling, and propose some practical limits for sampling the illumination spectrum.Additionally, in this work, we propose a new physically-based spectrum approximation method, which significantly reduces computation time at a cost of less than 0.25nm residual image-CD error from the fully-sampled image calculation. In addition to aerial image, we compare 45nm-node calibrated resist models and latent image results for 0.92NA dry and 1.2NA immersion processes using measured illumination profiles and lens aberrations. Finally, we consider the laser bandwidth sensitivity of 2D lineend patterns and typical post-OPC designs for a logic gate-process.