High flux solar simulators, comprising high-intensity-discharge lamps coupled with elliptical reflectors, have been widely employed to study concentrated solar thermal energy systems in a controlled manner. However, little information is available of the influence of the assumptions about the properties of the arc on the accuracy of the prediction of the heat flux at the focal plane. This paper presents a concentric multilayer model of the arc that is developed to predict the spatial distribution of the heat flux at the focus. Measurements were performed of both the time-resolved and time-averaged spatial distribution of the discharge arc from a commercial metal halide lamp with a 23 mm spacing between the two electrodes. The efficacy of various alternative simplified approaches to model the arc using a commercial Monte Carlo ray-tracing code were assessed, which include models of cylinder, monopole sphere, dipole-sphere and three types of compounds of these three shapes. These predictions were validated with measurements using a CCD camera and a heat flux gauge, which shows that the predicted profiles from the three compound models agree with the experimental results to similar extent, with no difference in the predicted maximum heat flux and within a difference of 11% for the predicted half-width. Crown Copyright (C) 2015 Published by Elsevier Ltd. All rights reserved.