As a detonation wave propagates from a confined tube to an unconfined space, it undergoes diffraction, which is characterized by a change of geometry from a quasi-planar wave to a cylindrical or spherical wave. Depending on a number of parameters, the detonation wave can be either extinguished (sub-critical regime) or re-initiated (super-critical regime). Although extensive numerical work has been performed on detonation diffraction, most numerical studies have been performed using an inviscid gas model (Euler equations) and global (1- or 2-step) kinetic schemes. In the present study, numerical simulations of detonation diffraction have been performed using a viscous gas model (Navier-Stokes equations) and accurate chemical and thermodynamic models. Two conditions have been numerically investigated, (i) a sub-critical case obtained for a stoichiometric H2-O2 mixture diluted with 70% of Ar at P1= 100 kPa (mixture 1); and (ii) a super-critical case obtained for a stoichiometric H2-O2 mixture diluted with 50% of Ar at P1= 57.5 kPa (mixture 2). In both cases, the initial temperature is 295 K. The numerical results were compared with the experiments of Pintgen, who performed simultaneous imaging of the shock front and reaction zone, using schlieren and 2D Laser Induced Fluorescence (PLIF). A direct comparison with the experimental PLIF images was allowed by post-treating the simulation with a 3-level LIF model. The numerical results demonstrate a reasonable agreement with previously obtained experimental and numerical data in terms of diffracting detonation behaviour and structure. Key features observed in the experimental LIF images were reproduced by the synthetic LIF visualization.