The aim of this research was to carry out a comparative analysis between scale model measurements and computer simulated daylight factor (DF) values for a square, four-sided, top-lit atrium. In a previous study, parametric changes were made to the distribution of diffuse surfaces reflectances in the well of a physical scale model that was placed inside an artificial sky. The artificial sky replicated the luminance distribution of a CIE overcast sky. The effects of these changes on the daylight levels at the centre, edge and corner of the floor of the atrium model were investigated. To achieve the variability in reflection distribution the atrium surfaces were painted in white (0.85% wall reflectance) and black (0.02% wall reflectance) alternating horizontal bands. For each of the experiments the widths of these bands were altered but the overall split between white and black bands was always maintained at 50% white and 50% black. Experiments were also conducted for atriums with all wall surfaces painted black (0.02% wall reflectance) and all wall surfaces painted white (0.85% wall reflectance). It was concluded from the study that atrium surfaces with wide bands of different reflectance values did affect the daylight levels at the base of the atrium. However, if these bands were narrow then the daylight levels were not significantly altered by different reflectance distributions. In this study, these experiments were repeated using Ecotect and Desktop Radiance. The study showed that Radiance always underestimated the daylight factor values in comparison with those obtained from physical scale model measurements. The Radiance values could therefore be multiplied by a factor of 1.2.