Luminance is a directional quantity, and its value is strongly determined by a material's reflectance/transmittance distribution function. If luminance is calculated using a Lambertian (i.e. diffuse) assumption, specular highlights and reflections are ignored and the result can easily be wrong by a hundred times or more. We cannot afford to lose directional information if we hope to use our simulation to evaluate visual performance, visual comfort and aesthetics.
A global illumination program is only as general as its local illumination model. The standard model of ambient plus diffuse plus Phong specular is not good enough for realistic image synthesis. Radiance includes the ability to model arbitrary reflectance and transmittance functions, and we have also taken empirical measurements of materials and modeled them successfully in our system .