This paper describes a recently developed, multi-chromatic lighting simulation model, known by the acronym DIM (Digital Illumination Model). DIM accepts a description of a zones geometry, surface finishes, contents and natural and artificial light sources. A multi-chromatic raytracking scheme is then employed to obtain the surface spectral luminance distribution corresponding to each light source. Outputs from the model include data on planar illuminance and coloured perspective images. The paper also describes the interaction between DIM and the ESP model for the simulation of a building's energy systems. In particular, the range of lighting control options, as implemented in ESP, are described. Finally the developments now underway to transform DIM into a polished design tool are outlined.