Lighting conditions can make the difference between success or failure of an architectural space. The vision of space-light co-design is that architects can control the impression of an illuminated space already at an early design stage, instead of first designing spaces and then searching for a good lighting setup. As a first step towards this vision we propose a novel method to calculate potential light source positions from a given user defined target illumination. The method is independent of the tessellation of the scene and assumes a homogeneous diffuse Lambertian material. This allows using a voting system that determines potential positions for standard light sources with chosen size and brightness. Votes are cast from an illuminated surface point to all potential positions of a light source that would yield this illumination. Vote clusters consequently indicate a more probable light source position. With a slight extension the method can also identify mid-air light source positions
