Abstract: We propose a novel painting interface that enables users to design an illumination distribution for a real room using an array of computer-controlled lights. Users specify which area of the room is to be well-lit and which is to be dark by painting a target illumination distribution on a tablet device displaying the image obtained by a camera mounted in the room. The painting result is overlaid on the camera image as contour lines of the target illumination intensity. The system then runs an optimization to calculate light parameters to deliver the requested illumination condition. We implemented a GPU-based parallel search to achieve real-time processing. In our system, we used actuated lights that can change the lighting direction to generate the requested illumination condition more faithfully than static lights. We built a miniature-scale experimental environment and ran a user study to compare our method with a standard direct manipulation method using sliders. The results showed that the users preferred our method for informal light control.