We propose an AR-based painting interface that enables users to design an illumination distribution for a real room using an array of computer-controlled lights. Users specify an illumination distribution of the room by painting on 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 runs an optimization interactively to calculate light parameters to deliver the requested illumination condition. In this implementation, we used actuated lights that can change the lighting direction to generate the requested illumination condition more accurately and efficiently 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 widgets. The results showed that the users preferred our method for informal light control.