This paper proposes LiBeamScanner, an indoor positioning system that utilizes only a single custom light bulb and a light sensor to achieve extremely high positioning accuracy. Single light bulb means that a dense deployment of illuminaries is unnecessary, and the use of a light sensor, instead of a camera, implies that even systems with strict energy constraints, such as wearable devices, can make use of our system. At the physical layer, we propose a novel transmitter design that allows information to be modulated onto very narrow light beams, projected to a large number of fine-grained locations simultaneously. More importantly, these light beams generate negligible interference to each other, as a result providing a high resolution for enabling accurate positioning. Evaluation results of our prototype show that most of the errors are less than 3cm and the average error does not exceed 5cm.