Abstract. A vehicle on a road or a robot in the field does not need a full-featured 3D depth sensor to detect potential collisions or monitor its blind spot. Instead, it needs to only monitor if any object comes within its near proximity which is an easier task than full depth scanning. We introduce a novel device that monitors the presence of objects on a virtual shell near the device, which we refer to as a light curtain. Light curtains offer a light-weight, resource-efficient and programmable approach to proximity awareness for obstacle avoidance and navigation. They also have additional benefits in terms of improving visibility in fog as well as flexibility in handling light fall-off. Our prototype for generating light curtains works by rapidly rotating a line sensor and a line laser, in synchrony. The device is capable of generating light curtains of various shapes with a range of 20-30m in sunlight (40m under cloudy skies and 50m indoors) and adapts dynamically to the demands of the task. We analyze properties of light curtains and various approaches to optimize their thickness as well as power requirements. We showcase the potential of light curtains using a range of real-world scenarios