Low-power Structured Light in the Outdoors

We introduce a compact structured light device that utilizes a commercially available MEMS mirror-enabled hand-held laser projector. Without complex re-engineering, we show how to exploit the projector's high-speed MEMS mirror motion and laser light-sources to suppress ambient illumination, enabling low-cost and low-power reconstruction of outdoor scenes in bright sunlight. We discuss how the line- striping acts as a kind of "light-probe", creating distinctive patterns of light scattered by different types of materials. We investigate visual features that can be computed from these patterns and can reliably identify the dominant material characteristic of a scene, i.e. where most of the objects consist of either diffuse (wood), translucent (wax), reflective (metal) or transparent (glass) materials.


"A low-power structured light sensor for outdoor scene reconstruction and dominant material identification"
Christoph Mertz, Sanjeev J. Koppal, Solomon Sia and Srinivasa G. Narasimhan
Proc. of IEEE Workshop on Projector-Camera Systems (PROCAMS),
June 2012.

Illustrations and Results

(Video Result Playlist)

Sensor: The sensor consists of a 120Hz camera and a 10 Lumens PicoP Microvision laser projector. The camera and projector are synchronized so each image captures one scan line illuminated by the projector.

Working principle of the PicoP projector: The projector consists of three lasers, red, green, and blue, that are optically combined into one beam. The color of the beam is controlled by changing the intensity of each laser. A MEMS scanning mirror steers the beam horizontally and vertically producing the projected image.

Ambient light removal: First, since the lasers have a narrow-bandwidth, an appropriate filter placed on the camera can block some of the unwanted ambient light. Second, if we block much of the remaining ambient light with a low image exposure, the very high horizontal frequency of the MEMS steering device still allows the detection of a horizontal line stripe. Finally, the vertical frequency of the MEMS mirror allows background subtraction at near real-time rates, further removing all visual input except the light from the projector.

Example result of ambient light removal: The strong sunlight overwhelms the projected image at the top. Notice the significant reduction of ambient light using our sensor and our approach.

Video of line scan after ambient (sun) light removal: This sensor line-striping video appears as if taken in the dark.

Video of 3D reconstruction from line scan