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.
Publications
"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.
[PDF]
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.
