High-Definition Time-of-Flight Imaging
Time-of-flight (TOF) three-dimensional (3D) imaging provides a complimentary fit for the LCPG steering technology. TOF cameras and flash lidars use a focal plane array (FPA) to simultaneously detect the return from thousands of locations in the receiver’s field of view (FOV). Large FOVs typically require diverging beams and wide-angle optics that reduce the amount of signal collected relative to background noise. Meanwhile, the angular resolution is limited by the resolution of the TOF FPA.
Using LCPGs, a TOF camera can concentrate illumination and signal collection over a narrow angle for high signal-to-noise ratio (SNR) and angular resolution, then non-mechanically scan both transmitter and receiver to regain a large FOV and high effective pixel count. We demonstrated this approach with a commercial TOF camera to boost range and resolution while reducing power consumption.
- Automotive Sensors
- Manufacturing Inspection and Factory Automation
- Aerospace + Defense
A TOF 3D imaging system generally illuminates a sizable area with modulated laser light and uses a FPA to simultaneously detect the return from thousands of locations in the receiver’s FOV. Thousands of points, from a QVGA (320x240) TOF detector for example, produce a dense 3D point cloud, but the 3D image’s spatial resolution is low if the points are distributed over a reasonably wide field of view. High-resolution point cloud generation over a wide angle presents problems for a TOF camera (or flash lidar), since more spatial resolution generally means more pixels within the array and thus less return signal per detector element. Also, wider view angles cause more ambient (background) light to be collected along with the TOF signal, which increases shot noise even if the ambient noise is rejected through electronic processing.
These signal-to-noise issues along with size, weight, power, and cost constraints usually force a tradeoff of resolution for coverage. Fortunately, a TOF camera (or flash lidar) can provide higher resolution and better SNR by narrowing the FOV of the camera, and the LCPG scanner can step both the transmit beam and receiver FOV to gain back area coverage.
A native QVGA TOF camera using 4 VCSELs covers a 60° × 45° FOV with 320 × 240 pixels.
Using LCPGs, the same TOF camera covers a 44° × 5.1° FOV with 2080 × 240 effective pixels. The modification boosts angular resolution by nearly 10×, reduces background noise by up to 78×, and reduces required laser power by 4×.
The power of this approach stems from the ability of LCPGs to scan both the transmitter and receiver paths of the sensor. This is enabled by the large apertures and angles provided by LCPG steering. Meanwhile, the tiling of a monochromatic sensor FOV provides a good match for the discrete steering nature of LCPGs.