How does it work?
3D Flash LIDAR cameras operate and appear very much like 2D digital cameras. 3D focal plane arrays have rows and columns of pixels, also similar to 2D digital cameras but with the additional capability of having the 3D "depth" and intensity. Each pixel records the time the camera’s laser flash pulse takes to travel into the scene and bounce back to the camera’s focal plane (sensor). A short duration, large area light source (the pulsed laser) illuminates the objects in front of the focal plane as the laser photons are "back scattered" towards the camera receiver by the objects in front of the camera lens. This photonic energy is collected by the array of smart pixels, where each pixel samples the incoming photon stream and "images" depth (3D) and location (2D), as well as reflective intensity. Each pixel has independent triggers and counters to record the time-of-flight of the laser light pulse to the object(s). The physical range of the objects in front of the camera is calculated and a 3D point cloud frame is generated at video rates (currently possible up to 60 frames/second).
Currently, twenty or forty-four analog samples are captured for each pixel per pulse allowing for accurate pulse profiling. Because of the physics involved with the velocity of light, the accurate range data is a direct and simple calculation (as opposed to stereoscopic camera systems whose range is interpolated based on lens disparity).
The 16,384 data points per single flash (frame) that ASC cameras capture allows for high-rate dynamic scene capture and 3D videos that LADAR scanners are unable to accomplish. Without moving or other mechanical parts to add weight or are subject to wear, make ASC cameras small, light and durable, without being subject to motion distortion.
As an emerging technology, 3D Flash LIDAR has a number of advantages over conventional point (single pixel) scanner cameras and stereoscopic cameras, including:
- Full frame time-of-flight data (3D image) collected with a single laser pulse
- Full frame rates (high) achievable with area array technology
- Unambiguous direct calculation of range
- Blur-free images without motion distortion
- Co-registration of range and intensity for each pixel
- Pixels are perfectly registered within a frame
- Ability to represent the objects in the scene that are oblique to the camera
- No need for precision scanning mechanisms
- Combine 3D Flash LIDAR with 2D cameras (EO and IR) for 2D texture over 3D depth
- Possible to combine multiple 3D Flash LIDAR cameras to make a full volumetric 3D scene
- Smaller and lighter than point scanning systems
- No moving parts
- Low power consumption
- Ability to "see" into obscurants (range-gating)
What are the advantages of 3D Flash LIDAR?
One significant advantage of 3D Flash LIDAR sensor array technology is that 3D movies can be acquired at the laser pulse repetition frequency, making 3D video a reality. This capability enables real time machine vision. High frame rates mean that topographical mapping can be obtained more rapidly than with point scan technology, decreasing the amount of flight time required to scan and capture an area. The inherent weight savings means a vehicle such as a UAV/UAS can stay aloft that much longer, and the pixel-to-pixel registration greatly reduces the need for precise pointing knowledge when stitching together images to create large maps. Another not-so-intuitive advantage is that single laser pulse Flash 3D images are generally immune to platform motion, platform vibration and object motion due to the speed-of-light capture of the data frame.
3D Mapping and Texturing
The value of 3D imaging and reconnaissance is evident in the images in Figure1, an example of hard target 3D images taken with ASC’s 3D Flash LIDAR camera. There are six 128x128 frames, taken with six laser pulses (1/5th of a second), automatically stitched together. Bore-sighted 2D visible data is overlaid as texture on the 3D point cloud data. Even though the frames are taken from a single camera position, the 3D image contains far more quantitative information than is present in the 2D image: all the dimensions and accurate spatial relationships among the objects in the image.
ASC 3D Flash LIDAR’s ability to create data sets for determining safe landing zones can be seen in Figure2. The images were taken at the JPL Mars Yard, and include rocks and a person holding a dual-hemisphere, cardboard target. The data is extracted from a single frame of data (a single pulse), and color-coded for range. For Moon or Mars landings, an on-board camera can be implemented so that the camera returns both point-clouds and contour maps with flat "Safe" landing zones. The position of these zones could then be updated 30 times a second allowing for real-time hazard identification and pilot response.