3-D gated viewing laser radar using 4 Picos intensified (ICCD) camera
The outstanding performance of 4 Picos intensified (ICCD) camera from Stanford Computer Optics enables fast three-dimensional (3-D) Laser radar with sup-millimeter range accuracy.
Topology of a space shuttle LEGO® model measured by the described 3D Laser scanner at a distance of 8m. The insert is a zoom of the cargo bay demonstrating the <1mm depth accuracy of the gated viewing 3D scanner. Reprinted figure with permission from J.F. Andersen, J. Busck and H. Heiselberg.
3D Laser scanner using the 4 Picos ICCD ultra high speed camera.
J.F. Andersen, et al. of the Danisch Defense Reasearch Establishment improved the accuracy of a 3-D Laser radar by taking advantage of the 4 Picos ultra high speed camera with a gating time down to 200ps rectangular. They build a non-scanning laser radar which is based on the gated viewing principle and applies a pulsed laser and a fast gated camera.
Gated viewing principle
The gated viewing principle is predicated on the fact that light reflected by an object needs some time travel to the detector, depending on the object-detector distance. However in our everyday environment this fact is not observable due to the continuously illumination by the sun and the slow detectors we are using like our eye or standard cameras. Even if we would use short Laser pulses to illuminate a street at night the light from a car directly in front of the detector and from the building down the street would be at the same image due to the slow detectors compared to the speed of light. Then the 3D information can not be directly determined by the image.
Nevertheless a 3D Laser radar can be realized using short Laser pulses and the 4 Picos ultra high speed camera, with a gating time of down to 200ps rectangular. Laser emits short pulses to illuminate the object and triggers the camera with a delay and a gate time. These times determine when and how long the camera gate opens. By scanning the delay time, images for different depth of field (gated-viewing-time) are taken from a 3D Object. A 3-D Model of the Object can be reconstructed by these 2D slice images.
Experimental setup of the 3D Laser scanner
J.F. Andersen, et al. used a diode pumped passively Q-switched Nd:Yag laser at 532 nm with a pulse width of ~200ps and a high pulse repetition frequency (RPF) of 32.4kHz. The image intensifier of the ICCD camera is triggered by each individual laser pulse with a delay and gate time. Since the 4 Picos camera is running with a 50-Hz read-out frequency, the CCD-Chip integrated the signal of all laser pulses during 20ms. This multiple exposure modi improved the Signal to Noise Ratio and between the individual time slices the gain and other setting can be arranged, which is very useful for suppression or enhancement of certain time slices.
With this arrangement the group from the Danish Defense Research Establishment in Copenhagen achieved a depth accuracy down to 0.2 mm. The advantages of this technique compared to different 3D laser scanner approaches is the speed, the lack of a scanner system, its compactness and the user-optimized handling.
Title: Submillimeter 3-D laser radar for Space Shuttle tile inspection
Author: J.F. Andersen, J. Busck and H. Heiselberg
Institute: Danisch Defense Reasearch Establishment, Copenhagen, Denmark