3-D imaging multiple target laser radar

Details 3-D imaging multiple target laser radar 3-D imaging multiple target laser radar

1999-11-24

2002-07-02

Hellner, Mark (Department: 3662)

Optics: measuring and testing

Range or remote distance finding

With photodetection

C250S332000

Reexamination Certificate

active

06414746

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a laser radar vision apparatus capable of producing three-dimensional images of distant targets located behind reflective or absorbing but penetrable barriers such as camouflage and obscuring smoke. In particular, this invention relates to a multiple pixel, electronic apparatus for capturing three-dimensional images of distant targets, within obscurants, at high-spatial and high-range resolution in the atmosphere or in space with a single laser pulse, using a laser-reflection generated trigger.
BACKGROUND OF THE INVENTION
This application is a continuation-in-part of U.S. patent application Ser. No. 08/665,738, Filed Jun. 19, 1995, 3D Now U.S. Pat. No. 6,133,989 which is a CIP of Ser. No. 08/015,623, Now U.S. Pat. No. 5,446,529 Imaging Laser Radar. Laser radars (ladars) determine range in the atmosphere by measuring the transit time of a laser pulse from the transmitter/receiver to a partially or fully reflective target and dividing by twice the velocity of light in the atmospheric medium. If there are more than one return pulse, only the first return pulse is used in the range processing. Range resolution in such devices is related to the accuracy of the transit time measurement. In the atmosphere, ranges are typically measured in kilometers, where range resolution can be smaller than 30 cm. A 3-D target image can be obtained with a conventional laser radar by rastering the laser beam across the target and measuring the transit time, pulse by pulse, where each pulse corresponds to a point on the target. The distance between points on the target determines the spatial resolution of the rastered image and defines the picture element (pixel) size; the number of pixels at the target determines the pixel-array size; the range resolution determines resolution in the third target dimension. Rastering is a slow process, particularly for large pixel-array sizes, and it requires cumbersome mechanical scanners and complex pixel-registration computer processing. In addition, if the first laser-pulse return is from a partially reflective obscurant, which is hiding the target, then the 3-D image does not reveal the nature of the real target.
U.S. patent application Ser. No. 08/665,738, Filed Jun. 19, 1995, by the present inventors disclosed a lightweight, small-size, multiplexing laser radar receiver, the LR-FPA, that could image an entire target, in the atmosphere or in space, at high-spatial and high-range resolution with a single laser pulse. Thus the necessity of laser rastering or using a multitude of laser pulses to obtain a three-dimensional image is avoided. The reflected laser pulse, from different portions of an object, stop independent clocks located in a two-dimensional array of pixels. The times at which the clocks are stopped are related to the third dimension of the object by the velocity of light and are stored in the pixels along with peak signal data. The time data and peak signal data is read out from the array between laser pulses and used to construct the three-dimensional image. Processing the peak signal amplitude with the time data increases the range resolution accuracy. More than one reflected pulse for each pixel is accommodated by separately storing the return time and peak signal of each reflection.
U.S. Pat. No. 5,446,529, issued Aug. 29, 1995 to the present inventors discloses a lightweight, multiplexing laser radar receiver (3DI-UDAR) that can generate a three-dimensional image of an entire object, in a light conducting medium, such as water or the atmosphere, with a single laser pulse. The imaging is accomplished by integrating and storing the reflected signals from a multitude of range resolution intervals (range bins), independently for each of a two-dimensional array of pixels; each range bin across the two-dimensional array corresponds to a range slice in three dimensions. After reading the range bin data out between laser pulses, the time of laser pulse returned from the object is determined for each range bin in the two dimensional array, by means of the integration clock and the start of integration. The three-dimensional image is constructed by the knowledge of the return time of each two-dimensional slice. Because there is return pulse amplitude information as a function of time rather than just the peak of the return pulse, more information can be derived concerning the character of the target. The first range bin begins storing information in response to a signal from the invention's drive electronics rather than from an external signal such as the first reflection from the light conducting medium (the surface of the water for example).
There is only a finite storage capacity for each of the pixels (typically 30 to 200 storage bins) in the 3DI-UDAR. For high spatial resolution in a medium that does not attenuate the light appreciably, the effective depth from which the information is coming from is only a small proportion of the entire range. For example, for 30 cm range resolution, 200 storage bins may only correspond to a depth of 60 m whereas the absolute range or the ladar may be many tens of kilometers. Turning on the range bin integration at the optimum range position (the target position) could involve a trial and error process requiring more than one laser pulse or another system which first finds the time delay to the target and then transfers that time delay to the drive electronics.
BRIEF DESCRIPTION OF THE PRESENT INVENTION
In the present invention a reflected laser pulse from one or more targets, or from an obscured target are integrated and stored in a sequence of range bins, independently for each of a two-dimensional array of pixels. The range bin integrations are turned on automatically, including a possible programmed delay, as the first reflection arrives at the receiver of the invention. Alternatively the integrations are occurring continuously, with the storage bins being filled by new data until the first reflection arrives. Storage then proceeds until a predetermined but adjustable number of storage bins are filled allowing the option of obtaining data prior to the first reflection and/or after it to be stored. By processing this data, preferably between laser pulses, one or more three-dimensional images of single or multiple targets or targets within obscurants can be generated.
A preferred embodiment of the sensor of the invention is a hybrid of two chips, an array of detectors (or collectors) directly connected to an array of processing-electronics unit cells. Each detector/collector, on one chip, is connected to its own processing-electronics unit cell, on the other chip, and defines a pixel in the image. Each processing-electronics unit cell, on the array, contains an identical and unique integrated circuit which can store the first reflected-pulse transit time, the first reflected peak amplitude and/or a sequence of range bins which contain amplitude information about the first reflected pulse and subsequent reflected pulses or amplitude information only about subsequent reflected pulses. Transit-time and pulse amplitude information for all pixels is read out preferably between laser pulses. Laser photons interact directly with the detector array generating a signal or laser photons are converted to electrons that then generate the signal in different embodiments of the invention.
It is the object of the present invention to provide a device for three dimensional imaging of obscured or unobscured objects using a single laser pulse, in transparent or semi-transparent media by a sequence of measurements on the returned pulse amplitude and to overcome the problems of prior systems associated with the need for first determining the range of the target. The device comprises a pulsed light source, means for projecting the light towards the object, optics for collecting the reflected light, improved sensors for detecting the reflected light, drive and output electronics for timing and signal conditioning of data from the sensors and a computer and software for converting

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