Optics: measuring and testing – For light transmission or absorption – Of fluent material
Reexamination Certificate
2001-09-26
2004-02-10
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
For light transmission or absorption
Of fluent material
C250S330000
Reexamination Certificate
active
06690472
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to systems for performing backscatter absorption gas imaging (BAGI), and more specifically, it relates to a pulsed linescanner for use in BAGI imaging.
2. Description of Related Art
BAGI is an existing and patented technique disclosed in U.S. Pat. No. 4,555,627, titled “Backscatter absorption gas imaging system”. Simply stated, the patent covers the use of infrared laser-illuminated imaging for the remote video visualization of gas plumes. It describes the coupling of an infrared laser to an infrared camera to produce an instrument that views a scene in the infrared as the laser illuminates the scene. The system produces, therefore, a laser-illuminated video picture of the scene. If a gas plume is present that can absorb light at the center wavelength, it creates a shadow in the picture that is essentially a video image of the gas plume. BAGI is currently being commercialized by Laser Imaging Systems (LIS), which offers systems operating in the 9-11 &mgr;m wavelength range based on the use of CO
2
lasers.
U.S. Pat. No. 3,317,730 discloses a method for determining atmospheric pollution by the detection of backscattered modulated infrared radiation.
U.S. Pat. No. 3,832,548 to Wallack shows a general infrared absorption detector in which infrared radiation first passes through a filter means having a plurality of positions for transmitting selected wavelengths, and then passes through a sample cell to a detector.
U.S. Pat. No. 4,204,121 to Milly shows a mobile detector comprising a vertical sampling array for quantifying emission rates from pollution sources.
U.S. Pat. No. 4,264,209 to Brewster shows a system for producing an indication of a concentration of a gas of interest in which the gas is illuminated and the output is filtered alternately with two filters, one at an absorption band of a gas to be detected, the other at a passband outside the absorption band.
U.S. Pat. No. 4,262,199 to Bridges, et al., shows a mobile infrared target detection and recognition system including an assembly of infrared detection elements which scan a field of view to produce a signal representative of the infrared level from point to point.
U.S. Pat. No. 3,829,694 to Goto discloses apparatus for detecting gases or particles using Mie scattering of pulsed light beams to detect resonance absorption.
U.S. Pat. No. 3,517,190 to Astheimer discloses a method for monitoring stack effluent from a remote position by illuminating the effluent across a broad spectral band and detecting the reflected illumination in two spectral regions: one in an absorption band and one outside the absorption band to determine the quantity of absorbing gas from the signal ratio.
The publication Kulp et al., “Development of a pulsed backscatter-absorption gas-imaging system and its application to the visualization of natural gas leaks”, Appl. Opt. 37 3912-3922 (1998), describes the development of a pulsed BAGI imager that uses full-field illumination at a laser pulse repetition rate of 30 Hz.
The publication Powers et al. “Demonstration of differential backscatter absorption gas imaging”, Appl. Opt. 39 1440-1448 (2000) described the development of a pulsed BAGI imager that uses full-field illumination at a laser repetition rate of 30 Hz and is capable of differential detection. It operates in a way that is adversely affected by system motion.
The publication of Imeshev et al. “Lateral patterning of nonlinear frequency conversion with transversely varying quasi-phase-matching gratings” Optics Letters 23 673-675 (1998) describes the use of periodically poled lithium niobate with lateral patterning to produce second harmonic frequency output beam with a flat-topped spatial profile.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a pulsed linescanner for use in a backscatter absorption gas imaging (BAGI) system.
It is another object of the invention to provide a BAGI imager that is capable of operation with pulsed laser sources. The term “laser” is intended to include lasers as well as any other light sources with spectral and brightness properties meeting the requirements presented in this teaching. For example such lightsources could include a laser followed by a frequency conversion device or an incandescent beam from a gas discharge source.
It is another object of the invention to provide a pulsed linescanner that is capable of differential imaging.
It is another object of the invention to provide methods for acquisition of images by a pulsed linescanner in ways that are immune to moderate camera motion (such as might be encountered in hand-held or vehicle-mounted operation).
It is another object of the invention to provide methods for acquisition of images by a pulsed differential linescanner in ways that are immune to moderate camera motion (such as might be encountered in hand-held or vehicle-mounted operation).
It is another object of the invention to provide means for achieving a linescanning BAGI imager that is capable of both single-wavelength and differential imaging.
It is another object of the invention to provide a pulsed linescanned imager that by concentrating the transmitted light in a small number of rows achieves a higher backscattered signal from a given target using a given laser pulse energy and repetition rate than can be obtained by a system employing full-field illumination.
These and other objects of the invention will be apparent to those skilled in the art based on the teachings herein.
The present invention is an active (laser illuminated) imaging system that is suitable for use as a BAGI imager. As in all BAGI systems, the present invention employs a laser, tuned to a wavelength absorbed by the gas to be detected, that is coupled to a suitable video camera. The laser illuminates the scene as the camera images it. Gases present in the imaged scene are visualized when they absorb the laser light, thus creating a dark region in the video picture. This allows the imager to be used to rapidly detect and pinpoint leaks of gases (such as hydrocarbons found in leaks at petroleum refineries or in natural gas pipelines) that absorb light produced by the laser employed. To maximize the attenuation, the spectral profile of the laser must be narrower than the target gas absorption linewidth and must be centered at the peak of the strongest absorption line that is not affected by interfering species. Operation away from the peak of the gas absorption or with lasers having a broader spectral width than the absorption feature is also possible with an associated reduction in detection sensitivity.
The invention described here uses a pulsed laser as its illumination source and creates images by linescanning. Plume visualization is accomplished in either of two modes—termed single-wavelength imaging and differential imaging. In single-wavelength imaging the scene is illuminated only with laser radiation having a wavelength absorbed by the gas. The gas image is produced when the gas plume attenuates the backscatter return from solid objects in the imaged scene. In differential imaging, the scene is illuminated by radiation at two different wavelengths; one strongly absorbed by the gas, termed the “on-wavelength”, and one that is not absorbed (or weakly absorbed), termed the “off-wavelength”. For every displayed frame, a backscatter signal is collected from each scene pixel at each wavelength. An image generated from the on-wavelength backscatter would be identical to the previously described single-wavelength image. An image generated from the off-wavelength backscatter would, on the other hand, contain no gas image (or only a weak gas image). In differential imaging both the on-and off-wavelength signals are processed to generate a differential image, in which the differences between the two frames are emphasized. An example of such processing is the log-ratio, where the logarithm of the ratio of the on-wavelength signal to the off-wavelength signal is displayed. The two wavelengths are selected to be
Bambha Ray P.
Kulp Thomas J.
Reichardt Thomas A.
Schmitt Randal L.
Evans Timothy P.
Rosenberger Richard A.
Sandia National Laboratories
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