Patent Application
20050026276
1. Field of the Invention
This invention relates generally to a system for remotely detecting and analyzing chemical and biological aerosols and, more particularly, to a detection system that includes a laser source for radiating a target cloud with a laser beam to heat the cloud and increase its temperature relative to background, and a spectrometer for detecting and analyzing chemical and biological aerosols in the cloud.
2. Discussion of the Related Art
It is known in the art to detect certain materials within a chemical cloud in the air by spectral analysis of the molecules making up the cloud. This type of chemical detection has many applications, including detecting natural gas leaks from underground pipes, chemical clouds from chemical spills, volatile organic vapor (VOC) from chemical processes, pollution from smoke stacks and the like, military chemical warfare agents, and other toxic materials present in the air. Typically, this type of spectral analysis of a chemical cloud is performed remotely, sometimes up to 10-20 km away, because the materials in the cloud may be toxic, and thus a threat to health, or it may not be possible to directly detect the chemical cloud. The distance the detecting instrument has to be from the cloud for this remote type of passive sensing depends on the particular application, and different systems exist for different applications.
To perform this type of detection and analysis, a spectrometer, such as a Fourier transform infrared (FTIR) spectrometer, is directed towards the cloud from a remote location, so that it passively receives emissions therefrom. If the chemical cloud is warmer than the background, such as sky, mountains, or other terrain, along the field-of-view of the spectrometer, target molecules in the cloud will exhibit emissions having energy greater than the background emissions from the sky. If the chemical cloud is the same temperature as the sky, the target molecules within the cloud are absorbing photons at the same rate that they are emitting photons, so that there is no net energy exchange between the cloud and the background, and thus, no difference relative to the background. As the temperature of the cloud increases, more photons are released from the materials in the cloud, which are available to be received by the spectrometer.
The spectral display generated by the spectrometerfrom the emissions provides emission lines and bands at certain wavelengths that are indicative of the atoms and molecules in the cloud. Because each material has its own spectral “fingerprint” representative of its molecules, the detected spectral display can be compared to a known “fingerprint” of a particular chemical to determine if that chemical exists in the cloud.
U.S. Pat. No. 6,531,701 issued Mar. 11, 2003 to Chou et al., assigned to the assignee of this application and herein incorporated by reference, discloses a system for the remote detection and analysis of chemical vapors in the air. A beam of electromagnetic radiation from an electromagnetic radiation source radiates a suspected chemical cloud. The radiation energy that is absorbed by the cloud is quickly thermalized as a result of the rapid collision energy transfer between the molecules that absorb the radiation and the surrounding air molecules. This collisional energy redistribution will result in heating the materials in the cloud. An increase in the temperature of the cloud will increase the emission intensity of the molecules against the background, resulting in an improvement in the detection of the materials therein.
A tracking telescope collects thermal emissions from the chemical vapors generated by the radiation. A spectrometer, such as an FTIR spectrometer, is used to resolve the emissions from the cloud that are enhanced by the radiation and to generate an emissions spectrum. The emissions spectrum is used to identify suspect chemical vapors in the cloud by comparing the detected emissions to the known “fingerprint” vibrational spectrum of the suspect chemical vapors.
There is a need in the art for the rapid detection and analysis of toxic bio-aerosols and chemical aerosols for both personal health applications and homeland security applications. The detection and analysis should be capable of providing surveillance over a large area. For personal health applications, the aerosol can include toxic molds and pollens, which are inside or outside of a building, such as a residential home. For homeland security applications, the aerosol can include liquid aerosols of chemical agents, industrial toxic chemicals, and bio-aerosols of bacteria, viruses and toxins.
There is currently no suitable method known in the art that is quick and accurate for the detection and analysis of liquid chemical or biological aerosols. Bio-aerosols can be analyzed by bioassay techniques, known to those skilled in the art. However, current bioassay techniques are relatively slow, and would be ineffective for emergency situations. Bio-aerosols can also be detected by ultraviolet (UV) florescence techniques. However, the known UV florescence techniques have a relatively poor selectivity in identifying man made bio-aerosols from naturally occurring bio-aerosols.
U.S. patent application Ser. No. 10/456,098, filed Jun. 6, 2003, title “Detection and Analysis of Chemical and Biological Materials,” assigned to the assignee of this application, discloses a system of detecting and analyzing bio-aerosols and chemical aerosols in a sample. The system disclosed in the '098 application employs a cooled or heated background relative to the target including the sample to provide a thermal contrast between the background and the target, thereby increasing the emissions or the absorption of the sample. However, the system disclosed in the '098 application has limitations because of the requirement of detection along a predetermined optical path, which is defined between a spectrometer and the cold or hot background. The system disclosed in the '098 application is also somewhat limited for wide-area surveillance.
In accordance with the teachings of the present invention, a system for the remote detection and analysis of chemical and biological aerosols within a sample, such as a cloud, is disclosed. A beam of radiation from an electromagnetic radiation source, such as a laser, is used to radiate the sample cloud. The radiation energy that is absorbed by the cloud is quickly thermalized due to a rapid collision energy transfer between the molecules that absorb the radiation and the surrounding air molecules. This collisional energy redistribution results in heating the materials in the cloud. An increase in the temperature of the cloud will increase the emission intensity of the molecules against the background, resulting in an improvement in the detection of the materials in the cloud.
A tracking telescope collects thermal emissions from the target molecules generated by the radiation. A spectrometer, such as an FTIR spectrometer, receives and resolves the emissions from the cloud into an emissions spectrum. The emissions spectrum is used to identify suspect molecules in the cloud by comparing the detected emissions to the known “fingerprint” vibrational spectrum of the suspect molecules.
The electromagnetic radiation can be microwave, millimeter-wave (MMW), infrared, visible or ultraviolet radiation. The wavelength of the electromagnetic radiation can be selected to be in resonance with the absorption lines of the molecules in the suspected aerosol, or of water vapor or oxygen molecules that are commonly present in the cloud. If the wavelength of the electromagnetic radiation is in resonance with the absorption lines of the aerosol molecules, the emission intensity, as a function of the excitation wavelength, provides an increased discrimination of the aerosol molecules against background materials.
Additional advantages and features of the present invention will become apparent from the following description and appended claims taken in conjunction with the accompanying drawings.
The following discussion of the embodiments of the invention directed to a system for the remote detection and analysis of chemical and biological aerosols is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.
The radiation source 12 can be any laser, millimeter-wave or microwave source suitable for the purposes discussed herein. In this embodiment, the radiation source 12 is a continuous wave (CW) CO2 laser that is tuned to an absorption band of a target aerosol within the cloud 12. Other applicable lasers include HF lasers, DF lasers, solid-state lasers and fiber lasers. In this embodiment, the telescope 16 is a Newtonian type telescope, including a collecting mirror 28 for receiving the emissions 26 from the cloud 12 and a turning mirror 30. However, the telescope 16 can be any suitable telescope for receiving and focusing radiation from a scene consistent with the discussion herein. The spectrometer 18 can be any type of spectral detecting device that provides a spectrum of the detected emissions in the frequency band of interest, such as the infrared frequency band. Suitable examples include Fourier transform infrared spectrometers, g rating tuned o r d ispersed spectrometers, acoustic-optic spectrometers, circularly variable filter spectrometers, linear variable spectrometers and MEMS spectrometers. Alternatively, an imaging spectrometer, such as a hyperspectral imager, can be used to obtain a spatially resolved spectrum.
The source 14, the telescope 16 and the spectrometer 18 can be mounted within a suitable housing, and can be included on a platform capable of scanning over a wide area for increased surveillance. Additionally, the system 10 can be made compact and portable to be readily moved from place to place.
The source 14 can be selected so that the wavelength of the radiation beam 20 is in resonance with a particular target molecule or molecules within the cloud 12 being detected. The wavelength of the radiation beam 20 can also be selected to be in resonance with the absorption lines of water vapor or oxygen molecules commonly present in air. The resonance causes the target molecules, water vapor or oxygen molecules to rotate or vibrate which causes their energy to increase. The radiation energy absorbed by the water vapor, the oxygen molecules or the target molecules in the cloud 12 is thermalized as a result of collision energy transfer causing inter-molecular relaxation. At atmospheric pressure, this thermalization is very rapid. This collisional energy redistribution results in heating the molecules in the cloud 12. An increase in the temperature of the cloud 12 will increase the emission intensity of the molecules in the cloud 12 against the background, resulting in an improved detection of the molecules.
If the wavelength of the electromagnetic radiation is selected to be in resonance with the absorption lines of the target molecules, the returned emission intensity, as a function of the excitation wavelength can be used to provide an additional way for discrimination against possible interference background chemicals. This is because the returned emission intensity from the target molecules should increase substantially as the excitation is tuned to the resonance absorption lines of the target molecules. In contrast, the emission intensity from background chemicals should not increase appreciably at these excitation wavelengths.
The telescope 16 collects the thermal emissions 26 returned from the cloud 12. The spectral content of the emissions 26 is then analyzed by the spectrometer 18. The emission spectrum, typical in the 8-14 micron region, is used to identify the molecules in the cloud 12 by comparing the detected emissions to the known “fingerprint” vibrational spectrum of predetermined molecules. The contrast from the spatially resolved spectrum can further be used to discriminate against any other interfering background chemicals that may be present.
A CW CO2 laser 48 generates a laser beam 50 that is directed by a folding mirror 52 through a transparent window 54 into the chamber 42. In this embodiment, the laser beam 50 is directed out of the chamber 42 through a transparent window 56 at an opposite end of the chamber 42. The windows 54 and 56 can be any transparent window suitable for the purposes discussed herein, such as polished salt windows, zinc selenide windows and other suitable windows having anti-reflective coatings. In one embodiment, the incident power of the laser beam 50 is about 1.5 W. An FTIR spectrometer 58 receives passive emissions 60 from within the chamber 42 through the window 54. The spectrometer 58 resolves the emission spectrum of the aerosol within the chamber 42 that is heated by the laser beam 50 against an ambient temperature background.
The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.
