Abstract:
An aerosol triggering device with an integrating sphere and direct air flow provides a simple and efficient biological aerosol trigger. A method for detecting biological aerosols using the aerosol triggering device also is disclosed.

Description:
GOVERNMENT INTEREST 
     The invention described herein may be manufactured, licensed, and used by or for the U.S. Government. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to biological aerosol detection. In particular, the present invention relates to a triggering device for warning of biological aerosol contaminants. Most particularly, the present invention triggers a suite of biological sample collectors with the positive indication of the presence of an aerosol biological contaminant. 
     2. Brief Description of the Related Art 
     With the threat of biological aerosol contaminants to military units and civilian communities, several systems have been developed to provide standoff detection of the biological agents. Although some of the systems possess substantial range capabilities, such as up to 100 kilometers, to detect generated biological aerosols, the systems are generally large and consume considerable power. In many circumstances, use of the large systems becomes problematic, such as being used in conjunction with a small mobile force, or in isolated areas. In these situations, smaller point sensors are needed. 
     Generally, smaller point sensors provide a reduced capability in detecting aerosol agents. One method of increasing the capability of the sensor was to develop an instrument that provided a trigger for a suite biological sample collectors. The triggering method was based on an increase in the concentration of a particular sized particle, however, the method proved in-effective for field operations. 
     One technology used to determine the presence or absence of biological contaminants includes an instrument called Laser Induced Fluorescence. All biologically based materials are composed of proteinaceous molecules, which auto-fluoresce when exposed to electromagnetic radiation, i.e. light, at an excitation wavelength. The excitation wavelength is any wavelength that couples into the absorption band of the biological compound sample. Once the compound adsorbs the radiation, the radiation is elastically (directly scattered) and inelastically (fluorescence) scattered. The inelastic scatter signature indicates whether the compound is biological in nature. 
     Previous three point sensors, based on laser induced fluorescence of biological materials, have experienced problems. In one design developed by Lincoln Labs of Concord, Mass., laser beams interact with the biological aerosol in a volume that is imaged onto two detectors with two large concave mirrors. Although the design appears to work well, it is very sensitive to misalignment and internal component contamination. Another design developed by Science and Technology Corporation of Hampton, Va. in conjunction with the Laser Standoff Detection Team of Chemical Biological Center, Aberdeen Proving Ground, Md. takes advantage of natural wind flow through the instrument and does not use mechanical pumps. As an open optical system, this design possesses the disadvantage of allowing solar radiation to enter the system, increasing system noise. Optical baffling may decrease the signal noise, but baffles also disrupt airflow through the system. Other point type triggers have not eliminated the problems of laser beam misalignment and instrument contamination. 
     Integrating spheres are optical instruments used primarily for optical calibration of detectors and sources. Integrating sphere are described in “A Guide to Integrating Sphere Theory and Applications” published by labsphere® of North Sutton, N.H., the disclosure of which is herein incorporated by reference. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is therefore an object of the present invention to eliminate the need to maintain alignment of the laser beam within the aerosol sample volume in relation to the point sensor detector elements. 
     These and other objects are achieved by the present invention which includes an aerosol triggering device, comprising a conduit forming a passage for an air flow, said conduit including an aerosol intake port and an aerosol exit port, wherein an air flow is capable of passing into said conduit through said aerosol intake port and exiting said conduit through said aerosol exit port; an optical chamber having at least two detectors capable of detecting an increase in the presence of a biological aerosol within an air flow, said optical chamber being in gaseous and optical communication with said conduit, wherein air flow entering said aerosol intake port is capable of entering said optical chamber prior to exiting through said aerosol exit port; a laser beam entrance window attached to said conduit and permitting entry of a laser beam into said conduit and said optical chamber; a laser beam exit window attached to said conduit capable of optical alignment with said laser beam entrance window; and, a laser beam system having a laser beam generation source capable of optically directing a laser beam of a selected frequency into said conduit and said optical chamber, wherein biological aerosol contaminants within an air flow emit elastic and inelastic scattering. The optical chamber comprises an integrating sphere. 
     The present invention further includes a method for detecting biological aerosols comprising the steps of providing an aerosol triggering device, comprising a conduit forming a passage for an air flow, said conduit including an aerosol intake port and an aerosol exit port, wherein an air flow is capable of passing into said conduit through said aerosol intake port and exiting said conduit through said aerosol exit port, an optical chamber having at least two detectors capable of detecting an increase in the presence of an aerosol within an air flow, said optical chamber being in gaseous and optical communication with said conduit, wherein air flow entering said aerosol intake port is capable of entering said optical chamber prior to exiting through said aerosol exit port, a laser beam entrance window attached to said conduit and permitting entry of a laser beam into said conduit and said optical chamber, a laser beam exit window attached to said conduit capable of optical alignment with said laser beam entrance window, and, a laser beam system having a laser beam generation source capable of optically directing a laser beam of a selected frequency into said conduit and said optical chamber, wherein biological aerosol contaminants within an air flow emit elastic and inelastic scattering; opening the aerosol intake port, wherein an air flow enters the conduit and into the optical chamber; energizing the laser beam system, wherein the laser beam generation source directs a laser beam into the optical chamber, causing an interaction between the laser beam and air flow; and, detecting the resulting interaction between the laser beam and air flow, wherein the presence of biological contaminants is determined. 
     Other and further advantages of the present invention are set forth in the description and appended claims. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 displays a cut away view of a preferred embodiment of the biological aerosol trigger of the present invention; and, 
     FIG. 2 displays a cut away view of a second preferred embodiment of the biological aerosol trigger of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is a system that is capable of indicating the presence of biological contaminants based on laser induced fluorescence of proteinacious compounds, The problems of alignment at the interface of the laser beam and aerosol are eliminated by providing an integrating sphere for directing the fluorescence and scatter onto the detectors, and contamination of the internal components is mitigated by providing a simple flow path of the aerosol or air flow. The sensitivity of tile system is unaffected by laser beam misalignment within the air flow by using an integrating&#39;sphere to couple the elastic and inelastic scatter onto the detectors. Inner components of the system are protected from contamination by confining the aerosol flow within a transparent tube. Additionally, the present invention eliminates aerosol losses in the system. 
     Referring to FIG. 1 , the biological aerosol trigger  10  of the present invention is shown. The biological trigger comprises an aerosol intake port  12 , aerosol/laser beam interface chamber  14 , optical chamber  16 , aerosol/laser beam exit chamber  18 , laser beam turning mirrors  20 , ultraviolet laser  22 , HEPA filters  24 , laser beam dump  28 , and control and signal acquisition electronics  32 . Air flow  102 , also referred to as an aerosol, within the present invention includes an air sample to be analyzed for biological aerosol contaminants. Generally, air is acquired from an open environment through the aerosol intake port  12 . The air flow  102  may contain contaminants in amounts of from about 10 particles per liter or more to be detected. 
     As seen in FIG. 1, an air flow  102  containing a possible biological aerosol contaminant flows into the biological aerosol trigger  10  through the intake port  12 , and passes into a gaseous channel, or conduit  30 , that is in communication with the aerosol/laser beam interface chamber  14 . The air flow  102  passes into the aerosol/laser beam interface chamber  14 , and within the aerosol/laser beam interface chamber  14 , the air flow  102  interfaces with a laser beam  100 . The aerosol/laser beam interface chamber  14  comprises an upper compartment which contains a laser beam entrance window  34  and clean air inlet  42 , and a lower compartment containing one or more aerosol inlets  44 , which couples to the upper compartment through a cone shaped nozzle  38 . As shown in FIG. 1, after the air flow  102  enters the biological aerosol trigger  10  through the aerosol intake port  12 , the air flow  102  passes through at least one aerosol inlet  44  into the lower compartment of the aerosol/laser beam interface chamber  14 . 
     Within the aerosol/laser beam interface chamber  14 , the air interacts with the laser beam  100  that originates from the ultraviolet laser  22 . The laser beam  100  is directed into the aerosol/laser beam interface chamber  14 , by several laser beam turning or steering mirrors  20 , through a laser beam entrance window  34 . The laser beam  100  enters through the laser beam entrance window  34  into the aerosol/laser beam interface chamber  14 , and proceeds through the center axis of the upper compartment of the aerosol/laser beam interface chamber  14 , through a cone shaped nozzle  38  and into the lower compartment of the aerosol/laser beam interface chamber  14 . Additionally, clean air is drawn through a clean air inlet  42  into the upper compartment of the aerosol/laser beam interface chamber  14 , and down through a small hole in the cone shaped nozzle  38  into the lower compartment. This action prohibits aerosols within the air flow  102  from backing into the upper compartment of the aerosol/laser beam interface chamber  14  and fouling the laser beam entrance window  34 . The air flow  102  is confined within the aerosol/laser beam interface chamber  14  to eliminate contamination of any optics and detectors of the system, particularly the laser beam entrance window  34  by contaminant aerosols. Once the air flow  102  enters the aerosol/laser beam interface chamber  14 , the air flow  102  proceeds along the axis of the aerosol/laser beam interface chamber  14  with the laser beam  100 , where the laser beam  100  and air flow  102  enter the optical chamber  16 . 
     Laser beam turning mirrors  20  direct the laser  100  from the ultraviolet laser  22 . The ultraviolet laser  22  may be any suitable laser that provides an appropriate wavelength for the detection of biological contaminants, with the type and power of the ultraviolet laser  22  being determinable by those skilled in the art. The intensity and wavelength of the laser beam  100  is controlled by the control and signal acquisition electronics  32 , with the laser beam  100  monitored by a laser beam power monitor  72  that provides a feedback to the control and signal acquisition electronics  32  for modulation and refinement of the laser beam  100 . 
     Within the optical chamber  16 , the air flow  102  continues to be exposed to the laser beam  100 , allowing laser scatter and fluorescence to be detected within the optical chamber  16 . The optical chamber  16  of the present invention, shown in FIG. 1, comprises an integrating sphere. The optical chamber  16  includes a direct illumination stop  48 , transparent tube  50 , optically filtered detectors  52 , and control and signal acquisition electronics  32 . The laser beam  100  and air flow  102  proceed down the axis of the biological aerosol trigger  10  into the optical chamber  16 . Within the optical chamber  16  the elastic and inelastic scatter resulting from the laser beam  100  and aerosol  102  interaction, referred herein as the interaction volume, is viewed by the detectors  52 . The signals generated by the detectors  52  are passed on to the control and signal acquisition electronics  32 . The detectors  52  may either be mounted at optical ports of the integrating sphere for free space operation, or coupled to the integrating sphere via fiber optics. By using the integrating sphere to optically couple the scattered light onto the detectors  52 , the need to align laser beam  100  within the aerosol interaction volume, relative to tile detector elements  52 , is eliminated. The integrating sphere is optically unique because all irradiance onto the sphere is uniform and independent of the location of the source. Regardless of where the laser beam  100  is located within the air flow  102 , the detectors  52  receive the same signal. As most trigger algorithms depend on the ratio from the resulting elastic and inelastic scatter signals, this aspect of the present invention provides a significant advantage. In systems having signal strength received at the detectors that vary as a function of laser beam location within the interaction volume, a small misalignment may lower sensitivity or even fail to indicate the presence of a biological aerosol. 
     As the aerosol  102  proceeds through the optical chamber  16 , the transparent tube  50  confines the air flow  102 . This eliminates contamination of the inner components of the optical chamber  16 , and significantly increases the usefulness of the system. A coated transparent tube  50  may be used to filter out some of the laser scatter to reduce detector  52  saturation. Laminar flow is maintained through the transparent tube  50  to allay aerosol deposits on the walls of the transparent tube  50 . As the transparent tube  50  becomes too contaminated to use, it may either be replaced or cleaned. In either case, the transparent tube  50  provides a less expensive, easier maintenance, and simpler replacement than other inner components of the biological aerosol trigger  10 . 
     Also shown in FIG. 1, once the aerosol  102  and laser beam  100  exit the optical chamber  16 , they pass into the aerosol/laser beam exit chamber  18 , which is similar to the aerosol/laser beam interface chamber  14  with the exception of the laser beam  100  and air flow  102  direction. On exiting the optical chamber  16 , the air flow  102  passes through the conduit  30  into the aerosol/laser beam exit chamber  18 , where it is drawn out of the biological aerosol trigger  10 . Like the aerosol/laser beam interface chamber  14 , there is a clean air inlet having HEPA filters  24  to eliminate fouling of the laser beam exit window  68 . The aerosol  102  proceeds through the upper compartment  56  of the aerosol/laser beam exit chamber  18  and is drawn out of the biological aerosol trigger  10  through aerosol outlets  62  by a vacuum pump  26 . A laser beam exit window  68  at the bottom of the aerosol/laser beam exit chamber  18  is attached to the conduit  30  and is in optical alignment with the laser beam entrance window  34 . The laser beam  100  proceeds through the upper compartment  56  of the aerosol/laser beam exit chamber  18  through a cone shaped nozzle  66 , and into a lower compartment  58 . Once the laser beam  100  passes through the aerosol/laser beam exit chamber  18  and out the laser beam exit window  68 , it enters the laser beam dump  28 . The laser beam dump  28  aids in reducing back-scattering of the laser beam  100  into the biological aerosol trigger  10 . The laser beam exit window  68  is kept clean by the barrier of clean air to the air flow  102 . 
     The HEPA filters  24  are in air flow communication with the conduit  30  and work in combination with the vacuum pump. Air flow  102  may be drawn through the biological aerosol trigger  10  by the vacuum pump, after which the air flow  102  passes outside of the biological aerosol trigger  10 . Clean air is drawn in through the HEPA filters  24  that purify the incoming outside air into the inside of the aerosol/laser beam interface chamber  14  and aerosol/laser beam exit chamber  18 . The clean air in turn prevents contamination of the laser beam entry and exit windows  34  and  68  from exposure to the air flow  102 . The laser beam  100  enters the aerosol/laser beam exit chamber  18  through a nozzle that restricts air flow  102  from access into the lower compartment  58 . The combination of the clean air and nozzle provides a barrier to the air flow  102  to come in contact with the laser beam exit window  68 . This barrier also occurs at the laser beam entry window  34 . This reduces the maintenance of the biological aerosol trigger  10 , and increases the reliability of the system. 
     A second embodiment of the present invention is shown in FIG.  2 . As seen in FIG. 2, the aerosol/laser beam interface chamber of FIG. 1 was eliminated, and a straight intake tube  12  was incorporated. The laser beam  100  exits at an aerosol exit/laser beam entrance chamber  128 . The second embodiment reduces aerosol  102  losses within the biological aerosol trigger  10 . The biological aerosol trigger  10  of the second embodiment incorporates within the aerosol exit/laser beam entrance chamber  128  a combination laser beam dump and weather cap. The functioning of the optical chamber  16 , ultraviolet laser  22  and related systems, and control and signal acquisition electronics  32  remain the same as the first embodiment of FIG.  1 . 
     The second embodiment of the present invention, incorporating minimal air flow disruptions, provides a simplified air flow  102  for the biological aerosol trigger  10  and reduces noise problems. As found in FIG. 1, an air flow  102  containing a possible biological aerosol contaminant flows into the biological aerosol trigger  10 , and interfaces with a laser beam  100 , allowing laser scatter and fluorescence to be detected within the optical chamber  16 . However, the present invention as seen in FIG. 2 reduces the number of changes in air flow  102  direction, while preserving the interaction between the laser beam  100  and air flow  102 . 
     Air flow  102  is acquired in the biological aerosol trigger  10 , shown in FIG. 2, from an open environment through the aerosol intake port  12  that includes a laser beam dump  128 , described below. The aerosol intake port  12  may be configured in the form of a weather cap, when desired. Once inside of the biological aerosol trigger  10 , the air flow  102  remains confined from the component parts of the biological aerosol trigger  10  to eliminate contamination of any optics and detectors of the system. The air flow  102  enters the biological aerosol trigger  10  through the aerosol intake port  12 , passes into a air intake tube or gaseous/optical conduit  30 , and continues into the aerosol/laser beam interface chamber  14 . In the aerosol/laser beam interface chamber  14 , the air flow  102  enters one side of the optical chamber  16 , as the laser beam  100  enters the optical chamber  16  from the opposite side. Within the optical chamber  16 , the air interacts with the laser beam  100 . After reaction with the laser beam  100 , the air flow  102  continues to the aerosol exit in the upper chamber  140  of the aerosol exit/laser entrance chamber  118 . Above the nozzle  138 , the air flow  102  exits the biological aerosol trigger  10 , being drawn out through the vacuum pump  26 . The laser beam  100  enters the aerosol exit/laser entrance chamber  118 , below the air flow  102  exit, at the laser entrance window  134 . Additionally, clean air is drawn through a clean air inlet  42 , after passing a HEPA filter  24 , and into the lower compartment  136  of the aerosol exit/laser entrance chamber  118 . The clean air enters between the laser beam entrance window  134  and the air flow  102  exit, passing in the opposite direction of the air flow  102  through the biological aerosol trigger  10 . The clean air passes up through a small hole in the cone shaped nozzle  138 , and enters the upper compartment  140 , thereby stopping the air flow  102  from entering the cone shaped nozzle  138 . This action prohibits aerosols within the air flow  102  from entering into the lower compartment  136  and fouling the laser beam entrance window  134 . The laser beam  100 , originating from the ultraviolet laser  22 , is directed into the optical chamber  16  by laser beam turning or steering mirrors  20 . On reflection from the steering mirrors  20 , the laser beam  100  passes through a laser beam entrance window  134 , conduit  30 , and into the optical chamber  16 . Once the laser beam  100  exits the optical chamber  16 , it passes through the conduit  30  and into the beam dump  128 . 
     In either embodiment, the transparent tube  50  provides a less expensive, easier maintenance, and simpler replacement than other inner components of the biological aerosol trigger  10 . The optical coupling scheme provided by the integrating sphere provides minimizes the importance of beam location and minimizes the required number of optical interface designs. By incorporating an integrating sphere, having an inner surface of a Lambertian reflector, radiation falling on the surface is diffusely reflected such that the radiance is not a function of angle and is given by: 
     
       
           I   s   =ρI/π,   (I) 
       
     
     where ρ is the surface reflectance and I is the irradiance on the surface. The unique trait of an integrating sphere is that the irradiance at the sphere surface is uniform and independent of the location of the source. With the detector placed on the sphere then the radiance on the detector is: 
     
       
           d φ=[ρφ s /(1−ρ4 πR   2 )] dA   (II) 
       
     
     where φ s  is the source radiance, R is the sphere radius and dA is the area of the detector. The entrance and exit ports provide a modified equation of: 
     
       
           d φ=[ρφ s /(1−ρ)(1 −f )4 πR   2   ]dA   (III) 
       
     
     where 
     
       
         ƒ=( A   i   +A   e )/4 πR   2   (IV) 
       
     
     and A i  and A e  are the areas of the entrance and exit ports respectively. As seen from equation (III), the radiance at the detector is not a function of source location or proximity. The optical efficiency of the sphere is only few percent, but with a high power laser and a large aerosol sampling volume, the low optical efficiency may be eliminated. Additionally, when using the integrating sphere, the source cannot directly illuminate the detector and all ports must be small compared to the surface of the integrating sphere. 
     Contamination is maintained at a minimum by limiting the flow to the center of the integrating sphere  46  through the tube  50 . The tube  50  comprises transparency parameters at the scatter and fluorescence wavelengths, such as a fused silica tube  50  having a transparency of from about 200 nanometers into the near infrared. Other tubes  50  of the present invention, include for example, a pyrex or transparent Teflon® tube. The laser wavelength used for the laser induced fluorescence technique of the present invention is 266 nanometers (the fourth harmonic of a Neodimium:Yittrium Aluminum Garnet (Nd:YAG) laser), although other wavelengths well known to those skilled in the art may be used to excite various constituents of biological aerosols. The tube  50  is kept clean by maintaining a laminar flow through the tube  50 . This keeps contamination to a minimum and allows easy tube replacement when it becomes contaminated. A design that permits easy replacement of the tube includes, for example, an insertion locking mechanism. 
     EXAMPLE 1 
     A biological aerosol trigger is used against low concentrations of biological simulants that are disseminated wet or dry. Algorithm parameters are adjusted for eliminating signal noise. Air flow is drawn into the system, which passes to the optical chamber. Signal noise interfered with parts of the test. Once biological contaminants are detected in the air flow, the system triggers a suite of detectors to identify the biological contaminant, with some false triggering events from system noise. 
     EXAMPLE 2 
     System tests using the system of the first embodiment indicated that the biological aerosol trigger breadboard improved the performance of the system. The flow tests were run using kaolin dusts and egg albumin and the data indicated a strong fluorescence for the egg albumin and negligible fluorescence with kaolin dust. The trials were conducted using a high concentration of dry aerosols. On disassembly after the trials, the instrument had an accumulation of dust in the lower part of the aerosol/optical interface chamber where the air flow funnels into the Pyrex tube. This was caused by turbulence where the air flow was constricted. Data acquisition and power equipment was mounted in rack beneath the system. The aerosol/optical interface chambers provided adequate protection to keep the optics clean. The laser windows on both the top and bottom were free of dust even after four weeks of trials in desert conditions. 
     EXAMPLE 3 
     A system test of the biological aerosol trigger of the second embodiment shows reduced dust accumulation at the completion of the testing. Performance problems of the biological aerosol trigger breadboard was attributed to an interrupted flow through the system causing a reduced signal level in the system. The second embodiment of the biological aerosol trigger reduces aerosol deposition through a straight-line flow, preventing particle losses in the system due to accumulation at bends in the flow path. 
     It should be understood that the foregoing summary, detailed description, examples and drawings of the invention are not intended to be limiting, but are only exemplary of the inventive features which are defined in the claims.