Patent Publication Number: US-2007097366-A1

Title: Optical system and method for detecting particles

Description:
BACKGROUND  
      Optical systems and methods are useful in detecting particles. One type of optical system is a fluorescent biological particle detection system. Particulate detection has certain security-related uses, such as, for example, ascertaining the introduction of potentially hazardous air-borne biological particles to an environment. Determining the size of air-borne particles can assist in identifying whether the particles are respirable or not. Further, air-borne particles may be subjected to a light source capable of inducing an emission of fluorescence from the particles. For example, fluorescence detected in the 400 to 540 nanometer (nm) range signals the presence of nicotinamide adenine dinucleotide hydrogen, which is indicative of biological activity or viability. See, for example, U.S. Pat. Nos. 5,701,012 and 5,895,922.  
      Optical particle detection also is used in commercial smoke detectors, where optical scatter detection is used to signify the presence of an airborne particle. Particle counters also are used in the semiconductor industry to monitor air cleanliness for the particle-sensitive photolithography step. By measuring the absorption of certain optical wavelengths, one also can measure the presence of specific chemicals, such as NO x , CO 2 , or carbon monoxide. Fourier-transform infrared spectroscopy (FTIR) detection can be used to identify the presence of ice and water vapor. In this sense, the term “particle” refers to any individual mass or collection of masses that can interact with energy—most typically electromagnetic energy.  
      Disadvantages have been noted in known particle detector systems. One disadvantage is that known detector systems have high noise to signal ratios, due primarily to stray light and a low particle detection cross-section. Known particle detector systems may utilize lasers or laser diodes as light emitting sources. Known fluorescent particle detector systems utilize a collimating lens prior to striking the target particles. Also, known particle systems utilize conduits that are not fully optically transparent.  
     SUMMARY  
      One embodiment of the invention described herein is directed to a particle detection system that includes at least one light emitting source for generating light, a non-collimating reflector for redirecting the generated light, an area through which a particle stream may be transmitted and into which the generated light is redirected, a collimating reflector, and at least one detector. At least a portion of energy formed by the redirected generated light striking one or more particles in the particle stream is directed to the collimating reflector and redirected to the detector(s).  
      Another embodiment of the invention is directed to an optical system for detecting particles that includes an air-sheath inlet through which a curtain of air is introduced, a conduit radially interior to the air-sheath inlet through which a particle stream is transmitted, and a pumping system consisting of a single pump positioned downstream of the air-sheath inlet and the conduit and configured to enable transmission of the particle stream and introduction of the curtain of air.  
      Another embodiment of the invention is an optical system for detecting particles that includes a plurality of light emitting sources for generating light and a light redirecting system consisting of a single reflector for redirecting the generated light. Each of the light emitting sources transmits generated light at the single reflector that redirects the generated light toward a stream of particles.  
      Another embodiment of the invention is a method for detecting particles. The method includes introducing a stream of particles into an enclosed container, transmitting light at a non-collimating reflector, redirecting the light to a focal point within the stream of particles, collecting incident light formed by the striking of the generated light upon at least one particle within the stream of particles, and transmitting the incident light to at least one detector.  
      These and other advantages and features will be more readily understood from the following detailed description of preferred embodiments of the invention that is provided in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1   a  illustrates a particle detection system constructed in accordance with an exemplary embodiment of the invention.  
       FIG. 1   b  is a partial view of a portion of the particle detection system of  FIG. 1   a  beneath the cover plate.  
       FIG. 2  is a cross-sectional view of a portion of the particle detection system of  FIG. 1   a  taken along line II-II.  
       FIG. 3  is a perspective view of the particle detection system of  FIG. 1   a.    
       FIG. 4  is a schematic view illustrating the modification of generated light to fluorescent light and then to reflected light within the particle detection system of  FIG. 1   a.    
       FIG. 5   a  is a schematic view illustrating generated light being transmitted into an excitation zone within the particle detection system of  FIG. 1   a.    
       FIG. 5   b  is a schematic view illustrating fluorescent light being transmitted from the excitation zone and reflected light being transmitted to a detector within the particle detection system of  FIG. 1   a.    
       FIG. 6  illustrates a process for identifying a particle type within a particle stream in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      Referring specifically to  FIGS. 1   a - 3 , there is shown an optical detection system  100  that includes an enclosure  102 , a detector  104 , a first reflector  106 , a second reflector  110 , an intake mechanism  113 , and a pump  136 . The optical detection system  100  may take the form of a fluorescent particle detection system. In certain embodiments, the first reflector  106  may be a non-collimating reflector, and in some embodiments the second reflector  110  may be a collimating reflector. The term “non-collimating” should be understood to refer to a reflective surface that does not have as a primary purpose the collimating of light, although some degree of collimation may nevertheless exist. First reflector  106  may include a coating  108 , while the second reflector  110  may include a coating  112 . The reflective coatings  108 ,  112  may be disposed on an inner surface (meaning a surface facing the interior  130  of the enclosure  102 ), thus serving to reflect any light striking such surface from within the enclosure  102 . Alternatively, the reflective coatings  108 ,  112  may be disposed on an outer surface (meaning a surface facing away from the interior  130  of the enclosure  102 ), thus serving to refract any light striking, respectively, the first reflector  106  or the second reflector  110  from within the enclosure  102 . In some aspects, the profile for the first and second reflectors  106 ,  110  may be curved, parabolic, spherical, holographic, or elliptical. As illustrated, the first reflector  106  has an elliptical profile, while the second reflector  110  has a spherical profile.  
      The intake mechanism  113  includes a pair of concentric inlets. Specifically, the intake mechanism  113  includes a particle inlet  114  having an opening  116  extending through a cover plate  120  and into the interior  130  of the enclosure  102  and concentric air inlet  122  disposed radially exterior to the particle inlet  114 . The cover plate  120  is attached to a surface of the enclosure  102  in such a way as to enclose the air inlet  122  underneath. An air filter  124  is attached to an open end  121  of the cover plate  120  to allow for filtered air to be transmitted through the air inlet  122 .  
      The air inlet  122  is concentric with the opening  116  of the particle inlet  114 . The particle inlet  114  may be attached to the cover plate  120 , in which case the air inlet  122  may extend completely around the particle inlet  114 . In other embodiments, and as illustrated in  FIG. 1   b,  the particle inlet  114  is attached to the enclosure  102 , and therefore the air inlet  122  does not extend completely around the particle inlet  114 . The openings for the air inlet  122  may be smooth-walled or they may be grooved to provide a spiral flow of air through the air inlet  122  and into the interior  130  of the enclosure  102 . In other embodiments, the air inlet  122  may be nonexistent and another optically transparent conduit may be utilized to segregate the particle stream  118  from the remaining environment of the interior  130  of the enclosure  102 .  
      Particles are introduced into the interior  130  of the enclosure within a particle stream  118  ( FIG. 2 ). Air is introduced into the interior  130  of the enclosure by passing an air stream  126  through the air filter  124  to produce a filtered air stream  128 . A filtered air stream  128  is advantageous in that it lessens the likelihood that particulates from the air stream can cause an erroneous fluorescence signature for the particle stream  118 . The pump  136  provides the pressure differential necessary to pull both the particle stream  118  and the filtered air stream  128  into the interior  130  of the enclosure  102 . Various factors are taken into account to enable the air stream  126  extending into the interior  130  of the enclosure  102  to serve as an air-sheath  132  to the particle stream  118 . Specifically, the pumping power of the pump  136 , the distance into the interior  130  that the particle inlet  114  extends, the initial velocity of the particle stream  118 , the size of the particle inlet  114 , and the size of the sheath flow inlet  122  all may be manipulated to ensure that the total flow of the air-sheath  132  is sufficiently less than the total flow of the particle stream  118  within the interior  130  to fully enshroud the particles within the particle stream  118 . Nonetheless, the velocity of the air-sheath  132  is greater than the velocity of the particle stream  118 . The difference in the velocities of the air-sheath  132  and the particle stream  118  within the interior  130  creates a pressure differential causing the particle stream  118  to remain within the air-sheath  132 . Further, the various factors are manipulated to ensure that the particle stream  118  has no turbulent flow within the air-sheath  132 . If either the velocity of the flow of air constituting the air-sheath  132  or the velocity of the radially inner particle stream  118  is too high, turbulence may be induced. Turbulence may coat the optical components of the optical detection system  100  and destroy optical sensitivity. In general, a turbulent flow is acceptable as long as particles do not coat optical surfaces, such as, for example, surfaces of a window  144 , an optical filter  140 , a beam dump  138 , or the coating  112 .  
      The air-sheath  132  serves as an optically transparent conduit serving to isolate the particle stream  118  from the remainder of the interior  130 . It should be appreciated that other optically transparent conduits may be utilized to isolate the particle stream  118 , such as, for example, poly ether ether ketone (PEEK), Teflon AF, fused silica, quartz, sapphire, or other transparent, low auto-fluorescent media capable of being formed into a conduit.  
      As the air-sheath  132  and the particle stream  118  extend closer to the pump  136 , the air-sheath  132  begins to collapse radially inwardly toward the particle stream  118 , and both streams  118 ,  132  exit the interior  130  through an outlet  134 , which is in fluid connection with the pump  136 . Through the use of the air-sheath  132 , the particle stream  118  is isolated from the environment through an optically transparent mechanism, thereby enabling a more accurate optical measurement of particles within the particle stream  118 .  
      An additional benefit of the air-sheath  132  is that it can assist in cleaning the interior walls of the enclosure  102 . Further, by ramping up the pump  136  intermittingly, a turbulent regime can be initiated to clean the interior  130  of the optical detection system  100 . Optionally, ultrasonic waves may be used to clean the interior walls of the enclosure  102 .  
      With specific reference to  FIGS. 4-5   b,  next will be described the optics of the optical detection system  100 . One or more light sources are located beneath the first reflector  106 . As illustrated in FIG. Sa, a first light emitting source  142  is disposed upon a surface  141 . An optional second light emitting source  242  is also shown disposed upon the surface  141 . It should be appreciated that more than two light emitting sources may be positioned beneath the first reflector  106 . The positioning of the light emitting sources  142 ,  242  is accomplished to ensure that light reflected, refracted or diffused from the first reflector  106  is transmitted into an excitation zone  150  that is located within the particle stream  118  within the interior  130 . Specifically, geometrical optics are utilized whereby upon determining the location of the target, i.e., the excitation zone  150 , the placement of the light emitting source(s) is accomplished by working backward, using known distances and angles. It should be appreciated that the excitation zone  150  should be located at a position within the particle stream  118  that is at a distance from the position at which the air-sheath  132  begins to collapse inwardly.  
      As illustrated, the first light emitting source  142  emits a light  146  which strikes the coated surface of the first reflector  106  and bounces into the excitation zone  150  at a focal spot  148 . The second (optional) light emitting source  242  emits a light  246  which strikes the coated surface of the first reflector  106  and reflects into the excitation zone  150  at a focal spot  248 . It should be appreciated that any suitable light emitting source  142 ,  242  may be utilized, such as, for example, light emitting diodes, including surface-emitting light emitting diodes, ultraviolet light emitting diodes, edge-emitting light emitting diodes, resonant cavity light emitting diodes, flip-chipped light emitting diodes, gas-discharge lamps, mercury lamps, filament lamps, black-body radiators, chemo-luminescent media, organic light emitting diodes, phosphor upconverted sources, plasma sources, solar radiation, sparking devices, vertical light emitting diodes, and wavelength-specific light emitting diodes, lasers, and laser diodes, and any other suitable light emitting device capable of emitting a sufficiently high intensity light of the desired wavelength. By “sufficiently high intensity light” is meant a light of sufficient intensity to induce an effective optical signal, such as particle fluorescence. The term “wavelength” should be understood to encompass a range of wavelengths and to refer to a spectral range of electromagnetic energy. Furthermore, the light emitting source  142 ,  242  may be pulsed to achieve the desired intensity of light without sacrificing reliability or lifetime. Another advantage of a very fast pulsed source, such as an LED, would be to synchronize the detector to the source for the purpose of improving the signal to noise ratio. A heat sink may be attached to the light-emitting source  142 ,  242  to enhance heat dissipation.  
      An optically transparent window  144  may be positioned between the first reflector  106  and the interior  130  of the enclosure  102 . The optically transparent window  144  may include an optical filter for lessening the amount of parasitic light that is in the range of the detection spectrum from entering the interior  130  of the enclosure and producing parasitic signals in the form of scattered light.  
      A particle  152  traveling within the particle stream  118  enters the excitation zone  150 . As the particle  152  encounters the focal spot  148 ,  248 , the redirected generated light  146 ,  246  strikes the particle  152 , creating an optical signal  154 ,  254 . It should be appreciated that the optical signal may be fluorescence, absorption, transmission, reflectance, and/or scattering. For ease of description, the optical signals  154 ,  254  will be described herein as being fluorescent in nature. Most of the fluorescent light  154 ,  254  scatters throughout the interior  130  of the enclosure  102 . This backscattered light eventually dissipates into a beam dump  138 . The backscattered light may be used to detect dirtiness within the interior  130  of the enclosure  102 . For example, a predetermined intensity of backscattered light may represent a certain threshold level of cleanliness within the enclosure  102 , and any backscattered light lacking that predetermined intensity to a certain degree may represent a dirtier interior  130 .  
      The remaining fluorescent light  154 ,  254  strikes the coated surface of the second reflector  110 . The second reflector  110  may be a collimating reflector. Reflected light  156 ,  256  is directed toward the detector  104 . The detector  104  may be a photoconductor, a photodiode, a photomultiplier tube, or an avalanche photodiode, or any photo detector capable of detecting single photons or collections of single photons. An optional optical filter  140  may be positioned between the second reflector  110  and the detector  104 . The optical filter  140  may be filtered to specific wavelengths, thus serving to eliminate one or more portions of the light spectrum to decrease the noise to signal ratio.  
      The first reflector  106 , the second reflector  110  and the detector  104  are all shown to be orthogonal to each other. Such an arrangement is advantageous in that neither reflector is in direct sight of the other, thereby lessening the reflection of direct light  146 ,  246  into the detector  104 . It should be appreciated, however, that absolute orthogonality may not be required, and the first reflector  106  may be somewhat less than or more than ninety degrees offset from the second reflector  110 , which in turn may be somewhat less than or more than one-hundred and eighty degrees offset from the detector  104 .  
      The light emitting sources  142 ,  242  and the detectors  104  may be tuned to the absorption and emission profiles of various particles. For example, at least one light emitting source  142 ,  242  may emit light at a first wavelength at which a predetermined particle fluoresces while another of the light emitting sources  142 ,  242  may emit light at a second wavelength at which a second predetermined particle fluoresces. It should be appreciated that certain particles fluoresce at more than one wavelength, and thus the first and second predetermined particles may indeed be the same particles. Alternatively, each of the light emitting sources  142 ,  242  may emit light at a wavelength at which several types of particles fluoresce and each of the detectors  104  is tuned to detect the fluorescent light at wavelengths differing from the other of the detectors  104 .  
      When several excitation wavelengths are employed and corresponding emission spectra are collected, this collection of spectra constitutes an excitation-emission map. Suitable methods for determination of fluorescence-excitation maps are provided in, for example, U.S. Pat. Nos. 6,166,804 and 6,541,264. Fluorescence excitation-emission maps are useful because they provide a more comprehensive spectral signature for a single species and provide a more detailed capability to reveal if more than one fluorescent species are present in a measured sample.  
      For example, a 280 nm UV source and 365 nm UV source can be turned on alternately such that an incoming particle stream is hit with one UV wavelength at a time. Bacteria will fluoresce primarily in the 340 nm range, due to protein fluorescence, upon exposure to 280 nm UV radiation. Bacteria will fluoresce primarily in the 430-550 nm range upon excitation with 365 nm UV light, due to NADH and flavin fluorescence. In contrast, many common fluorescent interferents, such as diesel soot and many vegetable oil aerosols, show significant fluorescence at only one of these excitation wavelengths. Thus, with one photo detector optically filtered at 340 nm and another photo detector optically filtered at 430-550 nm, a sufficient algorithm can be developed for discriminating airborne bacteria from common interferents. Table 1 provides a summary of fluorescence ranges for bio-agents and common interferents exposed to light at various wavelengths.  
                           TABLE 1                               λ excit  =           Agent   λ excit  = 280 nm   340/365 nm   λ excit  = 405 nm                  Vegetative   Tryptophan   NADH + Flavins   Flavins       Bacteria   (320-360 nm);   (430-600 nm)   (500-600 nm)           Flavins           (500-600 nm)       Spores   Tryptophan &amp;   Possible NADH,   Flavins           Flavins   but dim   (500-600 nm)       Viruses   Tryptophan &amp;   Non-detectable   Non-detectable           Flavins       Toxins   Tryptophan   Non-detectable   Non-detectable       Vegetable Oil   Non-detectable   400-550 nm   450-500 nm       Diesel Soot   Dim 380-500 nm   Dim 380-500 nm   410-650 nm       Fluospheres   Dim 280 nm   400-500 nm   Non-detectable       Road Dust   Non-detectable   Non-detectable   Non-detectable                  
 
      With specific reference to  FIG. 6 , next will be described a method for analyzing a particle stream to ascertain the presence of predetermined particles of interest. At Step  200 , at least one light emitting source, such as light emitting sources  142 ,  242 , is positioned such that a focal spot  148 ,  248  for light emitted from the light emitting sources is positioned within the particle stream  118 . Such positioning may utilize geometrical optics by working backward from the desired location of the focal spot to the appropriate location of the light emitting source.  
      At Step  205 , a pair of reflectors, such as reflectors  106 ,  110 , is located within an enclosure  102 . The reflectors are placed relative to one another such that direct light from the first reflector  106  does not impinge directly upon the second reflector  110 . For example, the reflectors  106 ,  110  may be placed orthogonal to one another. At Step  210 , at least one detector, such as detector  104 , is located relative to the two reflectors. Specifically, the detector  104  is placed so as to receive light directly from the second reflector  110  but be out of direct sight of the first reflector  106 . For example, the detector  104  may be placed directly opposite the second reflector  110  and orthogonal to the first reflector  106 .  
      At Step  215 , a pump, such as pump  136 , is engaged to induce a pressure differential within the enclosure  102 . At Step  220 , a particle stream is introduced into an environmentally isolated location. As described with reference to  FIGS. 1   a - 5   b,  a particle stream  118  is introduced through a particle inlet  114  into the interior  130  of the enclosure  102  and concentrically within the air-sheath  132 . The pump serves to pull both the air-sheath  132  and the particle stream  118  through the enclosure  102 .  
      While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. For example, while the enclosure  102  is illustrated as being cubic, it should be appreciated that the enclosure  102  may take any suitable configuration. Further, while optional optical filters have been described with reference to the detector  104  and the window  144 , it should be appreciated that each light emitting source may itself incorporate an optical filter. Also, while the velocity of the illustrated air-sheath  132  is described as being greater than the velocity of the particle stream  118 , it should be understood that the velocity of the air-sheath  132  can be any velocity relative to the particle stream  118  velocity. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.