Abstract:
A plastic particle detector for detecting biological and other fluorescent materials is disclosed. The detector detects the fluorescence and scattering signals from these materials using deep UV excitation. The detector is fabricated using plastic materials and exploits the properties of lower manufacturing costs, lower materials costs, light weight, ruggedness and assembly ease offered by plastics, while eliminating stray fluorescence signals ordinarily generated by plastic materials.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured, used, and/or licensed by or for the Government of the United States of America. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to particle detectors and more particularly to particle detectors made with plastic material and having lower cost and improved ruggedness. 
     BACKGROUND 
     UV (ultraviolet) induced fluorescence continues to be one of the most promising techniques for the real time detection of biological agents and other particles. A number of detectors developed around this technique have shown that UV induced fluorescence provides a means to rapidly and accurately detect biological agents at very credible limits of detection. Among these devices are the Biological Agent Warning Sensor (BAWS) developed by MIT Lincoln Laboratory, along with several others described by U.S. Pat. Nos. 5,701,012; 5,895,922 and 6,831,279. Additional devices are described in U.S. Pat. Nos. 5,999,250; 6,885,440; 6,967,338; 7,375,348; 7,567,391 and 7,852,469. 
     These detectors work under the principle that aerosolized biological agents will fluoresce and scatter light when excited with UV light. The UV light source can be a laser, LED with optics, or any other emission source that can produce a beam of light which can then be pointed towards aerosol particles. The excitation wavelengths are typically in the 405 to 266-nm wavelength range but are not limited to that range. 
     Aerosol particles will fluoresce when hit with an excitation beam due to biochemicals, specifically fluorophores, contained within the biological agents. The fluorescence is at a wavelength longer than the excitation wavelength. Tryptophan, for example, a common component of biological materials, has a peak fluorescence in the 350-nm range when excited with 266-nm light. The scattering wavelength is the same as the excitation wavelength, in this case 266-nm. Both the fluorescent and scattering light are detected using optical detectors such as a photomultiplier. 
     The relative amount of fluorescent and scattering light emitted from the biological aerosols can be characterized by the scattering and fluorescent cross sections ofthese materials. In general, the scattering cross sections are several orders of magnitude greater than the fluorescent cross sections. 
     The basic principles of detector operation are described in connection with  FIG. 1A  which shows a top view of a particle detector and  FIG. 1B  which shows a side view of a detector. Referring to  FIG. 1A , a source  10  of UV light generates a UV beam  12  which intersects at  14  with a stream  16  of particles being pulled into the detector. Intersection  14  occurs within a mirrored chamber  18 . A beam dump  20  captures excess light from beam  12  so that it is absorbed and removed. As shown in  FIG. 1B , the intersection  14  between UV beam  12  and particle stream  16  generates scattered and fluorescent light beam  22 . Beam  22  enters the region of the particle detector that transmits the scattering and fluorescence signals from the particle to the photomultiplier optical detectors. This includes beam splitter  24  which divides the beam into scattered component  26  and fluorescent component  28 . Filter  30  removes extraneous scattered light from fluorescent component  28 . Photomultiplier  32  records the intensity of scattering component  26  while photomultiplier  34  records the intensity of fluorescent component  28 . While this device is described using photomultipliers; other optical detectors such as avalanche photodiodes (APDs) may be used.  FIGS. 1A and 1B  depict a general overview of particle detector operation; many other components could be used as needed for a specific situation. 
     A great deal of design effort has been employed to attenuate the amount of stray excitation light that can inadvertently make it to photomultipliers  32  and  34  via the optical train. The optical train of a particle detector includes all internal surfaces that have a direct or indirect path between light source  10  and photomultipliers, or optical detectors  32  and  34 . 
     Stray light within the optical train can have two effects. First it can be falsely recorded as a scattering signal given that stray light and excitation light are both at the same wavelength. This signal will then appear as a scattering signal in scattering photomultiplier  32 . Second, and more importantly, the scattered light can cause other objects and materials in the overall optical train to fluoresce. This signal will then appear as a fluorescent signal in the fluorescent photomultiplier  34 . 
     Two main ways in which stray light can be attenuated are by using spatial filters and by eliminating reflective surfaces. In the prior art, reflective surfaces have been reduced by applying absorptive coatings or by increasing the volume of the optical train and placement of its components to the point that any reflected light would have an unlikely probability of reentering the pathway leading to the optical detector. 
     Those knowledgeable in the art of reducing stray light within optically based sensors and detectors understand that this represents a significant design challenge and, in general, the best solution is often a compromise as opposed to a perfect solution. This is caused by the fact that the sensor and detector design options are usually bounded by size, weight, and cost constraints. In principle, the entire optical train could be produced from any materials and coatings that result in an end product that addresses the need to attenuate the stray light and optical train fluorescence. In practice, however, this is often limited by cost and manufacturing constraints. 
     Prior art particle detectors working in the deep UV region and designed to detect biological materials typically avoid the use of uncoated plastics within the detector&#39;s optical train. This is especially true in the 266-nm region where plastics are known to fluorescence. This auto-florescence is significant and may easily mask the fluorescence from biological materials at the same wavelength. Plastics may also have reflective properties that causes undesired scattering of light throughout the optical train. In this case, they can appear as reflective surfaces and effectively act as shiny surfaces. 
     However, it is recognized that the plastics offer several benefits over non-plastic approaches. Compared to alternative approaches such as machined metals, plastics provide lower manufacturing and materials costs. They are also lighter, more rugged, and easier to assemble. However, the auto-florescence properties of available plastics have limited the ability to exploit these advantages. 
     While plastics offer some significant manufacturing, size, weight and cost advantages for an optical train, this choice of material has not been pursued in an integrated detector due to their reflective and fluorescent characteristics. Many plastics are inherently smooth and very reflective. There are, however, available techniques such as sanding to reduce this feature. A bigger impediment to the use of plastics in detectors is the fact that they inherently fluoresce when excited with UV light. Optically based detectors have been designed around the fluorescent quality of plastics. For example, Deep-UV LED and Laser Induced Fluorescence Detection and Monitoring of Trace Organics in Potable Liquids. WO 201204052 A2, teaches using detectors similar to those described in the patents listed above to detect a few parts per trillion of plastic resins in bottled drinking water and river plumes. 
     These and similar studies have generated a position within the detector development community that plastics cannot be used within UV based detectors, especially in regions where they could interact with the excitation light. The only noted exceptions were in applications where special plastics were used as a non-moving, solid support. The plastics were coated with immobilized binding ligands or similar materials. These coatings are known to produce an optical response when interrogated with UV light. In these cases, the plastic was not used or applied to the optical train in either the generation of the excitation source or the collection of the fluorescent and scattered light. The use was limited to an interrogated surface. For example. Ha Kim, et al. “Reusable low fluorescent plastic biochip: WO 2000055627 A1, teaches a non-auto-fluorescent solid support that is an alternative to glass that is suitable for the construction of biochips that can be employed in high-sensitivity, fluorescence detection and other methodologies. It identifies a UVT (Ultra-Violet Transmitting) Acrylic Ultraviolet Transmitting plastic, Glasfex® (now made by Spartech Polycast®) but does not teach if this material would function in the deep UV (&lt;266-nm or lower) range necessary for the detection of biological constituents such as tryptophan. Data collected on similar acrylics in the deep UV would suggest that these materials would have unusable fluorescence levels at this and lower wavelengths. Regardless, even if it had been shown that UVT Acrylic Ultraviolet Transmitting plastic would function in the deep UV, Kim does not teach or suggest that such or similar material can be applied to the design and construction of the detector&#39;s optical train. 
     In principle, the adverse fluorescent properties of plastics could be overcome by applying a protective coating on the plastic thus shielding it from any light. The common practice of applying a metal coating would not, in itself, suffice given that this would introduce a higher level of reflected light into the optical train and adverse increase scattering signals. In addition, a coating with the proper reflective and fluorescent properties to use with a practical plastic optical train is not known, and it would certainly add cost and complexity to the design and manufacturing of plastic detector. 
     Thus, a need exists for an optical train in a particle detector that benefits from the reduced cost, lighter weight and ruggedness of plastics but does not have the disadvantages of reflectivity and fluorescing in deep UV wavelengths. 
     SUMMARY 
     The invention in one implementation encompasses an apparatus. The apparatus comprises a plastic-based detector for biological and other fluorescent materials that detects the fluorescence and scattering signals from these materials using deep UV excitation is disclosed. The detector is fabricated using plastic materials and exploits the properties of lower manufacturing costs, lower materials costs, light weight, ruggedness and assembly ease offered by plastics. 
     The invention in a further embodiment encompasses a particle detector, having an excitation region including a first plastic housing and an excitation source for generating light to excite particles passing through the particle detector; an interrogation region including a second plastic housing, for producing an emission beam from one or more particles excited by light from the excitation source; and a detection region for receiving the emission beam and including one or more optical detectors configured to determine light scattering and fluorescence properties of particles, and a third plastic housing. 
     In a further embodiment, the excitation region includes one or more lenses to collect light from the excitation source fitted into said first housing adjacent to the excitation source; and one or more filters fitted into the opposite end of the first housing from the excitation source to remove stray light from the excitation source. 
     In a further embodiment, the interrogation region includes a cavity within said second housing, said first housing attached to one side of said second housing and said third housing attached to said second housing at a position perpendicular to said first housing; and a beam dump attached to said second housing opposite said first housing. 
     In a further embodiment, the second housing includes embedded attachment points that mate with those on the first and third housing; embedded attachment points that mate with those on inlet and exhaust pitot tubes on opposite sides of said cavity; and embedded attachment points that mate with those on the beam dump. 
     In a further embodiment, the detection region includes a beam splitter for splitting the emission beam into two portions; a first optical detector configured to determine radiation scattering properties of particles passing through the particle detector according to a first portion of the emission beam; and a second optical detector configured to determine fluorescence properties of the particles passing through the particle detector according to a second portion of the emission beam. 
     In yet another embodiment, the invention encompasses a method of manufacturing a plastic particle detector including the steps of forming a first housing from plastic, said first housing including molded structures to hold an excitation source and one or more optical devices for focusing light from the excitation source on a particle; forming a second housing from plastic, said second housing including molded structures wherein said focused light from said excitation source intersects with said particle and generates an emission beam, said first housing attached to one side of said second housing; and forming a third housing from plastic, said third housing attached to said second housing perpendicularly to said first housing, said third housing receiving the emission beam and including molded structures for attaching two or more optical detectors; wherein said first, second and third plastic housings contain embedded fasteners so that the housings can be assembled to form the particle detector without additional hardware, glue or welding. 
     In a further embodiment, the method includes the step of forming a beam dump from plastic, said beam dump attached to said cavity opposite said first housing, without requiring the use of additional fasteners. 
     In any of the above embodiments, the plastic housings of the particle detector are formed from an uncoated plastic that does not interact with light from the excitation source to produce fluorescent light beams or scattering light beams that will be detected by the one or more optical detectors for example, a polyamide 66 resin with glass and carbon black fill. 
     In any of the above embodiments, the plastic housings of the particle detector are injection-molded. 
     In any of the above embodiments, the plastic housings of the particle detector contain embedded fasteners so that the housings can be assembled to form the particle detector without additional hardware, glue or welding. 
     In any of the above embodiments, the one or more optical detectors are photomultipliers or avalanche photodiodes. 
     In any of the above embodiments, the excitation source generates light in the deep UV wavelength range. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Features of example implementations of the invention will become apparent from the description, the claims, and the accompanying drawings in which: 
         FIGS. 1A and 1B  are top and side views illustrating the basic operation of a particle detector. 
         FIG. 2  is a perspective view of a particle detector according to the present invention. 
         FIG. 3  is an exploded perspective view of excitation and interrogation regions of the particle detector of  FIG. 2 . 
         FIG. 4  is side cross section view of a particle detector according to the present invention. 
         FIG. 5  is a cross section view of the particle detector of  FIG. 2 . 
         FIG. 6  is an exploded perspective view of the detection region of the particle detector of  FIG. 2 . 
         FIG. 7  is view of stray light generation in the particle detector of  FIG. 2 . 
         FIG. 8  is view of stray light suppression in the particle detector of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     If used and unless otherwise stated, the terms “upper,” “lower,” “front,” “back,” “over,” “under,” and similar such terms are not to be construed as limiting the invention to a particular orientation. Instead, these terms are used only on a relative basis. 
     A particle detector  100  according to an embodiment of the invention is shown generally in  FIG. 2 . It can be broken down into three basic regions, excitation region  35 , interrogation region  36  and detection region  37 . Excitation region  35  typically includes an excitation light source  116 , housing  114  and one or more lenses and filters inside housing  114  as shown in more detail in  FIG. 4 . Excitation region  35  is also known as the front end assembly due to the fact that it can be assembled as a standalone part that can be attached or removed from the overall particle detector as a single, integrated part using, for example, pins  117  and keyhole slots  119 , explained in more detail below. 
     Interrogation region  36  is where the excitation light interacts with the particles to produce fluorescent and scattering light. Interrogation region  36  typically includes a housing  132  for holding a mirrored chamber, inlet and outlet pitot tubes, and a beam dump to remove any light that does not hit an aerosol particle, as shown in more detail in  FIGS. 4 and 5 . 
     Finally, detection region  37  typically includes lenses, windows, filters to direct an emission beam generated when excitation light hits a particle to optical detectors such as photomultipliers as well as a housing  152  as shown in more detail in  FIGS. 4 and 6 . The detection region is also known as the back end assembly due to the fact that it can be assembled as a standalone part that can be attached or removed from the overall particle detector as a single, integrated part using tabs  151  and slots  153  described in more detail below. 
       FIG. 3  is an exploded perspective view of the excitation and interrogation regions of particle detector  100 . Excitation region  35  includes excitation source  116  and housing  114 . Flange  115  includes two or more pins  117  used to attach housing  114  to housing  132  of the interrogation region using keyhole attachments  119 . Although a specific embodiment for attachment has been depicted, one of ordinary skill in the art would recognize that several different methods of attaching housings  114  and  132  could be used. 
     Excitation region  35  further includes spatial aperture assembly  236 , which includes tip  122 , holder  121  and filter  120 . Secure connection between the various components throughout particle detector  100  is provided by O rings  123  as would be understood by one of ordinary skill in the art. 
     Interrogation region  36  includes housing  132  which connects to several other elements of particle detector  100 . Particles are drawn into mirrored chamber  133  through inlet pitot tube  162  by means of a pump (not shown) attached to exhaust pitot tube  164 . A lid  134  that is also mirrored fits into housing  134  to form the rest of chamber  133 . Filter  138  and cap  139  are attached to lid  134  to form the rest of interrogation region  36 . Beam dump  130  is connected to housing  132  opposite excitation region  35 . Secure connection between components is provided by O rings  123 . 
     This invention describes a novel plastic-based particle detector design. A design using nylon (polyamide) 66 resin with glass and carbon black fill or similar product produces a functional biological detector. Zytel® ST801AW BK195 by DuPont® is an example of a plastic that has acceptable fluorescence and scattering characteristics. Zytel® will be used as an example of nylon (polyamide) 66 resin with glass and carbon black materials for the balance of this specification but any plastic with similar properties could be used. For balance of this document, the term novel plastic will be used to denote such a plastic. 
     An optical train made of this plastic has very low fluorescence and scattering, thus providing a particle detector using uncoated plastic components that can detect biological and other materials that fluoresce in the deep UV. 
       FIG. 4  shows a cross section view of a particle detector  100  along line B of  FIG. 2 . Excitation region housing  114  holds a light source  116  which generates deep UV light. In an embodiment, light source  116  is a Light Emitting Diode (LED) but a variety of UV light sources could be used. Housing  114  also holds a series of one or more lenses  118  to collect the light. Spatial aperture holder  121  fits over one end of housing  114  to hold short pass filter  120  and a spatial aperture tip  122  which is used to remove stray light. The result is a focused optical beam  124  containing a desired excitation wavelength. In an alternative embodiment, a laser or other techniques for to produce an excitation light beam may be used. Excitation region housing  114  can be injected molded or fabricated from the novel plastic described above. 
     In this design, an aerosol particle  126  is drawn into detector  100  (as explained in more detail in connection with  FIG. 5 ) such that the excitation light beam  124  intersects with it in a mirrored chamber  133  which forms the main part of interrogation region  36  (as shown in  FIG. 2 ). The interior surfaces of mirrored chamber  133  are coated with a reflective material such as aluminum thus giving them a mirrored surface. Mirrored chamber  133  could be injected molded or otherwise formed from the novel plastic described before. The detector is designed so that any excitation light  128  not hitting the aerosol particle  126  passes into a beam dump  130  where it is absorbed and removed. 
     The mechanism for drawing particles into the particle detector will now be described. A cross section view of particle detector  100  along line A of  FIG. 2  is shown in  FIG. 5 . Aerosol particles  160  are drawn into detector  100  from ambient air via inlet pitot tube  162  by means of a pump (not shown) attached to exhaust pitot tube  164 . The flow stream  168  carries aerosol particles through the point where the stream intersects with the excitation beam  124 . As also shown in  FIG. 2 , excitation light  128  not hitting the aerosol particle  126  passes into a beam dump  130  where it is absorbed and removed. Beam dump  130  attaches to housing  132 , as does inlet pitot tube  162  and exhaust pitot tube  164  as explained in more detail below. In addition, the parts defining the detection region and housed within detection region housing  152  of  FIG. 2  also attach to housing  132  as explained in connection with  FIG. 6 . Housing  132  can be injected molded or fabricated from the novel plastic described above. 
     Returning to  FIG. 4 , when excitation beam  124  hits aerosol particle  126 , light  136  is emitted from the particle. This light will typically have both scattering and fluorescence components. The scattering component will be at the same wavelength as the excitation wavelength. The fluorescent component will be at a longer wavelength. If the excitation wavelength is in deep UV range, such as 266-nm, and the aerosol contains UV fluorophores, such as tryptophan, the emission beam  136  will have a fluorescent component, typically in the 350-nm range. Mirrored chamber  133  in housing  132  collects emissions over a collection angle defined by 4 pi steradian. This light increases the intensity of the overall emission  136 . 
     Emission beam  136  passes into detection region  37  (shown in  FIG. 2 ) through filter  138 . Filter  138  is held in place by cap  139  and can also be a clear window, such as quartz, that passes all scattering and fluorescent wavelengths. Beam  136  then passes through collection optics  140 , beam splitter  142 , and filter  144 , then finally arrives at photomultiplier  146  which records the intensity of the emission. With the proper selection of these components based on considerations such as their optical band pass and transmissivity, the light reaching photomultiplier  146  contains only the fluorescent component of the total emission from the aerosol particle. Although a specific embodiment has been shown, one of ordinary skill in the art would understand that the filters, optics and beam splitter can be arranged and configured in several different ways. 
     A portion of emission beam  136  that impacts beam splitter  142  is redirected as beam  148  which arrives at photomultiplier  150 . Proper selection of these components based on considerations such as their optical band pass and transmissivity, ensures that the light reaching the photomultiplier  150  contains the scattering component of the total emission from the aerosol particle. It is possible to add filters along the path of beam  148  to attenuate its intensity or remove undesired wavelengths prior to detection by photomultiplier  150 . As an alternative, other optical detectors such as avalanche photodiodes can be used in place of one or both of photomultipliers  146  and  150 . 
     Detection region housing  152  captures and holds all the optical parts within the detection region as shown in more detail in  FIG. 6 . The actual placement of these components can be adjusted as needed and is critical to insure that the focuses and incident angles are optimized to achieve the greatest signal throughput to the PMTs and proper band pass filtering of the scattering and fluorescent light. Detection region housing  152  also forms a pathway by which the emission beam  136  and scattering beam  148  have an open channel to their respective photomultipliers. As shown in  FIG. 6 , housing  152  is split into two halves  152   a  and  152   b . These halves snap together to hold optical elements  140 ,  141 ,  142 ,  144  and  145  as will be described in more detail below. Photomultipliers, or optical detectors,  146  and  150  are also held by housing  152 . Although tabs and slots are shown as a connection mechanism, the specific design and alternatives would be apparent to one of ordinary skill in the art. Housing  152  can be injected molded or fabricated from the novel plastic described above. 
     There are a number of ways stray light, i.e., light not resulting from impact with an aerosol particle, can be generated in a particle detector. The housings and other materials of the particle detector may have characteristics of fluorescence and scattering independent of light interaction with particles. Several types of undesirable light generation and the solution provided by the present invention are now discussed. 
     As shown in  FIG. 7 , light source  116 , and especially if it is an LED, often generates stray beams of light  180 . This stray light can hit, for example, internal surface  182  of front end housing  114  and result in one or more additional beams  184 . Beam  184  could be a scattered light at the same wavelength as that generated by LED  116 , one at a longer wavelength due to fluorescence resulting from impact with internal surface  182 , or a combination of these. 
     The selection of materials for excitation region housing  114  is important to minimize the effects from scattering and fluorescence. Prior art designs employ a housing made from metal with a black absorptive coating. According to the present invention, excitation region housing  114  is manufactured from a plastic material that has low and acceptable scattering and fluorescence properties. In a preferred embodiment, Zytel ST801AW BK195 provides an injection moldable means of producing a low cost housing that inherently also forms a means to capture and hold optical components including lenses and filters. This material offers all the manufacturing and cost advantages of using plastic over metal without the impact of high and unusable levels of scattering and fluorescence produced from other plastics. For example, a plastic particle is much lighter than an equivalent particle detector made of metal. 
     Another common source of stray light arises from the optics of an aerosol detector, also illustrated in  FIG. 7 . Optic devices such as mirrors and lenses typically have some level of inherent scattering. While it is always the goal to maximize the direct beam paths described in  FIG. 4 , other beams will often exist. Beam  186  illustrates such a beam. The beam can originate anywhere in the optics. Eventually it will impact a surface as shown, for example, as point  188 . Another beam  190  will be produced from this impact that can travel to photomultiplier  146 . This signal is normally denoted as a clean-air background signal and, if large enough, can drastically impact the ability to detect the desired emission beam signal  136  triggered by beam  124  of FIG.  4 . This is especially true if the impact at point  188  results in fluorescence. Likewise, stray light may be detected as a scattering signal by photomultiplier  150 . 
     As explained above for excitation region housing  114 , prior art detectors use a detection region housing  152  fabricated from metal with a black absorptive coating. According to the present invention, detection region housing  152  is manufactured from a plastic that has low and acceptable scattering and fluorescence properties. In a preferred embodiment, Zytel ST801AW BK195 provides an injection moldable means of producing a low cost housing that inherently also forms a means to capture and hold optical components to include lenses and filters. This material offers all the manufacturing and cost advantages of using plastic over metal without the impact of high and unusable levels of scattering and fluorescence produced from other plastics. 
       FIG. 8  illustrates the functionality that can be achieved by using Zytel ST801AW BK195, for example, as the basis for detection region housing  152 . This material reduces the scattering and fluorescence signals from stray light beams  192  and  194  to the point that the detector can detect aerosol particles  126  (shown in  FIG. 4 ). The attenuation of the scattering and fluoresce signals has been demonstrated to the point that single sub micron particles have been detected. In contrast, others knowledgeable in the art of detection of biological and other fluorescent aerosols using deep UV light have not used uncoated plastic parts in their designs. This, in part, has been due to the fact that they have not identified a plastic and design that could produce satisfactory scattering and fluorescence characteristics. 
     There are several benefits realized by the use of the all-plastic detector of the present invention. For example, the various housings of particle detector  100  are designed to snap together using embedded fasteners that are integrally formed with the housings. This reduces assembly time and assembly cost when compared to other materials that may require fasteners. Further, injection molded parts can be designed with intricate details. These details can hold lenses, mirrors and filters. They can also generate spatial filters and baffles. Such details would be costly to reproduce in machined part thus, providing them as part of an integral plastic part also reduces assembly time and cost. 
     For example,  FIGS. 2 and 3  show a connection between housing  114  of excitation region  35  and housing  132  of interrogation region  36  that is formed by pins  117  and keyhole slots  119 . Another type of connection is illustrated in  FIGS. 2 and 6 , which show catches  151  on housing  152  which are inserted into slots  153  on housing  132 .  FIG. 6  also shows a plurality of catches  155  which are used to attach photomultipliers  146  and  150  to housing  152 . At least one catch  157  is inserted into slot  159  so as to attach the two halves of the housing,  152   a  and  152   b , to each other. Although specific embodiments have been shown, variations in shape and design would be understood by one of ordinary skill in the art. 
     A further benefit of using plastic for a particle detector is that injection molded parts with intricate details to capture optics reduces the number of parts. A lens, for example, could be inserted into a slot designed into the plastic part. It would not require a separate lens hold. This reduces cost while improving ruggedness due to the decrease in the number of parts that can become loose and lose optical alignment. Plastic parts eliminate the potential for corrosion and can also be over-molded to incorporate critical metal parts. This provides a means to insert metal parts into critical regions of the detector with minimum impact to the cost, size and weight. This may be necessary if the detector design requires the use of unique surfaces or structures that can only be achieved on a non-plastic part. Plastic provides a medium by which metal and plastic parts can be easily fused together into a single over-molded part. 
     Numerous alternative implementations of the present invention exist. For example, this invention can be applied to any optical system requiring a means to reduce stray optical background signal. That includes the telescopes, microscopes and binoculars. It also includes laboratory and office equipment using optical processes, for example, fluorometers, atomic emission spectrometers, Raman spectrometers, optical scanners, and optical readers. 
     The particle detector  100  in one example comprises a plurality of components which can be combined or divided in the particle detector  100 . The particle detector  100  in one example comprises any (e.g., horizontal, oblique, or vertical) orientation, with the description and figures herein illustrating one example orientation of the particle detector  100 , for explanatory purposes. 
     The steps or operations described herein are just for example. There may be many variations to these steps or operations without departing from the spirit of the invention. For instance, the steps may be performed in a differing order, or steps may be added, deleted, or modified. 
     Although example implementations of the invention have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the following claims.