Abstract:
An improvement is provided for a system that identifies particles such as microorganisms in fluid by directing a laser beam ( 52 ) forwardly through a tiny detect zone ( 46 ) in the fluid and detecting the pattern of light scatter by a particle as it passes through the detect zone. The improvement includes a holographic optical element ( 60 ) positioned forward of the detect zone to intercept light scattered in multiple directions by the particle. The holographic optical element is divided into discrete areas, or sections, that each directs intercepted scattered light toward a selected photodetector ( 74, 90, 92 ) of a linear array ( 62 ) of photodetectors. A converging lens ( 106 ) reduces the required diffraction angles of the sections of the holographic optical element. This arrangement avoids the need to custom mount and connect numerous individual photocells, and enables simplified high speed readout of the photodetectors.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Applicant claims the benefit of U.S. Provisional patent application Ser. No. 60/372,684 filed Apr. 12, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     Microscopic particles such as particular species of bacteria lying in a fluid such as water or air, can be identified by detecting their pattern of light scatter when they pass through a light beam such as a laser beam. A plurality of photodetectors can be positioned to detect light scattered in different directions from a small detect zone lying along the laser beam. The outputs of the photodetectors are delivered to a computer that compares the pattern of light scatter for an unknown particle that is passing through the detect zone, to the patterns of a list of known species of particles, usually microorganisms, to determine whether the unknown particle is a member of one of the listed species. 
     Previously, applicant custom mounted the multiple photodetectors on a frame that could position photodetectors to detect light scattered in different directions and at different angles from the forward direction of the laser beam. Such a frame and detectors can be awkward and expensive to build and connect to. Furthermore, such a setup can result in a rat&#39;s nest of wires extending from the multiple photodetectors to a cable leading to the computer. The large number of custom terminated wires can result in reduced reliability of electrical connections and considerable signal losses along some of the wires. A system that avoided the need for such difficult mounting and such rat&#39;s nest of wires, would be of value. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, an apparatus and method are provided that improve operation of a system for identifying microscopic particles in a fluid by directing a laser beam through the fluid and detecting light scattered in multiple directions by a particle passing through the detect zone. Instead of mounting individual detectors on a frame, so each detector can detect light scattered in a predetermined direction from the detect zone, and providing multiple wires leading from each detector to an amplifier that connects to a computer, applicant provides a holographic optical element and at least one linear array of photodetectors such as CCDs (charge coupled devices). The holographic optical element is constructed with multiple areas, or sections, that are each constructed to direct light received from the direction of the detect zone, toward a selected one of the photodetectors of the linear array. A converging lens preferably lies immediately forward or rearward of the holographic optical element. Only a single holographic optical element is required to intercept light scattered in multiple directions within a wide angle from the detect zone. By directing the scattered light to selected photodetectors of a linear array, applicant can use available linear arrays of photodetectors such as CCDs. The linear array not only simplifies mounting of photodetectors and avoids multiple custom connections and a rat&#39;s nest of wire which all degrade performance, but enables rapid readout. 
     The holographic optical element can be constructed with sections that each intercept light scattered in a particular circumferential direction and at a particular angle to the forward direction of the laser beam, to mimic the detection of light by individually mounted photodetectors of applicant&#39;s prior systems. In another arrangement, the holographic optical element takes advantage of the fact that the element can direct light from an area of any shape on the hologram to a selected photodetector, to make detections that facilitate the identification of the unknown particle. In one arrangement, the holographic optical element is divided into sections that are each in the shape of a narrow ring. In another arrangement, one part of the hologram forms sections that are parts of rings, while another part forms multiple pie-shaped sections. In another arrangement, the holographic optical element is divided into multiple small sections, or areas that each direct intercepted light to a different photodetector. Substantially all locations in each small area lie within about 5° of its center. The photodetector outputs can be combined to simulate rings, pie-shaped sections, etc. 
     A linear array of CCDs forms an image scanner, to deliver the outputs of the detectors of the array sequentially to the computer. This simplifies connection of the linear array to the computer, especially if a large number of detectors of the linear array are used. 
     The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of a prior apparatus that applicant has used to identify microscopic particles in a fluid. 
     FIG. 2 is an isometric view of a system of the present invention, that uses a holographic optical element, a linear array of photodetectors and an image scanner to enable the identification of microscopic particles in a fluid. 
     FIG. 3 is a sectional view of a portion of the system of FIG. 2, but modified to include a converging lens. 
     FIG. 3A is a sectional view of a portion of the system of FIG. 2, but with the converging lens positioned forward of the holographic optical element instead of behind it. 
     FIG. 4 is an isometric view of a portion of a system similar to that of FIG. 3 with a modified holographic optical element which enables the detection of the scatter angle of light from the detect zone with respect to the forward direction of a laser beam, for each of multiple angle. 
     FIG. 5 is an isometric view of a portion of a system similar to that of FIG. 4, but with only half of the hologram used to identify the angle of scatter light from the beam direction, with the other half of the hologram being used to detect the circumferential direction of light scatter regardless of the angle from the beam direction. 
     FIG. 6 is a diagram for identifying scatter angles. 
     FIG. 7 is an isometric view of a portion of a system similar to that of FIG. 3, with a modified holographic optical element divided by perpendicular lines into multiple small sections. 
     FIG. 8 is an isometric view of a portion of a system similar to that of FIG. 3, with a modified holographic optical element divided by lines radial and circumferential to the axis into multiple small sections. 
     FIG. 9 is an isometric view of a portion of another system, wherein the holographic optical element is a layer lying on the spherical lens. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a portion of a system  10  that applicant previously designed for the detection and identification of microscopic particles, and especially microorganisms, in a fluid such as water or air. The system includes a laser  12  that directs a laser light beam  14  in a forward direction F through a quantity of water that is flowing in a downward direction D. A small volume along the laser beam is designated as a detect zone  20 . Multiple photodetectors  22  are provided that are aimed at the detect zone  20 , to each detect only light originating from the detect zone. FIG. 1 shows sixteen photodetectors labeled DA through DP that are mounted on two rings  24 ,  26  of a frame  30 . Each photodetector detects lights within an angle of about 2° that originates from the detect zone  20 . The outputs of the sixteen photodetectors are delivered through wires (not shown) to a computer. The computer compares the pattern of light scattering from an unknown particle entering the detect zone  20 , to light scatter patterns for a known group of species of microorganisms, and can indicate if the light scatter pattern for the unknown particle indicates that it is one of the group of microorganisms for which the computer has been programmed. Applicant uses statistical pattern recognition to establish that an unknown particle is a member of a class of particles of interest (e.g. species of microorganisms). 
     It can be appreciated that the custom frame  30  involves some complexity in its construction and accurate mounting, to assure that each photodetector detects only light originated from the detect zone  20 . Also, it can be appreciated that if four or five wires are connected to each photodetector and the wires are extended to the computer, that this would create a “rat&#39;s nest” of wires near the detectors. Such a large number of wires which are connected to the photodetectors by custom connections such as solder joints, results in lower reliability and the possibility that there are significant signal losses along the wires leading to the computer. 
     FIG. 2 illustrates a complete system  40  in accordance with one embodiment of the invention. Fluid such as water which carries the microscopic particles, moves along a passage  42  of a glass carrier  44 . The detect zone  46  lies along the passage  42 , so that particles can pass through the detect zone. A laser  50  directs a laser beam  52  in a forward direction F along a beam axis  53  to pass through the detect zone  46  and to a trap  54  that traps substantially all light that has not been scattered (and that usually constitutes more than 99% of the laser beam energy). In one example, the laser beam  52  has a horizontal width of 0.5 mm and an average vertical thickness of 0.1 mm, and the detect zone occupies only a portion of the laser beam that extends along a length of 1 mm. 
     Instead of placing multiple photodetectors to intercept the scattered light beams, applicant uses a holographic optical element  60  to intercept the scattered light, and uses a linear array of photodetectors  62  to detect the light and generate an electrical signal having an amplitude corresponding to the amplitude of the detected light. Such an array has at least five separate photodetectors, and usually many more. CCDs are readily available with between 512 and 2048 pixels, each forming a photodetector. FIG. 2 shows a narrow light ray  70  scattered from the detect zone  46  by a particle therein. The holographic optical element has an active section  72  that is constructed to divert light received from the direction of the zone  46 , to a selected photodetector  74  of the linear array  62 . The holographic optical element has additional sections such as  80  and  82  that direct light beams  84 ,  86  respectively to photodetectors  90 ,  92 . Areas between the active sections are inactive in that they do not direct light received from the detect zone to one of the photodetectors. Such inactive sections may be opaque. The outputs of the photodetectors of the array  62  exit the array in a rapid sequence, and the outputs pass through an amplifier  100  and an analog-to-digital converter  102  to a computer  104  that indicates whether the detected particle is a member of a known group of species of particles, especially of microorganisms. 
     FIG. 3 shows some details of a system  40 A similar to that of FIG.  2 . Applicant constructs the carrier  44  with a largely spherical lens  110 . The center of curvature of the lens is located at or slightly behind the detect zone  20  so the lens serves as neither a converging nor diverging lens (although a converging lens could be useful). If the front surface of the carrier  44  were flat, as with a continuation of the flat top and bottom surfaces at  112 , then light scattered from the detect zone  20  at an angle E from the forward direction of the beam  52 , would be internally reflected at the interface of the carrier surface  112  and air  114  in the environment, for an angle E of more than about 41°. The spherical lens allows light scattered at any angle to the lens, to pass out into the atmosphere where the holographic element is located. Also, the largely spherical lens  110  makes the path of the light easier to determine. 
     In FIG. 3, applicant has added a condensing, or converging lens  106  between the lens  110  and the holographic optical element. A converging lens reduces the angle of spread, or collimates, or converges, light rays. FIG. 3 shows that the light beam  70  is refracted by the lens  106  toward the forward direction, and is refracted by the section  72  of the holographic optical element  60  to a selected photodetector of the linear array  62 . Although the system can be used without the converging lens, this results in the element sections being required to refract light by large angles (e.g. about 80° for beam  70 ). This requires finer resolution for the holographic sections, and such holographic optical element is more difficult to make. With the lens  106 , the holographic sections refract light at smaller angles (e.g. about 30° for beam  70 ). FIG. 3A shows a system where the converging lens  106 A lies forward of the holographic optical element  60 A instead of behind it. 
     The holographic optical element  60  of FIG. 2 is shown constructed so each refracting section  72  refracts, or redirects light from one path  70  to another  71  to fall on the linear array of photodetectors, only for light emanating from the detect zone  46  within about 2° of the axis  70  of the light beam. As shown in FIG. 3, the light intercepted by the holographic section  72  subtends a narrow angle G on the order of 4°. FIG. 2 shows that the particular holographic optical element  60  has sixteen of such sections similar to section  72 , that each diffracts light received within a small angle to one of the detectors of the linear array  62 . The rest of the element  60  is opaque or directs light away from the linear array  62 . The purpose of this arrangement is to have the element  60  mimic the detections by the sixteen photocells  22  of FIG.  1 . 
     With the use of a holographic optical element, applicant can make detections in different ways that would not be practical with an arrangement such as shown in FIG. 1 where a limited number of narrow acceptance angle photodetectors  22  are spaced apart and each individually mounted on a frame. 
     FIG. 4 shows a holographic optical element  130  which takes advantage of the fact that, for some particles, the most distinguishing feature is the angle H with respect to an axis  132  of the laser beam, at which light is scattered from the detect zone  46 , regardless of the circumferential direction C around the laser beam  52 . The holographic optical element is formed with ten ring sections (of 360° each), or rings  140 - 149 . Each ring is concentric with the beam direction axis  132  and refracts light scattered within a certain angle H from the beam direction  52  (through a condensing lens, not shown in FIG. 4) to a selected one of the detectors  160  of the linear array  162 . In one example, ring  149  intercepts and diffracts light falling within an angle of 10° to 16° from the beam direction  52  to a selected photodetector  164 . The outermost diffracting ring  140  diffracts scattered light received within an angle of 62° to 70° from the beam direction  52  to another photocell  166 . Each of the other rings  141 - 148  intercepts and diffracts light within a range of 6° to a selected one of the detectors. Each ring has a radial width J between its inner and outer ring edges K, L, which is no more than 20% of the radial distance M to the outer ring edge, for rings that lie beyond ring  146 . 
     FIG. 6 is a diagram showing a laser beam direction F which extends into the paper, and two angular coordinates. A first coordinate is the angle H of FIG. 4, from the axis or beam direction F. This determines the radius, or distance R at which scattered light falls on the holographic optical element (which is spaced a known distance from the detect zone  46 ). The other coordinate is the circumferential angle B in the circumferential direction C from a ray  166  of zero angle extending perpendicular to the beam axis. 
     FIG. 5 illustrates another holographic optical element  170  which enables the detection of scattering at different axial angles H from the direction of the laser beam, and which also enables the detection of scattering in different circumferential directions. The element  170  has its upper half divided into ten half-rings  180 - 189  that each extends about 180° about the axis  132  that is coincident with the laser beam  52 . The element has a lower half that is divided into thirty-six pie-shaped radial sections  192 , each section extending in a different circumferential direction and each section subtending an angle D′ of no more than about 12° about the axis  190 . Thus, all light scattering onto the particular section  194  is diffracted (through a condensing lens, not shown in FIG. 5) into a particular detector  200  or group that includes a limited number of detectors, of the linear array of detectors  202 . The holographic optical element  170  of FIG. 5 therefore enables the detection of the angle of scattering of light from the axis  132  along ring-shaped section areas  180 - 189 . The element also enables detection of the circumferential direction of scattering of light by the thirty-six elements  192 . 
     The construction of a holographic optical element can be accomplished in many ways. A traditional method suggested by early experimenters, is to split a coherent beam, such as is obtained from a laser, into two beams parts, one of which originates from the detect zone  46  and which illuminates a particular section of the element, and to direct another part of the split beam from the location of the desired photodetector on the linear array, at the same section of the element. The element may comprise a film or glass plate with a photosensitive coating or film on it. The portion of the film outside the section that is to diffract light toward a particular photodetector is masked. This is continued for all other sections of the element that are to diffract light. The photographic plate is developed, and can be photographically duplicated. At present, holographic optical elements are most easily created by a computer-controlled illumination source, which creates the desired fresnel patterns. 
     U.S. Pat. No. 6,313,908 by McGill. et al., owned by NASA, describes a holographic optical element that detects a wide beam of light containing many different wavelengths (a spectral distribution of light), so each wavelength is focused onto a different point. The points may be an array of CCDs. 
     FIG. 7 illustrates a holographic element  210  that is divided by horizontal and vertical lines  212 ,  214  into numerous sections  216 . Each section receives light scattered within a small angle of no more than about 4°, and preferably no more than 2° that originates at the detect zone, and may be referred to as a hologram pixel. Each section directs the light (preferably through a condensing lens) to one of the detectors  220  of an array  222 . The outputs of selected detectors can be combined to represent the output of one photodetector that receives light from a section (composed of a plurality of hologram pixels. For example, the output of all detectors that detect light from sections such as  330 , represent a ring extending 45° about the axis  340 . The outputs of all detectors that detect light from sections close to line  242  represent the output of a pie-shaped section. FIG. 8 illustrates a holographic optical element  250  divided into similar small sections by circumferential lines  252  and radial lines  254 . 
     FIG. 9 illustrates a holographic optical element  260  in the form of a layer lying on the surface of a convex lens  262 . The lens can be similar to the lens  110  of FIG. 3 which forms the front of a carrier. A converging lens  264  lies in front of the holographic layer at  260 . 
     Although a single holographic optical element can be used, such as shown in FIGS. 2,  4 - 5  and  7 - 8 , it is possible to use more than one holographic element. One or more additional of such elements can be useful to diffract light scattered at a large angle, such as more than about 70° or 80° to the beam direction, to detectors of an array, or even to diffract light scattered backward, that is, more than 90° to the forward direction of the laser beam. It is noted that applicant has illustrated a single linear array of detector elements, which is most likely implemented by a CCD (charge coupled diode) array. In such array, all CCD elements lie substantially in a single plane, and in substantially a single line. Such a linear array can be scanned at high rates of more than one thousand CCD elements per second. It is noted that applicant usually requires fewer than 1024 photodetectors, and this can be accomplished by taking the outputs of perhaps ten to fifty CCD elements as the output of a single photodetector. If necessary, two or more linear arrays can be used, whose outputs may be sequentially scanned. It is also possible to use a two dimensional array of CCD elements, all lying in substantially a single plane, but a scanning rate for such elements is commonly only 60 scans per second. In applicant&#39;s apparatus, where water may flow at a substantial velocity such as 8 cm/second, it is desirable to be able to detect particles that pass through the detect zone during a period of {fraction (1/1000)}th second. 
     Thus, the invention provides an improved apparatus and method for identifying microscopic particles in a fluid by detecting scatter patterns of particles. The improvement includes a holographic optical element that is positioned so different sections of it intercept light scattered in different directions from a detect zone through which a laser beam and particles to be detected pass, and each section directs (refracts or reflects) the intercepted light toward a different photodetector. A converging lens preferably lies forward or rearward of the holographic optical element. At least one linear array of photodetectors, such as a CCD with line array of CCD detectors, charge injection device, discrete silicon detector array, image intensified detector, etc. is positioned in the vicinity of the holographic optical element. The linear array can be progressively scanned to provide a sequence of signals representing the outputs of the different detectors of the array, to an analog-to-digital converter, whose output is delivered to the computer. The output of a CCD is automatically sequential. In one arrangement, the holographic optical element has isolated largely circular small sections mimicking the beams detected by the use of individual photodetectors that are separately mounted on a frame. In another arrangement, the holographic optical element has sections in the forms of rings concentric with the axis of the element. The rings may occupy only a fraction of a 360° continuous ring, but each preferably occupies at least 45° of such continuous ring. The holographic optical element may also include pie-shaped sections for detecting lights scattered all in the same circumferential direction from the detect zone, but at different angles to the beam direction. Other holographic optical elements may be divided into multiple squares, a pattern of rings divided by radial lines, etc. to provide small sections that each extend no more than about 4° from the middle of the section. The outputs of selected groups of detectors can be added together so each group represents a ring, etc. 
     Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.