Abstract:
An absorption spectroscopy apparatus including an elliptical mirror centered on the midpoint between a source/detector and a mirror. The cavity between the elliptical mirror and the source/bolometer and mirror defines an interior volume of a sample cell. Electromagnetic radiation from the source/detector travels along a multi-segment path starting from the source/bolometer toward the elliptical mirror, reflecting off of the elliptical mirror and traveling toward the mirror, reflecting off of the mirror and traveling back toward the elliptical mirror and finally reflecting off the elliptical mirror for a second time and returning toward the source/bolometer. The multiple reflections combined with the focusing effects of the elliptical mirrored surface result in an efficient sampling device. Among other aspects and advantages, the apparatus of the present disclosure is able to use incoherent, non-collimated light sources while maintaining high optical throughput efficiencies.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of Provisional Application No. 60/923,387 filed Apr. 13, 2007 and Provisional Application No. 60/931,463 filed May 22, 2007, each of which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure relates generally to spectrophotometric techniques for analyzing the content of a given fluid and, more particularly, to optimizing the sensitivity of such analysis. Even more particularly, the present disclosure relates to a new and improved multi-pass sample cell for use in absorption spectroscopy and an absorption spectroscopy apparatus incorporating the cell. 
       BACKGROUND OF THE DISCLOSURE 
       [0003]    Spectroscopy systems and methods are used to analyze the composition of various types of materials, including gases, liquids and the like. Spectroscopy is based on the fact that different chemical compositions absorb energy, e.g., light, at different frequencies, so that measuring the intensity versus the frequency of the light passed through a sample can be used to identify which light frequencies were absorbed by the sample and which were not. Thus, the chemicals present in the sample can be readily identified. Spectroscopy systems and methods also can identify the amount of light absorbed by the sample at each given frequency. Thus, the quantity of each chemical present in the chemical composition can be determined. In addition, such analysis can be performed with any one of various different ranges of light such as infrared, ultraviolet and the like, each of which pertains to a separate range of frequencies. 
         [0004]    An absorption cell (or resonator), which holds the gas or liquid sample through which light is passed, is used to perform spectroscopy analysis in conjunction with suitable spectroscopy equipment, such as a collimated laser light and a light detector. It has long been realized that to increase the sensitivity in providing both quantitative and qualitative analyses, the collimated laser light must be passed through a very large percentage of the available sample. Thus, absorption cells have been provided with “folded” light paths, in which mirrors reflect the light back and forth within the cell, such that the light makes multiple passes through the sample. The folded light path increases the optical path length between the laser and the light detector to thereby increase the sensitivity of a spectroscopy system incorporating an absorption cell producing a folded light path. Examples of existing “multi-pass” absorption cells are shown in U.S. Pat. Nos. 4,322,621; 4,749,276; 5,220,402; 5,440,143; 5,459,566; 5,485,276; 5,714,759; 5,731,583; 5,726,752; 5,818,578; and 5,949,537. 
         [0005]    U.S. Pat. Nos. 6,373,056 and 6,528,792, commonly owned by the assignee of the present application, disclose a spectroscopic sensor system whereby the hot filament source also acts as its own bolometric sensor. This requires the emitted light to be returned to the source/bolometer itself, a unique condition not found in other spectroscopic systems. The optical cell detailed in both filings disclose an apparatus containing at least three functional elements, a source/bolometer, a return reflector, and a driver/detector where the optical cell occupies the space between the source/bolometer and the return reflector. The return reflector in both filings is either a flat or contoured mirror that returns at least some of the light back to the source. Therefore the light passes twice through the optical cell, once on its way from the source/bolometer to the return reflector and a second time on its way back from the return reflector to the source/bolometer. 
         [0006]    What is still desired is an improved multi-pass sample cell for use in absorption spectroscopy that returns the light back to the source. This would be compatible with the hot self bolometer apparatus as detailed in U.S. Pat. Nos. 6,373,056 and 6,528,792 but would have applicability with other optical systems as well. Among other advantages and aspects, the new and improved multi-pass sample cell preferably will allow an absorption spectroscopy apparatus incorporating the cell to use light sources that are not collimated. In addition, the new and improved multi-pass sample cell preferably will minimize the difference (dispersion) in path lengths experienced by angularly divergent light rays traversing the sampling cell. The new and improved multi-pass sample cell preferably will also maintain or improve path length and throughput efficiency. Moreover, the new and improved multi-pass sample cell preferably will allow very high gas flow through the sampling cell while causing minimum disruption and turbulence to the gas flow. Furthermore, an absorption spectroscopy apparatus incorporating the new and improved multi-pass sample cell preferably will have a smaller total package size, including the source/bolometer, the optical cell, the driver/detector and the return reflector in order to fit into small footprint industry standard packaging. 
       SUMMARY OF THE DISCLOSURE 
       [0007]    The present disclosure provides a new and improved multi-pass sample cell and an absorption spectroscopy apparatus incorporating the cell. An exemplary embodiment of the sample cell includes an elliptical mirror, centered on the midpoint between a source/detector and a mirror. The cavity between the elliptical mirror and the source/bolometer and mirror defines the optical sample cell. The light travels in a four segment path starting from the source/bolometer to the elliptical mirror, reflects off the elliptical mirror and travels to the mirror, reflects off the mirror and travels back to the elliptical mirror and finally reflects off the elliptical mirror and returns to the source/bolometer. 
         [0008]    Among other benefits, the improved absorption spectroscopy apparatus of the present disclosure enables light to be passed through a substantial percentage of a sample passing through the sample cell. The cell is constructed so as to be small and compact while nonetheless enabling the light to make successive passes through the sample. The elliptical mirror minimizes differences (dispersion) in path lengths experienced by angularly divergent rays traversing through the sampling cell by refocusing the divergent light, while increasing path lengths and increasing throughput efficiency. Moreover, the sample cell of the present disclosure greatly relaxes the alignment tolerance between the source/bolometer and the multi-pass sample cell, a common problem in multi-pass absorption cell designs. The sample cell of the present disclosure improves the path length, efficiency and apparatus size of the hot self bolometer spectroscopic system. 
         [0009]    According to one embodiment of the sample cell of the present disclosure, the mirror is larger than the source/bolometer and the degree to which the mirror is larger defines the alignment tolerance required between the source/bolometer and the multi-pass sample cell. According to another embodiment, the elliptical mirror can be the degenerate case, a spherical mirror. 
         [0010]    In an additional embodiment of the sample cell the mirror can be mounted on a window that separates the source/bolometer from the multi-pass cell. 
         [0011]    In an additional embodiment of the absorption spectroscopy apparatus the apparatus contains multiple source/bolometers. The multi-pass cell independently returns light from each source/bolometer back to that same source/bolometer without crossing light from one source/bolometer onto another source/bolometer. The plurality of source/bolometers can be arranged along the long axis of the elliptical mirror but other arrangements are possible. 
         [0012]    In an additional embodiment of the absorption spectroscopy apparatus the cell is confined into a cylinder containing source/bolometer, elliptical mirror and the mirror. 
         [0013]    In another aspect, an absorption spectroscopy apparatus includes a sample cell defining a sample volume. The sample cell includes an elliptical contoured mirror and a source/bolometer disposed opposite to the elliptical contoured mirror. The source/bolometer is positioned to emit electromagnetic radiation toward the elliptical contoured mirror. Another mirror is disposed opposite to the elliptical contoured mirror. The other mirror is configured to reflect electromagnetic radiation received from the elliptical contoured mirror back toward the elliptical contoured mirror. In operation, light emitted by the source/bolometer travels along more than one optical path through the sample volume, the multiply reflected light being reflected back toward the source/bolometer. 
         [0014]    These and other advantages and aspects of the present disclosure will become apparent to those skilled in the art after a reading of the following description of exemplary embodiments when considered with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]    The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In addition, x, y, and z coordinates are illustrated in some of the drawings for reference only. 
           [0016]      FIG. 1  is a schematic diagram illustrating a longitudinal cross-section view of an exemplary embodiment of an improved absorption spectroscopy apparatus constructed in accordance with the present invention. 
           [0017]      FIG. 2  is a schematic diagram illustrating a longitudinal cross-section view of an alternative embodiment of an improved absorption spectroscopy apparatus constructed in accordance with the present invention, illustrating an optical path of the light through the cell. 
           [0018]      FIG. 3  is a schematic diagram illustrating a longitudinal cross-section view of an exemplary embodiment of an absorption spectroscopy apparatus showing a substantial alignment tolerance. 
           [0019]      FIG. 4  is a schematic diagram illustrating a longitudinal cross-section view of another alternative embodiment of an absorption spectroscopy apparatus including multiple source/detector elements. 
           [0020]      FIG. 5  is a schematic diagram illustrating a cross-section view of a spherical embodiment of absorption spectroscopy apparatus including a window separating the source/bolometer from the sample cell and the mirror mounted on the window. 
           [0021]      FIG. 6  is a schematic diagram illustrating a cross-section view of an additional exemplary embodiment of an improved absorption spectroscopy apparatus including an optical filter between the source/bolometer and the multi-pass cell. 
           [0022]      FIG. 7  is a schematic diagram illustrating a perspective view of an embodiment of an improved absorption spectroscopy apparatus including multi-pass cell including a semi-elliptical cylinder. 
           [0023]      FIG. 8A  is a top perspective view of an absorption spectroscopy apparatus adapted for integration in a cylindrical cavity. 
           [0024]      FIG. 8B  is cross-sectional view of the device of  FIG. 8A  along A-A′. 
           [0025]      FIG. 8C  is top perspective view of a cylindrical cavity adapted to house the absorption spectroscopy apparatus of  FIG. 8A . 
           [0026]      FIG. 9  is a top sectional view of an exemplary embodiment of an absorption spectroscopy apparatus according to the invention. 
           [0027]      FIG. 10  is a top perspective view of an exemplary embodiment of the absorption spectroscopy apparatus of  FIG. 9  shown in cross section. 
           [0028]      FIG. 11  is a top view of an exemplary embodiment of an absorption spectroscopy apparatus according to the invention with at least some of the surfaces shown in transparency. 
           [0029]      FIG. 12  is an elevation view of the absorption spectroscopy apparatus shown in  FIG. 11  with at least some of the surfaces shown in transparency. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0030]    Referring to  FIG. 1  of the drawings, there is shown a schematic representation of an exemplary embodiment of a new and improved absorption spectroscopy apparatus  20  constructed in accordance with the present disclosure. The apparatus  20  includes a source/detector  26 , a first elliptical reflector  22  defining an elliptical mirrored surface  24 , and a second reflector  28 . As shown, the source/detector  26  is positioned along one half of a plane bisecting the elliptical reflector  22 . A second reflector  28  is positioned along the other half of that plane. Electromagnetic radiation is emitted from the source/detector  26  into a broad region. Three exemplary rays are shown emanating from the source/detector  26  covering an angle φ. With the source/detector  26  positioned at or near a first focus point F 1  of the elliptical reflector  22  and the second reflector  28  positioned at a second focus point F 2  of the same elliptical reflector  22 , the divergent rays are substantially focused onto the second reflector  28 . In some embodiments, the second reflector  28  is a planar reflecting surface. 
         [0031]    An alternative embodiment of a multi-pass sample cell  30  is shown in  FIG. 2 . The sample cell  30  again includes a source/detector  36  and second reflector  38  positioned at respective focal points F 1 , F 2  of an elliptical reflector  31 . In some embodiments, the device  30  includes an enclosing wall  33  forming an enclosed sample volume together with the elliptical reflector  31 . One or more fluid ports  32 ,  34  can be provided to allow access to the enclosed sample volume. For example, an inlet port  32  and an outlet port  34  provide a flow path for continuous fluid or gas flow through the sample volume. 
         [0032]    As also shown in  FIG. 2 , electromagnetic radiation (e.g., light) is emitted from the source/detector  36  and travels through the sample cell  30  along an exemplary ray R 1  impinging upon the elliptical reflective surface  31 . The light is reflected off of the elliptical reflective surface  31  along a once-reflected ray R 2  and is refocused onto the second reflector or mirror  38  after traveling through the interior volume of the sample cell (optical sample cell). The once-reflected light is reflected off of the mirror  38  along a twice-reflected ray R 3  traveling again through the interior volume of the optical sample cell  30  to the elliptical reflective surface  31 . Finally, the twice-reflected light reflects off of the elliptical reflective surface  31  along a thrice-reflected ray R 4  being ultimately refocused back on the source/detector  36 . The rays R 1 , R 2 , R 3 , R 4  illustrated in  FIG. 2  are representative of one of many optical paths that the electromagnetic radiation travels within the cell  30  between the same source/detector  36  and mirror  38 . The source/detector  36  can be a source/bolometer, such as an incandescent light source with large angularly divergent emission pattern. 
         [0033]    The elliptical mirrors  22 ,  31  shown in  FIG. 1  and  FIG. 2  are shown in cross section, each mirror  22 ,  31  being a three dimensional object. In some embodiments, the contour of the mirror is a section of an ellipsoid defined by the following generic equation where  2   a  is the long axis of the ellipse,  2   b  is the short axis of the ellipse and  2   h  is the distance between foci. To generate a three-dimensional surface of the mirror, this contour is rotated about the long axis. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         x 
                         2 
                       
                       
                         a 
                         2 
                       
                     
                     + 
                     
                       
                         y 
                         2 
                       
                       
                         b 
                         2 
                       
                     
                   
                   = 
                   1 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     a 
                     2 
                   
                   = 
                   
                     
                       b 
                       2 
                     
                     + 
                     
                       h 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0034]    Among other aspects and benefits, the new and improved absorption spectroscopy apparatus  20 ,  30  of the present disclosure enables electromagnetic radiation, such as visible and/or infrared light, to be passed through a substantial percentage of a sample passing through the sample cell  20 ,  30 . In some embodiments, the cell  20 ,  30  is constructed to be small and compact (e.g., not more than about 10 cm in diameter), while nonetheless enabling the radiation to make successive passes through the sample between emission and detection at the source/detector  26 ,  36 . The elliptical mirror  22 ,  31  minimizes the differences (dispersion) in path lengths experienced by angularly divergent rays traversing through the sampling cell  20 ,  30  by refocusing the light, while at the same time increasing path lengths and increasing throughput efficiency. Moreover, the sample cell  20 ,  30  of the present disclosure can capture and return light from a highly disperse source and does not require a collimated source. 
         [0035]    In general, an ellipsoidal surface is a locus of all points disposed about two foci F 1 , F 2 , where the sum of the distances from each point along the surface to F 1  and F 2  is a constant. With an ellipsoidal surface, radiation from a radiation source (collimated or wide angle) from F 1  toward the surface is reflected by the surface and is focused at F 2 . The propagation path length from all angles of the radiated light is substantially equal. Thus, an equal amplitude wide angle source can cover substantially the entire volume of the cell, thereby improving sampling efficiency of a sample within the cell. 
         [0036]    In a preferred form of the invention, a source/detector (bolometer) is provided at F 1  and a mirror is provided at F 2 , such that substantially all radiation incident on the mirror at F 2  is reflected back to the source/bolometer at F 1 . Thus, only a single source/bolometer is necessary. In some embodiments, a second source/bolometer is placed at F 2  in place of the mirror. The two source/bolometers can each be tuned to the same wavelength and act in pairs: the first source/bolometer at F 1  radiates to the second source/bolometer at F 2 ; whereas, the second source/bolometer at F 2  radiates to the first source/bolometer at F 1 . Preferably, the emission detection characteristics of each of the first and second source/bolometers are substantially matched. Such matching can be accomplished by fabricating each device on the same substrate. The substrate can be severed to facilitate separation of the two source/bolometers. In some embodiments, the first and second source/bolometers are tuned to different wavelengths. 
         [0037]    As shown in  FIG. 3  an improved absorption spectroscopy apparatus  40  provides a very high tolerance to misalignment. The degree of misalignment or alignment tolerance, dx, can be best represented by equation 3 where L represents the size of the mirror  48 , h is the distance from the center of the ellipse  42  to one of the two foci and P is the size of the source/bolometer  46 . When L is much larger than h and P the alignment tolerance dx is large. In the extreme case, L can be the entire distance between the center and the pole of the ellipse.  FIG. 3  is shown in two dimensions, but this relationship holds in three dimensions. 
         [0000]    
       
         
           
             
               
                 
                   dx 
                   = 
                   
                     L 
                     - 
                     h 
                     - 
                     
                       P 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0038]    In an exemplary embodiment of the invention shown in  FIG. 5 , the elliptical mirror is a spherical mirror, the degenerate case when h=0. For this case, the equation 3 for the alignment tolerance dx still holds true. As illustrated, a source/bolometer  65  is positioned substantially at the center of a spherical reflector  62  and adjacent to a mirrored reflector  68 . Each of the source/bolometer  65  and mirror  68  can be disposed along a respective side of a central point, equidistant to the locus of points along the spherical reflector  62 . In some embodiments, the source/bolometer  65  represents a first pixel  66   a , positioned adjacent to a second pixel  66   b . The second pixel  66   b  can be used as a reference. In some embodiments, the second pixel  66   b  can be covered, or “blinded” from the sample volume to serve as a reference for identifying and removing device contributions from measurement results. As illustrated, the second pixel  66   b  can be blinded by the mirror  68 . 
         [0039]    As shown in the exemplary embodiment of  FIG. 5 , the source/bolometer  66   a ,  66   b  can be separated from a sample volume of the sample cell  60  by a window  67 . This configuration  60  prevents sampling fluids or gasses from contacting the source/bolometer  66   a ,  66   b  directly. In some embodiments, the window  67  can serve as a mounting surface for a mirror  68 . 
         [0040]    In another exemplary embodiment of the invention, referring now to  FIG. 6 , an optical filter  61  is positioned in the optical path between the source/detector  26  and the sample volume in order to filter the electromagnetic radiation emitted/sensed by the source/detector  26 . For example, the optical filter  61  can be positioned just above the source/detector  26 . In other embodiments, the optical filter  61  can be positioned anywhere throughout the optical path. When appropriately tuned, the filter  61  facilitates selection of a narrow spectral band for specific spectroscopic chemical analysis. 
         [0041]    In the exemplary embodiment of the invention, referring now to  FIG. 4 , multiple source/bolometers can be positioned in one half of the bisecting plane of an elliptical mirror  52 . In a two pixel example, the first and second source/bolometers  56   a ,  56   b  are positioned along a central axis of the elliptical mirror  52 . This representation is one of many possible configurations and was chosen to best demonstrate the functional state. Light from each of the source/bolometers  56   a ,  56   b  radiates into the interior volume of the sample cell  50  and onto the elliptical mirror  52 . The elliptical mirror  52  reflects the light and refocuses that light down onto the mirror  58  where an inverted image forms. The image reflects off of the mirror  58  and passes back through the interior volume of the sample cell  50  and onto the elliptical mirror  52 . Once again the light reflects off the elliptical mirror  52  and is refocused back onto the array of source/bolometers  56   a ,  56   b . Because a positive image forms in the bisecting plane, light from each individual source/bolometer  56   a ,  56   b  returns specifically back to that same source/bolometer  56   a ,  56   b  without significant crossover to neighboring source/bolometers  56   a ,  56   b . This maintains specificity between the individual source/bolometers  56   a ,  56   b . If each source/bolometer  56   a ,  56   b  is tuned to a different spectral waveband, different spectroscopic measurements can be made in parallel at the same time in the same sample cell  50  on the same sample fluid. 
         [0042]    In some embodiments, a cylindrical sample cell can be formed having an elliptical reflective surface defining at least a portion of an elliptical cylinder. Such an embodiment can be formed by extending the reflective surface of any of the embodiments shown in the preceding figures along an axis perpendicular to the page. A perspective view of a semi-elliptical cylindrical cavity  70  is shown in  FIG. 7 . An a reflective surface  72  extending for a distance along the z axis, forms an elliptical cross section parallel to the x-y plane. One or more source/detectors  76  can be located along a first line extending along a first focus F 1  of the elliptical surface. The source detector  76  can be an extended linear (i.e., line) source or single pixel, or an array of more than one source detectors, or pixels  76   a ,  76   b ,  76   c ,  76   d  disposed along the focal line F 1 , as shown. A mirror  78  is disposed along a second line extending along a second focus F 2  of the elliptical surface  72 . The mirror  78  need not be confined to the line as shown, but can extend over a more substantial region to improve device manufacturing tolerances. Electromagnetic radiation from the source/detector  76  reflects once off of the elliptical surface  72  onto the mirror  78 , where it is reflected back onto the elliptical surface  72 , where it reflects again toward the source/detector  76 . This arrangement can include a section of a right circular cylinder in which the source/detector  76  and mirror  78  are each disposed about a central axis of the cylinder. 
         [0043]    In some embodiments, the multi-pass sampling apparatus can be configured to fit into a confined volume. Referring to  FIG. 8A ,  FIG. 8B  and  FIG. 8C , an exemplary embodiment of the invention  80  is adapted to fit within a sample cylinder  82 . In this configuration, the boundary of an elliptical mirror  81  is defined at least in part by a lateral wall  83  of the cylinder  82 . A surface of the elliptical mirror  86  is positioned in a vertical direction within the cylinder  82  such that the curvature of the elliptical mirror  86  fits within the curvature of the walls  83  of the cylinder  82 . A source/bolometer  85  and the mirror  87  are positioned on the opposite side of the cylinder  82 . A top opening  89  of the cylinder  82  provides an inlet for sample fluids to diffuse in and out of the sample cell  80 . 
         [0044]    Referring to  FIG. 9 , a top sectional view of an alternative exemplary embodiment of an absorption spectroscopy apparatus is shown. A gas or fluid sample cell  100  includes a housing  101  including an elliptical mirror having an interior mirrored surface  102  and a supporting frame  104 . The supporting frame  104  is preferably formed from a rigid material, such as a plastic or metal having sufficient thickness to ensure uniformity of the elliptical mirrored surface  102  during use in its intended environment. The supporting frame  104  can be formed of plastic using an injection molding process, with a mirrored surface  102  applied in a separate thin film process. The mirrored surface  102  can be applied by a painting process, using thin film deposition, or otherwise attaching a reflective surface. Intended environments can include high temperatures or substantial temperature variations. To ensure rigidity, one or more structural supporting members or ribs can be included. The exemplary embodiment includes a vertical rib  106  extending along a rear vertical dimension of the center of the elliptical mirror. 
         [0045]    The device  100  includes a second frame member  108  placed at least partially in apposition to the mirrored surface  102 . An emitter sensor  110  is fixedly attached with respect to the second frame member  108 . To ensure precise positioning of the emitter sensor  110 , at least within a design tolerance, the frame member  108  provides an alignment function. Thus, the frame member  108  ensure the relative positioning of the emitter/sensor  110  is maintained with respect to the mirrored surface  102 . In some embodiments, the frame member  108  includes an emitter/sensor mount  108   c  and lateral frame extensions  108   a ,  108   b . The lateral frame extensions  108   a ,  108   b  extend from the emitter/sensor mount  108   c  to the mirror supporting frame  104 . Thus, proper positioning of the emitter/sensor  110  and mirrored surface  102  can be controlled to some degree by manufacturing processes of the frames  104 ,  108 . In some embodiments, the frame  108  is formed having vertical walls to laterally contain a sample volume together with the mirror frame  104 . 
         [0046]    One or more electronic modules can be positioned in a volume defined between exterior surface of walls of the frame member  108  and adjacent interior surface of the housing  101 . In the exemplary embodiment, a first drive/sense electronics module  112  is positioned behind a right frame extension  108   a , a drive/sense electronics module  114  is positioned behind a left frame extension  108   b , and emitter sensor electronics  116  is positioned behind the emitter/sensor mount  108   c . As shown, the frame member  108  includes posterior frame extension clips  118   a ,  118   b  adapted to keep the emitter sensor module in a fixed relation to the emitter/sensor mount  108   c.    
         [0047]    Referring to  FIG. 10 , a top perspective view of an exemplary embodiment of the gas or fluid sample cell  100  is shown in cross section. The mirror supporting frame  104  includes a first top support member  120  and a bottom support member  122 . The frame members  104 ,  108  are seated on a bottom wall  124 , defining a bottom surface of the sample volume. In some embodiments, the bottom wall  124  is a printed circuit board. In other embodiments, the bottom wall  124  is an end-cap filter, adapted to keep particulates out of the sample volume. The bottom wall  124  can cover the entire bottom surface of the housing  101  as shown. The bottom support member  122  of the mirror supporting frame  104  provides a secure footing to position the mirrored surface  132  with respect to the bottom wall  124 . The first top support member  120  defines at least part of a diffusive gas exchange port  126 . The frame member  108  includes a second top support member  128  coplanar with the first top support member  120 . The second top support member  128  also defines a portion of the diffusive gas exchange port  126 . In some embodiments, a bottom ledge  130  extends below the emitter/sensor  110  between the emitter/sensor mount  108   c  and the bottom wall  124 . The bottom ledge  130  is a sloping ledge remaining out of view from the emitter/sensor  110 . One or more electrical contacts  132  extend through and beyond the bottom wall  124  providing external access to electronic assemblies  112 ,  114 ,  116 . In some embodiments, a flex-print circuit  134  provides contact between the one or more electrical contacts  132  and the electronic assemblies  112 ,  114 ,  116 . 
         [0048]    Referring to  FIG. 11  and  FIG. 12 , top and side views of an exemplary embodiment of the gas or fluid sample cell  100  are respectively shown having at least some of the surfaces illustrated in transparency. In some embodiments, the lateral frame extensions  108   a ,  108   b  respectively include alignment pins  136   a ,  136   b  (generally  136 ). The alignment pins  136  are disposed to abut an adjacent surface of the mirrored surface  102  or frame  104 , the abutment providing a desired spacing between the emitter/sensor mount  108   c  and the mirrored surface  132 . 
         [0049]    The present disclosure, therefore, provides an improved “multi-pass” sample cell that causes light to pass through a very large percentage of a sample contained in the cell. The improved sample cell prevents the loss of significant amounts of light through ends of the cell and increases the throughput of the cell. In addition, the improved cell is compact, robust, and relatively easy to manufacture. 
         [0050]    Various embodiments of improved sample cell devices and methods of sampling have been described herein. These embodiments are given by way of example and are not intended to limit the scope of the present invention. It should be appreciated, moreover, that the various features of the embodiments that have been described may be combined in various ways to produce numerous additional embodiments. Moreover, while various materials, dimensions, shapes, source/detection and mirror locations, etc. have been described for use with disclosed embodiments, others besides those disclosed may be utilized without exceeding the scope of the present disclosure as recited in the following claims.