Patent Publication Number: US-6903329-B2

Title: Cooled mounting for light detector

Description:
PRIORITY 
     The present application is a continuation-in-part of U.S. patent application Ser. No. 09/934,272, filed Aug. 21, 2001, now U.S. Pat. No. 6,744,516 entitled Optical Path Structure for Open Emissions Sensing, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to open path emissions measurement systems. More particularly, the present invention relates to an apparatus for transmitting, reflecting, and detecting light in an open path sensing system such as a vehicle emission sensing system, having use in detecting and/or measuring one or more components of the air through which the light passes. 
     BACKGROUND OF THE INVENTION 
     Current methods of determining whether a vehicle is compliant with emission standards include open path and closed path emissions measurement systems. In a closed path system, an emission sensor is directly connected to the exhaust of the vehicle, such as by insertion into a tailpipe. An open path vehicular emissions measurement system collects data by a means other than a direct connection to the tailpipe, such as a remote sensor that analyzes the individual components of emissions. Open path vehicle emission systems are often preferable to closed path systems because they can be used in numerous locations and do not require the vehicle to stop for testing. 
     Various open path emission sensing systems have been known. One such device uses a radiation source on one side of a roadway that projects a beam across the roadway to be received by a detector. The radiation source and the detector are located on opposite sides of the roadway. The radiation source emits light spectra that may be used to detect an emission signature by way of absorption of light, or which alternatively may be used to excite emission components so as to cause the components to emit light. The detected emission signature can then be used in various applications, such as the measurement of a vehicle&#39;s compliance with emission limits and the determination of the type of fuel that a vehicle is using. 
     A disadvantage of many known arrangements is that the radiation sources and detectors must be placed on opposite sides of the roadway from each other. Since both the detectors and radiation sources require power to operate, this means that a separate power supply must be provided on each side of the roadway. 
     Furthermore, current open path embodiments are unable to maintain stability of measurements throughout the diurnal pattern of daytime-nighttime temperatures. Part of the reason for instability is a lack of effective thermal control of the detecting components of the emissions measurement system. Frequent recalibrations of the instrumentation are required, due to a baseline shift (zero drift) of the measuring system, caused at least in part by thermal instability of the detecting components of the system. For many systems, an increase in detector temperature can result in lowered sensitivity to light, which is seen in data as a rising baseline of measurement, and therefore causing data to move in the negative direction (negative bias). The opposite is true for falling temperatures. 
     Additionally, open path instrumentation in particular is susceptible to increased noise for each measurement, with increasing detector temperature. This is a problem especially for measurements of very small concentrations of gases of interest, where the concentration of gas may be within the noise of the measuring instrumentation. Ideally, detectors are chilled to close to absolute zero, however this is not practical or safe for portable instrumentation. 
     Some systems rely on metal to air heat transfer for cooling. The problem with this approach to cooling is that ambient air can be tainted with dust and other contaminants that depose onto the optical components of the measurement system, reducing the effectiveness of the system in determining a concentration of a gas or particles of interest, and requiring more frequent periodic maintenance. 
     Accordingly, it is desirable to provide an improved optical transmission, reflection, and detection system that can measure particulate matter and gaseous emissions measurements, along with an improved correlation opacity measurement as herein disclosed. 
     SUMMARY OF THE INVENTION 
     The present invention provides an optical transmission, reflection and detection system having a bisected mounting for a light detector which includes a baseplate, an insulating member attached to the baseplate, and a detector side attached to the insulating member. The insulating member thermally isolates the baseplate from the detector side. 
     The bisected mounting can also include thermal conductors on the baseplate or a cooling device located on the detector side such as a thermoelectric cooling device or a liquid cooling device. 
     The baseplate can be made of aluminum or be coated in areas that are not in contact with the insulating member with a powder coating, or an anodized coating. 
     For further thermal isolation, the bisected mounting can include fiber washers located between the insulating member and the baseplate, and between the insulating member and the detector side. Thermal grease can also be used between the insulating member and the baseplate, and between the insulating member and the detector side. 
     In an alternate embodiment of the invention, a method for keeping a detector side of a bisected mount of a light detector cool includes the steps of detecting a signal on the detector side of the light detector, transmitting the signal to a baseplate side of the light detector that is thermally isolated from the detector side by an insulating member. 
     The baseplate can be cooled by using thermal conductors on the baseplate. The detector side can be cooled using cooling devices such as a thermoelectric cooler or a liquid cooling device. 
     The baseplate can be made of aluminum or coated in areas that are not in contact with the insulating member using a powder coating or an anodized coating. 
     Thermal isolation can furthered by using fiber washers between the insulating member and the baseplate, and between the insulating member and the detector side. Thermal grease can also be used between the insulating member and the baseplate, and between the insulating member and the detector side. 
     In another embodiment of the invention, a system for keeping a detector side of a bisected mount of a light detector cool includes a means for detecting a signal on the detector side of the light detector, a means for transmitting the signal to a baseplate side of the light detector that is thermally isolated from the detector side by an insulating member. 
     The system can also include a means for cooling the baseplate using thermal conductors on the baseplate. The detector side can be cooled using a cooling device such as a thermoelectric cooler or a liquid cooling device. 
     The baseplate can be made of aluminum or have a coating in areas that are not in contact with said insulating member such as a powder coating or an anodized coating. 
     The baseplate can have fiber washers located between the insulating member and the baseplate, and between the insulating member and the detector side. Thermal grease can be located between the insulating member and the baseplate, and between the insulating member and the detector side. 
     There have thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto. 
     In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
     As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a preferred embodiment of a source unit of the present invention including housing with window, light sources, filter wheel, beam splitter/combiner, and reflector. 
         FIG. 2  illustrates a preferred embodiment of a reflection unit of the present invention. 
         FIG. 3  illustrates a preferred embodiment of a detection unit of the present invention including housing with window, reflector, beam splitter/combiner, detector and spectrometers. 
         FIG. 4  illustrates an exemplary filter wheel that may be used in accordance with one embodiment of the present invention. 
         FIG. 5  illustrates an alternate embodiment of a detection unit of the present invention including housing with window, reflector, beam splitter/combiners, spectrometers, spinning reflector, monolithic ellipsoidal mirror, filter array with gas cells, focusing reflector, and a single infrared detector. 
         FIG. 6  illustrates several elements of an exemplary computer of a type suitable for carrying out certain functions of the present invention. 
         FIG. 7  illustrates a detection unit using multiple spectrometers and a single detector. 
         FIG. 8  illustrates the properties of an ellipsoidal reflector. 
         FIG. 9  is a conceptual diagram of some basic components of the present invention, including light source, reflection unit, detection unit, and processor. 
         FIG. 10  illustrates the addition of reflectors to the components of FIG.  9 . 
         FIG. 11  illustrates the properties of a paraboloidal reflector. 
         FIG. 12  further illustrates the properties of a paraboloidal reflector. 
         FIG. 13  illustrates the addition of multiple light sources with beam splitter/combiners to the components of FIG.  10 . 
         FIG. 14  illustrates a modification of the embodiment shown in  FIG. 13  illustrating the arrangement of opposed sources. 
         FIG. 15  illustrates the rear view of a bisected mounting for a light detector with integral cooling within the mounting. 
         FIG. 16  illustrates the front view of a bisected mounting for a light detector with integral cooling within the mounting showing a fine adjustment mechanism for alignment of the detector within the optical path and showing the front face of a typical detector. 
         FIG. 17  illustrates the top view of a bisected mounting for a light detector with integral cooling within the mounting, highlighting the two sections, hot and cold, with an exemplary thermal electric cooler between the hot and cold bisected portions providing the desired cooling. 
         FIG. 18  illustrates thermal gradient modeling of the hot side of the bisected cooled light detector mount. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     A preferred embodiment of the present invention provides an improved optical source, reflection, and detection system for gas component analysis. A preferred embodiment includes a light source unit, which preferably includes one or more of infrared, visible, and ultraviolet light sources; a reflection unit; and a light detection unit. Preferably, light sources and detectors are contained within a housing. The light is transmitted through a gas, such as air containing vehicle emissions, reflected, and detected for analysis and measurement of the amount of absorption that has occurred at known wavelengths of the light. The amount of absorption may be used to determine concentrations of gases corresponding to the specific wavelengths. 
     In a preferred embodiment of this invention, infrared, visible, and ultraviolet radiation is combined into one beam, directed across a path such as a road along which vehicles travel and generate exhaust, reflected back across the path, collected and concentrated, separated again, and received by one or more discrete detectors and/or spectrometers. In order to be able to separately analyze each range of wavelengths, the infrared light passes through a sequence of filters and/or gas cells either before or after traversing the path of light across the road. The filters are preferably narrow band pass filters and the gas cells contain known concentrations of gases of interest, such that each filter or combination of filters and gas cells is specific to a gas of interest. In one embodiment, a spinning wheel holds the filters and passes each filter in front of the infrared light source in sequence, before the light traverses the road. In an alternate embodiment, the infrared light, after traversing the road, is distributed by a spinning reflector, such as a mirror, into a stationary array of filters and/or gas cells in sequence to an ellipsoidal mirror or an array of ellipsoidal mirrors that focus the light into a single detector. The visible and ultraviolet light is directed to one or more spectrometers that can analyze the desired wavelength ranges directly. 
     A portion of a preferred embodiment of the present inventive apparatus is illustrated in FIG.  1 .  FIG. 1  illustrates a possible light source component of the present invention. The light source component shown includes an infrared light source  10 , a source of visible light  11 , and an ultraviolet light source  12 . The infrared light  14  emitted by the infrared source  10  passes through a filter wheel  16 , more completely described in FIG.  4 . Then it is reflected by a beam splitter/combiner  18 , and follows an optical path  20  until it reaches a reflector  26  such as an off-axis paraboloidal mirror or spherical mirror. An off-axis paraboloidal mirror is preferred over a spherical mirror due to the aberrations in light that occur with spherical mirrors, however production economics may dictate the use of spherical mirrors. The reflector  26  reflects the infrared light along a path  22 , through a protective window  25  in the housing  27 , leading to a reflection unit illustrated in FIG.  2 . 
     The reflector  26  and other optical components described in this embodiment are protected by a window  25  that allows the transmission of all of the wavelengths of interest. This window  25  is attached to the housing  27  of the entire source unit. Preferably, the light sources and detectors are included within a single housing. However, the light sources and the light detectors may optionally be provided in more than one housing. Also preferably, the housings are sealed to prevent contaminants such as soot, road dust, and other road debris from damaging or coating the internal components and thus degrading the light signal received and/or transmitted by them. Also preferably, the sealed housings contain windows to allow light of the wavelengths of interest to leave and enter the housings as required for the light to travel along the desired optical path. These windows are preferably made of a material such as calcium fluoride (CaF 2 ), sapphire, or other material that will pass light of all wavelengths of interest with little or no attenuation. Optionally, the windows may be coated by a particular type of coating such as an anti-reflection coating or other suitable coating to enhance the transmission of light of the wavelengths of interest. 
     The infrared light source  10  may be any source that emits a sufficient intensity of light of the wavelengths of interest. The reflectors and optical path length determine the size of the spot from the infrared source that contributes to the light beam. Preferably the source is chosen, such that the light emitting area of the filament is as close to that spot size as possible for minimum power consumption. 
     Preferably, the filter wheel  16  is a spinning wheel that is powered by a motor  15  that spins the wheel  16  about an axis  19 . Also preferably, a synchronization device  58  is provided to track the position and rotational speed of the filter wheel  16 . Features of the filter wheel  16  are more completely illustrated in FIG.  4 . 
     In addition, visible light from source  11  is focused by an optical element  13  to bring diverging light rays back into a focus through the center of ultraviolet source  12  where it is combined with the ultraviolet light from source  12  into a combined beam  24 . The combined visible and ultraviolet light  24  passes through the beam splitter/combiner  18  such that it also follows optical path  20  to the reflector  26 , where the light is reflected to also follow path  22  out window  25  toward the reflection unit illustrated in FIG.  2 . The visible light source  11  may be a light emitting diode (LED), which emits light in a narrow range of wavelengths, or another visible source such as a halogen lamp that emits a broader range of wavelengths. The advantage of passing the visible light through the ultraviolet light source  12  is eliminating the need for another beam splitter/combiner, saving optical power that would otherwise be lost by the inefficiency of the beam splitter/combiner, in addition to saving space within the enclosure  27 . However, if it is desirable to have an ultraviolet source  12  of a design that does not allow for pass-though of the visible light, then alternatively, the visible source  11 , and ultraviolet source  12  may be reconfigured to take, for example, positions  146  and  144  as illustrated in an arrangement of sources in  FIG. 13  that will be discussed further below. 
     The visible light source  11  is not required for gaseous measurements, however visible light is used to measure particulate matter and potentially opacity and lubricating oil elements. Particulate Matter having a diameter of 2.5 microns and smaller (PM 2.5 ) can be measured by an absorption technique at a wavelength of 500 nanometers, using a spectrometer such as in  FIG. 3 , element  42 . Ideally, a PM 2.5  measurement would best be taken at 530 nanometers, however when measuring vehicle exhaust, there are interferences caused by gaseous species such as nitrogen dioxide (NO 2 ) that also absorb at 530 nanometers that would preclude obtaining a precise measurement of vehicular exhaust. This is especially true when measuring diesel exhaust as diesels emit a significant amount of NO 2  and particulate matter. PM 2.5  measurements at 500 nanometers include only elemental carbon and will therefore miss roughly 30% of the total concentration of diesel PM 2.5 , but will not have any significant interferences with the precise measurement of PM 2.5  at this wavelength. Despite the penalty of missing 30% of the total diesel PM 2.5 , this measurement can be scaled to provide a more accurate measurement when compared to other methods of detection. 
     Furthermore, the 500 nanometer wavelength was selected because of the desire to collect information about particulate mass for measurement of particulate emissions, in order to be consistent with the Federal reference method as summarized in BACKGROUND above. Focusing on the total mass measurement is done at the expense of measuring the total count of particles in exhaust, however the particles that are missed being measured by this embodiment are the smallest particles, and therefore do not contribute much to the total mass of the particulate sample. 
     This same PM 2.5  information can be used to determine whether a gasoline-powered vehicle is in a cold start mode. Cold start is when the engine of the vehicle being tested is not up to its normal operating temperature. A gasoline-powered vehicle in cold start mode will emit a much greater amount of particulates, on par with the amount of particulate emissions from diesel-powered vehicles, than a vehicle up to normal operating temperature. Cold start information is very useful for open-path emissions testing equipment, as it is important when enforcing air pollution laws not to falsely incriminate a tested vehicle for excess emissions when the vehicle is merely not operating in a normal mode. It is not possible to directly interrogate the driver of a tested vehicle using a non-intrusive method of sampling vehicle exhaust such as with an open-path method of a preferred embodiment. There is no means for stopping the vehicle to interrogate the engine&#39;s operating temperature or mode. The operating mode has to be deduced from several pieces of information, and cold start information is one element of this. 
     A second visible source in the approximate position of visible light source  11  can be added to the system to provide the ability to measure opacity, if no singularly suitable light source  11  can be obtained. The Society of Automotive Engineers J1667 opacity test, also known as the “Snap Acceleration Test”, measures opacity concentrations in the range of 562 through 568 nanometers. This embodiment can include measurement of exhaust opacity per the apparatus requirements of the J1667 specification, with the variation being that the measurements occur in an open path configuration. Using a visible light source  11  in combination with a spectrometer means of detection  42  provides for detection of opacity over the entire range of wavelengths as specified in J1667, as opposed to current art that has a much narrower field of view spectrally. 
     As an alternative to having two visible light sources, the visible light source  11  can be selected with a sufficiently broad output of spectra such that PM 2.5 , opacity, and even blue smoke can be measured. Blue smoke may be an indication that the vehicle is excessively burning lubricating oil, and therefore is in need of internal engine repairs to reduce emissions. This blue smoke plume comes as a result of lubricating oil combustion, and contains elements that are in the lubricating oil. Principal elements of lubricating oil that show up in the exhaust plume of a vehicle include sulfur, zinc, magnesium, copper, calcium, and phosphorus. The very high temperature combustion that occurs inside of a vehicle&#39;s engine temporarily causes these elements to appear in gas phase, when they can be viewed through an optical absorption technique. The wavelength of absorption for each of these elements is listed in Table 1. 
     Because of absorption interferences with gaseous emissions emanating from the same vehicle, some elements of the lubricating oil are better for observing than others. For instance, zinc absorbs ultraviolet light at 213.9 nanometers, which is unfortunately in the same general absorption vicinity as 1,3 butadiene and nitrogen monoxide. Both of these gases are present in tailpipe exhaust. Phosphorus however has an absorption wavelength in the visible spectra away from significant gaseous emissions interferences and is therefore a preferred method of determining a vehicle that is excessively burning its engine lubricating oil. 
     It is not essential to get a precise measurement of the amount of a lubricating oil element in the exhaust. The mere presence of the element, in significant concentrations that it is detected by the embodiment, is sufficient to provide probable cause that the tested vehicle is excessively burning lubricating oil. For this reason, there is no need to include the lubricating oil elements into a combustion equation that accounts for exhaust dilution. 
     Given that these elements have very narrow wavelengths of absorption on the order of 0.2 nanometers, it is preferred to use a laser source  11  directed to a discrete detector in place of a spectrometer  42  within this embodiment, as opposed to using a broadband source  11  and a visible spectrometer  42 . However, a spectrometer  42  with sufficient resolution of the grating and enhancements to other supporting parameters such as the slit opening to the spectrometer  42 , can provide a method for determining the above named elements that are present in combusted lubricating oil. For a visible spectrometer embodiment, an economy can be achieved by using the same spectrometer  42  for lubricating oil elements detection as is used for the J1667 equivalent test detection, though the J1667 test wavelengths do not require the small increment gradient as desired with the lubricating oil element detection. A broadband visible light source  11  can be used, when in combination with a spectrometer  42  for detection, to emanate light at wavelengths for lubricating oil elements that absorb in the visible spectra, along with emanating the J1667 wavelengths as disclosed above. 
     The ultraviolet light source  12  is preferably an ultraviolet lamp such as deuterium lamp, a xenon lamp, or another lamp that has ultraviolet light emission characteristics broad enough to include wavelengths of interest, ideally to emit light for at least all of the ultraviolet wavelengths of interest as listed in Table 1. 
     As  FIG. 1  illustrates, where multiple light sources such as components  10 ,  11 , and  12  are provided, the emitted beams preferably follow substantially the same optical path  20  toward the reflector  26 . The reflector  26  is positioned such that light sources  10 ,  11 , and  12  are near the focal point of the reflector  26  and the reflected light  22  is parallel to its axis of rotation. The angle between the incoming  20  and reflected light  22  and the focal length are determined by the design of the reflector  26  and may be chosen based on considerations of component layout and F-number. (F-number of an off-axis paraboloidal mirror is defined as the diameter of the mirror divided by its effective focal length.) Thus, light  20  transmitted to the reflector  26  is reflected in a direction  22  that is away from the original light sources  10 ,  11 , and  12 . In addition, if beam splitter/combiner  18  is a neutral density filter, it is preferably chosen so that the proportion of visible and ultraviolet light passed and the proportion of infrared light reflected are balanced according to the requirements of the detection unit. Optionally, a beam splitter/combiner  18  that is sensitive to different wavelengths such as a dichroic beam splitter may be used instead of a neutral density filter for beam splitter/combiner  18 . In order to use some types of beam splitter/combiners, the positions of the infrared  10  and visible/ultraviolet sources  11 ,  12  may be reversed. 
       FIG. 2  illustrates an exemplary reflection unit, which in an embodiment used to detect vehicle emissions is preferably placed across the road from the light source and detector components, creating an open-path emissions testing system. The reflection unit includes a retro-reflective system, preferably a vertical system, and preferably comprising three mirrors positioned to form 90° angles with respect to each other. A vertical orientation of the mirror assembly is preferred in order to adequately capture the emissions of vehicles of all profiles and heights. Referring to  FIG. 2 , incoming light  22  is reflected by a first mirror  30  and a second mirror  32 . The first and second mirrors are adjacent or substantially adjacent to each other to form a 90° angle. The light reflected by the first and second mirrors is transmitted to a third mirror  34 . As  FIG. 2  illustrates, the flat reflective portion of third mirror  34  forms a 90° angle with the flat reflective portions of both first mirror  30  and second mirror  32 . It is not important to have mirrors  30 , 32  on top of mirror  34 , as this orientation could be reversed without any change to the quality of reflection of light. Light  36  that is reflected by third mirror  34  is then transmitted to the detection unit and travels in a direction that is parallel to the incoming light  22  in a configuration as illustrated in  FIG. 9  to be discussed later in this text. The incoming light  22  and/or the reflected light  36  pass through an air component that is to be measured, such as vehicle emissions. 
       FIG. 3  illustrates an exemplary detection unit that receives the light that is generated by the source component of  FIG. 1 , and reflected by the reflection unit of FIG.  2 . Referring to  FIG. 3 , incoming light  36  passes through a protective window  35  that has similar characteristics to the window of the source unit illustrated in  FIG. 1 , is reflected by a reflector  38  such as an off-axis paraboloidal mirror or spherical mirror that reflects light along an optical path  40  at an angle relative to the incoming light  36 . The light transmitted along the optical path  40  is reflected by a beam splitter/combiner  44  that directs infrared light  48  toward infrared detector  50 . Preferably, the infrared detector  50  is positioned within the focal volume so that the light will over-bathe the detector&#39;s active area so that system vibrations will not adversely affecting measurements by causing a portion of the detector&#39;s active surface to temporarily not have light exposure. Focal volume is defined as the three-dimensional volume of light, in which the light is focused to its maximum intensity, in this instance infrared light  48 , that travels to the detector  50 . Maximum intensity of light occurs when all lights rays are concentrated into the smallest cross-sectional area of the focal volume. This cross-sectional area is not necessarily located at the focal point of the reflector  38 , but is located farther away from the reflector  38  than the focal point. 
     Small, economical, durable, and versatile spectrometers  42 ,  43  are commercially available for most ranges of wavelengths of interest in the visible and ultraviolet regions. In the infrared region, however, spectrometers are less practical than individual detectors optimized for particular ranges of wavelengths. These infrared detectors are expensive and require cooling and complicated electronics for support. It is therefore a great advantage to use only a single infrared detector  50  in the detection unit. If separate detectors are used to detect the intensity of each wavelength or band of wavelengths of interest, the calibration problem caused by the different sensitivities of the different detectors must be addressed. This problem is further compounded because sensitivities change with time and temperature and can be different for each detector. Therefore a system using only a single infrared detector  50  is much simpler and is preferred. 
     The infrared detector  50  is preferably composed of mercury-cadmium-telluride (MCT), preferably utilizing at least three-stage thermal electric cooling. However, a lead-selenide or other composition detector can be used, and with greater or lesser staged cooling. A liquid cooled detector could also be utilized in this embodiment provided there is supporting equipment to accommodate the liquid cooling. Another possibility for cooling the detector is by Stirling Engine cooling, however this adds cost and complexity. Furthermore, the mounting to which the detector is attached may require cooling if the manufacturer of the detector specifies a maximum allowable mounting temperature in order to maintain thermal stability of the detector. Such a mount cooling system is disclosed later in this specification. 
     The MCT composition detectors offer a more compatible electronic biasing consistent with reduced noise than other composition detectors. Other factors considered for single detector selection is the detectivity, commonly expressed in terms of “D*”, responsivity to light, the timing of the pulses of light to which the detector is exposed, and the saturation level. 
     This embodiment also prefers the economy of a photoconductive type of single detector as opposed to the more expensive photovoltaic detector. While photovoltaic detectors comparably offer less noise in lower pulse frequencies, this is not an issue for this embodiment as it is desirable to stimulate the detector with as high a frequency that the spinning filter wheel illustrated in  FIG. 1  item  16 , or spinning reflector illustrated in  FIG. 5  item  62  will allow. 
     Lastly, a detector needs to be selected to respond to light consistent with the range of desired wavelengths. A range of mid-infrared wavelengths for this embodiment can be viewed in Table 1 which suggests a detector sensitivity range of wavelengths between roughly 3-5 microns. However, if alternative wavelengths are used for such embodiment to measure the gases of interest, the desired range of wavelengths to which the detector is sensitive may have to be adjusted. 
     If the range of infrared wavelengths of interest is too broad for a standard detector, a dual substrate detector may be used. A commercially available dual substrate detector contains two different semiconductor compounds, each sensitive to slightly different ranges of wavelengths. They are mounted in a single detector package, one in front of the other so that their active areas nearly coincide. Thus the combination performs as if it were a single detector with sensitivity to a broader range of wavelengths than would otherwise be possible. 
     The beam splitter/combiner  44  may comprise any reflective or transmissive device, such as a neutral density filter, which transmit a specified fraction of the incident light and reflect almost all of the rest, treating a broad range of wavelengths equally, or dichroic beam splitter/combiner that can be designed to reflect almost all of the incident light of a specific range of wavelengths, and transmit almost all of the rest. The beam splitter/combiner  44  passes all or portions of visible and/or ultraviolet light  46  so that the visible and ultraviolet spectra may be measured by one or more spectrometers  42 ,  43 . The light which passes through beam splitter/combiner  44  is split off and carried to the respective spectrometers in one of two ways. The first, illustrated in  FIG. 3 , is to focus light onto the end of a Y-shaped optical fiber cable  41  that first receives the light in a single open end of the fiber optic cable, then divides the light within the cable sending a portion of the light to each spectrometer. 
     An alternative method of splitting the light to two or more spectrometers, illustrated in  FIG. 7 , is to use separate beam splitter/combiners  44  and  162  to split light beam  40  twice. Beam splitter/combiner  44  first splits beam  40  into beams  170  and  172 . Beam  170  is focused directly into the opening of spectrometer  43  while beam  172  continues on to beam splitter/combiner  162 . Beam splitter/combiner  162  then splits beam  172  into beams  174  and  176 . Beam  174  is focused on spectrometer  42  while beam  176  continues on to be focused on the infrared detector  50 . In either embodiment, whether cable splitting of light as illustrated in  FIG. 3  or multi-beam splitting method of  FIG. 11 , the light slightly over-bathes the opening to the optical fiber cable ( FIG. 3  item  41 ) or the light orifice of the spectrometer  42 , 43  for resistance to vibration and coincident reduction of light intensity with the vibration for similar reasons as expressed above for the infrared detector  50 . 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 List of Some Example Tailpipe Emissions 
               
               
                 Channels and their Wavelengths 
               
            
           
           
               
               
               
            
               
                   
                 Component 
                 Wavelength 
               
               
                   
                   
               
               
                   
                 Carbon Monoxide (CO) 
                  4.65 μ 
               
               
                   
                 Carbon Dioxide (CO 2 ) 
                  4.30 μ 
               
               
                   
                 HC 1  (Alkane series hydrocarbons) 
                  3.45 μ 
               
               
                   
                 Methane (CH 4 ) 
                  3.31 μ 
               
               
                   
                 HC 2  (Alkene series hydrocarbons) 
                  3.17 μ 
               
               
                   
                 HC 3  (Alkyne series hydrocarbons) 
                  3.01 μ 
               
               
                   
                 H 2 O (V)   
                 2.90 μ; 2.64 μ 
               
               
                   
                 Phosphorus (P) 
                 0.6400 μ 
               
               
                   
                 Elemental Carbon of PM 2.5   
                  0.500 μ 
               
               
                   
                 Calcium (Ca) 
                 0.4227 μ 
               
               
                   
                 Copper (Cu) 
                 0.3247 μ 
               
               
                   
                 Magnesium (Mg) 
                 0.2852 μ 
               
               
                   
                 Nitrogen Monoxide (NO) 
                  0.226 μ 
               
               
                   
                 Zinc (Zn) 
                 0.2139 μ 
               
               
                   
                 1,3 Butadiene (C 4 H 6 ) 
                  0.210 μ 
               
               
                   
                 Ammonia (NH 3 ) 
                  0.208 μ 
               
               
                   
                 Reference 
                  3.90 μ 
               
               
                   
                   
               
            
           
         
       
     
     In a preferred embodiment, the transmission and detection of light at the wavelengths of mid-infrared listed in Table 1 is accomplished by using a spinning filter wheel as the filter component (referred to in  FIG. 1  as item  16 ).  FIG. 4  illustrates an exemplary spinning filter wheel. Referring to  FIG. 4 , the spinning filter wheel contains light filters such as  52  that correspond to wavelengths associated with individual emission components, such as those illustrated in Table 1. One of the filters  54  must correspond with a wavelength at which no gaseous absorption takes place. Such a filter is known as a “reference” filter  54 . The light intensity measured from the reference filter  54  is used to normalize the light intensity measured from each of the gaseous filters  52 , so that concentrations of those gases may be calculated by a processor ( FIG. 6  item  92 ).  FIG. 4  illustrates a wheel having eight filters  52 ,  54  each utilizing one of the mid-infrared wavelengths of Table 1, however fewer and/or additional filters, corresponding to fewer and/or additional vehicular exhaust constituents, may be used in alternate embodiments. Each filter  52  is designed to allow light of a specific range of wavelengths to pass through it. 
     Another innovation regarding the filters  52 , 54  is that they are quadrants of an industry standard 25 millimeter optical filter. The round, 25 millimeter diameter filters are cut into four pie shapes allowing for filters to cost one-fourth of what they would otherwise cost if an entire industry standard sized filter were to be inserted in each of the open positions on the filter wheel  16 . In addition to cost, there is a savings in the amount of rotating mass by quartering the industry standard sized filters that the wheel  16  would have if the filters were installed whole. Lastly, special slots exist in the wheel  16  to allow for a two-piece optical filter  52 , 54 , should this be necessary. There are occasions when a filter manufacturer will supply two filters in order to provide the desired band pass of wavelengths to measure a gas of interest. The wheel  16  has the capability to accept these two-piece filters. 
     In addition, the filter wheel preferably will have one or more synchronization marks  56  that may be detected by a synchronization unit  58  to define either the exact filter or the start of a sequence of filters that will be in the optical path. The wheel  16  must have an opaque area  60  between each filter. The opaque areas  60  prohibit source light ( FIG. 1  item  10 ) from getting to a detector when the opaque areas  60  pass in front of the infrared source ( FIG. 1  item  10 ) transforming the incident light beam into a sequence of pulses ( FIG. 1  item  17 ). In operation, the wheel spins about an axis  19  at high speeds, preferably at least 12,000 rotations per minute, to form a sequence of infrared light pulses ( FIG. 1  item  17 ). Faster rotational speeds are even more preferable since they increase the sampling rate of the emission medium. The increased pulse rate to the detector  50  provides a higher signal to noise response. The synchronization unit ( FIG. 1  item  65 ) allows the processor ( FIG. 6  item  92 ) to associate a wavelength of interest, and corresponding gas of interest, with each pulse of light seen by the detector ( FIG. 6  item  90 ). This combination overcomes disadvantages of prior art, which require discrete detectors for each wavelength. 
     In accordance with an alternate embodiment of the present invention the light source unit illustrated in  FIG. 1  may omit the spinning filter wheel assembly  15 , 16 , 19 , 58 . In this embodiment, an alternate detector unit is provided as illustrated in FIG.  5 . Incoming light  36  transmitted from the source unit of FIG.  1  and reflected by the reflection unit of  FIG. 2  passes through window  35  that has similar characteristics to window of source unit illustrated in  FIG. 1 , and is reflected by a reflector  38 , which directs the light beam  40  onto beam splitter/combiners  44 , 45  which direct portions  46 , 47  of the light to the spectrometers  43 , 42 . The rest of the light  61  is focused on spinning reflector  62 . Reflector  62  is a single faceted flat mirror with a reflective surface that is optimized for the infrared light wavelengths of interest, such as an enhanced gold reflective surface or other suitable reflective surface. Alternatively, a multifaceted spinning mirror may be used, however the geometry of the rest of the layout would have to be modified from what is illustrated in FIG.  5 . The spinning reflector  62  splays the light in sequence around a stationary array of filters  52 , 53 , 54  and gas cells  70  by directing the beam  64  into the side of monolithic ellipsoidal mirror  80  which reflects the light  66  into the array, consistent with the splaying of the light. After passing through each stationary band pass filter  52 , 53 , 54  and gas cell  70 , the light beam  72  is redirected to and focused on a single infrared detector  50  by a reflector  74  such as a spherical mirror. The reflective surfaces of reflectors  80  and  74  are optimized for the wavelengths of interest in the same way as the surface of spinning reflector  62 . The single infrared detector sees a sequence of pulses of light  76  that are essentially the same as those illustrated as  FIG. 3  item  48 . Each filter  52 , 53 , 54  of this array substantially limits the passage of light to a predetermined spectral wavelength or range of wavelengths. Some filter center wave specifications are listed in Table 1. Each gas cell  70  of this array substantially limits the passage of light of a particular spectral pattern of wavelengths absorbed by the known concentration of the gas of interest that the cell  70  contains. 
     Another advantage of this embodiment is that there is much less rotating mass in the spinning reflector  62  than in the spinning filter wheel illustrated in FIG.  4 . Therefore the spinning reflector  62  can be spun at a much faster rate than the spinning filter wheel illustrated in FIG.  4 . Faster spin rate corresponds to a higher sampling rate that can contribute to lower electronic and optical noise levels, and provide better time resolution of a plume of vehicle exhaust constituents. 
     is instructive to refer to the illustration of  FIG. 8  to further the understanding on why an ellipsoidal mirror ( FIG. 5  item  80 ) is chosen to distribute light. An ellipsoidal mirror  200  has two focal points or foci  206 , 208 . Such mirrors have the property that all light rays  202  diverging from a small spot near one focal point  206  are reflected in such a way that those rays  210  are again focused into a small spot near the other focal point  208  of the mirror  200 . Given the unique layout of the alternative embodiment of  FIG. 5 , and commensurate need for a dual foci reflective device for light distribution through a full 360° of rotation of the spinning reflector ( FIG. 5  item  62 ), an ellipsoidal mirror is the best choice for this alternative embodiment. 
     An alternative embodiment replaces the monolithic ellipsoidal mirror  80  with individual ellipsoidal mirrors and may place the filters  52 , 53 , 54  and gas cell  70  array before the individual ellipsoidal mirrors if layout and construction is simplified. This alternative can provide the advantage of the system suffering less light loss through use of individual mirrors as opposed to the monolithic ellipsoidal mirror  80 . The disadvantage is that there may be more adjustments required in order to have the system of  FIG. 5  properly aligned such that all light through the system is optimized. 
       FIG. 6  illustrates several elements of a computer processing device that may be used in accordance with a preferred embodiment of the present invention. Referring to  FIG. 6 , the detection unit  90  delivers emissions-related data to a processor  92 . The detector may be any of the detectors or spectrometers as illustrated in  FIGS. 3 and 5 , or any device that receives or contains information collected by such detectors or spectrometers. Such detector systems for the purpose of discussion in  FIG. 6  include a means for amplifying and converting the detector signals into digital signals that can pass to the processor  92  via a direct link such as a parallel data bus  94 . 
     In this embodiment, the detection unit  90  is part of the unit that contains the processor  92 , and the delivery is performed over a parallel bus  94  such as that which can be found in AT, ATX, EBX, and other motherboard styles upon which computers are based. However, the processor  92  and detection unit  90  may be separate, such as with the remote detector  96  illustrated in FIG.  6 . Where a remote detector is used, the data may be delivered to the processor  92  by a communications link  100  that delivers the data to an input port  98  such as a communications port. A wireless communications link  102  and receiver  105  for such a wireless communication are also illustrated in FIG.  6 . The communications link  102  may be a direct wire, an infrared data port (IrDA), a wireless communications link, global communications network such as the Internet, or any other communications medium. 
     The system illustrated in  FIG. 6  also includes a memory  104  which may be a memory device such as a hard drive, random access memory, or read only memory. A portion of this memory  104  can contain the instructions for the processor  92  to carry out the tasks associated with the measurement of vehicular emissions. Preferably, concentrations of gases may be derived using the Beer-Lambert Law, however other tests and formulae may be used in alternate embodiments. 
     The Beer-Lambert Law, as disclosed in other art, relates absorbance of light to a concentration of gas where an amount of change in light intensity at a known wavelength is proportional to the concentration of a gas of interest at the wavelength of light where the gas is absorbed. The Beer-Lambert Law is expressed in terms of transmittance in Equation 1. 
     Equation 1: Beer-Lambert Law 
       2−Log 10 (%  T )=ε Cl   
     Where:
         % T is the amount of light transmitted through open air and the emissions sample expressed in percent units;   ε is the absorption coefficient for the gas of interest at a corresponding wavelength of absorption;   C is the concentration of the gas of interest expressed in parts-per-million (ppm)   l is the path length expressed in meters.       

     Transmittance is further expressed as the amount of light that passes through the gas of interest in proportion to the amount of light that was originally emanated from the light source unit as illustrated in Equation 2. If a broadband optical filter is used in conjunction with a detector, there will be some residual light remaining that arrives at the detector even though the gas or emission of interest is at sufficient concentration to be at 100% absorbance. This is due to the fact that a broadband filter will pass light of wavelengths outside of the wavelengths of interest that are associated with a gas or emission of interest. For this embodiment, the transmittance equation is modified to subtract the amount of residual light at 100% absorbance of the gas or emission of interest. The correction for residual light most likely is not necessary for embodiments that utilize Tunable Diode Lasers or other similar methodology, as this methodology can measure in narrow enough wavelengths to not have residual light at 100% absorbance of the gas or emission of interest. Background transmittance of light can also be accounted for in Equation 2 in order to account for variations in background concentrations, and their associated absorbances. Furthermore, source variations can and should be accounted for, as a simple change in light intensity from a light source could be misinterpreted as a concentration of a gas or emission of interest. 
     Equation 2: Transmittance as Expressed in Percent 
     
       
         
           
             
               % 
               ⁢ 
               
                   
               
               ⁢ 
               T 
             
             = 
             
               
                 
                   I 
                   p 
                 
                 
                   I 
                   o 
                 
               
               × 
               100 
             
           
         
       
     
     Where:
         I p  is amount of light left after passing through the gas sample of interest   I o  is the amount of light that was originally sent through the entire sample path and not absorbed by the gas of interest       

     The specific application of Beer-Lambert Law for this embodiment is found in Equation 3. Equation 3 is an algebraic substitution of transmittance “% T” (Equation 2), and subsequent manipulation of Beer-Lambert Law of Equation 1 to solve for a concentration of a gas in an open path, as this is the unknown for which this embodiment measures. 
     Equation 3: Application of Beer-Lambert in this Embodiment 
     
       
         
           
             C 
             = 
             
               
                 2 
                 - 
                 
                   
                     Log 
                     10 
                   
                   ⁢ 
                   
                     ( 
                     
                       
                         
                           I 
                           p 
                         
                         
                           I 
                           o 
                         
                       
                       × 
                       100 
                     
                     ) 
                   
                 
               
               
                 ɛ 
                 × 
                 l 
               
             
           
         
       
     
     The concentrations calculated in Equation 3 are expressed in units of parts per million (ppm) for gaseous measurements, or micromoles/mole for particulate measurements. The correlation coefficient is empirically derived per acceptable methods of empirical establishment of a correlation coefficient for each gas of interest and PM 2.5  absorption. Equation 4 illustrates the conversion needed to go from a measurement in units of micromoles/mole to micrograms per cubic meter (μg/M 3 ) at Standard Temperature and Pressure (STP), the standard units for a typical PM 2.5  measurement. Temperature measurements of the measurement path are read or converted in the preferred embodiment to degrees Kelvin (° K.) or other suitable temperature scale which has a lower limit at absolute zero. Pressure measurements of the measurement path are read directly or converted in the preferred embodiment to atmospheres (atm). The units conversion preferably takes place in the processor  92  immediately after the PM 2.5  measurement has been taken, however this is not essential to measurement accuracy. 
     Equation 4: Units Conversion for PM 2.5  Measurements 
     
       
         
           
             
               
                 
                   
                     Concentration 
                     ⁢ 
                     
                         
                     
                     [ 
                     
                       uMoles 
                       Mole 
                     
                     ] 
                   
                   = 
                     
                   ⁢ 
                   
                     
                       
                         12.01 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         g 
                         × 
                         1 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Mole 
                         × 
                         1000 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         l 
                         × 
                         
                           10 
                           6 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         ug 
                       
                       
                         1 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         Mole 
                         × 
                         22.4 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         l 
                         × 
                         1 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           M 
                           3 
                         
                         × 
                         1 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         g 
                       
                     
                     × 
                   
                 
               
             
             
               
                 
                     
                   ⁢ 
                   
                     
                       
                         Temp 
                         amb 
                       
                       
                         Temp 
                         
                           @ 
                           STP 
                         
                       
                     
                     × 
                     
                       
                         Press 
                         
                           @ 
                           STP 
                         
                       
                       
                         Press 
                         amb 
                       
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                   ⁢ 
                   
                     
                       5.36 
                       × 
                       
                         10 
                         8 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       ug 
                     
                     
                       M 
                       3 
                     
                   
                 
               
             
           
         
       
     
     Other memory devices  106  and  108  such as additional hard disk storage, a CD-ROM, CD-RW, DVD, floppy drive, ZIP® drive, compact flash compatible device such as that which conforms to IBM Microdrive™ specification, or other memory device may also be included. An internal memory device  106  can be used to extend the number of emissions tests that can be conducted and retained by this preferred embodiment. A removable memory device  108  can be used to make the emissions data portable to allow for the emissions data to be further processed in a centralized location. The device also optionally and preferably includes a display  110  and/or a transmitter  112  for providing output to a user or another device. 
     Utilizing a computer processor  92 , the intensity measured by the detector unit  90  at a wavelength of interest is compared by the processor  92  to the intensity of light detected by the detector unit  90  at a reference wavelength where no absorption of gases occurs. This method of detection is commonly known as Differential Optical Absorption Spectroscopy (DOAS). This DOAS methodology is a simple, inexpensive means of determining a concentration of a gas of interest emanating from a vehicle tailpipe in open air, and has examples in other art and fields of invention. 
     Alternatively, again using a computer processor  92 , the intensity measured by a detector unit  90  at a desired wavelength for an interval of time, followed by measuring light at the detector unit  90  for an interval of time at the same desired wavelength with additionally a gas cell of known concentration of gas that absorbs light of the same wavelength can also be used as a methodology to determine a concentration of a gas of interest. This method of detection is commonly known as Gas Filter Correlation Radiometry (GFCr), and is documented in other art. GFCr has the potential to provide improved precision &amp; accuracy of measurements due to the fact that the methodology allows for the constant referencing of a measurement to a known concentration of the gas of interest. 
     A preferred embodiment of  FIG. 5  shows both DOAS and GFCr methods of determining a concentration of a gas of interest contained within the same embodiment. For example, an optical filter  53  can be optimized for sampling carbon dioxide (CO 2 ). Another filter  54  can be optimized to pass wavelengths of light where no absorption of CO 2  or other gases exist; such a filter is used for reference to assess the amount of light that passes through the sample path without CO 2  influence. As the amount of CO 2  concentration increases, the amount of light that the detector  50  observes through filter  53  will decrease, while the amount of light that the detector  50  observes through the reference filter  54  will remain unchanged. This is the fundamental of the DOAS methodology by comparing the amount of light (I p  in Equations 2 and 3) off from the CO 2  filter  53  to the amount of light (I o  in Equations 2 and 3) from the reference filter  54 . Switching the light paths between the CO 2  path, created by filter  53  to detector  50 , and reference path, created by reference filter  54  to detector  50 , is accomplished by the spinning reflector  62  that splays the light for periods of time between the two mentioned paths and other paths that exist in this embodiment. 
     DOAS methodology is also provided in the embodiment illustrated in  FIG. 1 , however the light path switching is performed by the spinning filter wheel  16  such that, for a moment in time, the filter wheel rotation exposes an optical filter ( FIG. 4  item  52 ) to light ( FIG. 1  item  10 ) for a gas of interest, then for a roughly equal interval of time, the filter wheel exposes a reference filter ( FIG. 4  item  54 ) to the same light ( FIG. 1  item  10 ). 
     The GFCr methodology is provided in this embodiment as well. Expanding on the DOAS example above, a CO 2  filter  53  can be paired with another similar characteristic CO 2  filter  52  with the difference that the CO 2  filter  52  has a windowed small cell  70  that contains a sample of CO 2  gas. The amount of gas in the cell  70  is chosen based on the amount of optical depth that is desired with which the non-celled optical path is compared. The CO 2  filter  53  must have balancing windows  78  of the same optical characteristics as the gas cell  70  in order to make the amount of light between both light paths roughly equivalent. An alternative embodiment to the balancing windows  78  can use a second gas cell  70  in place of the balancing windows  78 , but with all air evacuated to a vacuum, or air replaced with nitrogen or other inert gas at partial pressure to provide the optical balance. If a gas is used to fill the balancing cell, the gas cannot have absorption characteristics similar to the gas of interest being measured. 
     The balancing windows  78  are added to create an optical balance for the two CO 2  detection paths in the example given, such that the only difference in intensity of light to the detector  50  between the two paths is a change in concentration of the gas of interest. For a period of time, the light travels through the CO 2  filter  52  with CO 2  gas cell  70  and reaches the detector  50 . In another time interval of approximately same length, the light will travel through the other CO 2  filter  53  with balancing windows  78  and on to the detector  50 . Since the gas cell  70  contains a known concentration and corresponding optical depth of a sample of CO 2 , the amount of light in the filter  52  to gas cell  70  to detector  50  path of light exists as a reference to which the amount of light from light path filter  53  to balancing windows  78  to detector  50  is compared. The amount of absorbance from each CO 2  light path is compared to determine a concentration of CO 2  in this example. As with the DOAS method of detection, light path switching is accomplished by the spinning reflector  62  that provides light to each mentioned path for a period of time in addition to making light paths for other gas sampling paths of this embodiment. 
     The unique advantage of GFCr is that any interferences to measuring a concentration of CO 2  in this example appear in both CO 2  light paths and therefore is commonly rejected among both light paths. Common mode rejection of interferences does not necessarily happen with the DOAS method of detection of gases, because of the use of a reference filter at a different wavelength, an interference could conceivably absorb light at the reference wavelength but not at the wavelength corresponding to the gas of interest. Also, the characteristics of the reference filter  54  are different from the other filters  52 , 53 , and create a situation where different filters  52 , 53 , 54  pass different wavelengths of light, to which the detector  50  will have greater or lesser sensitivity to such wavelengths. With proper optimizations, these effects may be minimized, but not eliminated. 
     It should be noted that it is not necessary to have both DOAS and GFCr methodologies utilized in an embodiment in order to obtain reasonable measurements of concentrations of gases of interest. However it is desirable to have both when economically feasible in order to provide for improved precision and accuracy of measurements. Furthermore, although an example was given here for CO 2 , it is possible to utilize GFCr for other gases including but not limited to carbon monoxide (CO), methane (CH 4 ), and any gas of interest that can be stored over long periods of time in a gas cell without the reference gas of interest degrading, attacking the walls of the cell and compromising the sample, or the reference gas combining with contaminants within the cell causing the reference concentration to no longer be known. GFCr methodology also is beneficial for speciation of hydrocarbons, as the gas cell  70  can be utilized as a sort of notch filter to indicate a particular gas of interest from a group of gases such as hydrocarbons. 
     Referring back to  FIG. 6  the processor  92  of the embodiment, coupled with the appropriate instruction set contained within memory  104 , can be capable of conducting either DOAS, GFCr, or simultaneously both methodologies of detection of concentrations of gases and then applying the concentrations to a combustion equation. Previous art in this field of invention has documented combustion equations that utilize ratioing concentrations of gases of interest relative to carbon dioxide (CO 2 ) to correct for any dilution effects in the exhaust stream of the vehicle being tested. The memory  104  can contain combustion equations unique to different fuels used to power vehicles that are tested by this preferred embodiment. Determination of the type of fuel used to power a tested vehicle can be done in the processor  92  at the time of measurement of the tailpipe emissions, or after emissions testing activities have concluded at the monitoring site in a centralized data processing facility. 
       FIG. 9  illustrates a preferred embodiment including a light source  120  capable of emitting at least one beam of light  122  having known emission intensities corresponding to one or more of infrared, visible, and ultraviolet spectra. The system also includes a reflection unit  124 , a detection unit  90  capable of receiving the beam and measuring received intensities corresponding to the light spectra, and a processor  92  capable of comparing received intensities and identifying a concentration of a gas of interest. The light  122  is transmitted through a gas, such as air containing vehicle emissions, reflected, then detected for analysis and measurement of the amount of absorption that has occurred at known wavelengths. The amount of absorption may be used to determine concentrations of gases and particulate matter corresponding to the specific wavelengths. 
     Preferably, as illustrated in  FIG. 10 , the system also includes a first reflector  130  positioned to receive the beam  128  from the light source  120  and reflect the beam  132  toward the reflection unit  124 . The reflection unit  124  is positioned to receive the beam  132  from the first reflector  130  and reflect the beam  134  toward a second reflector  136 . Also preferably, the second reflector  136  is positioned to receive the beam  134  reflected by the refection unit  124  and reflect the beam  138  toward the detection unit  90 . In a preferred embodiment, each reflector  130 , 136  comprises an off-axis paraboloidal mirror, however a spherical or other similar mirror could be used. 
     Referring to  FIG. 11 , a paraboloidal mirror  180  has the property that light rays  182  emitted from and diverging from a small spot of a light source  184  placed near the paraboloidal mirror  180  focus  186  are reflected into a beam of rays  188  nearly parallel to the axis of rotation  190  of the mirror. 
     Conversely, as illustrated in  FIG. 12 , a beam of light rays  192  traveling nearly parallel to the axis of rotation  190  of a paraboloidal mirror  180  become rays  194  reflected toward and concentrated into a small spot near the paraboloidal mirror focus  186 . The significance of a light beam of nearly parallel rays  192  is that the intensity of the light beam changes very little over a great distance, a desirable trait for long path, open-path gas detection systems. Off-axis paraboloidal mirrors have the advantage that the light source or detection unit may be located to the side of the reflected beam instead of in its midst. This means that the full diameter of the mirror can be used for the optical measurements. Layout of the source and detector components is also simplified. Spherical mirrors are more “fuzzy” at the focus if the spherical mirror is angled, the angle causing incoming/outgoing light rays to not be nearly as parallel as with the parallel rays  192  of the paraboloidal mirror  180 . Light rays that do not travel in the parallel path are lost from the optical path and as a consequence, are part of the reduced efficiency of an optical system that utilizes spherical mirrors. Nonetheless, other factors such as availability of product, production cost, etc. all factor in the decision whether to use the preferred paraboloidal mirror  180  for sending/receiving light in the embodiment, or utilize spherical mirrors in their place. 
     Returning to  FIG. 10 , a beam of light travels along an optical path  128 ,  132 ,  134 , and  138  from the light source  120 , to the first reflector  130 , to the reflection unit  124 , to the second reflector  136 , to the detection unit  90 . In this embodiment, the system also includes, as seen in  FIG. 13 , one or more additional light sources  144 , 146 , each capable of emitting a beam of light  148 , 152  having known emission intensities corresponding to one or more of infrared, visible, and ultraviolet spectra, as well as one or more beam splitter/combiners  140 , 142 , if necessary, positioned to direct beams  148 , 152  from the additional light sources  144 , 146  along essentially the same optical path  154 ,  132 ,  134 , and  138  as illustrated in FIG.  10 . The beam splitter/combiners  140 , 142  may be neutral density filters, or alternatively they may be wavelength sensitive beam splitter/combiners, such as dichroic beam splitter/combiners. 
     In another embodiment, illustrated by  FIG. 14 , the light sources  10 , 12 , beam splitter/combiners  140 , 160 , infrared detector  50 , and spectrometer  43  are positioned so that ultraviolet light beam  212  from source  12  is traveling along essentially the same optical path, but in the opposite direction from infrared light beam  14  from source  10 . This innovation is referred to herein as “opposed sources”. An embodiment using opposed sources may eliminate the need for additional expensive, light attenuating components. For instance, if ultraviolet light  212  is directed towards, instead of away from, the infrared detector  50 , the signal from the infrared detector  50  can degrade. If light  212  from an ultraviolet source  12  is traveling in the opposite direction from the light  14  emanating from the infrared source  10 , the ultraviolet light  212  is naturally kept away from the infrared detector  50  without the use of additional wavelength dependent filters or beam splitter/combiners. Light sources  12 , 10  and detectors  43 ,  50  need to be matched with optical components of corresponding F-numbers for efficient light transmission. An embodiment using opposed sources, and first and second reflectors  130 , 136  of significantly different F-number, allows the sources or detectors requiring a higher F-number to be matched with the reflector with the higher F-number, and the sources and detectors requiring a lower F-number to be matched with the reflector with the lower F-number. This eliminates the need for additional optical components for F-number matching. Finally, opposed sources may significantly simplify component layout and reduction of thermal and electrical interference among components. 
       FIG. 13  shows one possible arrangement of three sources  120 ,  144  and  146 . In one preferred configuration, the source  120  is an infrared source, the source  144  is a visible light source, and source  146  is an ultraviolet light source. In this example, ultraviolet light reflects off splitter/combiner  142  but does not pass through any splitter/combiners. The infrared light passes through two splitter/combiners. However, the arrangement of these sources may be interchanged in any combination, and one or more source types may be omitted entirely. 
       FIG. 14  depicts an ultraviolet source  12  and an infrared source  10 . The ultraviolet source  12  could also be combined with a visible light source in a manner similar to the combination shown in  FIG. 1 , either using a pass through ultraviolet source or by providing an additional splitter/combiner to combine the ultraviolet and visible light. 
     A thermal electric cooler within a detector  50  will attempt to dissipate heat out the base of the detector. Furthermore, depending on operating and environmental conditions, several watts of heat are continuously added to the base of a detector  50  from other components as shown in  FIG. 14  by a non-thermally isolated detector mount. Traditionally, the detector base is mounted onto a heat sink and ambient or cooled air is circulated through the heat sink via a cooling fan. However, it is undesirable to circulate ambient air through an open path optical system because of the difficulty in removing dust and contaminants from the cooling air. Optical components such as internal mirrors  130 , 136 , beam splitters  140 ,  160 , and other components must be kept free of dust and contaminants in order for effectiveness to be maintained. It is also undesirable to mechanically chill the entire enclosure (FIG.  1 : 27 ) for cost and mass reasons. However, failure to keep the detector within the temperature specifications results in a baseline drift of measurements of gases of interest, as well as an increase in noise. 
     FIG.  15  through  FIG. 17  illustrate three views, front, back, and top views respectively, of a special bisected mounting for a detector of light, be it a discrete detector sensitive to a broadband of infrared radiation as described above, a spectrometer as described above, or a means for detecting the amount of light radiation in the visible range. The bisected mounting is preferably composed of aluminum, for its lightweight yet good heat transfer properties. Since one preferred embodiment is a portable system, aluminum is desired over other metals with better thermal transfer properties, due to the weight savings advantage that aluminum provides. For example, a weight advantage exists with aluminum over copper despite requiring less copper material due to the improved thermal transfer properties of copper over aluminum. A typical aluminum alloy has a heat transfer rate of 180 watts/meter-° K. with a density of 2700 Kg/M 3 , compared to copper at 390 watts/meter-° K. but with a density of 8900 Kg/M 3 . 
     The purpose of a bisected mounting is to thermally isolate a detector from a baseplate, onto which all light measuring and optical components are attached as shown in  FIG. 14 , and to which an enclosure or housing (FIG.  1 :item  27 ) also attaches and surrounds. As shown in  FIG. 17 , this isolation allows for a detector  50  to be mounted in thermal isolation from the baseplate  341 , something that may otherwise contribute to heating of the detector mount. Furthermore, the isolation positively affects the detector  50  by additional cooling provided by a thermal electric cooler (TEC)  390 , working on the principle of a Peltier cooler, placed in between the hot and cold blocks  320 ,  300  of the bisected mounting. A TEC  390  is sandwiched between the cold  300  and hot  320  blocks such that the TEC  390  pumps heat away from the cold block  300  into the hot block  320  when voltage is applied to the TEC  390 . Some designs of TEC can achieve a temperature differential between the cold and hot sides of as much as twenty or more degrees centigrade. Alternatively, other means of cooling the mounting, such as sterling or liquid cooling, can be substituted to where the preferred embodiment has placed a TEC  390 , however there are cost advantages to using a TEC  390 , despite the inherent lower operating efficiency. There is also the advantage of using a TEC over liquid cooling in that there is no need for a dewar and associated plumbing, nor is there need for added safety handling issues of working with liquids that are close to absolute zero in some cases. 
     In a preferred embodiment, the TEC  390  is supplied by Melcor of Trenton, N.J., part number CP0.8-127-05L EP. TEC&#39;s from other suppliers could be used, provided they transfer an amount of heat from the cold block  300  to the hot block  320  in a sufficient amount to maintain a cold block  300  temperature recommended by the detector  50  manufacturer, for a given enclosure (FIG.  1 : 27 ) temperature. In a preferred embodiment, the enclosure temperature in a portable application of the embodiment can reach 55° C. in the hot summer sun, and therefore serves as a maximum temperature for which cooling capacity is selected. One mid-infrared detector from Judson Technologies requires the base of the case of the detector  50  kept below 30° C. in order for the internal thermal electric cooler of the detector  50  to be able to maintain the light sensitive detector substrate at the desired temperature of −65° C. TEC&#39;s are usually chosen for the amount of heat transfer, in terms of wattage, that can be achieved by the device. A thirty watt TEC will therefore meet the cooling challenge required by the detector for the given enclosure temperature in this instance. Furthermore, while only one TEC is shown in this embodiment, TEC&#39;s can be cascaded or stacked together to provide multistaged cooling if, for example, a higher enclosure temperature will be experienced, or the detector manufacturer requires a lower detector base temperature than the example temperatures given in this paragraph. Each stage may provide up to twenty degrees centigrade of cooling. 
     Referring to  FIG. 16 , a cooling method is used to cool the block  300  onto which the detector  50  is mounted, in addition to thermally isolating the detector  50  from the baseplate  341 . A material with high thermal conductivity, and a large cross sectional area are chosen for the cold block  300  so that the thermal gradient along the plate is kept to an acceptable level. More about thermal gradients will be discussed below in conjunction with FIG.  18 . Heat from the hot side  320  is ducted to the baseplate  341  of the entire apparatus through the base of the bisected mounting  340 . If there is appreciable thermal resistance between any of these pieces, due to improper design or poor construction, the hot side block  320  will get too hot and the heat transfer cooling will not be able to keep the cold side block  300  at a low enough temperature. 
     In a preferred embodiment, an aluminum-oxide thermal grease, in particular part number TG003 from Melcor of Trenton, N.J., is applied to any joint in metal where effective heat transfer from one part to another would be compromised due to air gaps, different materials, and other reasons. This type of thermal grease works best when under a pressure of 21 kg/cm 2 . Other thermal transfer enhancing materials can be applied, such as zinc-oxide based greases, and with or without silicon, with varying success in effective thermal transfer from one part to another. 
     Since the baseplate  341  is targeted for dissipation of much of the heat transferred to it from the bisected mounting base  340 , the baseplate  341  is designed to have sufficient thermal mass and sufficient contact with the environment such that its temperature will only rise a few degrees above the temperature within the enclosure (FIG.  1 : 27 ). It would be counterproductive to create a mounting that solves an excess heat problem for a detector, only to create a problem for other components ( FIG. 14 ) of the total optical system, hence the need to minimize the impact of heat onto the baseplate  341  transferring to other optical components. 
     Since a detector in an open path system must hold an optimal position relative to other components in the total open path optical system, there needs to be a means for adjusting the position of the detector relative to the optical path created by all of the components ( FIG. 14 ) in the system. These position adjustments need to be fine incremental movements with a great deal of precision. Returning to  FIG. 16 , the hot side of the bisected mounting  320  includes the ability to adjust the position of the mounting in the X and Y directions of the mounting relative to the optical path. Fine adjustment is provided by XY stage  350  which is connected to the hot side  320  by an angle bracket  370 . The hot block  320  is held tightly to the mounting base  340  by three retaining clamps  360 , operating in groove  325  cut in the bottom of the hot block  320 . When adjustment is needed, clamps  360  are first loosened, then a position adjustment is made by the XY stage  350 , and finally clamps  360  are tightened again. Thermal grease is also used between block  320  and base  340 . An XY stage can be obtained commercially from OptoSigma Corporation of Santa Ana, Calif., or can be custom made. 
     Heat is thermally conducted from the cold side  300  of the bisected mounting, to the hot side  320 , down to the mounting base  340 , and ultimately to the baseplate  341 . Some heat is conducted down the path of the angle bracket  370  to the XY stage  350 , down to the mounting base  340 , however this is not relied upon for primary thermal conduction, as this path is not a thermally efficient transfer path. Also, the heat transfer path from the hot block  320  to hold down clamps  360  via the groove  325  in the hot block  320 , has much thermal resistance, primarily due to the lack of contact over much of the bottom surface of the hold down clamps  360 . This lack of contact is designed into the mounting, because the groove  325  is set a bit higher than the height of the clamps  360  so that the desired hold down pressure of the thermal grease can be applied to the hot block  320  when the clamps  360  are secured tightly to the mounting base  340 . 
     For large incremental changes of detector position relative to the optical path, coarse longitudinal adjustment slots  345  are provided in base  340 . Extensive application of thermal grease is used between the mounting base  340  and the main baseplate  341 . 
     It should be noted in  FIGS. 15-17 , a Judson model J15TE35-66C-510M in a TO-66 case is modeled and positioned in the place of a detector  50 . However other suitable detectors in different sized cases down to the TO-3 specification would benefit from the bisected mounting with cooling. Furthermore, small spectrometers, such as the S2000 series of spectrometers from Ocean Optics of Dunedin, Fla., would also benefit from mounting to the cold side block  300 , where instead of mounting a discrete infrared detector  50  to the cold side block  300 , a spectrometer would be directly mounted to the cold side block  300 . Mount cooling would be desired for a spectrometer in particular due to gradient shift caused by higher temperatures. Correction for gradient shift is done by application of a polynomial, something that may not be practical for applications where there is insufficient computing power to conduct such a correction. Therefore, for simplicity of discussion, while not implying limiting language of applicability of the bisected cooled mounting, this text discusses the features of a bisected cooled mounting to provide extra cooling for an infrared detector  50  contained within a TO-66 case. 
       FIG. 17  shows that detector  50  is mounted in a bored-out pocket in the cold block  300  so that the cross section of the cold block  300  can be thick, yet allow enough length of the pins of detector  50  to protrude from block  300  such that proper electrical connections may be made. The detector  50  is mounted to the cold block  300 , held in place by retaining ring  380 . The retaining ring  380  provides even pressure across the base of detector  50 , without deforming the detector enclosure in any way. Thermal grease is applied between the detector  50  and cold block  300 . A similar installation could be done for a spectrometer, though the boring out may not be necessary, as there typically aren&#39;t electrical connectors that must pass through the bottom of the spectrometer and therefore through the cold block  300 . 
     Thermal grease is also used between plate  300  and the TEC  390  and between the TEC  390  and block  320 . Screws  400  provide the necessary pressure for the thermal grease to be effective. Fiber washers  410  with good thermal insulating properties provide a measure of thermal isolation between the cold block  300  and the hot block  320 . Note that the screw holes of the hot block  320 , as shown in  FIG. 17 , are a larger diameter than the screw shaft. This is done to add an element of thermal isolation between the cold and hot blocks  300 ,  320 . The fiber washers also serve to limit movement of the two blocks  300 ,  320  relative to each other, though some movement will still occur. 
     In order to restore rigidity lost from having oversized screw holes, an insulating block  330  is used to strengthen the mounting by holding the cold block  300  in a fixed position relative to the hot block  320 , while maintaining thermal isolation. Without the insulating block  330 , vibration to the bisected mounting could cause variations in the alignment of the detector  50  relative to the total optical path of FIG.  14 . Preferably, the insulating block  330  is made from norel, a plastic with low thermal conductivity and very low thermal expansion. 
     A tiny hole  310  allows for mounting a thermistor used in feedback control of the temperature of the cold block  300 . The ideal position for the thermistor is near the center of the cold side of TEC  390 . Thus the thermistor can react very quickly to changes in the temperature of the cold side of TEC  390  without causing wide temperature shifts to the detector  50 . Locating the thermistor too close to the detector  50  results in greater temperature swings than if the thermistor is located close to the thermal centroid of the TEC  390 . An electronic controller, not shown, senses the temperature of plate  300  using the thermistor in hole  310 . It then adjusts the voltage to TEC  390  as needed to bring the temperature of plate  300  back to its set point. Suitable thermistors can be obtained from Oven Industries of Mechanicsburg, Pa., or other sources. 
     The cold block  300  to which the detector  50  is attached must also be electrically grounded so that electrical noise from TEC  390  activity does not interfere with detector performance. However, the electrical grounding of the cold block  300  must be done with a wire that does not have appreciable thermal conductivity. Electrical noise can emanate from the TEC  390  if the TEC is controlled in more or less a digital fashion, i.e. turned on full power or turned off (no power), instead of continuous operation. One such TEC controller can be obtained from Oven Industries of Mechanicsburg, Pa., and is suitable for usage in a preferred embodiment, provided grounding of the cold block is done. A Custom-made controller can alternatively be built. 
     A more analog style of control of the TEC  390  is desired, such that the TEC is always on, but in varying states of power (current regulation), initiated by the TEC controller. One such analog controller can be obtained from Hytek Microsystems of Carson City, Nev., or a controlled can be custom built. This analog style of control of the TEC minimizes electrical interference with a detector  50 . 
     The distance between detector  50  and TEC  390  is held to a minimum. This means that for the detector  50  base to be kept at or below the 30° C. maximum in our example above, the part of the cold block  300  that touches the TEC  390  needs only to be cooled to a few degrees cooler than the targeted temperature for the detector  50 . This concept of close proximity of the detector relative to the TEC or other cooling is best demonstrated in the following example on thermal gradients within a material. 
       FIG. 18  illustrates the concept of the thermal gradient as illustrated from thermal transfer software. It represents a simplified thermal model of the hot block  320  of the bisected mounting under a typical heat load and severe ambient conditions within the enclosure (FIG.  1 : 27 ). For illustration purposes, the mounting base  340  is held at 56.9° C., a perfect thermal transfer by the thermal grease between the hot block  320  and mounting base  340  is assumed, and the hot block  320  is composed of aluminum with a cross section of 44.45 mm by 12.7 mm. Additional boundary conditions include the TEC being centered at point  420 , which is 69 mm from the mounting base  340 , adding thirty watts of heat to the hot block  320  from the TEC (not shown in FIG.  18 ), and the environment around the hot block  320  is also at 56.9° C. As illustrated, the temperature at the top of the hot block  320 , shown as item  430 , is 74.6° C. The temperature varies continuously down the length of the block to the base at 56.9° C., with temperature gradients illustrated by different shades of gray, darker shades of gray indicating cooler temperatures. 
     It should be noted that the thermal gradient modeling of  FIG. 18  does not take into account any coating of the material composing the hot block  320 . Coatings such as powder coating or anodizing will affect the heat transfer from the hot block  320  to the mounting base  340 , even though the thermal gradients within the hot block material will be unaffected. For this reason, in a preferred embodiment, anodized aluminum parts will have the anodizing removed from the portions of the hot block  320  that make contact with the TEC, as shown in  FIGS. 15-17 , and contact with the mounting base  340 . Experimentation has shown that there is a significant reduction in thermal transfer efficiency when the hot block  320  is coated in some way. However, it is not to be construed to mean that the hot block  320  or other parts should not be coated for protection against oxidation. Only the portions of mounting components that make contact with other components for the purposes of heat transfer are the portions that should not have any coating applied, or should have coating removed in the event that the entire component is coated in an industrial process. 
     Thus, the many features and advantaged of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Furthermore, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.