Abstract:
An analyzer for depicting a characteristic of a fluid with the alignment and nonalignment of transmission peaks of a light source and absorption lines of the fluid. A radiation source may emit light through broad band and narrow band filters, respectively. An output the narrow band filter having transmission peaks goes through a cell having the fluid to be examined. The fluid has absorption lines. The optical path of the narrow band filter varies so as to affect the alignment of the transmission peaks and the absorption lines which results in different magnitudes of the light from the cell which may imply a quantity or characteristic of the fluid. The analyzer may be made with MEMS techniques.

Description:
BACKGROUND  
         [0001]    This invention relates to the field of infrared fluid analysis and particularly to a device in which light is transmitted through a fluid sample at discrete frequencies correlated with the absorption spectrum of a constituent of the fluid to detect and quantitatively measure the constituent. “Fluid” is a generic term that includes liquids and gases as species. For instance, air, CO, water and oil are fluids.  
           [0002]    In the apparatus conventionally used for infrared fluid analysis, a beam of infrared radiation having an emission spectrum embracing the absorption spectrum of the fluid to be analyzed goes through a fluid sample to a transducer. The output signal from the transducer is compared with that produced by passing the beam through the series combination of the sample and a reference fluid of the type selected for analysis. A signal intensity differential, produced by absorption in the sample, is converted to a detectable signal and displayed.  
           [0003]    One problem with such analyzers is the difficulty of analyzing quantities of fluid constituents present in the low parts per million range. The signal intensity differential represents a relatively small change in a large signal and is frequently obscured by spectral interference between absorption spectra of the constituent being analyzed and absorption spectra of coexistent constituents. Another problem is the lack of inexpensive approaches for manufacturing numerous analyzers.  
         SUMMARY OF THE INVENTION  
         [0004]    The present invention transmits light through a fluid sample at discrete frequencies that may be correlated with the absorption spectra of a constituent of a fluid to detect and measure the constituent. It is a device that may be made with the techniques of micro electro mechanical system (MEMS) fabrication techniques for attaining inexpensive manufacturing advantages. The analyzer design is capable of accurately analyzing fluid in a small number of parts per million range. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIG. 1 is a block diagram of a fluid analyzer;  
         [0006]    [0006]FIG. 2 is a schematic of a fluid analyzer;  
         [0007]    [0007]FIG. 3 illustrates an absorption spectrum of an illustrative fluid example;  
         [0008]    [0008]FIG. 4 is a diagram of fluid analyzer of another configuration;  
         [0009]    [0009]FIGS. 5, 6 and  7  are graphs showing various aspects of alignment of transmission peaks and absorption lines of an illustrative fluid;  
         [0010]    [0010]FIG. 8 is a cross-section view of a narrow band filter;  
         [0011]    [0011]FIG. 9 is a diagram showing some characteristic modifications that radiation encounters as it passes through components of an analyzer;  
         [0012]    [0012]FIG. 10 shows a graph of thermal pulses versus wavelength for a filter layer;  
         [0013]    [0013]FIG. 11 shows an implementation of a filter of an analyzer in an illustrative example of monolithic form;  
         [0014]    [0014]FIG. 11 a  is a graph of thermal pulses in a filtering layer;  
         [0015]    [0015]FIG. 12 reveals a layer having a current loop in place among magnetic components;  
         [0016]    [0016]FIG. 13 shows the components of FIG. 12 proximate to a housing mount;  
         [0017]    [0017]FIG. 14 is an exploded view of an example analyzer; and  
         [0018]    [0018]FIG. 15 is an external view of a housed analyzer.  
     
    
     DESCRIPTION  
       [0019]    The light from the infrared frequency region is transmitted through a sample of fluid material at discrete frequencies correlated with the absorption spectrum of a molecular species thereof to detect and quantitatively measure the species. In FIG. 1, a fluid analyzer  10  may have a light source  12  for generating incoherent infrared radiation. A primary filter  16  may be adapted to receive the light and selectively transmit light having a frequency range in the region of an absorption band for the molecular species to be detected. A secondary filter  18 , adapted to receive the filtered light, may transmit light at a plurality of discrete frequencies forming a plurality of fringes which provide a detectable signal. Secondary filter  18  may have interference producing mechanism for providing a plurality of transmission windows regularly spaced in frequency. The frequency spacing between adjacent windows may be adjusted to equal substantially the product of the frequency difference between adjacent spectral lines of the absorption spectrum for the molecular species to be detected and the factor (n/n′), where n and n′ are integers and n does not equal n′. Under these circumstances, the interference producing device may form a comb filter. Secondary filter  18  also may have a scanning device for causing the transmission peaks for adjacent n′th orders to coincide substantially with the spectral lines of such absorption spectrum. A cell  20  may be provided for transmitting the detectable signal through the fluid material, thereby the intensity of the detectable signal changes in proportion to the concentration of the molecular species. The intensity change of the detectable signal may be converted to a measurable form by a signal conditioner  22 , and the magnitude thereof may be indicated by detector  24 .  
         [0020]    Further, an approach for detecting and quantitatively measuring a molecular species of fluid material in a sample to be analyzed, may include generating light in the form of incoherent infrared radiation; collecting, collimating and transmitting the light; filtering the light so as to selectively transmit light having a frequency range in the region of an absorption band for the molecular species to be detected; interferometrically filtering the filtered light and transmitting light at a plurality of discrete frequencies to form a plurality of fringes which provide a detectable signal by directing the light through a plurality of transmission windows regularly spaced in frequency, the frequency spacing between adjacent windows being equal substantially to the product of the frequency difference between adjacent spectral lines of the absorption spectrum for the molecular species to be detected and the factor n/n′, where n and n′ are integers and n does not equal n′, and scanning the light to cause the transmission peaks for adjacent n′th orders to coincide substantially with the spectral lines of the absorption spectrum, the detectable signal having an intensity substantially equal to the sum of the fringes; transmitting the detectable signal through the fluid material, whereby the intensity of the detectable signal changes in proportion to the concentration of the molecular species; and detecting and indicating the intensity change of the signal.  
         [0021]    Several filters may be used with the above apparatus. Secondary filter  18  may be a Fabry-Perot interferometer (FPI) having a mirror separation, d, adjusted to transmit the filtered light at a plurality of discrete frequencies correlated with the absorption spectrum of a molecular species of the fluid material. This condition may be obtained when  
           d =( n′/ 4 μBn )  
         [0022]    where d is the mirror separation of the Fabry-Perot interference filter, μ is the index of refraction of the medium between the mirrors. B is the molecular rotational constant of the species n and n′ are integers and n does not equal n′. For a given molecular species, the rotational constant B is a unique quantity. Thus, identification of the species having a particular absorption spectrum may be made by adjusting the mirror separation of the interference filter such that the discrete frequencies transmitted coincide substantially with the absorption lines of the molecular species to be detected. Advantageously, the intensity of the detectable signal should not be affected by molecular species other than the species appointed for detection and the intensity differential represents a relatively large change in a small signal. Spectral interference may be minimized and no reference fluid is needed. The sensitivity of the apparatus is increased and highly sensitive forms and combinations of detectors, sources, filters and control systems generally are unnecessary. As a result, this device may permit fluid constituents to be detected more accurately and at less expense than systems wherein the emission spectrum of light passed through the sample contains a continuum of frequencies.  
         [0023]    Again in FIG. 1, fluid analyzer may be for detecting and quantitatively measuring a molecular species of fluid material. Analyzer  10  may have light source  12  for generating light  15  containing incoherent infrared radiation. A light conditioner  14  may collect, collimate and transmit light  15  to a primary filter  16 . Primary filter  16  may be adapted to receive light  15  and selectively transmit light  17  having a frequency range in the region of an absorption band for the molecular species to be detected. Secondary filter  18 , adapted to receive the filtered light  17 , may transmit light at a plurality of discrete frequencies forming a plurality of fringes which provide a detectable signal  30 . Detectable signal  30  may be transmitted through fluid material in cell  20 . A signal conditioner  22  may convert to measurable form, intensity changes created in signal  30  by the molecular species of the fluid material in cell  20 . The magnitude of the intensity change may be indicated by detector  24 .  
         [0024]    More specifically, as shown in FIG. 2, primary filter  16  may be a narrow band pass filter composed of multiple layers of dielectric thin films, and secondary filter  18  may have interference producing device for providing a plurality of transmission windows regularly spaced in frequency. In addition, secondary filter  18  may have scanning device for variably controlling the frequency of each order. The interference producing device may be adjusted so that the frequency spacing between adjacent windows equals substantially the product of the frequency difference between adjacent spectral lines of the absorption spectrum for the molecular species to be detected and the factor (n/n′, where n and n′ are integers and n does not equal n′. Under these circumstances, detectable signal  30  transmitted by secondary filter  18  may have an intensity substantially equal to the sum of the fringes. Moreover, the intensity of signal  30  should not be affected by molecular species other than the species appointed for detection, referred to hereinafter as the preselected species.  
         [0025]    Upon transmission of detectable signal  30  through fluid material in cell  20 , its intensity may change in proportion to the concentration of the preselected species. Such intensity change may be converted to measurable form by signal conditioner  22 . The latter may have a modulator  26  for modulating the phase difference between interfering rays of light transmitted by secondary filter  18  so as to shift the frequency of each fringe transmitted thereby. Signal conditioner  22  also may have synchronous (e.g., phase sensitive) detector  28  for detecting the intensity variation of signal  30 , whereby the magnitude of the intensity change can be identified by detector  24 .  
         [0026]    Several kinds of filters may be used as the secondary filter  18 . For an illustrative example, secondary filter may be a Fabry-Perot interferometer having a mirror separation, d, adjusted to transmit filtered light from primary filter  16  at a plurality of discrete frequencies correlated with the absorption spectrum of the preselected species. The transmission function of an FPI (I t ) can be given by the Airy formula: I t =T 2 [1+R 2 −2cos ø] −1 (I o ) where T+R+A=1, I o  is the intensity of the incident light, and the phase difference ø is expressed as ø=4πμωd for rays normal to the FPI mirrors. The symbols A, R and T represent, respectively, the absorbance, reflectance and transmittance of the FPI mirrors, μ is the refractive index of the medium between the FPI mirrors, d is the FPI mirror separation, and ω is the frequency of the incident light expressed in wavenumbers. When cos ø is equal to unity, transmission maxima for I t  may occur. Hence, ø=2 μm, where m takes on integral values and represents the order of interference. The transmission maxima for I t  may be referred to in the specification and claims as transmission windows. For a specific value of the mirror separation, d, the FPI provides a plurality of transmission windows regularly spaced in frequency. The frequency spacing, Δf, between adjacent windows (or spectral range) of the FPI is Δf=(2μd) −1 . For a simple diatomic molecule such as carbon monoxide, the frequency spacing between adjacent absorption lines of the infrared rotation-vibration absorption spectrum is approximately equal to 2B. By varying the mirror spacing, d, of the FPI, Δf can be adjusted to substantially equal the frequency difference between adjacent spectral lines of part or all of the absorption spectrum for the preselected species. That is, continuous scanning of the FPI in the vicinity of  
           d= 1/4 μB    
         [0027]    may produce an absorption interferogram having a plurality of fringes corresponding to a superposition of substantially all the absorption lines of the preselected species. When Δf=2B, the transmission peaks for adjacent orders may coincide substantially with the adjacent spectral lines of the absorption spectrum so as to produce a one-to-one correspondence therewith, and the amplitude of the signal from fluid sample  20  is a minimum. For values of Δf slightly different from 2B, the transmission peaks for adjacent orders would not perfectly coincide with the absorption lines and the amplitude of the signal from fluid cell  20  will decrease.  
         [0028]    Other absorption interferograms may be produced for values of the interferometer mirror separation.  
         d≈n′/(4μBn)  
         [0029]    where n and n′ are integers and n does not equal n′. These absorption interferograms are produced when Δf is equal to certain multiples of the rotational constant, B. The principal interferograms may be produced when every absorption line coincides with a different transmission window of the FPI. Such principal interferograms may be obtained for values of interferometer mirror separation  
         d≈n′/4μBn  
         [0030]    where n is equal to 1 and n′ is an integer greater than 1. More specifically, for values of interferometer mirror separation d=n′/(4μB) where n′ is an integer greater than 1, the principal interferograms may be obtained. For example, with n′=3, radiation may be transmitted by the interferometer not only at frequencies corresponding with those of adjacent absorption lines of the molecular species to be detected but also at two discrete frequencies located between each pair of the absorption lines. Secondary interferograms may be obtained when every other absorption line or every third absorption line (and so on) coincides with the transmission peaks of the FPI. Such secondary interferograms may be obtained for values of the interferometer mirror separation  
         d≈n′/4μBn  
         [0031]    where n is an integer greater than 1 and n′ is equal to 1. More specifically, for values of interferometer mirror separation d=(1/4μBn) where n is an integer greater than 1, the secondary interferograms may be obtained. For example, with n=3, radiation may be transmitted by the interferometer at frequencies corresponding with those of every third absorption line of the molecular species to be detected.  
         [0032]    Use of infrared fluid analyzer  10  may be exemplified in connected with the detection of a diatomic molecule such as carbon monoxide. Carbon monoxide (CO) has a vibration-rotation absorption band in the wavelength region of about 4.5-4.9μ, with its band center at about 4.66μ. This absorption band corresponds to transitions from the ground vibrational state (v=0) to the first vibrational state (v=1). As shown in FIG. 3, the absorption band may consist of two branches: an “R-branch”  96  corresponding to rotation-vibration transitions for which the rotational quantum number J changes by +1 and a “P-branch”  97  corresponding to rotation-vibration transitions for which the rotational quantum number J changes by −1. The frequencies, in units of wavenumbers, of the rotational transitions for the R an P branches are given by the formulas  
         ω R =ω O +2 B   1 +(3 B   1   −B   O ) J +( B   1   −B   O ) J   2    
         [0033]    with J=0, 1, 2, . . .  
         ω P =ω 0 −( B   1   +B   0 ) J +( B   1   −B   0 ) J   2    
         [0034]    with J=1, 2, 3, . . .  
         [0035]    The quantities ω O , B 0  and B 1  represent the absorption band center frequency, the ground state rotational constant and the first vibrational state rotational constant, respectively. The rotational constants B 0  and B 1  may be related according to the equation  
           B   0   =B   1 +α e    
         [0036]    where α e  is the rotation-vibration interaction constant. Values for the rotational constants of carbon monoxide appear to be:  
         [0037]    B 0 =1.9225145 cm −1    
         [0038]    B 1 =1.9050015 cm −1    
         [0039]    α e =0.017513 cm 1    
         [0040]    The intensity distribution for the R and P branches may be given by the equation  
           I   abs =(2 C   abs   ω/Q   R ) S   J  exp [− B   0   J ( J+ 1) ( hc/kT )] 
         [0041]    where C abs  is a constant factor, Q R  is the rotational partition function (≈kT/hcB), ω is the frequency, in wavenumbers, of the individual rotation-vibration absorption lines, h is Planck&#39;s constant, c is the speed of light, k is the Boltzmann constant, T is the absolute temperature and the line strengths S J  are:  
           S   J   =J+ 1 for the R-branch  
         S J =J for the P-branch  
         [0042]    Using these equations for line positions and intensities, a schematic representation of the CO absorption spectrum shown in FIG. 3, may be constructed. The representation may be termed schematic as, in reality, each rotational absorption line of the spectrum has a small but finite width.  
         [0043]    In order to utilize a Fabry-Perot interferometer to provide discrete frequencies of light at the frequencies of the absorption lines of the band, it may be necessary to determine the effect of the non-periodic spacing of the rotational absorption lines on the operation of analyzer  10 . For this purpose the Fabry-Perot interferometer may be adjusted such that the J=6 and J=7 R-branch rotational absorption lines coincide exactly with two adjacent discrete frequencies from the Fabry-Perot interferometer. These two rotational absorption lines appear to be the strongest lines in the band. Their frequencies are:  
         [0044]    ω R (J=6)=2169.169975 cm −1    
         [0045]    ω R (J=7)=2172.734796 cm −1    
         [0046]    The wavenumber difference between these lines may be 3.564821 cm −1 . The free spectral range of the interferometer may be adjusted to be equal to this wavenumber difference between adjacent lines. In order to determine the manner in which the mismatch of the light frequencies from the interferometer and the individual rotational absorption lines occur, the quantity Ω R =ω R (J+1)−ω R (J) may be calculated. The quantity Ω R  may be evaluated as follows:  
         Ω R =ω R ( J+ 1)−ω R ( J )=(3 B   1   −B   0 )−α e [( J+ 1) 2   −J   2 ]=(3 B   1   −B   0 )−α e (2 J+ 1).  
         [0047]    Therefore, the frequency difference between adjacent rotational absorption lines in the R-branch may change in direct proportion with the rotational quantum number J and the rotation-vibration interaction constant α e . The halfwidth, A, of the Fabry-Perot transmission windows may be given by the equation  
           A =(1− R )/(2 μdπR   1/2 )  
         [0048]    where R is the reflectivity of the Fabry-Perot mirrors and μd is the optical path length between the mirrors. Assuming that the reflectivity R=0.85, then A=0.185 cm 1 . The frequency mismatch with the ω R (J=5) line is about 0.035 cm −1 , which is well within the transmission halfwidth of the Fabry-Perot interferometer. The frequency mismatch with the ω R (J=3) line may be 0.210 cm −1 , which is just slightly larger than the FPI halfwidth. The frequency mismatch with the ω R (J= 10 ) line may be 0.210 cm −1 , which is also just slightly larger than the FPI halfwidth. Therefore, the R-branch lines from J=3 to J=10 may coincide substantially with the discrete frequencies from the FPI and therefore be most effective in the operation of analyzer  10 . The absorption line positions can be determined relative to the FPI transmission windows. From the equation for Ω R , the non-periodicity of the absorption line positions may be given by the term α e (2J+1). Equating this to the FPI transmission halfwidth yields  
           A=α   e (2 J   R +1)  
         [(1 −R )/(2 μdπR   1/2 )]=α e (2 J +1)  
         [0049]    Since (1/2μd)=free spectral range, it may be set to be equal to the product of the periodic contribution in the equation for Ω R , namely, 3B 1 −B 0 , and the factor n/n′ (n/n′) (3B 1 −B 0 ) [(1−R)/(πR 1/2 )]=α e (2J R −1)  
         [0050]    Solving for J R    
           J   R   ={[n (3 B   1   −B   0 )/(2 α   e   n ′)]/[(1− R )/π R   1/2 )]}−1/2  
         [0051]    The equilibrium value of the rotational constant B e  may be given as  
           B   e   =B   v +α e ( v+ 1/2)  
         [0052]    where B v  is the rotational constant of the v-th vibrational state.  
         [0053]    Hence, 3B 1 −B 0 =2B e −4α e , and  
           J   R   =n/n ′{[( Be/αe )−2][(1 −R )/(π R   1/2 )]}−1/2  
         [0054]    For CO, B e =1.931271 cm −1  and assuming a FPI mirror reflectivity of 0.85,  
           J   R =5.6  n/n′− 0.5.  
         [0055]    Similarly, for the P-branch  
         Ω P =ω P ( J+ 1)−ω P ( J )=−( B   1   +B   0 )−α e (2 J+ 1)  
         [0056]    and the same reasoning yields  
           J   P =( n/n ′){[( B   e /α e )−1][(1 −R )/(π R   1/2 )]}−1/2  
         [0057]    Since B e /α e &gt;&gt;1, J R =J P . The values of J R  and J P  can be denoted by J opt . Therefore, the optimum bandwidth of primary filter  16  should be equal to approximately 2B e J opt  and no greater than 4B e J opt . The value of J opt  for the principal interferograms having n′=3, for example, may be equal to 1.4. Thus, it may be always possible to match the transmission windows of the interferometer with at least two absorption lines of the species appointed for detection.  
         [0058]    For the principal interferogram of CO with n′=3, the interferometer may transmit radiation through transmission windows corresponding to the frequencies of at least two of the absorption lines appointed for analysis and, in addition, through two extra tramission windows spaced at equal frequency intervals with and between the absorption lines appointed for analysis. In situations where the absorption lines of the fluid being analyzed are relatively narrow and exist in a frequency region that does not contain interfering absorption lines from other fluids, one may use of principal inteferograms of the type wherein n′=3 provides increased sensitivity. The increase in sensitivity may be produced by the better match created between absorption linewidth and the widths of the interferometer&#39;s transmission windows. The decrease in sensitivity otherwise resulting from the presence of additional FPI transmission windows may be offset by the increase in sensitivity achieved by reducing the width of the FPI transmission windows. The increase in sensitivity which is realized in a particular situation may depend on the value of n′ selected, which, in turn, is governed by the experimental conditions associated with the fluid sample under investigation. It may be significantly greater than that produced by increasing the reflectivity of the FPI mirrors. The latter approach may permit narrowing the width of the FPI transmission windows without introducing additional radiation not absorbed by the fluid, and would, at first, appear to be a better way to improve the match between absorption linewidth and FPI transmission linewidth. In practice, however, for high mirror reflectivities the transmissivity of the FPI may be decreased by small absorption and/or scattering losses in the FPI mirrored surfaces. This reduction in transmissivity may result in a decrease in sensitivity that is greater than the sensitivity loss produced by introduction of additional transmission windows discussed above. Further, the use of lower reflectivity FPI mirrors with high transmissivity may result in a device that can be used for a larger number of experimental applications.  
         [0059]    For the secondary interferograms of CO with n=3, a value of 16 may be obtained for the quantity J opt . This value for J opt  indicates that absorption of radiation transmitted by the FPI may occur over a frequency range that contains approximately 16 absorption lines. In use of a secondary interferogram having n=3, FPI transmission windows may occur at every third absorption line, so that absorption will take place at only five absorption lines. The usefulness of these secondary interferograms is anticipated for the cases where fluid mixtures are being analyzed. In such cases strong absorption lines from a fluid other than the one appointed for analysis may interfere with the measurement of the fluid selected for analysis. This interference may be reduced or eliminated by selecting a secondary interferogram which does not provide radiation at the absorption frequencies of the interfering fluid.  
         [0060]    As previously noted, modulator  26  may modulate the phase difference, ø, so as to vary the intensity of transmitted signal  30 . In order to obtain the maximum modulated signal, the modulating range may be adjusted to approximately 1/2 the frequency spacing between adjacent fringes. The modulating range can, alternatively, be restricted to preselected portions of the absorption spectrum of the preselected species in order to increase the intensity of the modulated signal. Generally speaking, the modulating range should be no greater than the frequency spacing between adjacent absorption lines of the preselected species.  
         [0061]    Resultant signal  30  from secondary filter  18  and fluid cell  20  may be focused in the plane of pinhole stop  32  by lens  34 . Lens  34  may be adjusted so that the center of the signal is positioned on pinhole  36 . The intensity of the portion of signal  30  passing through pinhole  36  may be detected by an infrared detector  38 . Phase sensitive detector  28 , such as a lock-in amplifier, may be adapted to receive the signal from infrared detector  38  and detect the intensity variation thereof. The output of phase sensitive detector  28 , representing the signal intensity change, may be displayed by an indicator and recorder  40 , which may have an oscilloscope and a chart recorder.  
         [0062]    [0062]FIG. 4 shows configuration  50  having a radiation source  51  that may emit radiation or light  57  through a broadband filter  52 . From filter  52  light  58  may go through a narrowband filter  53 . Filter  52  may be designed to limit light  57  to light  58  of interest. Filter  53  may have somewhat reflective surfaces  61  and  62  to form an interference space or cavity between the surfaces. Filter  53  may be a layer  65  of silicon with reflective surfaces  61  and  62  on both sides of layer  65 . As light  59  comes through filter  53 , it may develop transmission peaks  63  resulting in a comb structure typical of a Fabry-Perot filter as shown in FIGS. 5, 6 and  7 . Light  59  may go through cell  54  which contains some fluid. For this illustrative example, carbon-monoxide (CO) is in cell  54 . This fluid may have absorption spectral lines  64 . These absorption lines may be based on rotational differences of the CO atom. These lines may constitute an absorption cross-section of the fluid, i.e., it is a characteristic of the fluid. The spacing of these lines may be rather equal or linear. These CO rotational absorption lines  64  are represented as circles in FIGS. 5, 6 and  7 . The graphs of these figures show transmittance from zero to 100 percent versus wavelength from 4500 to 4660 nanometers. Transmission peaks  63  of light  59  may or may not be aligned with rotational absorption lines  64 . When there is an alignment, the amplitude of light signal  59  may be reduced and the light may exit as light  60  from cell  54  to detector  55 . With this alignment, there is absorption. When there is not alignment, light  59  may be pretty much all transmitted through cell  54  and exiting as light  60 . The index of refraction or optical path length of filter  53  may be modulated, dithered or changed by tilting layer  65  about 7 degrees other appropriate amount in an alternating manner or other fashion, or by heating and cooling layer  65  about 20 degrees or other appropriate amount in an alternating manner or other fashion. A dither/modulator  56  may be connected to filter  53  to provide such modulation, dithering or changing of the layer&#39;s angle or temperature. Device  56  may be also referred to as an optical path varying mechanism. A processing and/or control electronics component  95  may connected to dither/modulator  56 , detector  55  and/or radiation or light source  51 , for reasons as may be desired. Component  95  may be a computer.  
         [0063]    The measure of inherent light strength or intensity ratio may be a light  60  intensity when there is alignment and absorption, divided by a light  60  intensity when there is not alignment and no absorption.  
           I   A       I   NA       =     I   R                           
 
         [0064]    Detector  55  may do a summation of the intensity (flux) peaks.  
         [0065]    Narrow transmission peaks  63  of light  59  may be moved relative to absorption lines  64  by changing the filtering characteristic of filter  53 . This characteristic may be affected by changing the optical thickness of the filter. Several ways to do this changing include heating the filter  53  material to change the optical thickness and rotating filter  53  material at an angle relative to the incident direction of light  58 . By changing optical thickness of filter  53 , one is slithering or modulating transmission peaks  63  in and out of absorption structure lines  64 . Filter  53  under this treatment may be regarded as an etalon.  
             I   R     =         I   A       I   NA       =     E     -   kx           ;     k   =       ln        (       I   A     /     I   NA       )       x         ,                         
 
         [0066]    where k is the absorption constant and x is the absorption path length, i.e., the path of light or radiation through the fluid in cell  54 . “k” indicates an amount of absorption at a particular band. “k” is proportional to the CO partial pressure which may be calculated as parts per million of CO in, for instance, a room. The thickness of a layer for filter  53  for a given fluid may be determined with an equation,  
         λ= m/nd,    
         [0067]    where m is an integer, n is the index of refraction and d is the thickness of the layer. One may assume m=1 and Δλ=λ 1 −λ 2 . Δλ is the distance between the transmission peaks  63 . A is the wavelength of the transmission and the absorption for alignment. One may note that  
         Δλ=1 /nd.    
         [0068]    Filter  53  may be a membrane formed from a full layer thickness. The thickness of the membrane should be about 417 micrometers to provide a comb transmission structure in the case of CO. The specific thicknesses of the structure of filter  53  are calculated for the graphs of FIGS. 5, 6 and  7 , may be 416.92 micrometers for a silicon layer  65 ,  810  nanometers for an SiO 2  layer  66  formed on the opposing surfaces of layer  65 , and 338 nanometers for an Si layer  67  formed on each layer  66 . Each set of layers  66  and  67  may form a mirror  61  and a mirror  62 . The structure of filter  53  may resemble the one in FIG. 8 which is not drawn to scale.  
         [0069]    [0069]FIG. 5 shows the transmission comb structure with peaks  63  of light  59  from filter  53  aligned with rotational absorption lines  64  (in circles) with filter  53  at an ambient temperature. Transmission peaks  63  may be shifted so that they are not aligned with absorption lines  64  as shown in FIG. 6 by heating structure or filter  53  to a temperature that is 20 degrees centigrade greater than the ambient temperature of filter  53  as represented in FIG. 5. In another way, transmission peaks  63  may be shifted so that they are not aligned with absorption lines  64  as shown in FIG. 7 by rotating structure or filter  53  about seven degrees in either direction relative to its original position as represented in FIG. 5. The original position of structure or filter  53  may be such that the surface of its mirror  61  is approximately perpendicular relative to the direction of incident light  58 .  
         [0070]    In FIG. 9, radiation or light  57  may enter a T-filter  52 . Light  57  may be well collimated. It may be an infrared light having a wavelength in the four micrometer range. Light source  51  may be a filament. Filter  52  may limit the range of wavelengths for light  58  which enters filter  53 . The peaks  63  and lines  64  may or may not be aligned. Filter  52  may cut off portions  68  of peaks  63  that could have been present in cell  54  if filter  52  were not in place. “x” is the travel length or absorption path length of light  59  in the fluid of cell  54 .  
         [0071]    A thermally tuned filter  53  that would be treated as an etalon may need a certain amount of temperature change as determined by the optical thickness of the filter  53  layer. FIG. 10 shows thin layer  71  to have thicker transmission peaks  73  that are farther apart from each other wavelength wise than peaks  74  from thick layer  72 . Obviously, thin layer  71  appears easier to warm up than thick layer  72 . However, with thin layer  71  having a limited amount of material, there is relatively less phase change via the index of refraction change for a given temperature change. Peaks  73  may be effected with a 120 degree centigrade change, and peaks  74  may be effected with a 20 degree centigrade change. Thus, a greater temperature change appears to be needed for thin layer  71  than for thick layer  72  to effect a reasonable index of refraction change. This means that such etalon may need thermal tuning, since temperature change would somewhat be determined by the thickness of a layer in a filter, such as filter  53 . FIG. 10 is not necessarily drawn to scale.  
         [0072]    The present invention may be implemented in a monolithic wafer-like structure or in MEMS. FIG. 11 shows an implementation of the comb etalon filter  53  in a MEMS monolithic form. The structure may be made from silicon or any other appropriate material. Filter  53  may change its index of refraction by changing its position relative to the direction of incident radiation  58 , or by changing its optical thickness. Filter  53  in FIG. 11 has a similar layer structure as layer  65  with mirrors  61  and  62  on the larger surfaces as opposite sides of layer  65 . A conductor  76  is on the perimeter of filter  53 . When electrical current is passed through conductor  76 , it may heat up layer  65  and cause a change of index of refraction sufficient enough to cause alignment and non-alignment of transmission peaks  63  with absorption lines  64 . The temperature change may be about 20 degrees Centigrade. The current may be turned on and off so as to pulse the filter thermally. FIG. 11 a  illustrates an example of thermal pulses in a layer versus time. The thermal time constant may be about ten milliseconds for a given silicon structure. The amount of degree change needed for appropriate dithering and the thermal time constant may be dependent on the filter  53  structure and material. The electrical current may be fed to an element or conductor  76  via bond wires  77  and  78 . Conductor  76  may be wire-, plate-, layer- or wafer-like. It may have other forms. Magnets  79  would be absent in the structure of FIGS. 11, 12,  13 ,  14  and  15  for the filter  53  heating configuration.  
         [0073]    In another configuration, with conductor  76  having a sufficient current in it, the index of refraction or optical path length of layer  65  of filter  53  may be changed by tilting layer  65  about seven degrees relative to its perpendicular position to incident light  58 . Rather than heating, the conductor may create a magnetic field that interacts with magnets  79  when yoke  80  and filter  53  are positioned on plate  81  as shown in FIG. 12. Magnetic forces between loop  76  and magnets  79  may be used to rotate or tilt layer  65  relative to the direction of light  58 . If such rotation or dithering is done at a certain rate, the rate may be such that it is in sync with the mechanical resonance of layer  65  and associated structures and components thus requiring smaller current. An example rate of dithering might be about one kilohertz. Plate  81  may have a slot  85  for the passage of radiation or light  58 . Magnets  79  may fit through slots  82  of filter  53 . In such structure the current may be about 0.5 ampere in a magnetic field of approximately one tesla. These values may vary according to structure and material. Lower field currents are required for resonance operation. Layer  65  may supported in yoke  80  by flexible serpentine springs or other kind of structure  83  so that layer  65  can tilt or move relative to yoke  80 . These springs  83  may be used to conduct current to loop  76  in lieu of bonding wires  77  and  78 . Small gaps  84  may exist between layer  65  and yoke  80  so as to let layer  65  be moveable relative to yoke  80 . Magnets  79  may be micromachined and be made from one or more of a variety of materials.  
         [0074]    [0074]FIG. 13 shows where filter  53 , yoke  80  and support structure  81  fit in with structures  86  and  87  that hold filter  52  and radiation or light source  51 , respectively. FIG. 14 is an exploded view of structure  90  that may house configuration  50  or other configurations of the fluid analyzer. In addition to the structure components shown in FIG. 13, structure  90  may have a component  88  that holds fluid cell  54  and a component  89  that holds detector  55 . A component  92  may support component  89  and may be a cap for structure  88 . Power and modulation or dither signals may be from dither/modulator electronics  56  housed in structure  86  and/or  87 ; or dither/modulation signals may be fed into port  91 . Port  93  may provide for communications to detector  55 . Fluid cell  54  may be a container inserted into component  88 , or component  88  may act as a fluid cell  54  with port  91  and/or  93  providing a way to put fluid in or to expel fluid from cell  54 . On the other hand, ambient air or fluid may be free to flow through the fluid cell. FIG. 15 shows structure  90  assembled together with its components. Structure  90  and its components may be made or fabricated with MEMS and/or other technologies from one or more of a variety of materials.  
         [0075]    Although the invention has been described with respect to at least one illustrative embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.