CA1036833A - Infrared gas analysis - Google Patents

Abstract

INVENTION: INFRARED GAS ANALYSIS INVENTOR: JOSEPH J. BARRETT ABSTRACT OF THE DISCLOSURE A method and apparatus for detecting and quantitatively measuring a molecular species of gaseous material in a sample to be analyzed are provided. Light containing incoherent infrared radiation is collected, collimated and transmitted by a light conditioning means to a primary filtering means. The primary filtering means selectively transmits light having a frequency range in the region of an absorption band for a molecular species to be detected. A secondary filtering means, adapted to receive the filtered light, transmits light at a plurality of discrete frequencies, providing a detectable signal, through the gaseous material. The intensity of the signal changes in proportion to the concentration of the molecular species. Means are provided for measuring and recording the magnitude of the signal intensity change. The intensity of the detectable signal is not affected by molecular species other than the species appointed for detection, and the intensity differential represents a relatively large change in a small signal. Hence, gaseous constituents are detec-ted in an accurate and economical manner.

Description

103~;833 INFRARED GAS ANALYSIS
.
BACKGROUND OF THE INVENTION
This invention relates to the field of infrared gas analysis and more particularly to a method and apparatus in which light is transmitted through a gas sample at discrete frequencies correlated with the absorption spectrum of a gaseous constituent thereof to detect and quantitatively measure the constituent.
DESCRIPTION OF THE PRIOR ART
In the apparatus conventionally used for non-dispersive infrared gas analysis, a beam of infrared radiation having an emission spectrum embracing the absorption spectrum of the gas to be analyzed is directed through a gas 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 gas of the type appointed for analysis. A signal intensity differential, produced by absorption in the sampls, is converted to a detectable signal and displayed.
One of the major problems with such analyzers is the difficulty of analyzing quantities of gaseou~ constituents present in the low parts per million range. The signal intensity differ-ential 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 with such analyzers is the decreased sensitivity which results unless the temperature and pressure of the reference gas are carefully controlled. To alleviate these problems, it has been necessary to provide the analyzers with highly sensitive forms and combinations of detec-tors, sources, filters, control systems and the like, which are relatively expensive. For the above reasons, gas analyzers of the type described have low sensitivity and high operating costs.

,~k ~036833 SUMMARY OF THE INVENTION
The present invention provides apparatus wherein light from the infrared frequency region is transmitted through a sample of gaseous material at discrete frequencies correlated with the absorption spectrum of a molecular species thereof to detect and quantitatively measure the species. Briefly stated, the apparatus has light source means for generating incoherent infrared radia-tion. A light conditioning means collects, collimates and trans-mits the light to a primary filtering means. The primary filter-~ng means i5 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 filtering means, adapted to receive the filtered light, transmits light at a plurality of discrete frequencies forming a plurality of fringes which provide a detectable signal. The secondary filtering means ; has interference producing means for providing a plurality of tran8mission windows regularly spaced in frequency. The frequency spacing between adjacent windows is adjusted to equal substantially the frequency difference between adjacent ~pectral lines of the ah80rption spectrum for the molecular species to be detected.
Under these circumstances, the interference producing means forms a comb filter. The secondary filtering means also has scanning means for causing the transmission peaks for adjacent orders to coincide substantially with the spectral lines of such absorption spectrum. Means are provided for transmitting the detectable signal through the gaseous material, whereby the intensity of the detectable signal changes in proportion to the concentrabion of the molecular species. The intensity change of the detectable signal is converted to a measurable form by a signal conditioning means, and the magnitude thereof is indicated by detecting means.
Further, the invention provides a method for detecting and quantitatively measuring a molecular species of gaseous ~03~i833 material in a sample to be analyzed, comprising the steps of generating light in the form of incoherent infrared radiation;
collecting, collimating and transmitting the light; filtering said 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 said 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 frequency difference between adjacent spectral lines of the absorption spec- -trum for the molecular species to be detected, and scanning said light to cause the transmission peaks for adjacent orders to coin-cide substantially with the spectral lines of said absorption spectrum, said detectable signal having an intensity substantially equal to the sum of said fringes; transmitting the detectable signal through said gaseous material, whereby the intensity of the detectable signal change~ in proportion to the concentration of the molecular species; and detecting and indicating the intensity change of the qignal.
Several known filtering means may be adapted for use with the above apparatus. Preferably, the secondary filtering means is a Fabry-Perot interferometer (FPI) having a mirror sepa-ration, d, adjusted to transmit the filtered light at a plurality of discrete frequencies correlated with the absorption spectrum of a molecular species of the gaseous material. This condition is-obtained when d 4~B

where d is the mirror separation of the FPI, ~ is the index of refraction of the medium between the mirrors and B is the molecular rotational constant of the species. For a given molecular species, ~036~33 the rotational constant B is a unique quantity. Thus, identifica-tion of the species having a particular absorption spectrum is made positively by adjusting the mirror separation of the FPI
such that the discrete frequencies transmitted coincide substan-tially with the absorption lines of the molecular species to be detected. Advantageously, the intensity of the detectable signal is not 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 inter-erence is minimized and no reference gas is needed. The sensiti-vity of the apparatus is increased and highly sensitive forms and combina~ions of detectors, sources, filters and control systems are unnecessary. As a result, the method and apparatus of this invention permits gaseous constituents to be detected more accu-rately and at less expense than systems wherein the emission spectrum of light passed through the sample contains a continuum of frequencies.
BRIEF DESCRIPTION OF THE DRAWINGS
~he invention will be more fully understood and urther advantages will become apparent when reference is made to the following detailed description of the preferred embodiments of the invention and the accompanying drawings in which:
Figure 1 is a block diagram showing apparatus for detecting and quantitatively measuring a molecular species of gaseous material;
Figure 2 is a schematic diagram of the apparatus of Figure l;
Figure 3 is a side view, partially cut away, showing means for modulating the secondary filtering means of Figures 1 and 2;
Figure 4 is a schematic diagram showing an alternate embodiment of the apparatus of Figure l; and ~0~33 Figure 5 illustrates the absorption spectrum of a parti-cular molecular species.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Figure 1 of the drawings, there is shown preferred apparatus for detecting and quantitatively measuring a molecular species of gaseous material. The apparatus, shown gener-ally at 10, has light source means 12 for generating light 15 con-taining incoherent infrared radiation. A light conditioning means 14 collects, colllmate8 and transmits the light 15 to a primary filtering means 16. The primary filtering means 16 is adapted to receive the light 15 and selectively transmit light 17 having a frequency range in the region of an absorption band for the mole-cular species to be detected. A secondary filtering means 18, adapted to receive the filtered light 17, transmits light at a plurality of discrete frequencies forming a plurality of fringes which provide a detectable signal 30. The detectable signal 30 is transmitted through gaseous material in sample 20. A signal con-dltioning means 22 converts to measurable form intensity changes cr~ated in the signal 30 by said molecular species of the sample 20. The magnitude of the intensity change is indicated by detec-ting means 24.
More specifically, as shown in Figure 2, the primary filtering means 16 is a narrow band pass filter composed of multi-ple layers of dielectric thin films, and the secondary filtering means 18 has interference producing means for providing a plura-lity of transmission windows regularly spaced in frequency. In addition, the secondary filtering means 18 has scanning means for variably controlling the frequency of each order. The inter-ference producing means is adjusted so that the frequency spacing between adjacent windows equals substantially the frequency difference between adjacent spectral lines of the absorption spectrum for the molecular species to be detected. Under these 10;~6833 circumstances, the detectable signal 30 transmitted by the second-ary filteri~g means 18 has an intensity substantially equal to the sum of the fringes. Moreover, the intensity of the signal 30 is not affected by molecular species other than the species appointed for detection, referred to hereinafter as the preselected species.
Upon transmission of the detectable signal 30 through ga~eous material in sample 20, its intensity changes in proportion to the concentration of the preselected species. Such intensity change is converted to measurable form by the signal conditioning means 22. The latter has modulating means 26 for modulating the phase difference between interfering rays of light transmitted by the secondary filtering means 18 so as to shift the frequency of each fringe transmitted thereby. Signal conditioning means 22 also has synchronous (e.g. phase sensitive) detecting means 28 for detecting the intensity variation of the signal 30, whereby the magnitude of the intensity change can be identified by detec-ting means 24.
Several known filtering means may be used as the second-ary filtering means with the apparatus 10. Preferably, the second-ary filtering means is a Fabry-Perot interferometer having a mirror separation, d, adjusted to transmit filtered light from the primary filtering means 16 at a plurality of discrete fre-quencies correlated with the absorption spectrum of the preselec-ted species. The transmission function of an FPI (It) can be given by the Airy formula: It = T211+R2-2cos~]~l IO where T +
R + A = 1, Io is the intensity of the incident light, and the phase difference ~ is expressed as ~ = 4~d for rays normal to the FPI mirrors. The symbols A, R and T represent, respectively, the absorbance, reflectance and transmittance of the FPI mirrors, ~ is the refractive index of the medium between the FPI mirrors, d is the FPI mirror separation, and ~ is the frequency of the ~036833 incident light expressed in wavenumbers. When cos ~ is equal to unity, transmission maxima for It occur. Hence, ~ = 2~m, where m takes on integral values and represents the order of interference.
The transmission maxima for It are 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, Qf, between adjacent windows ~or spectral range) of the FPI is ~f = ~2~d)-1. By varying the mirror spacing, d, of the FPI, ~f 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 scan-ning of the FPI in the vicinity of d 4~B

produces an absorption interferogram having a plurality of fringes corresponding to a superposition of substantially all the absorp-tion lines of the preselected species. When ~fz2B, the trans-mission peaks for ad~acent orders coinaide substantially with the adjacent spectral lines of said absorption spectrum so as to pro-duce a l-to-l correspondence therewith, and the amplitude of the signal from gas sample 20 is a minimum. For values of ~f slightly different from 2B, the transmission peaks for adjacent orders will not perfectly coincide with the absorption lines and the ampli-tude of the signal from gas sample 20 will increase.
Use of the apparatus 10 for infrared gas analysis may be exemplified in connection 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 - 4.9~ with its band center at about 4.66~. This absorp-tion band corresponds to transitions from the ground vibrational state (v = 0) to the first vibrational state (v = 1). As shown in l036e~3 Figure 5, the absorption band consists of two branches: an "R-branch" corresponding to rotation-vibration trans~tions for which the rotational quantum number J changes by +l and a "P=branch"
corresponding to rotation-vibration transitions or which the rotational quantum number J changes by -1. The frequencies, in units of wavenumbers, of the rotational transitions for the R and P branches are given by the formulas ~ R Z ~0 + 2Bl + (3Bl-Bo)J + (31 Bo)J
with J = 0, 1, 2, ....
and ~p (~)o (Bl+Bo)J + (sl~tsO)J2 with J = 1, 2, 3, The quantities ~0, Bo and Bl represent the absorption band center frequency, the ground state rotational constant and the first vibrational state rotational constant, respectively. The rota-~ional constants Bo and Bl are related according to the equation Bo = Bl + ae where ~e is the rotation-vibration interaction constant. Values for the rotational constants of carbon monoxide listed in American Institute of Physics Handbook, Third Edition, p. 7-173, are:
Bo = 1.9225145cm~l Bl = 1.9050015cm l ae = O . 017513cm~l The intensity distribution for the R and P branches is given by the equation 2Cabs~ hc Iab = QR SJ exp[-BoJ(J+l)kT]

where Cabs is a constant factor, QR is the rotational partition function (~kT/hcB), ~ is the frequency, in wavenumbers, of the individual rotation-~ibration absorption lines, h is Planck's constant, c is the speed of light, k is the Boltzmann constant, T is the absolute temperature and the line strengths SJ are:

1036~33 SJ = J + 1 for the R-branch SJ = J for the P-branch Using these equations for line positions and intensities, a sche-matic representation of the CO absorption spectr~m shown in Figure 5, was constructed. The representation is termed schematic as, in reality, each rotational absorption line of the spectrum has a small but finite width.
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 is necessary to determine the effect of the n~n-periodic spacing of the rotational absorption lines on the operation of the apparatus 10. For this purpose the Fabry-Perot interferometer is 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 interfero-meter. These two rotational absorption lines are the strongest lines in the band. Their frequencies are:
~R(J~6) = 2l69.l69975cm-l ~Rt~'7) - 2172.734796cm 1 ~he wavenumber difference between these lines is 3.564821cm~l.
The free spectral range of the interferometer is 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 tke quantity Q~=~R(J+l)-~(J) is ca~u~te~ Ihe quantity QR may be evaluated as follows:
QR=~R(J+l)-~R(J)=(3Bl-Bo)-e[(J+l)2-J2l~(3Bl-Bo)-ae~2J+l).
Therefore, the frequency difference between adjacent rotational absorption lines in the R-branch changes in direct proportion with the rotational quantum number J and the rotation-vibration interaction constant ae. The halfwidth, A, of the Fabry-Perot transmission windows is given by the equation _g_ 1036~
A = l-R

2~d~
where R is the reflectivity of the Fabry-Perot mirrors and ~d is the optical path length between the mirrors. Assuming that the reflectivity R~0.85, then A = 0.185cm 1. The frequency mismatch with the ~R(J=5) line is 0.035cm~l, which is well within the transmission halfwidth of the Fabry-Perot interferometer. The frequency mismatch with the ~R(J=3) line is 0.210cm~l, which is just slightly larger than the FPI halfwidth. The frequency mis-match with the ~R(J=lO) line is 0.210cm~l, which is also just slightly larger than the FPI halfwidth. Therefore, the R-branch lines from J=3 to J=10 will coincide substantially with the dis-crete frequencies from the FPI and therefore will be most effec-tive in the operation of the apparatus lO. The absorption line positions can be de~mir~edrelative to the FPI transmission windows.
From the equation for QR, the non-periodicity of the absorption line positions is given by the term ~e(2J+l). Equating this to the FPI transmission halfwidth yields A ~ ~e(2JR+l) ~ l-R
~ ~ = ae(2JR+l) Since ~ - free spsctral range, is set to be equal to the periodic contribution in the equation for ~R, namely, 3Bl-Bo, (3Bl-Bo) ~ )= ae(2J~l).

Solving for JR

t3Bl-B0) (l_R~ -l/2 2 ae ~r ~J

The equilibrium value of the rotational constant Be is given as Be = Bv + ~etV+l/2) where Bv is the rotational constant of the v-th vibrational state.
Hence 3Bl-Bo = ~Be-4ae, and ~0~68~

~ ( ) For CO, Be = 1.931271cm~l and assuming a FPI mirror reflectivity of 0.85 yields JR = 5.107~5. Therefore, the first 5 rotational absorption lines of CO would overlap substantially with the trans-mission halfwidths of the FPI.
Similarly, for the P-branch Qp = ~p(J+l)-~p(J) = -(Bl+Bo)-ae(2J+l) and the same reasoning yields (Be ~ ( R) -1/2 Since Be/~e >> l, JR~Jp. The values of JR and Jp can be denoted by Jopt- Therefore, the optimum bandwidth of the primary filtering means 16 should be equal to approximately 2BeJopt and no greater than 4BeJopt As previously noted, the modulating means 26 modulates the phase difference, ~, so as to vary the intensity of the trans-mitted signal 30. In order to obtain the maximum modulated signal, the modulating range i9 adjusted to approximately l/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.
The resultant signal 30 from secondary filtering means 18 and gas sample 20 is focused in the plane of pinhole stop 32 by lens 34. Lens 34 is adjusted so that the center of the signal is positioned on the pinhole 36. The intensity of the portion of signal 30 passing through the pinhole 36 is detected by an infra-red detector 38. Phase sensitive detection means 28, such as a lock-in amplifier, is adapted to receive the signal from infrared ~03G~33 detector 38 and detect the intensity variation thereof. The output of the phase sensitive detection means 28, representing the signal intensity change, is displayed by an indicating and recording means 40, which can comprise an oscilloscope and a chart recorder.
In Figure 3, the secondary filtering means 1~ ~nd the modulating means 26 are shown in greater detail. The secondary filter~ng means shown is a Fabry-Perot interferometer (FPI) ~hich 1s scanned by varying the phase difference, ~, between inter-fering beams of light in a conventional way. Scanning methods such as those wherein the pressure of gas between the mirrors of the FPI is altered so as to change the optical path therebetween can also be used. Accordingly, the secondary filtering means 18 shown in Figure 3 should be interpreted as illustrative and not in a limiting sense. Such means has cylindrical air bearings 56 and 58 which normally operate at about 30 psi and collectively support a hollow metal cylinder 60 approximately 35 cm. long and aonstructed of stainless steel or the like. The outer diambter of the cylinder 60 is centerless ground to about 4 cm. The inner diameter of the cylinder 60 is about 3.5 cm. Each of the air bearings 56 and 58 is about 8 cm. long and has outer and inner diameters of about 5 cm. and about 4 cm., respectively. The separation between centers of the air bearings is approximately 20 cm. One of the mirrors 62 of the secondary filtering means 18 is fixedly mounted on end 64 of cylinder 60 as by a suitable adhesive or the like. The plane surface of the mirror 62 is sub-stantially perpendicular to the translational axis of the cylinder.
The other mirror 66 is fixedly mounted to the modulating means 42 as hereinafter described. Each of the air bearings 56 and 58 rests in precise v-blocks of a base plate (not shown) treated so as to dampen external vibrations. Filtered light 17 from primary filtering means 16 enters the secondary filtering means 18 at end 68 of cylinder 60. A carriage 70 caused to move hori-zontally by means of a precision screw 72 and having a coupling arm 82 fixedly secured thereto by mechanical fastening means, such as screws 88, and to cylinder 60 as described hereinafter provides the cylinder 60 with the linear motion needed to scan the secondary filtering means 18. Precision screw 72 is coupled to a digital stepping motor 74 through gear assembly 76. The scan rate of the interferometer is controlled either by changing the gear ratio of as~embly 76, as by means of magnetic clutches or the like, or by varying the pulse rate input to the digital stepping motor 74. With apparatus of the type described, the s~an rate can be varied over a range as great as 106 to l or more.
In order to transmit precisely the linear motion to cylinder 60, a collar 78 having glass plate 80 adhesively secured thereto, is fixedly attached to the cylinder 60. The coupling arm 82 has a ball 86 comprised of stainless steel, or the like, associated with an end 84 thereof. A permanent magnet 90 is attached to end 84 of coupling arm 82 near the ball 86. Due to the magnetic attraction between the collar 78 and the magnet 90, the ball is held in contact with the glass plate 80. A low fric-tion contact point is thereby provided. The contact force pro-duced at such contact point by linear movement of the carriage 70 can be adjusted either by varying the separation between the magnet 90 and the collar 78, or by decreasing the strength of the magnet 90.
A sectional view of one form of modulating means 26 is shown in Figure 3. Other forms of the modulating means 26 can also be used. Preferably, the modulating means 26 has a hollow cylindrical body 92 of piezoelectric ceramics. The inner and outer wall 94 and 96 of the cylindrical body 92 are coated with an electrically conductive material such as silver or the like.

-~o~a Insulating members 98 and 100 comprised of an insulating material such as ceramic or the like are securea to the cylindri-cal body 92, at ends 102 and 104, respectively, by a suitable adhesive such as an epoxy resin. Mirror 66 is fixedly attached to insulating member 98 by an adhesive of the t~pe used to secure mirror 62 to the end 64 of cylinder 60. In order that mirror 66 be maintained in parallel with mirror 62, the insulating member 100 is adhesively secured to face 106 of holding member 108. The outer face 110 of the holding member 108 has connected thereto a plurality of differential screw micrometers 112, which can be adjusted in the conventional way to provide for precise angular alignment of the mirror 66. Electrodes 114 and 116 are attached to the inner wall 94 and the outer wall 96, respectively. Vol-tage having a wave form such as a sine wave or a square wave impressed thereon i9 applied from a high voltage low current power ~upply 101 to the electrodes 114 and 116. Upon application of the voltage the cylindrical body 92 is caused to modulate in a linear direction, whereby the intensity of signal 30 is varied.
When the voltage applied from power supply lOl to eleatrodes 114 and 116 has the form of a square wave, the voltage limits of the wave form can be adjusted so that the intensity of the signal 30 alternates between its maximum and minimum values. A synchronous detection means is provided for determining the difference in amplitude between the maximum and minimum values of the signal 30 for each cycle of the square wave to produce an electrical output signal proportional to the maximum and minimum values of the signal 30. AS a result, the accuracy of the detecting means and hence the sensitivity of the apparatus 10 is increased by a factor in the order of about 100 or more.
The apparatus lO which has been disclosed herein can, of course, be modified in numerous ways without departing from the scope of the invention. For example, secondary filtering means 10368~3 18 can be a fixed etalon tuned by controlling the temperature thereof. One type of fixed etalon which is suitable is comprised of optically transparent material, such as potassium bromide, potassium chloride, lithium fluoride, magnesium fluoride, calcium fluoride, cesium bromide, sodium bromide, cesium iodide, barium fluoride, sodium chloride, and the like, having high transmission characteristics in the frequency region of the absorption band of the preselected species. In addition, such an etalon has opposed surfaces which are polished, flat, parallel and coated with silver, dielectric material or the like for high reflectivity at a preselected frequency region. For a preselected species such as carbon monoxide, having an absorption spectrum in the frequency region of about 2050 to 2250 wavenumbers, preferred optically transparent materials include potassium bromide, lithium fluoride and magnesium fluoride. The thickness of the solid etalon can be ~hosen so that the free spectral range of the etalon corresponds approximately to the frequency difference between spectral compo-nents of the given absorption spectrum. Fine tuning of the solid etalon i9 affected by providing means for controlling the tempera-ture, and hence the~optical path length, thereof so as to cause the transmission peaks for adjacent orders to coincide with the compo-nents of the given absorption spectrum. Lenses 14 and 34 can be replaced with off-axis parabolic mirrors (not shown) to enhance the optical through-put of the apparatus 10. As shown in Figure 4, the signal intensit~ change can be determined without modula-ting the phase difference of the secondary filtering means 18 by transmitting a second beam of light from source 12' through con-ditioning means 14', primary and secondary filtering means 16' and 18', sample 20, lens 34', pinhole 36' of pinhole stop 32' to infrared detector 38' and indicating and recording means 40'. In the latter embodiment, the secondary filtering means 18 and 18' each receive light having the same frequency range and are adjusted -~0368~
to transmit the light at two different sets of discrete frequen-cies, the first set of frequencies being shi$ted from the second set of frequencies by an amount no greater than the frequency spacing between adjacent absorptionllines of the preselected species, and preferably about 1/2 of such frequency spacing.
Other similar modifications can be made which fall within the scope of the present invention. It is, accordingly, intended that all matter contained in the above description and shown in the accompanying drawings be interpreted as illustrative and not in a limiting sense.
In operation, light 15, containing incoherent, infrared radiation, is collected, collimated and transmitted by light con-ditioning means 14 to primary filtering means 16. The primary filtering means 16 receives the light lS, selectively separates therefrom light 17 having a frequency range in the region of an absorption band for the preselected molecular species, and sends the separated light 17 to the secondary filtering means 18. The secondary filtering means 18 receives the light 17 and transmits light having a plurality of discrete frequencie~ which provides a detectable -qignal 30. The detectable signal is transmitted through gaseous material in sample 20, whereby the intensity of the signal changes in proportion to the concentration of the pre-selected species. A modulating means 26 operates to modulate the phase difference of the secondary filtering means so as to vary the intensity of the signal 30. The intensity variation of the signal 30 is detected by a phase sensitive detection means 28.
The resultant signal from the phase sensitive detection means 28 is displayed by the indicating and recording means 40.
Having thus described the invention in rather full detail, it will be understood that these details need not be strictly aahered to but that various changes and modifications may suggest themselves to one skilled in the art, all falling within the scope of the present invention as defined by the subjoined claims.

Claims (14)

I claim:

1. Apparatus for detecting and quantitatively measur-ing a molecular species of gaseous material in a sample to be analyzed, comprising:
(a) light source means for generating incoherent infrared radiation;
(b) light conditioning means for collecting, colli-mating and transmitting said light;
(c) primary filtering means adapted to receive said light and selectively transmit light having a frequency range in the region of an absorption band for the molecular species to be detected;
(d) secondary filtering means adapted to receive said filtered light and transmit light having a plura-lity of discrete frequencies forming a plurality of fringes which provide a detectable signal, said secondary filtering means having interference producing means for providing a plurality of trans-mission windows regularly spaced in frequency, the frequency spacing between adjacent windows being adjusted to equal substantially the frequency difference between adjacent spectral lines of the absorption spectrum for the molecular species to be detected, and scanning means for causing the transmission peaks for adjacent orders to coincide substantially with the spectral lines of said ab-sorption spectrum, whereby said detectable signal has an intensity substantially equal to the sum of said fringes;
(e) means for transmitting said detectable signal through said gaseous material, whereby the inten-sity of said detectable signal changes in proportion to the concentration of said molecular species;
(f) signal conditioning means for converting to measurable form said intensity change; and (g) detecting means for indicating the magnitude of said intensity change.

2. Apparatus as recited in claim 1, wherein said signal conditioning means includes modulating means for modulating the phase difference between interfering rays of said light so as to shift the frequency of each fringe, the modulating range being no greater than the frequency spacing between adjacent orders, and synchronous detection means for detecting the intensity variation of the signal, whereby the magnitude of the signal intensity change can be identified.

3. Apparatus as recited in claim 2, wherein said modu-lating means has a modulating range of about 1/2 the frequency width of each fringe.

4. Apparatus as recited in claim 3, including indica-ting and recording means for displaying said signal.

5. Apparatus as recited in claim 3, wherein said modu-lating means is a piezoelectric cylinder and said synchronous detection means is a phase sensitive detection system.

6. Apparatus as recited in claim 1 wherein said secondary filtering means is a Fabry-Perot interferometer.

7. Apparatus as recited in claim 1 wherein said second-ary filtering means is a solid etalon having temperature control means associated therewith for adjusting the optical path length thereof.

8. Apparatus as recited in claim 5 including means for applying to said cylinder a voltage having a square wave form, the limits of said voltage being adjusted so that the intensity of said detectable signal alternates between its maximum and minimum values, means for determining for each cycle of said voltage the difference in amplitude between said maximum and minimum values of said detectable signal to produce an electri-cal output signal proportional to the maximum and minimum values of the detectable signal.

9. Apparatus as recited in claim 7 wherein said solid elaton is composed of optically transparent material selected from the group consisting of potassium bromide, potassium chloride, lithium fluoride, magnesium fluoride, calcium fluoride, cesium bromide, cesium iodide, barium fluoride, sodium chloride and sodium bromide.

10. Apparatus as recited in claim 3, wherein said modu-lating means is a piezoelectric cylinder and said synchronous detection means is a lock-in amplifier.

11. A method for detecting and quantitatively measuring a molecular species of gaseous material in a sample to be analyzed, comprising the steps of generating light in the form of incoherent radiation;
collecting, collimating and transmitting the light;
filtering said 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 said 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 frequency difference between adjacent spectral lines of the absorption spectrum for the molecular species to be detected, and scanning said light to cause the transmission peaks for adjacent orders to coincide substantially with the spectral lines of said absorption spectrum, said signal having an intensity substantially equal to the sum of said fringes;
transmitting said detectable signal through said gaseous material, whereby the intensity of said detectable signal changes in proportion to the concentration of said molecular species; and detecting and indicating the intensity change of said signal.

12. A method as recited in claim 11, including the steps of modulating the phase difference between interfering rays of said light so as to vary the intensity of the signal, the modu-lating range being no greater than the frequency spacing between adjacent absorption lines of said molecular species.
13. Apparatus for detecting and quantitatively measuring a molecular species of gaseous material in a sample to be analyzed comprising:
(a) light source means for generating incoherent infrared radiation, said light source means being adapted to emit light having a frequency range in the region of an absorption band for the molecular species to be detected (b) light conditioning means for collecting, collimating and transmitting said light;
(c) filtering means adapted to receive said light and transmit light having a plurality of discrete frequencies forming a plurality of fringes which provide a detectable signal, said filtering means having interference producing means for providing a plurality of transmission windows regularly spaced in frequency, the frequency spacing between adjacent windows being adjusted to equal substantially the frequency difference between adjacent spectral

Claim 13 continued lines of the absorption spectrum for the molecular species to be detected, and scanning means for causing the transmission peaks for adjacent orders to coincide substantially with the spectral lines of said absorption spectrum, whereby said detectable signal has an intensity substantially equal to the sum of said fringes;
(e) means for transmitting said detectable signal through said gaseous material, whereby the intensity of said detectable signal changes in proportion to the concentration of said molecular species;
(f) signal conditioning means for converting to measureable form said intensity change; and (g) detecting means for indicating the magnitude of said intensity change.

14. Apparatus as recited in claim 13 wherein said light source means is a light emitting diode.