DE2700422A1 - METHOD AND DEVICE FOR MEASURING A TYPE OF GAS MOLECULE - Google Patents

Description

Dr. Hans-Heinrich Willrath t

Dr. Dieter Weber Dipl.-Phys. Klaus Seiffert

PATENT LAWYERS

D-62 WIESBADEN Jan. 4, 1977 P.O. Box ■■ 6145

Gustav-Freyug-Stra & e IS We / Wh

«(06ItI) 37I7JO

Telegram address: WILLPATENT Telex: 4-186 247

7OOO-1235

Allied Chemical Corporation, Morris town, New Jersey 07960, USA

Method and device rtli the In

feiZr Hess ii * fe / »er

Priority; Serial No. 647 388 dated January 8, 1976 in USA

Additional registration to P 23 40 862.O

(Patent) and P 25 25 787.8

(Patent)

Γ -] r. j '. j ■ '-. I.
* ·· ι , i
' ' 1 ν ■ ' r. 0 ν ί IO C.

The invention relates to the field of infrared gas analysis and more specifically, a method and apparatus by which light passes through a gas sample at discrete or isolated frequencies is allowed through or transmitted, which is associated with the absorption

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tion spectrum of a gas component are related in order to record the component and measure it quantitatively.

Conventional for non-dispersive infrared gas analysis The device used is a beam of infrared radiation with an emission spectrum which is the absorption spectrum of the gas to be analyzed, directed through a gas sample to a transmitter. The output signal from the transmitter it is compared with the signal which is generated when the beam passes through the series combination of the sample and a reference gas of the type selected for analysis is generated. A signal intensity differential which is determined by the absorption generated in the sample is converted to a detectable signal and displayed.

One of the main problems with such analyzers is the difficulty Analyze quantities of gas components that are present in the lower millionth range. The signal intensity differential represents a relatively small change in a large signal and is often caused by spectral interference between the absorption spectra of the constituent to be analyzed and the absorption spectra of the constituents present at the same time darkened or blurred. Another problem with such analyzers is the decreased resulting sensitivity, unless the temperature and pressure of the reference gas are carefully controlled. It was to alleviate these problems necessary the analyzers with extremely sensitive forms and combinations of detectors, sources, filters, control systems and the like, which are relatively expensive. For the reasons mentioned above, the gas analyzers of the

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described type low sensitivity and high running costs.

The present invention provides an apparatus in which light of the infrared frequency range passes through a gas sample is transmitted or transmitted at discrete frequencies which are related to the absorption spectrum of a type of molecule of the gas stand to record the species and measure them quantitatively. In short, the device assigns a light source Generation of incoherent, infrared radiation. A light conditioning device collects, collimates and transmits the light to a first filter device. This is set so that it receives the light and selectively transmits or transmits light that has a frequency range in the range of a Has absorption band for the types of molecules to be detected. A second filter device is for receiving the filtered Light is set up and transmits the light at a multitude of discrete frequencies forming multiple stripes, the provide a detectable signal. The second filter device has interference-generating devices for creating several transmission windows, which are arranged at regular intervals with respect to the frequency. The frequency spacing between neighboring ones Window is set so that it is substantially equal to the product of the frequency difference between adjacent ones Spectral lines of the absorption spectrum for the to be recorded

η ^is
Molecular type and the factor —j-, where η and n ¹ are integers and η is not equal to n ¹ . Under these circumstances, the interference generating device forms a comb filter. The second filter device also has scanning devices to the

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"'' 27-22

to cause transmission peaks for neighboring n-th orders, essentially with the spectral lines of such an absorption spectrum to collapse. Means are provided for the transmission or for the passage of the detectable Signal through the gas, the intensity of the detectable signal changing in relation to the concentration of the type of molecule. The change in intensity of the detectable signal is converted into a measurable form by a signal conditioning device, and their size is determined by a detector device shown.

The invention also provides a method for detecting and quantitatively measuring a type of molecule of a gaseous material in a sample to be analyzed, and this method consists in emitting light in the form of incoherent infrared radiation generated, the light is collected, collimated and transmitted, the light is filtered in such a way that selectively light with a frequency range in the range of an absorption band for the type of molecule to be detected is transmitted, this filtered Light is filtered interferometrically and light is transmitted at a multitude of discrete frequencies, with several stripes

erfen which provide a tangible signal by sending the light through a plurality of transmission windows which are regularly spaced with respect to frequency, the frequency spacing between adjacent windows being essentially equal to the product of the frequency difference between adjacent spectral lines of the absorption spectrum for the to Molecular type and the factor - ^ - is, where η and n 'are integers and η is not equal to n ¹ , and that

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Light is scanned in such a way that the transmission peaks of neighboring nth orders causes them to essentially coincide with the spectral lines of this absorption spectrum, this detectable signal having an intensity substantially equal to the sum of these fringes, the detectable signal is sent through the gaseous material, the intensity of the detectable signal being in proportion to the concentration the type of molecule changes, and the change in intensity of the signal is detected and recorded.

Various known filter devices can be adapted for use with the apparatus described above. Preferably the second filter device is a Fabry-Perot interferometer (FPI) with a mirror separation d that is so set is that filtered light is transmitted at a variety of discrete frequencies that coincide with echoing. This condition is obtained when d -, where d is the

sorption spectrum of a type of molecule of the gas

Mirror separation or separation of the FPI is, u is the refractive index of the medium between the mirrors, B is the molecular rotation constant of the type, η and n ¹ are integers and η is not equal to n '. For a given type of molecule, the rotation constant B is a unique quantity. Thus, the identification of the species with a particular absorption spectrum is good by setting the mirror separation of the FPI in such a way that the transmitted or transmitted discrete frequencies essentially coincide with the absorption lines of the molecular species to be detected. In an advantageous manner, the intensity of the detectable signal is not due to other types of molecules than

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27,422 AO

influences the species selected for recording and presentation or disturbed, and the intensity differential represents represents a relatively large change in a small signal. Spectral interference is kept to a minimum and no reference gas is required. The sensitivity of the device is increased, and very sensitive forms and combinations of detectors, sources, filters and control systems are not necessary. Consequently allow the method and the device according to the invention a more precise detection of gaseous components with less Effort as systems in which the emission spectrum of light passing through the sample contains frequencies continuously.

Other advantages, features and uses of the present Invention emerge from the present description in conjunction with the drawings. Show it Fig. 1 is a block diagram showing the device for

Detection and quantitative measurement of a type of gas molecule, FIG. 2 a schematic diagram of the device of FIG. 1, Figure 3 is a side view, partially cut away, showing means for modulating the secondary filter of Figs. 1 and 2,

Fig. 4 is a schematic diagram showing another embodiment of the device of Figs Fig. 5 shows the absorption spectrum of a particular type of molecule.

The following are preferred embodiments of the invention described. In Fig. 1, a preferred device for the detection and quantitative measurement of a gas molecule type is shown. The device shown generally at 10 has a light source 12 for generating light 15 with incoherent infrared radiation. A light conditioning device 14 collects, collimates and

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transmits the light 15 to a first filter device 16. This is set up in such a way that it receives light 15 and selectively transmits or transmits light 17 with a frequency range in the region of an absorption band for the type of molecule to be detected. A secondary filter device 18 is set up to receive the filtered light 17 and allows light to pass through at a large number of discrete frequencies, which form a large number of strips which provide a detectable signal 30. The latter is transmitted through a gas in the sample 20. A signal conditioning device 22 converts changes in intensity into measurable form which are caused in the signal 30 by the type of molecule in the sample 20. The size of the change in intensity is indicated by the detector device 24.

In particular, as shown in Fig. 2, the primary filter means 16 is a narrow band pass filter composed of a plurality of layers of dielectric thin films, and the second filter means 18 has interference generating means for providing a plurality of transmission windows which are regularly spaced in frequency are. In addition, the second filter device 18 has a sampling device for variably controlling the frequency of each order. The interference-generating device is set so that the frequency spacing between adjacent windows is essentially equal to the product of the frequency difference between adjacent spectral lines of the absorption spectrum of the molecular species to be detected and the factor - ^ V, where η and n ¹ are integers and η is not equal to n ¹ is. Under these circumstances, the detectable signal 30, which is from the second filter means

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18 was transmitted, an intensity that was essentially is equal to the sum of the strips. Further, the intensity of the signal 30 is not determined by any kind of molecule other than that specific selected affected or disturbed for the detection, hereinafter referred to as the selected type.

After the detectable signal 30 has passed through the gas in the sample 20, the intensity changes proportionally to the concentration of the preselected type. This change in intensity is converted into a measurable form by the signal conditioning device 22. The conditioning device 22 has a device 26 for modulating the phase difference between interfering Rays of the light transmitted by the second filter means 18 to the frequency of each thereby the strip that has passed through. The signal conditioner 22 also has a synchronous (i.e. phase sensitive) Detector device 28 for detecting the change in intensity of the signal 30, whereby the size of the change in intensity can be identified by the detector device 24.

Various known filter devices can be used as the second filter device in the device 10. Preferably the second filter device is a Fabry-Perot interferometer with a mirror separation or separation d, the to transmit or let through filtered light from the primary Filter device 16 is set at several discrete frequencies, which match the absorption spectrum of the preselected Kind of related. The transfer function of a FPI (I.) can be given by the Airy formula:

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I = Τ ² / ΐ + R ² - 2COS0 / " ¹ * I _Q , where T + R + A = 1, I _{Q is} the intensity of the incident light and the phase difference is 0, expressed as 0 = 4 1TuWd for rays that The symbols A, R and T represent the degree of absorption, reflection and transmittance of the FPI mirrors,, u is the refractive index of the medium between the FPI mirrors, d is the FPI mirror separation and LO is the frequency of the incident light, expressed in wave numbers. If cos0 = 1, transmission maxima occur for I. Consequently, 0 = 2ifm, where m assumes whole sizes and represents the order of interference In the description and in the claims referred to as the transmission window. For a special value of the mirror separation d, the FPI provides several transmission windows that are evenly spaced in frequency range) of the FPI is Δί = (2, ud). For a simple diatomic molecule such as carbon monoxide, the frequency spacing between adjacent absorption lines of the infrared rotational vibration absorption spectrum is approximately equal to 2B. By changing the mirror spacing d of the FPI, Af can be adjusted so that it is essentially equal to the frequency difference between adjacent spectral lines of the part or the entire absorption spectrum for the selected species. That is, the continuous scanning of the FPI in the vicinity of d - -: - ~ produces an absorption interf erogram with several stripes, which corresponds to a superposition of essentially all absorption lines of the preselected type.

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When Af = 2B, the transmission peaks of adjacent orders substantially coincide with adjacent spectral lines of the absorption spectrum to produce a one-to-one correspondence therewith, and the amplitude of the signal from gas sample 20 is a minimum. For values of Δί slightly different from 2B, the transmission peaks of adjacent orders do not exactly coincide with the absorption lines and the amplitude of the signal from the gas sample 20 is increased.

Other absorption interferograms are used for values of the interfer-

n '
rometer mirror distance d - -. —5—, where η and n ¹ are integers and η is not equal to n '. These absorption interferograms are generated when Δί = certain multiples of the rotation constant B. The main interferograms are generated when each absorption line coincides with a different transmission window of the FPI. Such main interferograms are obtained for values of an interferometer mirror separation ■ w η '

1 where η = 1 and n 'is an integer greater than 1

is. More specifically, for values of the interferometer mirror spacing d = n '/ (4, µB), where n ^{1 is} an integer greater than 1, the main interferograms are obtained. For example, with n '= 3, radiation is transmitted through the interferometer not only at frequencies corresponding to those of the neighboring absorption lines of the type of molecule to be detected, but also at two discrete frequencies in each case between a pair of the absorption lines. Secondary interferograms are obtained when every further absorption line or every third absorption line (etc.) coincides with the transmission peaks of the FPI. Such secondary

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270-22

Interferogranune is obtained for values of the interferometer mirror distance d ^ ₍ where η is an integer greater than 1 and η ¹ = 1. More specifically for values of the interferometer mirror from -— = -

Standes d = -j - = - , where η is an integer greater than 1.

the secondary interferograms are obtained. For example with η = 3 is radiation from the interferometer at frequencies accordingly that of every third absorption line of the type of molecule to be detected.

The use of the device 10 for infrared gas analysis can be explained 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 range from about 4.5 to 4.9, u, with the center of the band being about 4.66, u. This absorption band corresponds to transitions from the basic oscillation state (v = 0) to the first oscillation state (v = 1). As shown in FIG. 5, the absorption band consists of two branches: an "R-branch" corresponding to the torsional vibration transitions for which the rotational quantum number J changes by +1, and a "P-branch" which corresponds to torsional vibration transitions for the the rotational quantum number J changes by -1. The frequencies, in units of wave numbers, of the rotational transitions for the R and P branches are given by the following formulas: <*> _R = U _Q + 2B ₁ + (3B ₁ - B _Q ) J + (B ₁ - B _Q ) J ² , where J = O, 1, 2 ... and ίθ _ρ = CJ _Q - (B ₁ + B _Q ) J + (B ₁ -B _Q ) J ² , where J = 1, 2, 3. .. The order of magnitude «J, B _Q and B ₁ represent the absorption band center frequency, the ground state rotation constant or the rotation constant for the first oscillation state. The rotation constants B_ and B ₁ are represented by the following equations

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in relation: B = B ₁ + ⁰ ^ / ^where ^ _{e is} the torsional vibration interaction constant. The values for the rotational constants of carbon monoxide given in the list in the American Institute of Physics Handbook, 3rd Edition, pages 7-173 are:

Γ ¹ Γ ¹

-1

B ₀ = 1.9225 145 cm
B ₁ = 1.9050015 cm "

= 0.017513 cm

The intensity distribution for the R and P branches is given

2C, U.

by the equation: I, = —I ^s = - S _T exp / -B ^ J (J + 1) £ § /. Here-

aos ** p - -

in is C, a constant factor, Q_ is the rotation distribution function

dDS K

tion (- kT / hcB), U is the frequency (in wave numbers) of the individual

the

nen rotation-oscillation absorption lines, h Planck's constant, c the speed of light, k the Boltzmann constant, T the absolute temperature, and the line widths S _T are:

S _T = J + 1 for the R branch j

S _T = J for the P branch j

Using these equations for the line positions and intensities, a schematic representation for the CO absorption spectrum shown in Fig. 5 is established. The representation is schematic; because in fact every rotary absorption line has of the spectrum has a small but finite breadth.

To use a Fabry-Perot interferometer and discrete To provide light frequencies at the frequencies of the absorption lines of the band, it is necessary to have the effect of the non-periodic To determine the distance between the rotary absorption lines during operation of the device 10. For this purpose the Fabry

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- yf -

Perot interferometer adjusted so that the rotational absorption lines of the J = 6 and J = 7 R-branches exactly with two adjacent discrete frequencies from the Fabry-Perot interferometer coincide. These two lines of rotational absorption are the strongest lines in the band. Their frequencies are:

CJ _n (J = 6) = 2169.169975 cm " ^{1 row}

(a) _d (J = 7) = 2172.734796 cm " ¹

The wavenumber difference between these lines is 3.564821 cm. The free spectral range of the interferometer is set so that it is equal to this wavenumber difference between the 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 β _R -w _R (J + 1) ω _ρ (3) is calculated. The size Q. _ can be calculated as follows:

% = ω _κ (J ₊ I) - (J ₁₁ (J) = (3B ₁ -B ₀ ) -a _e [(J + l) ² ~ J ² ] = (3B ₁ -B ₀ ) -a _e ( 2Y + 1).

Therefore, the frequency difference between neighboring ones changes Rotational absorption lines in the R branch directly proportional to the rotational quantum number J and the torsional vibration interaction constant C ^ . The half-width A of the Fabry-Perot transmission window is given by the following equation;

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where R is the reflectivity of the Fabry-Perot mirrors and / ud is the optical path length between the mirrors. Assuming that the reflectivity R "^ 0.85, then A = 0.185 cm. The frequency mismatch at the ψ _ο (J = 5) line is 0.035 cm", which is well within the transmission half-width of the Fabry-Perot interferometer . The frequency mismatch in the V _R (J = 3) line is 0.210 cm ", which is only slightly larger than the FPI half- _{width. The frequency mismatch in the W n} (J = IO) line is 0.210 cm, which is also only slightly Therefore, the R-branch lines from J = 3 to J = IO substantially coincide with the discrete frequencies from the FPI and are therefore extremely effective in operating the device 10. The absorption line positions can be relative to the FPI The non-periodicity of the absorption line positions is given by the expression ζ4 (2J + 1) from the equation for i} _

A = a _e (2J _R + l) lP. Λ

Since 2-3 " ⁼ the free spectral range, this is set equal to the product of the periodic contribution in the equation for -Q _R , namely 3B ₁ - B _Q and the factor ~, - ^ (3B-, -B) = d _e (2j +1). If we solve this for J _R , we get.

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~ 27Π0Α22

A3

η (3Β - B), _R

j = 'P-H- (I- ^) - 1/2. The final value of the rotation constant B

^R ^ e "* ^{R e}

is given as B = B + ^ (v + 1/2), where B is the constant of rotation

Schwing is in the vth state. Therefore 3B., - B_ = 2B - 4 <<

_B 1 O ee

and J _D = Λ (^ - 2J (I ^ S) - 1/2. For CO is B, - 1.931271 cm " ¹ , ^R ^ e ^ ***

and assuming an FPI specular reflectance of 0.85, J _R = 5.6 n / n '- 0.5. Similarly, for the P-branch Λ = W (J + 1) - W _n (J) = - (B ₁ + B _Q ) - <* _e (2j + 1), and with the same reasoning we get J _p = -τ - (: τ— ~ 1) (^ 7 = :) ~ 1/2. Since B _Q / dL is much larger than 1, J _n - J _n . The values of J_ and J can be denoted by J. Therefore, the optimal band width for the first filter device 16 should be approximately 2BJ. and no greater than 4BJ. be.

e opt e opt

The value of J for the main interferograms, for example with n ¹ = 3, is 1.4. It is always possible to have the transmission window of the interferometer match with at least two absorption lines of the type of molecule to be detected.

For the main interferogram of CO with n '= 3, the interferometer transmits radiation through transmission windows corresponding to the frequencies of at least two absorption lines used for the analysis and also through two further transmission windows that are spaced at equal frequency intervals from and between the absorption lines intended for the analysis. In situations where the absorption lines of the gas to be analyzed are relatively narrow and in a frequency range that does not contain interfering absorption lines from other gases, the use of main interferograms of the type where n ¹ = 3 provides increased sensitivity. The increase in sensitivity is due to the better combination

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ao

men fit between the absorption line width and the widths of the transmission window of the interferometer caused. The decrease in sensitivity, which otherwise results from the presence of additional FPI transmission windows, is compensated for by the increase in sensitivity which is achieved by reducing the width of the FPI transmission window. The increase in sensitivity that is achieved in a particular situation depends on the selected value for n ¹ , which in turn is determined by the experimental conditions associated with the gas sample to be examined. It is much larger than that created by increasing the reflectivity of the FPI mirrors. The latter approach allows the width of the FPI transmission window to be narrowed without introducing additional radiation not absorbed by the gas and appears at first glance to be a better way of reducing the match between the absorption line width and the FPI transmission line width.

with

improve. In practice, however, mirror reflectivity increases the transferability of the FPI due to small absorption and / or scattering losses in the mirrored surfaces of the FPI. This decrease in portability leads to a Decrease in sensitivity greater than the loss of sensitivity is generated by the introduction of additional transmission windows discussed above. It also leads the use of lower reflectivity FPI mirrors with high transferability to a facility used for a greater number of experimental applications can be.

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27-022 U

For the secondary interferogranune of CO with η = 3 gets you get a value of 16 for size J .. This value for

opt

J shows that absorption of radiation transmitted by the FPI occurs over a range of frequencies comprising about 16 absorption lines contains. When using a secondary interferogram with η = 3, FPI transmission windows are added every third absorption line, so that absorption takes place at only 5 absorption lines. The usefulness of this secondary interferograms is impaired in cases where gas mixtures are to be analyzed. In such cases strong absorption lines from a gas other than the one to be analyzed can be observed interfere with the measurement for the gas to be analyzed. This disturbance can be reduced or eliminated by one selects a secondary interferomgram that does not contain radiation supplies at the absorption frequencies of the interfering gas.

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As noted above, the modulating device 26 modulates the phase difference 0, uir the intensity of the transmitted Signal 30 to change. In order to obtain the maximum modulated signal, the modulation range is set to about 1/2 of the frequency spacing set between adjacent strips. On the other hand, the modulating range can also be set to preselected Parts of the absorption spectrum of the preselected type can be restricted in order to increase the intensity of the modulated signal. Generally speaking, the modulation range should not be greater than the frequency spacing between adjacent absorption lines be of the selected type.

The resulting signal 30 from the second filter device 18 and the gas sample 20 is in the plane of the perforated diaphragm 32 focused through lens 34. The lens 34 is adjusted so that the center of the signal is located on the hole 36 will. The intensity of the part of the light 30 passing through the hole 36 is detected by an infrared detector 38. the phase sensitive device 28, such as a synchronous detector, is for receiving the signal from infrared detector 38 and set to record its change in intensity. The output signal of the phase-sensitive detector device 28, which represents the change in signal intensity, is represented by a display and recording device 40, which may include an oscilloscope and a card writer.

In Figure 3, the second filter device 18 and the modulating

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device 26 shown in greater detail. The second filter device shown is a Fabry-Perot interferometer (FPI), which is achieved by changing the phase difference 0 between interfering light beams is scanned in a conventional manner. Scanning methods, such as those in which the Gas pressure being changed between the mirrors of the FPI to change the optical path between them can also be used will. Consequently, the second filter device 18 shown in Figure 3 should be illustrative and not in the sense of Limitation to be interpreted. Such a device has cylindrical air bearings 56 and 58, usually at

2
operate about 2.11 kg / cm (30 psi) and collectively support a hollow metal cylinder 60 about 35 cm in length made of stainless steel or the like. The outside diameter of the cylinder 60 is ground to be centerless to about 4 cm. The inside diameter of the cylinder 60 is about 3.5 cm. Each air bearing 56 and 58 is about 8 cm long and has an outside and inside diameter of about 5 cm and about 4 cm, respectively. The separation between the centers of the air cylinders is about 20 cm. A mirror 62 of the second filter device 18 is fixedly attached to one end 64 of the cylinder 60, for example by means of a suitable adhesive or the like. The flat surface of the mirror 62 is substantially perpendicular to the translational axis of the cylinder. The other mirror 66 is fixedly attached to the modulator 42, as will be described below. Each air bearing 56 and 58 rests in precise V-blocks of a plate, not shown, which is treated to dampen external vibrations. Gefil-

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Third light 17 from the first filter device 16 enters the second filter device 18 at the end 68 of the cylinder 60 a. A carriage 70 made to move horizontally by means of a precision screw 72 and a fixed one Coupling arm 82 attached by mechanical fasteners such as screws 88, as well as on the cylinder 60, as will be described below, ensures that the cylinder 60 with the necessary linear movement second filter device 18 scans. The precision screw 72 is via a gear 76 on a digital stepper motor 74 coupled. The sampling rate or speed of the interferometer is controlled either by the The gear ratio of the assembly 76 is changed, e.g., by means of magnetic clutches or the like, or by change of the pulse rate input to the digital stepper motor 74. In the apparatus described, the sampling rate can be varied over a range of about 10 to 1 or more.

In order to precisely transfer the linear movement to the cylinder 60, For example, a collar or collar 78 is fixedly attached to the cylinder 60 with a glass plate 80 attached to the collar is glued on. The coupling arm 82 has a ball 86 made of stainless steel or the like, the one. Late 84 of the arm is assigned. A permanent magnet 90 is attached to the end 84 of the coupling arm 82 near the ball 86. Due to the magnetic attraction between the cuff

- 19 -

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27-022

as

78 and the magnet 90, the ball is in contact with the Glass plate 80 held. This creates a low friction point of contact. The touch that caused at this point of contact by the linear movement of the carriage 70 can either be adjusted thereby be that one changes the separation or separation between the magnet 90 and the sleeve 78, or the Magnet 90 strength decreased.

A sectional view of an embodiment of the modulating device 26 is shown in FIG. Other embodiments of modulators 26 can also be used. The modulating device 26 preferably has a hollow cylindrical body 92 made of piezoelectric ceramic. the inner and outer walls 94 and 96 of the cylindrical body 92 are coated with an electrically conductive material such as e.g. Silver or the like coated. Insulating bodies made of an insulating material such as ceramic or the like 98 and 100 are attached to the cylindrical body 92 at its ends 102 and 104, respectively, by a suitable adhesive, such as epoxy resin. A mirror 66 is firmly attached to the insulating body 98 by means of the adhesive used, to secure the mirror 62 to the end 64 of the cylinder 60. So that that mirror 66 is parallel to mirror 62 can be held, the insulating body 100 is glued to the surface 106 of the holding body 108. The outer surface 100 of the holding body 108 has a plurality of differential micrometer screws 112, which are connected to the surface and

- 2o -

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can be adjusted in a conventional manner to a to provide precise angular alignment of the mirror 66. Electrodes are attached to the inner wall 94 and the outer wall 96, respectively 114 and 116 attached. A voltage with a waveform, such as a sine wave or a square or Square wave, is applied to electrodes 114 and 116 from a high voltage, low current power source 101. After the voltage is applied, the cylindrical body 92 ν is made to modulate in a linear direction, Wodirch the intensity of the signal 30 is changed. When the from the power supply unit 101 to the electrodes 114 and 116 applied voltage has the shape of a square wave, the voltage limits of the waveform can be adjusted be that the intensity of the signal 30 alternates between its maximum and minimum values. A synchronous one Detector device is used to determine the amplitude difference between the maximum and minimum values of the signal 30 is provided for each period of the square wave to produce an electrical output signal which is proportional is the maximum and minimum values of signal 30. Consequently, the accuracy of the detection device and hence the sensitivity of the device 10 is increased by a factor of the order of 100 or more.

The device 10 described here can of course can be modified in numerous ways without departing from the spirit of the invention. For example, can the second filter device 18 is a fixed etalon (etalon)

- 21 -

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which is tuned by controlling its temperature. One type of solid etalon which is suitable consists 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, the transmission properties in the frequency range of the absorption band of the selected type. In addition, such an etalon has opposite Surfaces that are polished, flat, parallel and covered with silver, dielectric material or the like for a high reflectivity in a preselected frequency range are coated. For a pre-selected type, such as carbon monoxide, with an absorption spectrum in the frequency range from about 2050 to 2250 wave numbers have preferred optical transparent Materials potassium bromide, lithium fluoride and magnesium fluoride id on. The thickness of the solid etalon can be selected so that the free spectral range of the etalon is approximately the frequency difference between spectral components of the given absorption spectrum. Fine-tuning the solid Etalons are effected by providing a means for controlling temperature and, consequently, the optical path length the same, around the transmission peaks of adjacent orders coincide with the components of the given absorption spectrum allow. Lenses 14 and 34 can be replaced by parabolic mirrors (not shown) with offset axes in order to achieve the optical To improve the permeability of the device 10. As shown in FIG. 4, the signal intensity can change without modulating the phase difference of the second filter device 18

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be determined by taking a second beam of light from source 12 'via conditioning device 14', primary and secondary filters 16 'and 18', sample 20, lens 34 ', hole 36 * of the aperture plate 32' to the infrared detector 38 ' passes or transmits and leads to the display and recording device 40 ·. In the latter embodiment takes the second filter means 18 and 18 · light with the same frequency range, and they are set so that They let the light through in two different groups of discrete frequencies, the first group of frequencies out of the second Frequency group is shifted by an amount that is not greater than the frequency spacing between adjacent absorption lines of the preselected type and preferably about 1/2 this frequency spacing. Other similar modifications can be made which fall within the scope of this invention.

During operation, light 15 is emitted with incoherent infrared radiation

-elt, collectively collimated by the light conditioning device 14 and transmitted to or transmitted to the first filter device 16. The first filter device 16 receives the light 15, selectively separates therefrom the light 17 with a frequency range in the range of an absorption band for the preselected type of molecule and sends the separated light 17 to the second filter device 18 Frequencies through, whereby a detectable signal 30 is provided. This is passed through the gas sample 20, whereby the intensity of the light changes proportionally to the concentration of the preselected type. A modulating device 26 modulates the phase difference of the second filter

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means to vary the intensity of the signal 30. The change in the intensity of the signal 30 is determined by a phase-sensitive Detector device 28 detected. The resulting signal from the phase-sensitive detector device 28 is by the display and recording device 40 shown.

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Claims (5)

Claims 1. ) Method for the detection and quantitative measurement of a type of gas molecule in a sample to be analyzed by generating light in the form of incoherent radiation, collecting, collimating and transmitting the light, filtering the light for the selective transmission of light with a frequency range in the range of a Absorption band for the type of molecule to be detected, interferometric filtering of the filtered light, transmission of the light at several discrete frequencies with the formation of stripes or lines that result in a detectable signal by passing the light through several transmission or Transmission window directs, transmission of the detectable signal through the gaseous material, the intensity of the detectable signal changing proportionally to the concentration of the type of molecule, and detection and display of the change in intensity of the signal according to patent (patent application P 23 40 862.0) and patent (Paten registration P 25 25 787.8), characterized in that transmission windows are used in which the frequency spacing between adjacent windows is essentially equal to the product of the frequency difference between adjacent spectral lines of the absorption spectrum of the type of molecule to be detected and the factor -τ-, where η and n ^{1 are} whole numbers and η is not equal to n ¹ , and that the light is scanned in such a way that the transmission peaks for neighboring nth orders are essentially combined with the spectral lines of the absorption spectrum. 709833 / 0S72 X 27-22 drops, the signal having an intensity substantially is equal to the sum of the stripes or lines. 2. Device for performing the method according to claim 1 with an incoherent infrared radiation generating device, possibly light with a frequency range in the range of an absorption band for the type of molecule to be detected emitted, a light conditioning device for collecting, collimating and transmitting the light, in the event that the Light source emits light with a frequency range in the range of an absorption band for the type of molecule to be detected, a filter device and otherwise with a primary filter device, which is transmitted from the light conditioning device Receives light and light with a frequency range in the range of an absorption band for the type of molecule to be detected transmits to a secondary filter device, the filter device or secondary filter device this Absorbs light with a frequency range in the range of an absorption band for the type of molecule to be detected and light transmits with several discrete frequencies forming several stripes or lines, which result in a detectable signal, and wherein the filter device or secondary filter device comprises an interference generating device comprising a plurality of provides transmission windows arranged at regular intervals in terms of frequency, a device for transmitting the detectable signal through the gaseous material, the intensity of the detectable signal being proportional to the concentration the type of molecule changes, a signal conditioning device for converting the change in intensity into measurable ones 709833/0572 3 27-0422 Form and a detector device for displaying and recording the magnitude of the change in intensity, characterized in that the frequency spacing between adjacent windows is set in the transmission windows so that it is essentially equal to the product of the frequency difference between adjacent spectral lines of the absorption spectrum for the type of molecule to be detected and the Factor - ^ - is, where η and n ¹ mean integers and η is not equal to n ¹ , and that a scanning device is provided which allows the transmission peaks for adjacent n-th orders to coincide essentially with the spectral lines of the absorption spectrum, the detectable signal has an intensity which is substantially equal to the sum of the stripes or lines. 3. Apparatus according to claim 2, characterized in that the signal conditioning device (22) is a modulating device for such modulating the phase difference between interfering Rays of light has that the frequency of each strip or each line is changed, wherein the modulation range is not greater than the frequency spacing between adjacent orders. and that a synchronous detector device (28) for detecting the change in intensity of the signal to identify the size of the change in signal intensity is provided. 4. Apparatus according to claim 2 and 3, characterized in that the frequency spacing between adjacent windows of the interference-generating Device is set so that it is essentially equal to the product of the frequency difference between adjacent spectral lines of the absorption spectrum for the type of molecule to be detected and the factor - _r is, where n ^{1 is} an integer greater than 1. 5. Apparatus according to claim 2 and 3, characterized in that the frequency spacing between adjacent windows of the interference-generating device is set so that it is essentially equal to the product of the frequency difference between adjacent spectral lines of the absorption spectrum for the type of molecule to be detected and the factor - _r where η is an integer greater than 1 and n ¹ is 1. 709833/0572