Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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
--7--
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:
--8--
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
--10--
~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
1036833
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
-15-
-
~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.
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