U.S. patent application number 13/141380 was filed with the patent office on 2012-01-12 for method and device for measuring the concentration of substances in gaseous or fluid media through optical spectroscopy using broadband light sources.
Invention is credited to Nils Damaschke, Martin Degner, Hartmut Ewald, Elfred Lewis.
Application Number | 20120006098 13/141380 |
Document ID | / |
Family ID | 41664703 |
Filed Date | 2012-01-12 |
United States Patent
Application |
20120006098 |
Kind Code |
A1 |
Degner; Martin ; et
al. |
January 12, 2012 |
Method And Device For Measuring The Concentration Of Substances In
Gaseous Or Fluid Media Through Optical Spectroscopy Using Broadband
Light Sources
Abstract
The invention relates to a method and to a device for using
partially non-stabilized broadband light sources to accurately
measure partially broadband-absorbing substances using referencing
measuring cells. In order to create a low-cost, high-resolution,
and at the same time fast spectrographic device for measuring
concentrations of substances in fluid or gaseous media that is also
suitable for harsh environments, the light radiated by the
broadband light sources (1) through light guiding optical systems
is fed through the measuring section of the self-referencing
measuring cell (20, 30, 40) or only partially through a measuring
cell (10) to a measurement detector (photoreceptor 11) and
partially through a reference path (optical waveguide 8) to a
reference detector (photoreceptor 15), and a mode coupler (5, 9,
14) is associated with each optical waveguide (2, 4, 7, 8) in order
to homogenize the radiation characteristic of the broadband light
sources (1), which varies over time and space.
Inventors: |
Degner; Martin; (Kropelin,
DE) ; Ewald; Hartmut; (Rostock, DE) ;
Damaschke; Nils; (Bargeshagen, DE) ; Lewis;
Elfred; (O'Brians's Bridge, IE) |
Family ID: |
41664703 |
Appl. No.: |
13/141380 |
Filed: |
December 10, 2009 |
PCT Filed: |
December 10, 2009 |
PCT NO: |
PCT/EP2009/066840 |
371 Date: |
September 21, 2011 |
Current U.S.
Class: |
73/31.05 |
Current CPC
Class: |
G01N 21/3504 20130101;
G01N 21/31 20130101; G01N 21/33 20130101; G01N 21/274 20130101;
G01N 21/0303 20130101; G01N 2201/0631 20130101; G01J 3/02 20130101;
G01J 3/0297 20130101; G01N 2201/08 20130101; G01N 2201/062
20130101; G01N 2021/0307 20130101; G01J 3/42 20130101; G01N
2201/0668 20130101 |
Class at
Publication: |
73/31.05 |
International
Class: |
G01J 3/42 20060101
G01J003/42 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 12, 2008 |
DE |
10 2008 064 173.1 |
Claims
1. A method for determining the concentration of substance(s) in
gaseous or fluid media using optical absorption spectroscopy with
broadband light sources, wherein the light of an optical
fibre-coupled broadband light source is guided by way of an optical
wave guide and a mode mixer after the light source directly via an
optical path to a free path-absorption measurement cell, is
partially absorbed therein and guided via an optical wave guide and
mode mixer after the measurement cell to a measurement detector,
and that the influences of the emission characteristics of the
broadband light source are referenced via a reference detector
which is connected via an optical wave guide coupler, a mode mixer
and an optical wave guide directly to the optical fibre-coupled
broadband light source, wherein the measurement detector and the
reference detector may be the same.
2. The method of claim 1, wherein for referencing the light source
and/or for determination of the concentration of further substances
several spectral different broadband optical fibre coupled light
sources are added to the optical path via mode mixer and optical
wave guide coupler, and the emission of the light sources are
separated from each other at the detector and reference unit.
3. The method of claim 1, wherein a part or the whole optical wave
guide outside the measurement cell is realized via free path
optics.
4. The method of claim 1, wherein instead of a reference detector a
self referencing measurement cell is used, where the path length of
the interaction between the irradiated light and the measuring
media is varied and thus a reference signal is generated, whereby
disturbances within the emission characteristics can be
suppressed.
5. A device for determining the substance concentration in gaseous
or fluid media via optical absorption spectroscopy with broadband
light sources according to the method of claim 1, wherein the light
of an optical fibre-coupled broadband light source (1) is guided by
way of optical wave guide (2) and mode mixer (3) after the light
source directly via an optical path with mode mixer (5) and optical
wave guide (6) into the free path absorption measurement cell (7),
it is partly absorbed therein, and is guided by way of an optical
wave guide (9) and mode mixer (10) after the measurement cell (7)
to a measurement detector (11), and that the influences of the
emission characteristics of the broadband light source (1) are
compensated via a reference detector (13) which is connected via an
optical wave guide coupler (4), mode mixer (14) and optical wave
guide (12) to the optical fibre-coupled broadband light source (1),
wherein the measurement detector (11) and the reference detector
(13) may be the same.
6. The device of claim 5, wherein for referencing the light source
and/or for determining the concentration of further substances
several spectrally different broadband optical fibre coupled light
sources (1A, 1B, . . . ) are added to the optical path via mode
mixer and optical wave guide (19), and the emissions from the light
sources are separated from each other at the detector unit and
reference unit.
7. The device of claim 5, wherein a part or the whole optical wave
guide outside the measurement cell (7) is realized in free path
optics.
8. The device of claim 5, wherein instead of the reference detector
a self referencing measurement cell (16) is installed, wherein the
path length of the interaction (17) between the irradiated light
and the media to be measured is varied and thus a reference signal
is generated, whereby disturbances within the emission
characteristics can be suppressed.
9. The device of claim 8, wherein the self referencing measurement
cell is realized as an inclining or rotating glass plate (33),
whereby the interaction path length of the measurement media is
varied.
10. The device of claim 8, wherein at the end of the measurement
cell a concave mirror is implemented as reflector.
11. The device of claim 8, wherein the light coupled into the self
referencing measurement cell (40) is partly reflected by a half
mirror (42) to a full mirror (46), and an additional rotor
propelled by the flow of the media and located within the optical
path between the half and the full mirror (42 and 46, respectively)
temporarily interrupts the reflection of the full mirror (46), and
thereby the interaction path length and thus the measurement signal
for referencing is varied.
12. The device of claim 8, wherein the strength of the interaction
is varied via the pressure, and the pressure is detected via a
sensor (18) or defined by way of an actuator.
Description
[0001] The invention relates to a method of referencing in the
optical absorption spectroscopy using broad band light sources for
determining the concentration of substance in gaseous or fluid
media through and to a device for measuring the concentration of
substance in gaseous or fluid media within the measurement path of
a measurement cell using absorption spectroscopy of light emitted
from broad band light sources via light guiding optics. The device
is used among other things for measuring carbon monoxide (NO),
carbon dioxide (NO.sub.2), suflur dioxide (SO.sub.2), ozone
(O.sub.3), as well as components in fluid media and others, for
combustion engines, especially in the online monitoring of diesel
combustion engines, in environmental measurement technique, in
medical technology, for instance for the measurement of respiratory
air and others.
[0002] The determination of the concentration of substances using
spectroscopic methods via broadband light sources and spectral
selective detectors, such as spectrometer of filtered optical
detectors, is well known. Further more it is state of the art, to
guide spectral selective sources, such as Laser or filtered
broadband light sources, via a measurement path to a filtered or
unfiltered detector, to thereby characterize for instance gases or
fluids. A logical conclusion of that is the utilization of already
spectral limited LED-light sources with and without optical
filters. LEDs are to understand here as broadband light sources,
because in opposite to narrow line with sources (such as Laser)
they emit a comparable broad frequency spectrum. The utilization of
light guiding optics such as optical wave guides (LWL), for
mechanical and thermal decoupling respectively for the spatial
separation of the measurement place and the source and receiving
unit is also well known in the sensor technology.
[0003] The basic measurement principle of the optical spectroscopy
is based on the measurement of light extinction that has passed a
measurement cell. The inference to a defined substance
concentration in the measurement cell is therewith only an indirect
method. A reliable measurement can be realized due to the usage of
additional so called reference wavelength, whereby the spectral
characteristic of the substance is utilized. These issues result
for example in a measurement setup that is shown in FIG. 5 in the
report of M. Degner and H. Ewald "Low cost sensor for online
detection of harmful diesel combustion gases in UV-VIS region"
[SPIE Photonics Europe 2006, Photonics in the Automobile 11, ISBN
0-8194-6254-3, FR Strasbourg, April 2006]. In addition to broad
band light sources very often tuneable Laser sources are used in
spectroscopy to reach high resolution. An absorption band of the
requested substance is there scanned with the Laser line. Here the
intensity of the detected light outside the absorption band is used
as a reference for the intensity at the place of the absorption
band, because the intensity is attenuated in the range of the
substance absorption band.
[0004] The disadvantage here is, that on the one hand a high
concentration resolution can be realized using the laser
spectroscopy, on the other hand the number of detectable substances
is limited due to the availability of an adequate Laser light
sources at the required interaction wavelength of the substance. In
addition such arrangements often are cost intensive, less robust
and therewith not suitable for mass production in the field of
sensors.
[0005] The implementation of broadband light sources in combination
with spectrometers leads likewise to cost intensive and in addition
not very sensitive measurement arrangements. In this case the
emission spectrum of the light broad band light source is compared
with the spectrum after the light pass through the measurement
cell. Filtered broadband band light sources and especially LEDs are
in opposite to that a more cost effective alternative. The general
problem of devices with broadband light sources is the spectral and
temporal changing of the light intensity respectively the emission
characteristics, whereby the resolution and especially the maximum
reachable accuracy is strongly limited. In addition high
measurement times are required to reach that high resolution
because of the resulting limited optical power density (except some
very specified LEDs).
[0006] Therefore the invention is based on the problem to provide a
low cost, high resolution and at the same time fast spectroscopic
method for the determination of the concentration of substances in
gaseous or fluid media as well as a device for implementation of
the method that is, respectively that are, robust versus exterior
influences.
[0007] The solution to this problem is obtained, according to the
characterizing features of the method claim, in that the light
emitted by the broadband light sources is guided partially through
the measurement path of a self referencing measurement cell to a
measurement detector, and only partially through a reference path
to a reference detector, wherein the measurement path and the
reference path are partially identical, and the influences of the
emission characteristics of the broadband light sources and of the
mode effect of the optical components are avoided by way of mode
couplers in the light paths. According to the characterizing
features of the device claim the solution to this problem is
obtained in that the light emitted by the broadband light sources
via light guiding optics is guided through the measurement path of
a self referencing measurement cell, or only partially through a
measurement cell to a measurement detector and partially via a
through a reference path to a reference detector, and that for
homogenizing the temporally and spatially varying emission
characteristics of the broadband light sources a mode coupler is
referenced to the light guide optics and to the light paths,
respectively. The Mode couplers should be dimensioned in a way,
that attenuation or scattering is as low as possible. There are
typically cost effective spectral selective broadband light sources
used, its light is guided via a light guiding system into the
measurement path and is spectral selective evaluated.
[0008] One problem with commonly used beam splitters in the
spectroscopy is the limited stability of the accuracy of the ratio
of the resulting beams. This is caused by the temporal changing of
the spatial fluctuation of the emitted light intensity. The
currently splitting of the light is there depending from the
actual, temporal changing inhomogeneous emission characteristic of
the broadband light source. According to the invention the
homogenisation of the emission characteristic is realized by the
use of the mode couplers. The afterwards splitting of the light is
therewith no more depending on the fluctuating intensity allocation
of the light source. The irradiated light principally "forgets"
where it comes from. In principle also scattering plates (milk
glass, diffusor) can be used but the attenuation of light intensity
would be quite high here. According to the invention the mode
coupling is realized within the mode couplers inside the light
guiding optics respectively in the light paths. Therefore optical
wave guides are well suited because these mode couplers can be
integrated there. There are different realisation possibilities
such as using a long fibre, taper, more dimensional bending
coupler. By using mode couplers an efficient optical component for
homogenization of the emission characteristic is utilized.
[0009] According to the invention the resulting temporal
fluctuations of the broadband light source, especially LEDs, are
compensated by use of a well suited reference arrangement, that
means: The disturbances due to the measurement arrangement and the
surrounding are compensated according the target value. Thus the
measurement certainty is much bigger and especially there through
higher measurement accuracy/resolution is achieved.
[0010] Due to the utilization of the mode coupler, realized for
instance as a ring coupler and the fibre coupler for light
separation a source independent, robust and thus exact referencing
is feasible to for instance measure gas concentration below 1 ppm
at a path length of some centimeters at a measurement time of some
milliseconds.
[0011] Furthermore the invention uses another embodiment utilizing
a self referencing measurement arrangement to reach on that way the
wanted accuracy. Also here the coupling and mode mixing of the
separated light sources with the help of fibre optics are used to
reach for all wavelengths preferably the same path through the
optical measurement cell. Therewith disturbances within the cell
e.g. on the absorption and reference wavelength are effecting in
the same way. In opposite to the previous described measurement
setup here there is no second receiving channel for referencing
required. Instead of this the effective absorption path length of
the measurement cell is changed (self referencing) and therewith
the measurement signal is modulated in a defined manner as it can
not be caused by disturbances. The due to the receiving part
detected .signal sequence can adequate be demodulated. There
through at least to signal are generated, that can be used for
referencing of each single wavelength. The advantage of this
arrangement is that the whole optical path outside the measurement
cell is identical and it is only within the measurement due to the
modulation modified. There through only one receiving unit is
required, there is no need for two identical receivers. Thus
receiving disturbances influence the reference and the measurement
signal in the same way. The disadvantage here is the higher
complexity of the measurement cell design.
[0012] In an example arrangement due to active switching or
inclining or rotating of a small plate of glass within the
measurement cell the path length through the detectable media can
be varied and thus a reference due to the measurement volume can be
realized. Through the possible usage of a concave mirror as a
reflector also wavelength depending disturbances of dispersion are
reduced.
[0013] In a further example arrangement of the self referencing
measurement cell a part of the light is reflected directly to the
receiver at a first mirror. A second path is transmitted and
reflected to the receiver at a second mirror in dependency of the
orientation of a rotor. This rotor is propelled for instance due to
the flow of the measurement media and modulates therewith the
effective path length. Also here there is an important advantage
compared to conventional reference arrangements due to the
referencing within the measurement cell that enables disturbances
are acting on the reference and measurement path in the same way
where through a strong suppression of disturbances is realized.
[0014] Especially for gaseous media a further kind of referencing
can be realized, through converting the indirect measurement
principle of the optical spectroscopy into an other direct sensor
effect. Through an additional modulation of the measurement
variable for instance through variation of pressure, the volume
concentration of the gas is changed and therewith the detected
extinction measurement values of the single sources. Is the
variation of this additional physical effect for instance due to
the use of conventional sensors (e.g. pressure) simultaneously
detected, so the effect of the modulation can be used to reference
the whole system very accurately, disturbances are suppressed by
this way and the real concentration values at normal pressure are
estimated. In many systems pressure changing is inert, thus this is
a very simple and effective method for self referencing of a
measurement cell.
[0015] As one example of the above described methods for
referencing and for the realization of low cost, precise
spectroscopic sensors, the exhaust gas sensor for combustion
processes respectively engines on the basis of novel UV-LEDs is
mentioned here. This sensor is also constructively thought and
probed for the usage in extremely rough environment, such as the
exhaust channel of a car (amongst others high temperature,
vibration, chemical aggressive media).
[0016] The device for measuring of substance concentrations in
gaseous or fluid media according the invention is explained in
detail in the following with the help of embodiments shown in the
drawings. It is shown:
[0017] FIG. 1: a principle presentation of the first
embodiment,
[0018] FIG. 2: a principle presentation of the second and third
embodiment,
[0019] FIG. 3: a principle longitudinal section through one self
referencing measurement cell according to the second in FIG. 2
shown embodiment using a inclining small plate of glass and
[0020] FIG. 4: a principle longitudinal section through a second
self referencing measurement cell according the second in FIG. 2
shown embodiment.
[0021] The in FIG. 1 shown first embodiment of a device for
measurement of substance concentration in media includes 1 . . . n
spectral selective broadband light sources 1, for instance LEDs,
their light is guided via light wave guides 2 to a first fibre
coupler 3, it is mixed and coupled to one light path there. One to
the first coupler 3 connected wave guide 4 is via a mode coupler 5
connected to a second to a second fibre coupler 6 (e.g. 75/25).
From this two further wave guides 7 and 8 start, the wave guide 7
leads via a mode coupler 9 to a measurement cell 10 that contains
the measurement media, further to a first photo detector 11, to an
A/D-converter 12 and than to a controller 13, e.g. to a .mu.C or a
DSP. The second wave guide 8 leads the light of the source via a
mode coupler 14 to a second photo detector 15, an A/D-converter 16
and than to the mentioned controller. Through an adequate signal
analysis e.g. referencing through compare of the both receiving
channels, a fibre optical referencing is realized. Thus a very
precise concentration measurement of the substances within the
measurement path of the measurement cell 10 is realizable. The
electronic control of the spectral selective light sources occurs
via the mentioned controller via the controller path 18, this is
advantages for a synchronous control and signal acquisition
according high measurement rates.
[0022] According the invention the homogenization of the emission
characteristics of the 1 . . . n spectral selective light sources 1
occurs through the utilization of the mode coupler 5, 9, 14. The
following splitting of the light therewith is independent of the
fluctuating intensity allocation of the light sources 1. The
irradiated light quasi "forgot" where it comes from. Here the mode
coupling is implemented via the mode coupler 5, 9, 14 within the
optical wave guides 2, 4, 7, 8. Therefore there are different
realization possibilities such as the use of long fibres, taper,
more dimensional bend coupler and others are usable. With the mode
coupler 5, 9, 14 an efficient optical component for homogenisation
of the emission characteristics is used.
[0023] The fluctuations of the filtered spectral selective
broadband light sources 1 . . . n, especially LEDs, are compensated
with the help of the reference arrangement, consisting of the light
path from the wave guide 8, with mode coupler 14 and the second
photo receiver 15 as well as the associated A/D-converter 16 with
adequate signal analyses in the controller 13. Disturbances from
the measurement arrangement and the surrounding are therewith
suppressed versus the target value. Therefore the measurement
reliability is substantially higher and in particular it is the
reason to achieve a better measurement precision and resolution. By
using of mode couplers 5, 9, 14, implemented e.g. as ring-couplers,
and the fibre-couplers 3, 6 for splitting the light, a robust and
therewith exact referencing is feasible which is independent from
the source, to enable for example the measurement of gas
concentrations with a resolution of less than 1 ppm at an
absorption path length of few centimetres at a measuring time of a
few milliseconds.
[0024] Second embodiment shown in FIG. 2 includes in the same way
as in first embodiment shown in FIG. 1 the 1 . . . n spectral
selective broadband light sources 1, the optical wave guides 2, the
fibre coupler 3, the mode coupler 5, the wave guides 4, 7, the
measurement cell 20 including the measuring path, the photo
detector 11, the A/D-converter 12 and the controller 13. In
opposite to the first embodiment regarding FIG. 1 the fibre optical
splitting of the light with the reference path is missing here.
[0025] In this case the measurement 20 cell is self referencing
realized; therefore the light path for instance regarding the
presentation in FIG. 3 respectively FIG. 4 or according the third
embodiment is modulated. For the detection of the modulation
regarding the last mentioned kind of modulation, the sensor 21 with
the measurement connection 19 is implemented and sends in the same
way as the photo detector measurement values to the controller.
Within the controller an adequate demodulation of the measurement
signals is realized, its results are used for the referencing.
[0026] In the second embodiment according FIG. 2 is thus a self
referencing measurement cell arrangement planed to reach a source
independency and thus the target resolution. It also uses the
coupling within the fibre coupler 3 and the light mode mixing of
the single light sources 11 utilizing fibre optics in the mode
coupler 5 to reach nearly the same optical path for all wavelengths
through the measurement cell 20. Therewith disturbances on for
instance the absorption--and reference wavelength within the
measurement cell 20 result in the same way.
[0027] In opposite to the first embodiment according FIG. 1 here is
no light splitting and no second receiving optical channel required
for referencing. Instead of this the effective absorption path
length of the measurement cell is modified (self referencing) and
therewith the measurement signal is modulated in a defined way, as
can not be affected by disturbances. The using the receiving unit
detected signal sequence can be adequate demodulated. There through
at least two signals are generated that are utilized for the
referencing of each single wavelength. The advantage of this
arrangement is that the whole optical path outside the measurement
cell 20 is identical and it is only within the measurement cell 20
modified due to the modulation. There through only one receiving
unit is required, there is no need for two identical receivers.
Thus receiving disturbances influence the reference and the
measurement signal in the same way. The disadvantage here is the
higher complexity of the measurement cell design 30, 40.
[0028] Especially for gaseous media a further kind of referencing
can be realized, through converting the indirect measurement
principle of the optical spectroscopy into an other direct sensor
effect, by using a measurement cell as it is described in the
application example 1. Through an additional modulation of the
measurement variable for instance through variation of pressure,
the volume concentration of the gas is changed and therewith the
detected extinction measurement values of the single sources. Is
the variation of this additional physical effect for instance due
to the use of conventional sensors 21 (e.g. pressure) detected, so
the effect of this modulation can be used to reference the whole
system very accurately, disturbances are suppressed by this way and
the real concentration values at normal pressure are estimated. In
many systems pressure changing is inert, thus this is a very simple
and effective method for self referencing of a measurement
cell.
[0029] The further embodiment according FIG. 3 shows the
measurement cell 30, where the target substances are included, with
in- and outlets 31, 32 for the from the light sources 1 emitted
light that is guided via the optical wave guide 4 and the mode
coupler 5, the measurement cell 30, the optical wave guide 7 to the
photo detector 11. Within the measurement cell 30 a glass plate
respectively a small glass plate 33 is integrated, due to its
inclining or rotating the effective wavelength through the
measurement media within the measurement cell 30 is
switched/varied. In position II of the little glass plate 33 the
light passes along the path shown by the arrow 38. In the position
I of the glass plate 33 the light moves along the arrows 39 also
via the in- and outlets 35, 36. Thus the reference is realised by
the interaction with the media in the measurement volume of the
measurement cell 30. It is an advantage, that the light has passed
on both ways the same optical components--in position I and II.
Through the usage of a concave mirror 34 as a reflector at the
right side of the shown measurement cell 30 (FIG. 3) wavelength
depending disturbances of dispersion are minimized. Also a plane
mirror could be used here; in this case the media bordering glass
plate 37 would be exchanged by a convex lens.
[0030] In the embodiment according FIG. 4 of the self referencing
measurement cell 40 one part of the light, that is coupled into the
cell via the inlet 31 and collimated at the collimating lens 41
into direction 45 is reflected at the half transmission mirror 42.
This reflected light passes again the collimating lens 41 and is
guided to the outlet 32 and finally to the receiving unit 11, 12,
13. A second part of the irradiated light passes according the
direction 46 the half transmission mirror 42 and reaches the second
measurement volume II. In dependency of the rotor 44 orientation in
the optical path, this mentioned part of the light is reflected at
a full mirror 43 (in FIG. 4, right) and reaches according 47 the
outlet 32 respectively the receiving unit 11, 12, 13.
[0031] The rotor 44 is propelled for instance by the flow of the
measurement media that is passing the measurement cell 40 in the
measurement volume II in the direction of the arrows 48. The blades
of the rotor 44 are used to interrupt the light transmission and
therewith it modulates the effective wavelength of the light
respectively the interaction strength of the light with the media.
Within the measurement volume I and II of the measurement cell 40
there is here the same measurement media included.
[0032] As one example of the above described methods for
referencing and for the realization of low cost, highly precise
spectroscopic sensors, the exhaust gas sensor for combustion
processes respectively--engines on the basis of novel UV-LEDs is
mentioned here. This sensor is also constructively thought and
probed for the usage in extremely rough environment, such as the
exhaust channel of a car (amongst others high temperature,
vibration, chemical aggressive media).
LIST OF REFERENCE NUMBERS
[0033] 1 Light source [0034] 2 Optical wave guide [0035] 3 Fibre
coupler [0036] 4 Optical wave guide [0037] 5 Mode coupler [0038] 6
Fibre coupler [0039] 7 Optical wave guide [0040] 8 Optical wave
guide [0041] 9 Mode coupler [0042] 10 Measurement cell [0043] 11
Photo detector [0044] 12 Analogue/digital converter (ND converter)
[0045] 13 Controller [0046] 14 Mode coupler [0047] 15 Photo
detector [0048] 16 A/D converter [0049] 18 Source controlling path
[0050] 19 Measurement path [0051] 20 Referencing measurement cell
[0052] 22 Sensor [0053] 30 Measurement cell [0054] 31 Light inlet
of non stabilized light source respectively disturbed inlet path
[0055] 32 Light outlet to the photo detector [0056] 33 (Small)
glass plate [0057] 34 Concave mirror as reflector [0058] 35 Inlet
for light back coupling [0059] 36 Outlet of light back coupling
[0060] 37 Optional glass plate [0061] 38 Direction of the first
light path [0062] 39 Direction of the second light path [0063] 40
Measurement cell [0064] 41 Collimating lens [0065] 42 Half
transparent mirror [0066] 43 Full mirror [0067] 44 Rotor [0068] 45
Direction of the first part of the light [0069] 46 Direction of the
second part of the light [0070] 47 Reflection of the second light
part [0071] 48 Direction of media flow
* * * * *