Note: Descriptions are shown in the official language in which they were submitted.
1~33Z76
Laser Scanning Apparatus
The present invention relates to an apparatus
for the remote monitoring of gases and in particular to
an apparatus for the remote monitoring of gaseous pol-
lutants in an atmospheric environment, for example in
chemical plants.
There is a need to carry out remote monitor-
ing of gaseous pollutants, for example toxic or inflam-
mable gases, within industrial locations, such as
- in chemical plants. In particular, there is a need to
obtain rapid and continuous measurements of the concen-
- tration of gaseous pollutants over a large area and to
ensure that the measurements are continually updated.
This is especially important with regard to plant safety
when toxic or inflammable gases are involved, for example
in the detection of ethylene leaks in a polyethylene
- plant where explosive mixtures of ethylene and air could
result. At present leaks may be detected by an array
of individual sensors distributed over the area, but
because gas at high pressure emerges as a narrow plume
from any small hole, the gas can miss all the detectors
until it is too late~
Most of the ~mdesirable gases absorb electro-
magnetic radiation at wavelengths which can readily be
.
,
11;33i~'76
matched by lasers, and various methods of measuring
small amounts of such gases using lasers have been pro-
posed for a variety of situations. However, none of those
methods or apparatus could be applied entirely satisfact-
5 orily to the continuous monitoring of pollutants over a
wide area, e.g. through lack of robustness, an inability
to identify where a leak is occuring within the wide area,
or through lack of sensitivity.
According to one aspect of the present invention
10 we provide an apparatus for remote quantitative
monitoring of one or more selected gases in a gaseous
environment,which comprises laser sources for generating
electromagnetic radiation capable of being tuned to give
at least one detection beam containing a specific absorp-
15 tion wavelength of the gas or gases to be monitored and
at least one reference beam having a wavelength that is
significantly less strongly absorbed by the gas or gases
to be monitored, means for modulating the amplitude of
each of the beams with different modulation frequencies
20 or phases, means for combining the modulated beams into
r a single combined beam in which the component modulated
beams are substantially coincident with one another,
scanning means to displace the combined beam angularly
through the gaseous environment so as to direct the com-
25 bined bea~. towards a plurality of locations sequentially
and repetitively, means for collecting at least a portion
of the radiation which is returned from each of the
locations, a detector for deriving electrical signals
corresponding to the intensity of the collected radiation,
30 means for isolating the electrical signals corresponding
to the intensity of radiation having the aforesaid
modulation frequencies or phases, means for obtaining
the ratio of the isolated signals corresponding to
radiation collected from a detection beam and a relat~d
35 reference beam thereby to provide a measure of the amount
~1~3'~'7~j
of the selected gas or gases in each beam path traversed
between the apparatus and the scanned locations, and
means for indicating the amount of gas detected.
While it may be possible to divide a single laser
beam to provide the sources for the detection and refer-
ence beams, it is preferred to use a separate laser for
- each beam.
The light emerging from a pulsed mode laser is
a~ready modulated in that it emerges in the form of
pulses of a predetermined frequency. In such lasers the
means for modifying the amplitudes of ~he beam thus forms
an integral part of the laser itself. The different
modulations for the detection and reference beams are obtained
by having lasers whose pulse-rates are different. ~owever,
we find that in the current state of the art, continuous
mode gas lasers are more able to withstand use over long
periods under the rugged conditions of industrial
environments, and accordingly they are preferred as the
laser sources for both detection and reference beams,
even though they require the provision of separate
modulating means. The lasers particularly preferred are
continuous mode gas lasers which are capable of being
tuned to selected discrete wavelengths in the infra-red
corresponding to absorption wavelengths of the gas or
gases to be monitored. Infra-red laser light has the ad-
vantage, as compared to visible laser light, of not
affecting the retina of the human eye and hence being safe
in this respect. Moreover, infra-red light, in having a
longer wavelength than visible light, penetrates better
through wet, smokey and dusty industrial environments, thus
giving infra red lasers a greater range, enabling them more
readily to monitor locations hundreds of metres away.
Suitable discretely tunable gas lasers include
carbon dioxide cl2o216, e.g. for monitoring ozone, ammonia,
35 vinyl chloride, fluorpcarb~ons and unsaturated compounds
~1~33Z'7~:;
-
such as ethylene; carbon dioxide C12o213 e.g. for themonitoring of sulphur dioxide; carbon dioxide C140216 e.g.
for phosgene or ethylene oxide; helium-neon (at 3.39 ~m)
e.g. for monitoring methane, and carbon monoxide lasers
for monitoring carbon monoxide. The only gases having no
absorption in the infra-red are homonuclear diatomic gases,
such as chlorine, and monatomic gases such as mercury, so
it will be appreciated that the wide range of tunabie gas
lasers available enables the apparatus of the invention
to be used to monitor a wide range of commonly occurring
pollutants in the atmosphere. The carbon dioYide lasers
are tunable to wavelengths which avoid the absorption
bands of water and carbon dioxide, and hence are a range
of lasers particularly suited to the present application.
In the plant environment where a variety of
scattering surfaces are encountered, it is preferable
that the wavelengths of the detection and reference beams
be close together in order to reduce the effect of dif-
ferential albedo. Thus the difference in wavelengths of
the two beams is preferably less than 0.1 ~m, where
suitable absorption bandstrengths are available.
The preferred means for modulating the beams from
continuous mode gas lasers is a rotatable sector chopper,
which, when rotated in the path of a laser beam, inter-
mittently interrupts the beam at a frequency dependenton its rate of rotation. The detection and reference
beams are preferably passed through different choppers
rotating at different speeds so as to modulate them with
different frequencies, or through a two-frequency chopper.
Alternatively, the two beams, while still separated, may
be passed through the same chopper so that they are
interrupted at different times, the beams thereby being
modulated at the same frequency, but with different
phases. Other ways of modulating laser beams include
the use of acousto-optic and electro-optic means.
Mcdulation of a continuous mode laser can also be carried
- ~ , .
3;~6
out within the laser, e.g. by modulating the discharge or
by using physical means within the laser rather than
` externally as described above.
` The beams, preferably one detection beam and one
reference beam, are conveniently combined using a slab of
infra-red transparent material, for example germanium (or
sodium chloride or zinc selenide etc.). This is prefer-
ably mounted such that the two laser beams will strike
opposite sides of the slab at the Brewster angle with
mutually perpendicular planes of polarisation, whereby
one beam will be transmitted and the other will be pre-
dominantly reflected to form two substantially coincident
beams distinguishable by their different modulations. It
is an advantage of this method of combining the beams
that, for a slab mounted in a vertical plane, about 99
of the horizontally-polarised beam may be transmitted
and about 70~ of the vertically-polarised beam reflected,
thereby allowing combination of the beams coaxially with
minimum loss of power. Thus each beam may be passed
along exactly the same path through the same absorption
materials and return to the same detector, preferably
to exactly the same portion of the detector. However,
the term "substantially coincident" may include beams
which are for example close together and parallel so that
- 25 they pass through or strike closely adjacent regions.
The simplest form of scan is one wherein the
locations are aligned and are sufficiently narrow to ~all
; within the width of the beam, the scan then being effected
simply by movement of the beam to and fro in a single
arcuate sweeping movement. This simple scan is suitable
for monitoring pipe lines, for example, which may run
transverse or parallel to the beam direction. However,
this does not fully utilize the potential scope of the
apparatus, and a preferred scanning means is one which
is moveable in at least two d~rections so as to direct
~1;33Z7~
the combined beam at locations which are spread over
an area whose dimensions in any two perpendicular
directions are greater than the width of the beam. The
area can then be covered by a raster scan.
The preferred scanning means comprises a mirror
mounted within the path of the combined beam and moveable
angularly relative to the incident combined beam so as to
direct the beam towards different locations when such
angular movement occurs. The mirror must have high
reflectivity at the wavelengths used, gold plated or
polished stainless steel surfaces being particularly
suitable for wavelengths in the infra-red. By moving
only a mirror, the mass of moveable material is much less
than if, for example, the lasers and ancillary equipment
were also moved, and scanning can therefore be achieved
more rapidly with less energy. It is preferable for the
scanning means to incorporate means for cleaning the
surface of the mirror in order to minimise back scatter
in a dusty environment. Suitable cleaning means include
windscreen wipers, heaters, electrostatic means and air-
blasts, for example. Where the detection and reference
beams are closely side by side rather than being exactly
aligned in the combined beams, s~parate mirrors for the
two components can be used, but the use of a single
mirror is mechanically easier and is preferred.
Radiation directed at various locations on a
chemical plant, will fall on a variety of surfaces,
including walls, ground, pipes and possibly trees for
example, and the various surfaces will affect the
radiation in different ways. In most cases it will be
scattered in all directions with a proportion absorbed,
and only a small portion of the scattered radiation will
be returned to the apparatus- ~or collection. In
some places, however, the radiation may fall onto a
specular reflector, which if correctly aligned, could
.
11332~7~;
reflect substantially all the radiation falling on it,
back in the direction of the apparatus. As will be
appreciated, it is the portion of the radiation which
is returned to the collecting means which is collected
whether this radiation is a small portion of scattered
radiation or radiation which has been directed at the
collector by a specular reflector, retroreflector or
other directional means. The collecting means prefer-
ably comprises a telescope to focus collected radiation
onto the detector. The telescope is preferably static
to minimise moveable mass, but can as an alternative be
angularly displaced in unison with the mirror or other
scanning means.
A variety of detectors may be used depending on
the particular laser sources being used. Suitable
detectors include photo-emissive, photo-conductive and
photo-voltaic detectors. When using infra-red laser
beams, the returned radiation is conveniently detected
by a photo-conductive or photo-voltaic detector, for
example a cooled cadmium mercury telluride or lead tin
telluride detector.
The components of the detector output in the
form of electrical signals corresponding to the intensity
of radiation having the aforesaid modulations are isolated,
conveniently by use of frequency and/or phase selective
amplifiers. Preferred amplifiers are lock-in amplifiers
each being locked into the modulation frequency of one
of the modulated beams by a signal derived from the
modulating means or from a sample of the modulated beam.
This enables the amplifier to follow any changes which
might occur in the modulation frequency e.g. through
drift. The detection system is able thereby to reject
all the background radiation and the amplifier output
gives the magnitude of the signals of interest, i.e.
those which are due to the collected laser radiation
at the two ~avelengths.
` 1~33'~7~
The two isolated signals are preferably smoothed,
conveniently by means of simple electrical circuits
which are normally provided as a feature of commercial
lock-in amplifiers, with a time constant which is longer
than the modulation cycle times.
The power output of either of the lasers may
alter without there being any change in the output of
the other, e.g. due to drift in the power supply. The
apparatus is therefore preferably provided with means to
compensate each of the isolated signals for any
fluctuations in the power output of the relevant laser
e.g. by sampling the laser output and taking the ratio~
of isolated signal to laser output for each channel,
e.g. using electronic ratiometers.
The ratio of the magnitudes of the two isolated
and possibly compensated signals may be obtained by
coupling them to an electronic ratiometer. In the
absence of any of the gas to be monitored, the two
signals fed to the ratiometer can be adjusted by an
electronic balance control to be equal, that is, the
ratio of the signals in this condition is unity. It will
be appreciated that the absolute intensity of the collected
radiation may differ widely depending on weather conditions
reflectivity of the location being probed etc. Whenever
the gas to be monitored is present, the detection beam
signal will be attenuated and the ratio of detection beam
to reference beam will be reduced.
What happens to the output of this latter ratio-
meter depends largely on the degree of sophistication
desired for the apparatus. Thus, for example, the
apparatus may only be required to sound an alarm when a
predetermined quantity of gas is detected. The indicator
means may then simply be an alarm system arranged to
- operate when the ratio of the signals exceeds a predete~ned
threshold, preferably with an indication of the position
;33'~:7~
g
of the scanning means (and hence of the location being
monitored) when the threshold is exceeded. This threshold
` can be varied over the area scanned, when appropriate,
e.g. to compensate for beam path length differences or
angles of incidence.
According to the Beer-Lambert law (which governs
the transmission of energy through an absorbing medium)
the logarithm of this ratio multiplied by a factor cal-
culated from the known extinction coefficients for the
two laser wavelengths, gives the mass of the gas being
monitored in the beam at any given time. The factor may
alternatively be found experimentally by placing in the
detection beam a cell containing known amounts of gas.
As it is the logarithm of the ratio which is proportional
to the mass of gas in the beam, a logarithmic ratiometer
is preferred, the output of which may be displayed on a
linear scale to give the mass o gas.
The preferred indicating means comprises
computing means having inputs of at least the ratio of
the isolated signals (preferably compensated for laser
fluctuations as described above) and the position of the
scanning means, and being programmed to derive from these
inputs an accessible correlation between the amounts and
the positions of the selected gas or gases detected while
continually updating this correlation as monitoxing
proceeds. The computing means preferably has a visual
display unit arranged to provide a continuously updated
; display of the amount of gas detected throughout the
area scanned. This is conveniently shown by super-
imposing an indication of the amount of gas detected
onto a plan of the area scanned, showing for example
the main topographical features of the area. The amount
of gas can readily be shown on the screen as a varying
level of brightness or symbols corresponding to different
amounts of gas.
~1332'~
:
According to a further aspect of the present
invention we provide a method for the remote quantitative
monitoring of one or more selected gases in a gaseous en-
. vironment which comprises the steps of generating electro-
. 5 magnetic radiation from laser sources to give at ieast one
detection beam containing a specific absorption wavelength
: of the gas or gases being monitored and at least one ref-
erence beam having a wavelength that is significant~y
- less strongly absorbed by the gas or gases being monitored,
10 modulating the amplitude of each of the beams with dif- -
ferent modulation frequencies or phases, combining the
modulated beams intG a single beam in which the component
modulated beams are substantially coincident~ with one
another, displacing the combined beam angularly through
- 15 the gaseous environment so as to direct the comjbined beam
towards a plurality of locations sequentially and repet-
itively, collecting at least a portion of the radiation
which is reflected from each of the locations, deriving
electrical signals corresponding to the intensity of the
: 20 collected radiation,isolating the electrical signals
corresponding to the intensity of the radiation having
the aforesaid modulation frequencies or phases, and ob-
taining the ratio of the isolated signals corresponding
to radiation collected from a detection beam and a
related reference beam thereby to provide a measure of
the amount of the selected gas or gases in each beam
path traversed by the collected radiation originating
from the laser sources.
In general the method is preferably carried out
by angularly displacing the combined beam continuously
so as to direct the beam at successive locations
in a pattern wherein the beam traverses
substantially parallel rows of locations sequentially
and in alternate directions r the rate at which the beam
traverses each row being varied with distance along the
li33Z76
11
row such that the beam executes simple harmonic motion.
This reduces the acceleration of the mirror, or other
moveable scanning means used to direct the combined beam,
at the ends of each row. It also compensates for the
increased range at the edges of each row. However,
although such a scan gives a good plan of the whole
area, in some cases it is possible to predict in which
locations, if any, the leaks will occur, e.g. where there
are spaced units of machinery handling compressed gases.
Under such circumstances, it may be more advantageous to
direct the beam at and stcp on those separate locations in
turn, or to pause on those locations during a full raster
scan so as to give a higher weighting to the monitoring
of those more-vulnerable areas.
It is further preferred to display the results
of the monitoring by showing on the screen of a visual
display unit, a plan of the area scanned with an
indication of the amount of detected gas superimposed.
The invention is illustrated by reference to two
specific embodiments thereof, shown in the accompanying
- drawings in which:
Figure 1 shows a prototype apparatus for monitoring ethy-
lene in the atmosphere around a polythene plant
and
Figure 2 shows a further apparatus which differs from
that of Figure 1 in a few details. Both drawings
are diagrammatic views, each showing the apparatus
in operation.
The apparatus of Figure 1 comprises two continuous
30 mode carbon dioxide C120216 lasers, 1, 2, each emitting
about 10 W, which are operated by a high voltage power
supply 3. One laser 1 is grating-tuned to a wavelength
of 9.227 ~m in the infra-red, this being a wavelength at
- which there is very little absorption by ethylene
35 (absorption coefficient = 0.046 cm~l bar 1), this laser
1~33~
12
is used for providing the reference beam. The other
laser 2 is grating-tuned to a wavelength of 10.258 ~m in
the infra-red, at which wavelength there is much stronger
absorption by ethylene (absorption coefficient 0.55 cm 1
bar 1), and this laser is used to provide the detection
beam. The reference beam is steered by gold mirrors 4,
5 and then focussed by an anti-reflection coated germanium
lens 6 through a rotating sector chopper 7 which mechan-
ically interrupts the reference beam at a frequency near
3kHz. The detection beam is steered by gold mirrors 8,
9 and then focussed by an anti-reflection coated
germanium lens 10 through a rotating sector chopper 11
which mechanically interrupts the detection beam at a
frequency near 3kHz, but which differs from the frequency
at which the reference beam is modulated. Typically,
the detection and reference beams are modulated at 3kHzand
2.7 kHz respectively. The blades of the choppers 7, 11
are gold plated to reflect the radiation which does not
pass between the blades,onto thermopile power meters 12,
13 which serve to monitor continuously the laser power.
The detection and reference beams are then com-
bined using a slab 14 of germanium mounted in a vertical
plane at the Brewster angle so that it transmits approx-
imately 99~ of the horizontally-polarised reference beam
and reflects approximately 70% of the vertically-
polarised detection beam, to give a combined co-incident
beam.
The combined beam then passes through a reverse
telescope comprising anti-reflection coated germanium
lenses 15, 16 which expand the beam. A removeable pin
hole 17 is used as a simple spatial filter to aid adjust-
ment. Typically, the telescope gives a beam divergence
of 10 mrad, whereby the spot diameter at 100 m distance
will be 1 m, and the average power level will be only
1 mW cm~2. This power level is much less than the infra-
~133;~76
^ 13
red radiation from a human body or even from the grounditself, and is well within the continuous exposure allowed
under British Specification 4803.
The beam is turned by an elliptical gold mirror 18
mounted in a telescope having a parabolic mirror 19, and
then directed towards the desired locations around the
polyethylene plant by a flat gold mirror 20 which is
steered by two motors (not shown) so that it can be
angularly displaced about both horizontal and vertical
axes. To aid initial alignment, a mirror 21 is provided,
which may be inserted temporarily into the beam, and a
telescopic gun sight 22 used to view the locations
towards which the beam is directed, using the same
optical system.
A small fraction of the radiation directed at the
various locations is scattered back in the direction of
the apparatus, and reflected by the gold mirror 20 onto
the parabolic mirror 19 which condenses the radiation via
mirror 18 and the focussing lens 23 onto a liquid-
- 20 nitrogen-cooled cadmium mercury telluride photoconductive
detector 24.
The electrical signal resulting from the detector
2~ is fed to a preamplifier 25, and the preamplified
signal resulting therefrom is fed to two lock-in
amplifiers 26, 27 which take a reference from choppers
7, 11 respectively, and are thereby made sensitive to
the part of the signal having the respective modulation
frequencies corresponding to the reference beam and the
detection beam. The two outputs of the lock-in
amplifiers 26, 27 are coupled to an electronic logarithmic
ratiometer 28 and its output is displayed on a meter 39.
As the ratiometer is logarithmic, the relation-
ship between its output and the mass of gas through
- which the beam has travelled is linear.
The apparatus shown in Figure 2 is very similar
113;3276
14
- to that of Figure 1 in overall structure. Two uprated
continuous mode carbon dioxide C120216 lasers 31, 32 are
used, each having its own integral power supply and each
being capable of emitting 40 ~I power. These provide
respectively a detection beam at 9.673 ~m tabsorption
coefficient = 2.15 cm 1 bar~l) and a reference beam at
9.619 ~m (absorption coefficient - 0.24 cm~l bar~l).
The beam is again passed through choppers 33, 34 with
light reflected from the back of the chopper blades
being directed to power meters 35, 36.
The power meters 35, 36 continuously monitor the power
output of the lasers 31, 32 and give out signals A and
B which are a measure of the laser power. The choppers
also give out signals C and D indicative of their rate~_
of rotation and hence of the frequencies at which the
respective beams are being chopped. The beams after
modulation by the choppers, are combined using a slab
of germanium set at the Brewster angle and the combined
beam is directed towards the various locations using
essentially the same optical system as that shown in
Figure 1 except that polished stainless steel mirrors
are used instead of gold mirrors. The large mirror 37
is positioned angularly with respect to the combined beam
by two drive motors 38, 39 which tilt the mirror about
! 25 a horizontal axis and about an axis perpendicular to that
horizontal axis, respectively. The angular displacements
are measured by shaft encoders on the drive units, and
these measurements are given out as signals ~ and F
respectively.
Instead of using a mirror to focus the scattered
radiation collected by the telescope (as in Figure 1)
this apparatus uses a lens 40, which focusses the light
directly onto liquid nitrogen cooled detector 41, having
automatic topping-up means 42 for the cooling liquid.
The output from the detector is divided and fed to two
11332~6
lock-in amplifiers 43, 44 taking signals C and D from
the choppers to provide a continuous reference for the
frequency selection. These amplifiers isolate the two
modulated signals, which are smoothed and ther. corrected
for power fluctuations in their respective originating
lasers, by ratiometers 45, 46 using signals A and B from
the power meters as their references. The corrected
signals are then compared in a further ratiometer 47,
and the ratio obtained is fed to a computer 48.
The computer effectively does two jobs. One is
to compare the ratio signal from the ratiometer 47 with
a preset standard representing the tolerable gas limit
and to sound an alarm 49 when the ratio drops below that
limit, i.e. when the mass of gas exceeds the tolerable
limit. The other purpose is to display on a visual
display unit 50, a plan of the area of the polyethylene
plant being scanned, superimposed with an indication of
the level of ethylene detected, with a continuous up-
dating of the display. To do this, the signals E and F
from the mirror drives are input into the computer as an
indication of the orientation of the mirror 37 and hence
of the location on the plant to which the signal from
the ratiometer 47 at any instant, relates.
The computer of this apparatus is also provided
with a store 51 in the form of a floppy disc which
records continuously the data fed to the visual display
unit, the data being stored for a period of 75 minutes
before being erased on the introduction of fresh data.
This predetermined period was found to be convenient
for this application in that it enabled a record of the
build up of any leak to be analysed later. The most
appropriate periods for storing the data depend on the
particular application, but generally lie within the
range of 0.5 to 1.5 hours. The store may also have means
to freeze the data in the store when a gas leak is
detected.
'
1~3276
16
The apparatus is mounted on a tall tower over-
looking the polyethylene plant, but outside the haz-ard
area. The total area scanned is about 100 m by 150 m,
and the beam is directed downwards as it scans this area
from the tower. Much of the radiation is scattered by
the ground, but other scattering surfaces include buildings
and pipework. The diameter of the beam when it hits the
ground or plant buildings is between 1 and 2 m depending
on range and there are roughly 3000 individual locations
to be scanned. The response time of the complete system
is about 3 ms. This enables the whole area to be scanned
in 30 seconds while giving a residence time for each
location in at least part of the beam of about 10 ms.
Even though heterodyne techniques cannot be used for
improving the signal to noise ratio with a scanning beam
of this type, the simple techniques employed here have
been found to be both rapid and sensitive in the detection
of escaping ethylene, and the apparatus has proved to be
both reliable and robust.
What we claim is:
