Note: Descriptions are shown in the official language in which they were submitted.
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SHORT RANGE LIDAR APPARATUS HAVING A FLAT SPATIAL RESPONSE
FIELD OF THE INVENTION
The present invention relates to optical measuring devices and more
particularly
concerns a LIDAR apparatus with a flat spatial response used for measuring
concentrations of particles at a short range.
DESCRIPTION OF THE PRIOR ART
Many industrial fields require the short range detection of particles such as
io aerosols, fog or clouds of chemical droplets such as pesticides,
insecticides and
the like. Some also require detection of suspensions, in liquids such as
water.
The best known current optical solution for spatially resolved aerosol
detection is
the use of a LIDAR (Llgth Detection And Ranging) system. In a LIDAR, a pulsed
is light signal is sent and is back-scattered by the particles. A temporal and
amplitude analysis of the back-scattered light determines the particle
concentrations along the path of the emitted beam of light. The scattering
phenomena observed by such devices may be instantaneous or delayed
according to fluorescent, luminescent or phosphorescent mechanism, and
20 accompanied or not by a 'wavelength shift. Most particle detection LIDARs
are
however designed and built for long ranges, usually for 200 meters or more.
These
highly coherent LIDARs use light beams with very small divergence and
detectors
with small diameters, of the order of less than 1 mm. This maximizes light
collection from long distances (over 1 km) and reduces detection noise levels.
The major impairment of using standard LIDARs for short range applications is
the
1/r2 .dependence of the signal, r being the distance from the target to the
receiving
optics. For short distances, this entails a huge signal variation; a factor of
10000
(40 dB optical) between 1 m and 100 m, if all the light gathered by the
receiving
optics falls on the detector. This places stringent requirements on the
detection
electronics. Moreover, for lower cost operation, the temporal resolution is
limited,
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usually between 10 and 20 nanoseconds, so that a data point represents the
average density of aerosols in some volume covering 1.5 to 3 m along the path
of
the emitted beam. In order to have an accurate value of the average density,
the
system's spatial response must not vary significantly over the volume' along
the
emitted beam over which the average is taken. A short range LIDAR thus
requires
that the system response be practically constant over large distances, that
is,
distances larger than the system's spatial resolution.
For these reasons, what comes to mind to those skilled in the art for
measuring
to particle concentrations in a specific spatial interval is to place a
standard LIDAR
far enough away from the zone of interest in order to make the measurements in
the "long range" mode of operation, the mode for which the spatial response of
the
LIDAR is practically constant over a distance equivalent to the system's
resolution.
This is not optimal. These LIDARs are bulky, power hungry and very costly.
They
were designed for very low light detection. For these applications, an
optimized
short range LIDAR is desirable.
LIDAR spatial responses are covered in a number of scientific articles, for
example:
- Giorgos Chourdakis, Alexandros Papayannis and Jacques Porteneuve,
"Analysis of the receiver response for a noncoaxial- lidar system . with fiber-
optic
output", APPLIED OPTICS, 20 May 2002, Vol. 41, No. 15, 2715-2723.
- Raffaele Velotta, Bruno Bartoli, Roberta Capobianco, Luca Fiorani, and
Nicola
Spinelli, "Analysis of the receiver response in lidar measurements", APPLIED
OPTICS, 20 October 1998, Vol. 37, No. 30, 6999-7007.
J. Harms, "Lidar return signals for coaxial and noncoaxial'systems with
central
obstruction", APPLIED OPTICS, 15 May 1979, Vol. 18, No. 10, 1559-1566.
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- J. Harms, W. Lahmann, and C. Weitkamp, "Geometrical compression of lidar
return signals", APPLIED OPTICS, 1 April 1978, Vol. 17, No. 7, 1131-1135.
- T. Halldorsson and J. Langerhoic, "Geometrical form factors for the lidar
function", APPLIED OPTICS, Vol. 17, No. 2, 15 January 1978, 240-244.
In these articles the spatial responses of coaxial and non-coaxial systems are
described in detail, both with direct detection and through fiber optics. In
all these
systems, calculations and simulations are for a single field of view (FOV)
receiver
io and a usually round, uniform and unique detection surface. Light
distributions in
the detection plane are shown. These authors show the spatial variation of the
system response without suggesting any equalization scheme, except, to a
certain
extent, Harms et al. (1978) for a uniaxial system. Harms computes overlap
factors
and uses them to retrieve and correct measured data, but never for very short
ranges (0-100 meters).
Uwe Stute, Michel Lehaitre and Olga Lado-Bordowsky, "Aspects of temporal and
spatial ranging for bistatic submarine lid&', Proceedings of EARSeL-SIG-
Workshop LIDAR, Dresden/FRG, June 16 - 17, 2000, 96-105, described an
underwater LIDAR using multiple field of views, but with independent detection
for
each FOV. A series of fibers are used, but for purposes unrelated to the
equalizing
of the spatial response.
U.S. patent no. 5,880,836 (LONNQVIST), entitled "Apparatus and method for
measuring visibility and present weather", teaches of a system for measuring
particles (fog, rain, snow and cloud ceiling) using a combination of systems.
LONNQVIST presents a method and apparatus for detecting particles at short
range using a pair of detectors in a half-bridge, a beamsplitter and a common
lens.
This patent however does not tackle the problem of large signal variation with
3o distance for short ranges, nor does it discuss it.
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U.S. patent no. 5,241,315 (SPINHIRNE), entitled "Micro-pulse laser radar",
describes
a low cost, compact, lightweight, reliable and eye safe LIDAR used for
atmospheric
measurements. SPINHIRNE does point out signal compression because of optical
geometry but does not tackle the problem of equalizing the short range spatial
response.
Finally, in U.S. Patent no. 5,116,124 ("Measurement system for scattering of
light"),
HUTTMANN describes a uniaxial system, somewhat like that of LONNQVIST,
capable of short range measurements, but again, without equalization of the
io response. HUTTMANN uses multiple fibers, but their purpose is not for
equalization
of the short range spatial response.
There is therefore a need for a LIDAR apparatus which can be used to measure
particle concentrations at a short range and having a generally flat,
equalized
is response in such a range.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a LIDAR apparatus for measuring
concentrations of particles with respect to a distance of these particles from
the
20 apparatus within a short range therefrom. The apparatus has a substantially
flat
spatial response, whereby a same concentration of particles at any distance
within
the short range will generate a signal of substantially the same intensity.
The apparatus first includes a source optical arrangement projecting an
excitation
25 light beam along an optical path. The excitation light beam is back-
scattered by the
particles within this optical path.
The apparatus further includes a light-collecting arrangement having a field
of view
intersecting the optical path along the short range. The light-collecting
arrangement
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includes light-receiving optics receiving the back-scattered light, a spatial
filter
receiving the back-scattered light from the light-receiving optics and
spatially filtering
the received back-scattered light so as to flatten the spatial response of the
apparatus, and a detector detecting the spatially filtered back-scattered
light from the
5 spatial filter.
The present invention advantageously uses optical propagation after the light-
receiving optics and detection geometry to render the response of a LIDAR as
flat as
possible with respect to distance, for short range detection such as from 1 m
to less
1o than 100 m or so. This new class of particle detection LIDARs can be
optimized for
low cost, low weight, low power consumption, amongst other parameters.
Other features and advantages of the invention will be better understood upon
reading of preferred embodiments thereof with reference to the appended
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. I is a schematic representation of an apparatus according to a preferred
embodiment of the invention.
FIG. 2 is a schematic representation of an apparatus according to another
preferred
embodiment of the invention.
FIGs. 3A to 3H shows the spatial light distribution in a detecting plane for
reflecting
planes respectively at 1 m (FIG. 3A), 2 m (FIG. 3B), 4 m (FIG. 3C), 8 m (FIG.
3D), 15
m (FIG. 3E), 30 m (FIG. 3F), and 50 m (FIG. 3H).
FIG. 4 illustrates the profile of a mask according to one embodiment of the
invention.
FIG. 5 is a graph illustrating the calculated spatial response of an apparatus
3o according to an embodiment of the invention.
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DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION
Referring to FIG. 1, there is schematically illustrated a LIDAR apparatus 10
having
an almost flat spatial response according to a preferred embodiment of the
present
invention.
The apparatus is intended for measuring, at short range, concentrations of
particles 11 in the air such as aerosols, fog, dust, clouds of chemical
droplets such
as pesticides, insecticides or the like, or suspensions in a liquid medium,
for
example in the course of analyzing the turbidity of water in waste water
settling
1o tanks.
The term "range" is used herein as referring to the distance between the
detected
particles 11 and the apparatus 10 itself. It is understood that the
designation of
"short range" for the apparatus 10 of the invention is by comparison to
traditional
aerosol detecting LIDAR devices, which are designed to operate at a range of
about 200 m or more; in the preferred embodiments, the apparatus 10 is
designed
to be operated within 100 m, preferably within 50 m. It will of course be
understood
by a person skilled in the art that the range of the apparatus also has a
lower
boundary close to the apparatus where the geometry of the system prevents the
measure of particle concentrations. Typically, this boundary may for example
be
around 1 m from the apparatus.
By "flat spatial response", it is understood that a same concentration of
particles at
any distance within the range of the apparatus will generate a signal of
substantially the same intensity. The spatial response of the apparatus may of
course deviate slightly from a strict constant value within the target range
to a
degree determined by the particular needs of a given application.
The apparatus 10 first includes a source arrangement 12 projecting excitation
light
3o beam 14, which is preferably pulsed or otherwise modulated. The source
arrangement 12 may for example include a pulsed laser source, such as typical
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low cost sources used for range finders such as for example pulsed high power
laser diodes 16 (such as those sold by OSRAM, Perkin-Elmer or Laser
Components, for example), a pulsed fiber laser (such as those sold by INO,
Keopsys, SPIOptics or OzOptics, for example), a pulsed solid state Q-switched
chip laser (such as those sold by JDS Uniphase, for example) or any other type
of pulsed laser generating an appropriate light beam. Preferably, the
excitation
beam 14 from the source is imaged on a diffuser 20 by an imaging lens 22; this
is
done in order to render the system eye safe, an important parameter for many
short range LIDAR applications. Any other appropriate means may 'of course be
1o used for such a purpose. An output lens 18 images the light beam from the
diffuser 20 along an optical path 19 traversing the target range of the
apparatus
10.
The presence of particles 11 in the optical path 19 of the excitation light
beam 14
will have the effect of scattering the light beam 14, and a portion 25 thereof
will be
back-scattered towards the apparatus 10. The term "scattering" is used herein
to
refer in the large sense to the dispersal of the light beam by the particles
as a
result of physical interactions therewith. The mechanisms involved may be
instantaneous, as is the case for "true" scattering, or according to
fluorescent,
luminescent or phosphorescent phenomena. Depending on the particular
application, the scattering may be without a wavelength change, or accompanied
by a small wavelength shift. It will be clear to one skilled in the art that
the data
processing of the detected light will depend on the type of scattering
observed.
For any distance along the optical path 19 of the excitation light beam 14
where
back-scattering occurs, there can be said to correspond an elementary
scattering
volume R delimited by the cross-section of the excitation light beam and a
very
small distance along the beam. There is therefore a plurality of parallel
elementary
scattering volumes R1, R2 ...Rn at different distances from the apparatus 10
within
its range of operation. The light reaching the apparatus is therefore a mix of
the
back-scattered light from each scattering volume along the optical path 19.
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The apparatus 10 further includes a light collecting arrangement 24 having a
field of
view intersecting the optical path 19 within the operation range of the
apparatus. The
light collecting arrangement 24 therefore receives the back-scattered light 25
within
its field of view. The light-collecting arrangement 24 includes a detector 26
and light-
receiving optics. The light-receiving optics may be embodied by any
appropriate
optical arrangement. In the illustrated embodiment, it includes an input lens
28
collecting the back-scattered light and propagating it towards at least one
detecting
plane D.
io The distribution of light in any detecting plane varies depending on 1) its
distance
from the components of the light-collecting arrangement, 2) the elementary
scattering
volume, or mix of elementary scattering volumes, from which it originates and
3) on
the degree of back-scattering from each scattering volume (in other words, it
depends
on the mix of particles and on their distribution within the excitation beam
path). The
first factor is fixed for a given system, and the third factor contains the
information to
be measured by the apparatus. The second factor, however, will be greatly
affected
by the spatial response of the apparatus, and this effect must be taken into
account
for the information measured to be significant.
The problem is best understood through an example. Let us suppose the spatial
resolution of the apparatus is 2 m, that is, it is impossible to determine
from where a
light signal originates within a 2 m long interval along the optical path. At
short range,
for example between 2 m and 4 m, it is impossible to distinguish between a
signal
produced by 4000 particles all located in a 1 mm slice around 2 m and a signal
produced by 16000 particles in a 1 mm slice around 4 m. If all the light
falling on the
light collecting arrangement is detected, both signals will be the same, and
yet the
actual concentrations of particles differ by a factor of 4.
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In order to take these short range effects into consideration and obtain
accurate
concentration values, the relative intensity of light received from each
volume R; if the
particle concentration was constant along the optical path has to be tailored.
This
relative intensity is for example illustrated in FIGs. 3A to 3H, which shows
the light
distributions in a same detecting plane for different scattering volumes (at
different
distances from the light collecting arrangement). In this example, the source
is a laser
diode imaged 50 m away from the apparatus. A scattering target is placed at
planes
respectively at 1 m, 2 m, 4 m, 8 m, 15 m, 30 m and 50 m from the apparatus,
and the
resulting back-scattered light is detected in a detection plane at 208 mm from
the
io back of the input lens, which in this case has a 202 mm back focal length.
Only the
light falling on a 6 mm x 6 mm surface is shown. The back-scattered light
therefore
corresponds to the signal which would originate from different elementary
scattering
volumes R; (replaced in this case by scattering surfaces, which is equivalent)
within
the operation range, all with the same concentration of particles (or the same
back-
scattering coefficient for the scattering surface). As can be seen, the image
from the
50 m reflecting surface (FIG. 3H) is sharp and has substantially the same
dimensions
as the effective source. Light distributions gathered from other reflecting
surfaces are
offset from the distribution at 50 m and become larger and larger as the
originating
surface comes closer to the lenses.
Referring back to FIG. 1, in order to compensate for the aforementioned short
range
effects, the light-collecting arrangement 24 of the apparatus 10 includes a
spatial filter
29, spatially filtering the back-scattered light 25 to flatten the spatial
response of the
apparatus. By spatially filtering, it is meant that portions of the light
incident on one or
more detecting planes are blocked so that only selected portions of the back
scattered light 25 are provided to the detector 26. These portions are
selected in
order for the spatial response of the apparatus to be substantially flat, that
is, that a
same concentration of particles at any distance within the operation range
would
generate a signal of substantially the same intensity.
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The spatial filter 29 may be embodied by any reflective, refractive or
diffractive
component or combination thereof accomplishing the desired spatial shaping of
the
back-scattered light. In the embodiment of FIG. 1, the spatial filter 29 is
embodied by
a mask 30 disposed along the detecting plane D. The mask 30 preferably has
5 openings therein which allow the above-mentioned selected portions of the
back-
scattered light therethrough. Alternatively, it may be used in reflection, in
which case it
may include reflecting and non-reflecting portions thereon defining the
filter. In the
preferred embodiment, these openings will accept all or a substantial portion
of the
light from the scattering volume at 50 m from the apparatus, and much less of
the
to light from the scattering volume at 1 m. The proportion of light accepted
from 1 m is
preferably close to 10000 times smaller than at 50 m, in order to account for
the 1/,2
dependence.
A possible shape for the mask 30 is shown in FIG. 4. The shape accepts the
right
amount of light from each scattering volume along the optical path of the
excitation
beam so as to render the system response as flat as possible. A corresponding
calculated response is shown in FIG. 5. As can be seen, the obtained response
is
substantially flat and, in particular, sufficiently flat for the needs of most
short range
applications. As will be readily understood by one skilled in the art, a great
number of
possible mask profiles could be used to provide the desired result. In order
to design
the mask, the light distributions in the plane of the mask should preferably
be known
for a number of reflecting surfaces along the optical path of the emitted
beam. The
light distributions can be deduced from simulations using optical design
software or,
preferably, they can be directly measured, for example with a pinhole and
detector or
with a digital camera. In this latter case, an appropriate optically
scattering target is
placed at different distances from the emitting lens. A measurement is done
for each
target distance, by moving the pinhole-detector pair in the mask plane or by
acquiring
a digital image with the camera detector in the mask plane. Once the light
distribution
from each target distance is known, as in FIGs. 3A to 3H, computer software
may be
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11
used to compute the spatial response for a number of different intuitively
determined
mask geometries until an acceptable design is found. The mask is then
fabricated
with an appropriate technique (such a laser micro-machining), tested and
optimized.
Referring back to FIG. 1, the detector 26 is seen disposed behind the mask 30
and
accepts the portions of the scattered light transmitted by this mask. The size
of the
detector is preferably adapted to the size, shape and configuration of the
spatial filter
29. In this preferred embodiment, the detector 26 is larger than the mask 30.
Light
impinging on the detector 26 generates an electronic signal that is preferably
to amplified and digitized in processing electronics 32. The processing
electronics 32
should have sufficient bandwidth for the purpose and digitization is done in a
proper
manner. Various possible hardware and software apt to embody the processing
electronics 32 are well known in the art and need not be described further.
The result
is a set of data points spaced by a time interval corresponding to a distance
interval,
each data point representing the amount of light back-scattered by a given
volume at
the data point location.
The apparatus 10 is preferably provided in a casing 34 optically isolating its
various
components. The source arrangement 12 and light-collecting arrangement 24 are
preferably optically isolated from each other by a panel 36.
Referring to FIG. 2, there is shown an apparatus 10 according to an
alternative
embodiment of the present invention. In this embodiment, the source
arrangement 12
includes a large core optical fiber 42 to which the laser diode 16 is butt
coupled. The
optical fiber core has an output 44 which defines a round and uniform source
of light.
This output 44 is imaged, with output lens 18, at a distance of the apparatus
10
generally corresponding to its operating range (preferably between 50 and 100
m).
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lla
In the light-collecting arrangement 24, instead of a mask as described above,
the
spatial filter 29 is embodied by a plurality of waveguides such as, but not
limited to,
optical fibers 38, each having an input 40 positioned on one of the detecting
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planes and an output 41 coupled to the detector 26. The inputs 40 of the
fibers 38
are strategically distributed so as to collectively receive the appropriate
portions of
back-scattered light 25 according to the principle explained above. This
ensemble
of optical waveguides will therefore preferably accept light in order to
compensate
for the l/r2 dependence; for example, it will detect all of the light
collected from a
target scattering volume or surface at 50 m (or another end-of-range target)
and
much less of the light from the same target at I m or less. In any event, the
ensemble should accept the right amount of light from any scattering volume so
as
to render the system response as flat as possible. The inputs 40 of the
optical
1o fibers 38 are preferably in the same detecting plane D, but not
necessarily. When
it is the case, the fibers 38 should all have the same length in order not to
distort
the time response of the system. The captured portions of the back-scattered
light
will be guided to reach the detector 26.
Advantageously, the embodiment of FIG. 2 allows a gain of flexibility in the
positioning of the detector and associated electronics. In addition, the use
of a
much smaller detector area improves the signal to noise ratio and lowers the
cost
of the detector.
It is interesting to note that the different configurations of source optical
arrangements and light-collecting arrangements shown in FIGs. I and 2 need not
be necessarily used in the illustrated combinations. For example, the source-
.
optical arrangement of FIG. 2 may be used with the light-collecting
arrangement of
FIG. 1, and vice versa.
In summary, the present invention provides a useful LIDAR apparatus for
detecting particles and measuring substantially accurate concentrations within
a
shorter range than traditional aerosol detecting LIDARs. It will be noted that
one
advantage of some of the embodiments of the invention is the reduction of the
signal from longer distances than those targeted by the apparatus. The mask
can
be shaped to receive all of the light from 50 m, but light from 100 m could be
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reduced by more than the 1/r2 dependence, and this reduction could be larger
for
larger distances between the axes of the output and input lenses. In other
words,
the apparatus of the present invention can be used to render only a part of
the
spatial response flat, the other parts falling more or less rapidly to a much
lower
level. The spatial response can be tailored to the needs of a given
application.
It should be clear to one skilled in the art that many modifications could be
made
to the embodiments described above. Different shapes of light sources are
possible, and the mask or detection profile (such as the waveguides of the
second
1o embodiment) could be designed in many ways in order to render the system's
response as flat as desired for a given application. In addition, the source
need not
be on the axis of the output lens, lenses need not be of same diameter or
focal
length and need not be in the same plane.
As for existing LIDARs, these LIDARs could be scanned to form 3-D plots of
aerosol, suspension and other particle concentrations.
Of course, numerous modifications could be made to the embodiments described
above without departing from the scope of the invention as defined in the
appended claims.
