Note: Descriptions are shown in the official language in which they were submitted.
201~31
APPARATUS FOR THE MEASUREM~NT OF AEROSOLS AND
DUST OR THE LIKE DISTRIBUTED IN AIR
The invention relates to an apparatus for the measurement of
aerosols and dust or the like distributed in air, the
apparatus comprising a light source for generating an
extended elongate scattering volume by means of a primary
beam directed into the medium to be measured, a collector
for scattered light and also at least one photoreceiver
connected after the collector for scattered light.
In an apparatus of this kind, which can for example serve
for the investigation of ~iquid suspensions and emulsions,
the scattered light emerging from the scattering volume is
first concentrated and subsequently detected.
Scattered light photometry is a proven method for the
measurement of aerosols and dust distributed in the air and
also for the investigation of liquid suspensions and
emulsions. The intensity of the scattered light and its
distribution over the scattering angle are dependent on the
particle size and particle concentration and can be used Eor
their determination.
Particularly when the particle size is known an angular
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resolution is not required for the scattered light
measurement in order to determine the partide concentration.
On the contrary, in order to increase the signal/noise ratio
in the range of scattering angles of interest it is
necessary to detect as large a solid angle of the scattered
light as possible. For this purpose a large detector surface
or a corresponding optical concentrator is necessary.
The use of a large area detector such as for example a PIN
photodiode array or a photomultiplier (see US-PS 4 597 666)
is however not only extremely costly but a large detector
surface necessarily also leads to a large electrical
capacity of the overall detector by which the noise of the
detector is incr~ased.
Scattered light photometers are, on the other hand, known in
which the scattered light is focussed by an image forming
optical system onto the detector. In this way only a
restricted solid angle can be detected even when using large
area Fresnel lenses.
With the in-situ measurement of dust concentrations in room
air, exhaust air or larger chimneys in particular, one is
mainly concerned with extended scattering volumes. In such
cases the use of an image forming optical system is no
longer possible simply because of the restricted depth of
focus. Finally, the detection of the full solid angle around
a scattering volume can fundamentally not be realised by an
image forming optical system.
The invention is based on the object of so further
developing the apparatus of the initially named kind that
the scattered light emerging from more extended scattering
volumes can in particular also be measured with the simplest
means over solid angle~ which are practically as large as
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desired.
The object is satisfied in accordance with the invention in
that the collector for scattered light comprises transparent
fluorescent material; and in that the photoreceiver is
optically coupled to the light exit region of the scattered
light collector which acts as a fluorescent light guide,
with the particular scattering angles which are to be
detected being determinable by a corresponding shape of the
collector for scattered light.
Here several photoreceivers are advantageously provided and
distributed uniformly over the light exit region of the
scattered light collector.
As a result of the use of a fluorescent collector in
accordance with the invention for the measurement of
aerosols and dust or the like distributed in air, the
collector can be matched without problem to the particular
scattering angle geometry of interest. Even with more
extended scattering volumes, such as can for example be
present during the in-situ measurement of dust
concentrations in room air, exhaust air or larger chimneys
the scattered light can be detected with the collector over
a solid angle which is practically as large as desired. With
a larger solid angle of this kind the signal/noise ratio of
the scattered light detection increases so that on the whole
a more precise measurement is possible.
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The invention is accordingly in particular based on theconcept, for the measurement of aerosols or of dust
distributed in the air, of simultaneously using transparent
fluorescent elements as collectors for the scattered light
which is to be detected and as optical concentrators, with
the invention exploiting the characteristic of the
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correspondingly shaped transparent fluorescent elements that
they concentrate at least a large part of the fluorescenk
light to a remaining light exit zone in the manner of a
light guide, practically independently of the particular
shape.
Finally, the deposition of dust ont.o the transparent
fluorescent collectors is unproblematic in contrast to dust
deposits on the lenses of an image forming optical system
since light incident on the dust grains is scattered on into
the collector at least to a large degree. The directional
independency of the collectors accordingly contributes in
particular to ensuring a measurement accuracy and
sensitiYity which is as high as possible, even when dust
deposits are present on the collector.
The mass concentration of the scattering medium and/or the
range of visibility present in the scattering medium can for
example be determined from the measured intensity of the
scattered light. Furthermore, it is also possible to
determine the size of the particles present in the
scattering medium from the dependence of the measured
intensity of the scattered light on the wavelength of the
light.
While it is preferably only the intensity of the forward
scattering which is measured, in particular for the
determination of medium mass concentrations it can be
expedient, in particular for the determination of lower mass
concentrations to measure the scattered light intensity over
at least substantially the entire solid angle.
A part of the light exit region of the scattered light
collector which acts as a fluorescent light guide can be
made specularly reflecting or provided with reflector
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surfaces, with the photoreceivers in this case being
optically coupled to the remaining non-specularly reflecting
part of the light exit region.
The mirrored or reflector surfaces not only prevent possible
light loss but also reduce the light exit area at which
photoelements are to be provided. The number of the
photoelements that are required can thus be kept extremely
small.
If for example it is only desired to detect the forward or
backward scattering then it is expedient to design the
scattered light collector in plate-like manner and to
arrange the photoreceiver or receivers at the edge of the
plate.
For the detection of the forward or backward scattering it
is of advantage when the plate-like scattered light
collector is arranged at least substantially perpendicular
to a preferably narrow light beam emerging from the light
source. In this arrangement the scattered light collector
can have a central opening through which the narrow light
beam originating from the light source passes.
In another variant it is however also possible to guide the
primary light originating from the light source sideways
past the scattered light collector. If several light sources
are provided then the scattered light collector is
praferably arranged between the preferably mutually parallel
light beams generated by the light sources. An arrangement
of this kind can for example be used for the measurement of
the back radiation.
For the determination of higher mass concentrations in
particular it is preferably only the intensity of this back
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scattering which is measured. For example, with an
arrangement of one or several light sources (~or example
light emitting diodes) at the center of a plate-like
fluorescent collector or around the latter with radiation
into tha scattering medium the collector will collect the
scattered light radiated back by the scattering medium.
An extremely simple arrangement for detecting the forward
scattering is obtained if the scattered light collector has
a spacing from the light source and includes a collector
surface which faces the light source and which is
illuminated by the forwardly scattered light.
A further variant is characterised in that the scattered
light collector is arranged in the region of the light
source and has a collector surface which detects the back
scattering.
Those collector surfaces which are not to deliver a
contribution to the measurement result can for example be
made specularly reflecting at the outside. Furthermore, when
using plate-like scattered light collectors it is mainly
expedient to make these of circular shape.
In particular for the determination of the range of
visibility by the measurement of the light scattered in the
atmosphere it is necessary to measure the scattered light
intensity either so far as possible over the entire solid
angle or to detect a fraction proportional to this. ~ith
unpolarised incident light in particular the scattered light
intensity is expediently measured with this arrangement over
at least substantially the entire range of scattered angles
from 0 to 180 over a constant azimutual aperture angle. A
geometry of this kind can be realised with an elongate
scattering volume in the incident light beam (for example
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laser beam) for example by a tube or by a half tube around
the primary beam or by a narrow collector strip parallel to
the primary beam.
If it is desired to measure not only the forward scattering
and the backward scattering but rather also the scattering
over as large a solid angle as possible, or to detect a
fraction of the scattered light proportional to this
provision is accordingly advantageously made for the
scattered light collector to be tubular or to have the shape
of a tubular section, and for the photoreceivers to be
provided at the ends of the tube, or at least at one tube
end and/or at one of the two elongate side edges of the
tubular section. If photoreceivers are only provided at one
tube end then the other tube end is again preferably
provided with reflector surfaces in order to avoid possible
light loss. Reflector surfaces can also be correspondingly
provided at the elongate edges.
When using a tubular scattered light collector or a
scattered light collector having the shape of the section of
the tube the narrow light beam generated by the light source
is preferably directed along the tube axis.
The scattered light collectors are preferably manufactured
from fluorescent plexiglass or from other organic or
inorganic glasses doped with fluorescent dye. By way of
example, with a collector plate consisting of plexiglass,
approximately 74~ of the fluorescent light does not emerge
from the plate surface as a result of total reflection. On
the contrary this proportion of the light passes as in a
light guide to the edges of this plate.
Relatively high degrees of efficiency can in particular be
achieved if the light source generates monochromatic light
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and if the fluorescent dye of the scattered light collector
is matched to the wavelength of the monochromatic light. The
wavelength of the fluorescent light: can preferably also be
matched to the maximum spectral sensitivity of the
photoreceivers.
In accordance with a further embodiment several scattered
light collectors, in particular scattered light collectors
arranged above one another, are provided with different
absorption wavelengths, with at least one light source which
transmits polychromatic light being associated with these
transparent fluorescent scattered light collectors. The
scattered light can in this case be simultaneously measured
at several wavelengths. Conclusions can then in particular
be drawn concerning the particle size in the scattering
medium from the dependency of the intensity of the scattered
light on the wavelength.
Further advantageous variants of the invention are set forth
in the subordinate claims.
The invention will now be described in the following in more
detail with reference to examples and to the drawing in
which are shown:
Fig. 1 an apparatus for the measurement of the intensit~ of
the forward scattering by means of a plate-like,
transparent, fluorescent collector for scattered
light,
Fig. 2 an apparatus for the measurement of the scattered
light intensity over approximately the entire solid
angle by means of a transparent fluorescent tube,
Fig. 3 a measurement apparatus comparable with the
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apparatus of Fig. 2 in which however a transparent
fluorescent half tube is used to detect the
scattered light, and
Fig. 4 an apparatus for measuring the intensity of the back
scattering, with a plate-like transparent
fluorescent collector agai;n being provided for
detection of the scattered light of interest.
The measurement arrangement shown in Figs. 1 to 4 serves for
the measurement of the intensity of the scattered light
which emerges from a scattering volume in which a scattering
medium 22 is illuminated by light by means of light source
12 and 12' and 12' respectively.
In the arrangements of Figs. 1 to 3 the scattering volume is
in each case generated by a narrow light beam 30 which is
transmitted by the light source and directed into the
scattering medium 22. In the measurement arrangement shown
in Fig. 4 two light sources 12' and 12" are provided to
generate two narrow light beams 30.
The scattered light is in each case detected by means of a
transparent fluorescent collector 14, 16, 18 or 20. Here a
transparent fluorescent collector acting as a fluorescent
light guide is in each case provided for the concentration
of the fluorescent light generated in the collector 14, 16,
18 or 20.
The scattered light concentrators or collectors 14, 16, 18
or 20 that are used can in particular be manufactured from
fluorescent plexiglass, or from another organic or inorganic
glass doped with a fluorescent dye.
The scattered light collectors 14-20 which detect the
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relevant scattered light component simultaneously operate as
optical concentrators as a result of the light conducting
characteristics for the fluorescent light. Only a small part
of the fluorescent light can namely emerge from the
collector surface as a result of the total internal
reflection. This small part of the light large passes to
restricted light exit regions 26, as in a light guide, where
the concentrated fluorescent light is finally detected by
means of photoreceivers 24.
The photoelements 24 can for example be PIN photodiodes
which are optically coupled to the light exit regions 26 of
the fluorescent light guides or scattered light collectors
14, 16, 18 or 20 for the detection of the fluorescent light.
Reflector surfaces are provided at the portions of the light
exit regions not occupied with photoreceivers in order to
avoid possible light loss. In the measurement arrangement of
Fig. 1 which is provided for the measurement of the mass
concentration of an aerosol only one light source 12 is
provided to generate a narrow light beam 30 which is
directed into the scattering medium 22. The scattered light
collector 14 of transparent fluorescent material is arranged
at a clear distance from the light source 12 and is formed
in the present case as a large area circular plate. The
plate-like scattered light collector 14 stands perpendicular
to the narrow incident light beam 30 which as the primary
beam generates an extended elongate scattering volume.
The plate-like scattered light collector 14 has a central
opening 28 through which this narrow light ~eam 30
originating from the light source 12 passes.
The surface of the scattered light collector 14 facing the
light source 12 detects the forward scattering emerging from
the scattered volums. The oppositely disposed surface of the
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scattered light collector 14 can be treated in such a way
that any back scattering which eventually occurs has no
effect on the generation of the fluorescent light.
The photoreceivers 24 are uniformly distributed over the
edge of the plate~ e scattered light collector 14. In
order to avoid a possible light loss the regions of the
plate edge between the photoreceivers can be provided with
reflector surfaces. Through appropriate dimensioning of a
plate-like scattered light collector 14 of this kind it can
be arranged that the scattered light captured within a
certain size range of the scattered particles (ca. 1 to 100
times the wavelength) is independent from the particle size
and proportional to the total mass of the scattering
particles.
The measurement arrangement of Fig. 2 is for example
suitable for the determination of extremely low aerosol or
dust concentrations where the largest possible part of the
scattered light is to be detected.
~or this purpose the transparent, fluorescent, collector 16
for scattered light is formed as a tube which surrounds the
narrow light beam 30 coming from the light source 12. The
primary beam or the narrow light beam 30 extends along the
tube axis through the scattering medium 22.
The photoreceivers 24 are optically coupled to the edges of
the two tube ends (to the end faces of the tube) and are in
turn uniformly distributed over the relevant edge. It is
however also basically possible to provide such
photoreceivers 24 at only one tube end and the edge not
occupied with photoreceivers can again be provided with
reflector surfaces. Moreover, the region between two
photoreceivers 24 can also again be mirrored or provided
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with reflector surfaces.
In this measurement arrangement the largest part of the
light scattered by the particles in the tube is absorbed by
the tube walls and is passed on in large part as fluorescent
light to the tube ends as a conse~lence of the light
conducting characteristics of the t:ube. With constant
particle size distribution the intensity of the ~luorescent
light emerging at the edges of the tube ends is proportional
to the dust or aerosol concentration in the tube.
Fig. 3 shows a measurement arrangement comparable to the
arrangement of Fig. 2 in which however the transparent
fluorescent scattered light collector 18 is simply formed by
a half tube. The narrow light beam generat~d by the light
source 12 again extends along the tube axis through the
scattering medium 22. Photoelements 24 are again optically
coupled to the edge (end face) of the tube ends, i.e. to the
light exit rPgion 26 of the scattered light collector which
acts as the fluorescent light guide. As the longitudinal
side edges also form a light exit region 26 reflector
surfaces are provided there to avoid possible light loss. It
is however also possible to arrange photoreceivers at these
longitudinal edges.
This arrangement is in particular suitable for the
determination of the range of visibility in the atmosphere
since for this purpose the scattered light intensity must
either be detected over the entire solid angle as far as
possible or a fraction proportional to this must be
detected. With non-polarised incident light this signifies
that the scattered light is to be detected with a constant
azimutual opening angle over the entire range of scattering
angles from 0 to 180. A geometry of this kind can however
be realised with an elongate scattering volume in the
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incident li~ht ~eam (for example laser ~eam) by a tube or a
half tube around the primary beam or by a narrow detector
strip parallel to the primary beam.
The measurement arrangement shown in Fig. 4 includes two
light sources 121, 12" which generates two at least
substantially mutually parallel narrow light beams 30. A
large area circular plate is again provided as a transparent
fluorescent scattered light collector 20 as in the case o~
the arrangement of Fig. 1.
The scattered light collector 20 is however arranged
directly in the vicinity of the light sources 12' and 12"
and between the two narrow light beams 30. This measurement
arrangement serves to detect the back scattering of the
medium 22 which, is in particular with extremely high dust
concentrations, represents a suitable measure for the dust
concentration. The back scattering is detected in the
present case by the surface of the plate-like scattered
light collector 20 which faces away from the light sources
12', 12". The surface of the scattered light collector which
faces the light sources can in turn be dealt with in such a
way that the forward scattering has no influence on the
genaration of the fluorescent light. Several photoelements
24 are again uniformly distributed over the edge or side
surface of the circular plate, i.e. the light exit region 26
of the plate~ e, circular collector 20 for scattered
light, and the concentrated fluorescent light is detected
via these photoelements.
In place of the illustrated outer light sources 12' and 12"
one can also provide one or more light sources in the
central region of the disk-like scattered light collector
20.
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An advantageous use of the measurement arrangement of the
invention is in particular always possible where priceworthy
large area receivers are re~uired with simultaneously large
apertures.
It is not only important that the fluorescent dye can be
adapted to achieve a maximum absorption of the measurement
wavQlength but rather also that the wavelength of the
fluorescent light can be matched to the maximum spectral
sensitivity of the particular photoreceiver (for example
silicon).
The following fields of application are in particular
conceivable: dust detectors in exhaust gases, dust
monitoring at the workplace or in the exhaust air from
halls, air conditionin~ systems, visibility measuring
apparatus etc. Here the shape of the transparent fluorescent
scattered light collector can in each case be matched
without problem to the required scattering angle geometry.
