Note: Descriptions are shown in the official language in which they were submitted.
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"PARTICLE CLASSIFICATION METHOD AND APPARATUS"
BACKGROUND TO THE INVENTION
THIS invention relates to a particle classification method and apparatus.
One of the major problems encountered in the use of a Raman
spectroscopy technique in the detection of a particular particle is the fact
that the Raman component of the radiation emitted by the particle can
be swamped, and hence rendered difficult to detect, by other secondary
radiation, typically reflection and luminescence. For accurate detection
of the Raman component it would accordingly be beneficial to eliminate
the other secondary radiation effects.
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SUMMARY OF THE INVENTION
According to a first aspect of the invention there is provided a method
of classifying a particle, the method comprising the steps of:
- irradiating the particle with a pulse of monochromatic light
capable of causing Raman spectral activation of particles of a
certain class,
- filtering light emitted by the particle to remove components
which are outside a wavelength band including a characteristic
Raman waqvelength for particles of the certain class,
- directing the filtered onto a first light sensitive switch which is
arranged to close on receipt of filtered light,
- on closure of the first light sensitive switch, generating an
electrical pulse and directing such electrical pulse towards a pulse
detector, and
- classifying the particle according to whether or not the pulse
detector detects an electrical pulse within a predetermined time
window.
Thus the particle is classified according to whether it has a Raman
response indicative of a certain class of particle.
The method preferably comprises the steps of:
- splitting the pulse of monochromatic light into a test portion and
a control portion;
- irradiating the particle with the test portion of the pulse of
monochromatic light; and
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- using the control portion of the pulse of monochromatic light to
activate a time gating means which operates to allow the pulse
detector to detect only an electrical pulse which reaches the pulse
detector within the predetermined time window.
The control portion of the pulse of monochromatic light may be directed
onto a second light sensitive switch which is arranged to close on receipt
of such light and thereafter to prevent an electrical pulse from reaching
the pulse detector. In one embodiment, the electrical pulse is diverted
to earth by the second light sensitive switch when that switch is closed.
In another embodiment, the electrical pulse is diverted into a closed
circuit by the second light sensitive switch on closure thereof.
In an application of the method, the particle is classified according to
whether it is a diamond or non-diamond particle. The invention is
applicable to on-line sorting of particulate, diamondiferous material, in
which case a plurality of particles are irradiated in turn and particles for
which the pulse detector detects an electrical pulse are sorted from other
particles for which the pulse detector detects no electrical pulse.
According to a second aspect of the invention there is provided a
particle classification apparatus comprising:
- irradiation means for irradiating the particle with a pulse of
monochromatic light capable of causing Raman spectral
activation of particles of a certain class,
- a filter for filtering light emitted by the particle to remove
components which are at wavelengths outside a wavelength band
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containing a characteristic Raman wavelength for particles of the
certain class,
- a first light sensitive switch arranged to receive light filtered by
the filter and to close on receipt of filtered light of
predetermined intensity,
- an electrical pulse generator arranged, on closure of the first light
sensitive switch, to generate an electrical pulse,
- an electrical pulse detector arranged to detect an electrical pulse
generated by the pulse generator, and
time gating means operating to prevent an electrical pulse
generated by the pulse from reaching the pulse detector unless
the pulse reaches the pulse detector within a predetermined time
window,
the response of the pulse detector being indicative of whether or not the
particles is a particle of the certain class.
The apparatus may comprise a light splitter for splitting the pulse of
monochromatic light into a test portion with which the particle is
irradiated and a control portion which activates the time gating means.
Preferably, the time gating means comprises a second light-sensitive
switch which is arranged to receive the control portion of the pulse of
monochromatic light and to close on receipt of such control portion.
The apparatus may be arranged to classify the particle according to
whether it is a diamond or a non-diamond particle. In this case, the filter
is arranged to filter out light components outside a band including a
characteristic Raman spectrum for diamond.
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The apparatus may be a sorting apparatus in which case means are
provided for locating a plurality of particles one by one in an irradiation
zone where they are irradiated in turn, and means for sorting diamond
particles, for which the pulse detector detects an electrical pulse, from
other particles, for which the pulse detector detects no electrical pulse.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail, by way of example
only, with reference to the accompanying drawings in which:
Figure 1 graphically illustrates the different rise times for
Raman, luminescence and reflected light spectra;
and
Figure 2 diagrammatically illustrates the present invention.
DESCRIPTION OF A PREFERRED EMBODIMENT
The graph seen in Figure 1 has relative intensity and time on the vertical
and horizontal axes respectively. The graph shows three curves 10, 12
and 14 which are respectively representative of the spectra for reflected
light, the Raman spectrum for a diamond particle irradiated with
monochromatic laser light, and the luminescence or fluorescence
spectrum for the same particle. It will be noted that the reflected light
spectrum rises more rapidly than the Raman spectrum and that the
Raman spectrum rises more rapidly than the luminescence spectrum.
As explained previously, a difficulty with particle classification based
upon detection of the Raman spectra of particles is the fact that the
luminescence spectrum can swamp the Raman spectrum if it is allowed
to rise to a high enough intensity level, i.e. if sufficient time is allowed
to go by before spectral analysis takes place.
Figure 2 illustrates an apparatus which exploits the different rise times
of the Raman and luminescence spectra to classify the particle as a
diamond particle or as a non-diamond particle. Referring to this Figure
a pulse 16 of monochromatic light capable of causing Raman spectral
activation of a diamond particle is directed at a beam sputter 18 which
transmits a test portion 20 of light and reflects a control portion 22 of
light. The test portion 20 is used to irradiate a particle 24 which it is
desired to classify.
The light emitted by the particle can take various forms. Firstly there
may be a reflected light spectrum. Secondly, there may, in the case of a
diamond particle, be a Raman spectrum. Thirdly, there may be a
luminescence spectrum. With a view to eliminating or reducing
secondary radiation effects, the emitted light is passed through a narrow
band pass filter 25 which filters out light components except those in a
bandwidth including a characteristic Raman wavelength for diamond.
Light passing the filter 25 is directed onto an optical switch 26 located
in an electrical circuit 28 between a pulse generating power source 30
and a pulse detector 32. Light which falls on the switch 26 and which is
accordingly indicative of the presence of a Raman spectrum, causes it to
close momentarily and complete the electrical circuit.
_g_
An electrical pulse therefore travels, from left to right in Figure 2, from
the pulse generating power source to the pulse detector. The apparatus
also includes a short circuit line 34 in which a second optical switch 36
is located and which, in the illustrated example, extends to earth.
The control portion of the original light pulse is directed by the beam
splitter 18 onto a mirror 38 which in turn directs it onto the optical
switch 36. Like the switch 26, the switch 36 closes when light falls on it.
It will be appreciated that it takes a first, finite and calculable period of
time for the test portion of light to travel from the beam sputter 18 to
the particle 24 and for the emitted radiation to travel from the particle
24 to the switch 26.
It takes a second, finite and calculable period of time, after closing of
the switch 26, for the electrical signal to travel from the power source
to the junction point 40. It takes a third, finite and calculable period of
time for the control portion of light to travel from the beam splitter 18
to the mirror 38 and from the mirror 38 to the switch 36.
It will also be recognised that if the third period of time is shorter than
the sum of the first and second periods of time, the switch 36 will close
before the electrical signal has reached the junction point 40. In this
situation, any electrical pulse from the power source is connected to
earth and does not reach the pulse detector 32. The pulse detector 32
therefore has a nil response. If, on the other hand, the electrical pulse
has reached the junction point 40 before the switch 36 closes, the pulse
detector will have a positive response.
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The operating parameters and particularly distances are set such that a
positive response is indicative of a diamond particle, i.e. so that the
intensity of the Raman component of the radiation emitted by the
particle reaches a level high enough for the switch 26 to close, and for
the electrical pulse to reach the point 40, before the switch 36 closes.
If the detector has a nil response, this is indicative of a non-diamond
particle for which the emitted radiation contains no Raman component
characteristic of diamond. In this case, the parameters are such that the
relatively slow rise time of the luminescence spectrum prevents the
electrical signal from reaching the junction point 40 before the switch 36
closes, with the result that the electrical pulse goes to earth.
Thus only those electrical pulses attributable to the diamond-
characteristic Raman spectrum are detected by the pulse detector. Those
pulses attributable to the slower luminescence spectrum, and which are
generated later in time, are shorted to earth along the earth line.
It will be appreciated that the success of the technique as described
above is dependent on the sensitivity and speed of closing of the
switches 26 and 36. Currently available solid state switches with pico-
second reaction times are capable of suitably rapid operation.
In one application, the apparatus and technique described above are
incorporated in an automatic sorting system. In such a system, a mass of
particles which is to be sorted is manipulated such that the particles pass
one by one through the test location seen in Figure 2, where irradiation
takes place.
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The particles may, for instance, fall in free flight through the test
location after being projected from the end of a conveyor belt.
Whenever a desired particle, such as a diamond particle in the example
described above, is detected a microprocessor connected to the pulse
detector classifies the particles as a desired particle, i.e. a diamond, and
causes that particle to be separated from other non-selected particles.
This may, for instance, be done using an air blast ejector which is
activated to issue a short duration blast of air, at the appropriate
moment, at the falling stream of particles, thereby to deflect the
diamond particle away from the general particle stream for collection
apart from the other particles.
In the above example, the electrical pulse from the source 30 is removed
by the earth connection if the switch 36 has closed before the pulse
reaches the point 40. In another embodiment of the invention, the earth
connection may be replaced by a closed circuit, such as that indicated by
the numeral 42, around which the electrical pulse will travel without
resulting in a positive pulse detection by the pulse detector 32.
It will be appreciated that the switch 36 acts, in essence, to control a
time window which only allows the pulse detector to detect a pulse from
the source 30 if that pulse is attributable to impingement, on the switch
26, of the rapidly rising Raman spectrum characteristic of a desired
particle, such as a diamond in the case of a diamond sorting apparatus.
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Although specific reference has been made to diamond, it will be
appreciated that the principles of the invention are applicable to the
classification and sorting of other particulate materials as well.
