EP1240496A1

EP1240496A1 - Device for measuring, by diffraction, the size of substantially spherical particles, in particular opaque drops

Info

Publication number: EP1240496A1
Application number: EP00985341A
Authority: EP
Inventors: Jean De Metz
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1999-11-29
Filing date: 2000-11-28
Publication date: 2002-09-18
Also published as: WO2001040766A1; JP2003515738A; FR2801671A1; FR2801671B1; US6850324B1

Abstract

The invention concerns a device for measuring, by diffraction, the size of substantially spherical particles, in particular opaque drops. Said device comprises means (46, 49, 50, 51) for receiving light diffracted by the particles (12) illuminated with a light beam, for separating said diffracted light into concentric annular zones and concentrating the parts of diffracted light, corresponding to said zones, into points (24, 26, 28) different from one another. Levels of luminous intensity are detected corresponding to said points and the size of the particles are determined on the basis of said levels of luminous intensity. The angular pattern of the concentric zones is selected in accordance with a distribution optimising the discrimination between the dimensions of the drops.

Description

DEVICE FOR MEASURING, BY DIFFRACTION, SIZES OF SUBSTANTIALLY SPHERICAL PARTICLES, IN PARTICULAR OF

OPAQUE DROPS

DESCRIPTION

TECHNICAL AREA

The present invention relates to a device for measuring, by diffraction, the sizes of substantially spherical particles.

It applies in particular to the measurement of opaque drop sizes but also has many applications in particle size.

PRIOR STATE OF THE ART

A commercially available device is already known, intended to measure, by diffraction, the sizes of opaque drops whose diameters range from 0.1 μm to 100 μm.

In this known device, a set of these drops is illuminated by means of a laser. The drops infinitely diffract the light emitted by the laser. For each drop, assumed to be spherical, the light thus diffracted admits the optical axis of the device as the axis of symmetry of revolution.

The diffracted light in the focal plane of a converging lens is measured. We know that, in this plane, the illumination E obeys the following law: where Z is 2π x R x oc / λ.

In the above equality, we have considered the illumination E obtained, in the focal plane, for N drops of the same radius R, of which the illumination is noted E ₀ and obtained with a light of wavelength λ (by example equal to 0.5 μm); the focal length F of the lens is for example 200 mm, α is the diffraction angle and Ji represents the Bessel function of order 1.

In order to explain the drawbacks of this known device, let us consider for example two families of drops, one comprising drops of 1 μm in diameter while the other comprises drops of 10 μm in diameter.

To avoid a cascading diffraction process, the number of drops in the volume illuminated by the laser must be limited.

To do this, a number N of drops of 1 μm in diameter is chosen such that these drops absorb 10% of the light received from the laser. Assuming that the latter illuminates an area of 1 mm ² , the cross section of the set of N drops is then equal to 0.1 mm ² and N is equal to 127000. With drops of the same mass (total) but of diameter 10 μm, there are 1000 times less drops (i.e. 1270 drops) which cause an absorption of 1%. In the focal plane of the device, place the end of an optical fiber whose core has a diameter equal to 100 μm and suppose that the laser provides a light power of 5W to illuminate the drops.

The variations in the light power P (expressed in watts), which is diffracted in the fiber (logarithmic scale), as a function of the diffraction angle α (expressed in radians), are shown in FIG. 1 of the appended drawings, and therefore depending on the position of the end of the optical fiber in the focal plane, for the drops of 10 μm in diameter (curve I) and for those whose diameter is 1 μm (curve II). Curve III is the "sum" of the curves

I and II.

It is noted that, below 0.06 radians, it is the drops of 10 μm in diameter which illuminate the most and that the reverse takes place beyond. If the two families of drops are present and if the measurement is stopped at 0.06 radians (abscissa of point A in FIG. 1), only the family of drops of 10 μm in diameter is highlighted with an excess 10% for the number of drops.

With families with various drop sizes, the "hollows" of light that are observed in FIG. 1 are filled and the diffracted light power decreases regularly as a function of α. For drops of 1 μm in diameter, it should be noted that a more precise calculation would require the use of Mie series which are well known.

The known measurement technique, which has just been explained, has the following drawbacks. The amount of light collected by the optical fiber, that is to say the diffracted intensity, is low and ranges from approximately 15 μ near the optical axis of the device up to approximately 50 n for an angle. diffraction equal to 0.5 radians. In addition, the drops are distributed randomly in the area illuminated by the laser.

Interferences between the light fluxes diffracted by each drop form peaks of light. The respective positions of these peaks, which depend on the positions of the drops, do not exhibit symmetry of revolution around the optical axis of the measuring device.

This makes it difficult to count the measurements aimed at determining the sizes of the drops.

STATEMENT OF THE INVENTION

The object of the present invention is to remedy the above drawbacks.

The invention aims to reduce, even eliminate, the effects of light peaks with a view to a more reliable determination of the various drop sizes and, more generally, of the various sizes of substantially spherical particles, in particular in the range from 0.1 μm to 1000 μm. The invention also aims to increase the amount of diffracted light that is collected to obtain a more sensitive device, or usable at a higher rate, than the known device, mentioned above.

Specifically, the subject of the present invention is a device for measuring the sizes of substantially spherical particles, this device being characterized in that it comprises: - a light source capable of providing a light beam intended to illuminate the particles, these beam diffracting particles,

- Optical concentration and separation means provided to receive the light thus diffracted and capable of separating this diffracted light into a plurality of concentric annular zones and of concentrating the parts of the diffracted light, which correspond respectively to these annular zones, in a plurality of points different from each other, and

photodetection means provided for detecting the light intensities corresponding respectively to these points, the particle sizes being determined as a function of these light intensities, a device in which the optical means for concentration and separation comprise:

optical concentration means capable of concentrating the diffracted light, the optical concentration means comprising an optical focusing having a flat entry face and an aspherical exit face, and - optical separation means comprising a plurality of annular portions of optical deflection means, these annular portions being provided for intercepting the light thus concentrated and deflecting the light thus intercepted in respective directions different from each other. According to a first particular embodiment of the invention, these optical deflection means are prisms.

Preferably, the angle of each prism is small, less than 20 °, to avoid geometric aberrations.

According to a second particular embodiment of the invention, these optical deflection means are light reflection means.

According to a third particular embodiment of the invention, these optical deflection means are diffraction gratings ("diffraction gratings").

In this case, use is preferably made of an optic of the kind described in the following document, incorporated by reference into the present description:

[1] French patent n ° 1550406 issued on November 12, 1968 (invention of Jean de Metz and François Millet) or better, in the following document incorporated by reference into the present description: [2] Certificate of addition No. 95885 to French patent No. 1550406, issued on October 4, 1971 (invention of Jean de Metz and François Millet).

More precisely, according to a first preferred embodiment, the focusing optic is a lens having a first planar face and a second aspherical face of curvature corresponding to a minimum coma, pierced along its axis with a blind hole opening into the first face of the lens, with a polished wall, and of a depth such that, when a light is sent towards the second face, the hole is crossed by the light rays reflected successively on the first face and the second face of the lens. According to a second preferred embodiment, the focusing optic is a dioptric system capable of focusing a monochromatic light beam substantially parallel to its focus and of a type which only interposes two successive diopters in the path of the rays of the beam, this dioptric system comprising a central lens having a first flat face and a second aspherical face of profile revolution corresponding to a minimum coma, pierced along its axis with a hole opening in the first face of the lens, with a polished wall, and of sufficient depth so that, when a light is sent towards the second face, the hole is crossed by the light rays successively reflected by the first and second faces of the central lens, and an annular lens surrounding the central lens, also having a first flat face and a second aspherical face, extending from the second face of the central lens of a length such that the light rays successively reflected on the first face and the second face of the annular lens have a pseudo-focusing outside the lenses.

Preferably, the device which is the subject of the invention further comprises means preventing the diffraction of light at the interfaces of the annular portions which comprise the optical means of concentration and separation.

In the invention, the number of concentric annular zones in which the light diffracted by the particles is separated is preferably equal to M + 1 where M is the considered number of different sizes of the particles.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood on reading the description of exemplary embodiments given below, purely indicative ^'and in no way limitative, with reference to the accompanying drawings in which:

"FIG. 1 shows the variations of the light power diffracted by drops in a fiber as a function of the diffraction angle, for two sizes of drops, and has already been described,

"FIG. 2 schematically illustrates the principle of the invention, "Figure 3 is a schematic and partial sectional view of a device useful for understanding the invention, using annular portions of converging lenses," Figure 4A is a schematic and partial sectional view of a device in accordance to the invention, using annular portions of prisms,

"FIGS. 4B and 4C schematically illustrate examples of focusing optics which can be used in the invention,

• Figure 5 is a schematic and partial sectional view of another device according to the invention, using annular portions of mirrors, "Figure 6 is a schematic and partial sectional view of another device according to l invention, using annular portions of diffraction gratings,

"FIG. 7 schematically illustrates the manufacture of these annular portions of diffraction gratings,

FIG. 8 schematically illustrates the possibility of forming chamfers on the annular portions of lenses of FIG. 3 to improve the operation of the corresponding measuring device, and

"Figures 9 and 10 show curves intended to explain the determination of the angular limits of rings used in the invention. DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The principle of a measuring device according to the invention is schematically illustrated by FIG. 2. This device is intended to measure the sizes of substantially spherical particles, for example the sizes of opaque drops whose diameters range from 0.1 μm to 100 μm.

These are for example drops of a molten metal, which form a cloud 2, this cloud 2 being generated by means not shown.

The device of FIG. 2 comprises a light source 4, for example a laser, which emits a light beam 6 with parallel rays. This beam 6 is focused by a converging optic 8 in a plane

10 which constitutes a focal plane of this optic 8.

In addition, as can be seen in FIG. 2, the light beam 6 illuminates a part 12 of the cloud 2 and it is sought to know the sizes of the drops which are in this part 12 of the cloud and diffract the light from the incident beam 6. More precisely, we look for the number of drops per size in this lit part 12.

The light diffracted by the drops from part 12 has the reference 16 in FIG. 2.

The region of intersection of this light 16 and the focal plane 10 (which is perpendicular to the axis X of the beam 6) is decomposed into a plurality of concentric annular zones which are circular and therefore have a symmetry of revolution around an axis perpendicular to plane 10 (the X axis in the example shown).

In the example of FIG. 2, there are two annular zones 18 and 20 and we also consider the central zone 22 delimited by the annular zone 18 which is the most internal and is thus included between the zones 20 and 22 .

Let us concentrate (by optical means not shown) the light of each zone at a point or more exactly a small zone, so that the points obtained are different from each other.

In the example of FIG. 1, there are thus three points 24, 26 and 28 corresponding respectively to zones 18, 20 and 22. Under these conditions, the sequence formed by the values of the quantities of light coming respectively from zones 18, 20 and 22, which have been defined in the focal plane 10, no longer include the interference peaks which hindered the analysis of the measurements in the prior art.

In addition, much more light is collected at large angles, where the light comes mainly from small drops and more difficult to exceed the level of background noise. We also see in Figure 2 photodetection means of the lights respectively concentrated at points 24, 26 and 28. In the example shown, these photodetection means consist of photodetectors 30, 32 and 34 whose number is equal to the number zones that we have defined. These photodetectors supply electrical signals representative of the intensities of the lights thus concentrated.

Electronic means 36 are provided for determining, from these signals, the various sizes of the drops which are found in the part 12 of the cloud 2.

This determination is made for example according to the method described in the following document: B. Arad, New method for studying debris from laser induced spall in metals, Review of Scientific Instruments, vol. 66 n ° 12, December 1995, p.5590 to 5597.

We also see means 38 for displaying the results with which the electronic processing means 36 are provided.

Figures 3 to 6 are schematic sectional views of various examples of the optical means of concentration and separation which are provided for receiving the diffracted light 16, separating this light into concentric annular zones and concentrating the parts of the diffracted light, ^" which correspond respectively to these annular zones, at points or foci 24, 26, 28 which are different from each other.

In the example of FIG. 3, useful for understanding the invention, the concentration and the separation take place at the same time. As can be seen in this FIG. 3, a glass ring 40 is used which is cut from a converging lens 41 seen in dotted lines and which concentrates the light in its focus 24. We also use another glass ring

42 which is cut from another converging lens

43 dotted view and which concentrates the light in its focal point 26. The section of the ring 42 is offset so that the focal point 26 is different from the focal point 24 as seen in FIG. 3.

The set of glass rings 40 and 42 is completed by a central portion of lens 44 whose focal point 28 is distinct from the focal points 24 and 26.

The assembly formed by the rings 40 and 42 and by the lens portion 44 is substantially arranged at the focal plane 10.

By way of example, an assembly of fifty rings can be produced in this way plus the central lens portion 44. This assembly can then be reproduced by molding.

In the example of FIG. 4A, the concentration and separation functions are carried out by different components.

The concentration function is performed by a converging optic 46 disposed substantially at the level of the focal plane 10.

Preferably, an optic glass-planar lens with an optical index of 1.618, of the type described in document [1] mentioned above, is used as the optic.

It will be recalled, with reference to FIG. 4B, that it is a single lens 46a having a plane outlet face 46b and an aspherical inlet face 46c of curvature corresponding to a minimum coma, pierced along its axis 46d with a blind hole 46a opening into the exit face of the lens, with a polished wall, and of a depth such that the rays reflected successively on the exit face and the face lens entry.

As a variant, an optic of the kind described in document [2] also cited above is used. It will be recalled, with reference to FIG. 4C, that it is a dioptric system intended to focus at its focal point a substantially parallel monochromatic light beam and of a type interposing only two successive diopters on the path rays of the beam. This dioptric system comprises a central lens 47 having a planar exit face and an aspherical entry face 47a of revolution 47b of profile corresponding to a minimum coma, pierced along its axis 47c with a hole 47d opening into the exit face of the lens, with a polished wall, and of sufficient depth for the rays reflected successively by the exit and entry faces of the central lens to pass through, and an annular lens 47e surrounding the central lens, also having a face flat exit 47f and an aspherical entry face 47g, projecting from the front face of the central lens of a length such that the rays 47h successively reflected on the exit face and the entry face of the annular lens have a pseudofocusing outside the lenses. In the present invention, the two optics described in these documents are reversed: the planar faces of the lenses are used as entry faces of the beam diffracted by the particles and the aspherical faces are used as exit faces so as to form a substantially parallel output beam.

Such optics lead to a minimum of geometric aberrations for a small image. In the example of FIG. 4A, the separation function is carried out by a set of prisms, namely two prisms 48 and 50 in the example shown, it being understood that, for each prism, only one part in the form of a ring is kept. The prisms 48 and 50 are shown in dotted lines and the rings corresponding respectively to the prisms 48 and 50 have the references 49 and 51.

It should be noted that the orientations of the prisms are different for obtaining the separate focal points 24, 26 and 28.

In addition, the angles of the prisms are small, less than 20 °, so as not to cause aberrations. In the example, the angles α and β of the prisms 48 and 50 are respectively 10 ° and 5 °. In FIG. 4A, the prisms are shown joined by one face but the rings, once made, are embedded in each other.

The dimensions of the prisms are chosen as a function of the distance from all of the portions 49 and 51 to the optics 46 to precisely cover the desired angular zones. In the example of FIG. 5, the separation function is carried out by a set of annular mirrors, namely two annular mirrors 52 and 54 in the example shown. The central hole of the set of mirrors lets in part of the light which is focused by optics 46 and corresponds to point 28.

The annular mirrors 52 and 54 deflect parts of light corresponding respectively to points 24 and 26.

It is specified that the annular mirrors have different inclinations relative to the axis Y of the optic 46 and are pierced with elliptical holes such that the light coming from this optic 46 is separated between circles.

In the example of FIG. 6, the separation function is carried out by a photoresist layer 56 which is deposited on the convex face of the optic 46 and in which annular diffraction gratings are formed. concentric, namely the two diffraction gratings 58 and 60 in the example shown, as well as a central diffraction grating 62, as seen in FIG. 6. These gratings 58, 60 and 62 are holograms intended to deflect the light which comes from optics 46 respectively towards points 24, 26 and 28.

To obtain these networks 58, 60 and 62, the zones of the resin are successively exposed photosensitive corresponding to these networks so as to obtain the holograms.

For each exposure, two laser sources are used which are formed - at the level of the zone 12 and at the level of the point 24 to form the network 58,

- at zone 12 and at point 26 to form the network 60, and

- At the level of the zone 12 and at the level of the point 28 to form the network 62 and which in each of these three cases create interference in the desired zone of the layer of photosensitive resin to form the corresponding hologram there. We choose a layer of photosensitive resin thick enough to have only one useful interference order in each case.

It is specified that the exposure of each zone of the layer 56 is made through two appropriate masks which prevent the exposure of the rest of this layer 56 by each of the two laser sources.

FIG. 7 diagrammatically illustrates, by way of example, the insolation of the zone of the layer

56 in which we want to form the central diffraction grating 62.

Two masks 64 and 65 prevent the exposure of the rest of the layer 56 by the beams coming respectively from the laser sources 70 and 68 which we see in FIG. 7. A single laser 66 is used to form the laser source 68 at the level from the cloud part of drops 12 and the other laser source 70 at the point or focus 28.

To do this, a semi-transparent mirror 72 and laser reflectors 74, 76 and 78 are used, which are conveniently arranged as seen in FIG. 7 and as is conventional in the field of holography.

After development of the exposed layer 56, the three networks - holograms which concentrate the light respectively at points 24, 26 and 28 are obtained.

In another particular embodiment (not shown), the photosensitive resin layer is formed on the flat face of the optics 46. In this case, the exposure method provides additional correction of the aberrations already reduced by using an optic of the kind described in document [1] or document [2] and makes it possible to obtain a good image of part 12 of the cloud of drops on each of the photodetectors which are placed respectively at points 24 , 26 and 28 as we saw above.

In another particular embodiment (not shown), the photosensitive resin layer is formed not on the optic 46 but on a glass plate spaced from this optic 46, the latter then being between the glass plate and the studied part 12 of the cloud of drops.

With regard to each of the devices, which are schematically and partially shown in Figures 3 to 6, it is specified that, to strip the measurements making it possible to know the various sizes of the drops, it is necessary to calibrate this device as for the luminosity produced by each ring and to check that each ring only lights a photodetector.

In order to computerize the measurements, it is preferable to use a network (“array”) of photodetectors, for example an array of photodiodes or a CCD type detector. In this case, the concentration and separation means are produced so that the various light concentration points (referenced 24, 26 and 28 in FIGS. 3 to 6) have positions allowing this use. For example, if you use a photodiode array, these points must be aligned. Let us return to the device of FIG. 3. Preferably, in order to have, at the outlet of the set of rings 40, 42, 44, only "useful" light and no stray light capable of being diffracted at level of the internal or external edges or blanks of each ring, diffracted light should be prevented from "licking" these edges.

To do this, for example chamfers are formed on these internal and external edges, on the side where the diffracted light arrives, and each chamfer thus formed is made opaque, for example by depositing a layer of black paint thereon.

This is illustrated by referring to FIG. 8 and by taking the example of the ring 40 in FIG. 3. FIG. 8 shows the internal edge 80 and the external edge 82 of this ring 40. We also see the chamfers 84 and 86 respectively formed on these edges 80 and 82 and the layer of black paint 88 formed on each chamfer.

Preferably, the procedure is the same in the case where annular portions of prisms are used (FIG. 4A).

Similarly, in the case where the diffraction gratings of FIG. 6 are used, areas absorbing light are preferably formed between the exposed areas. To do this, we put a ring of black paint on it.

Let us now consider the determination of the angular limits of the rings.

We have seen (figure 1) that the light comes at the small angles of the large drops and at the large angles of the small drops.

Let us draw, with the help of the formula (1) mentioned above, a series of curves (Figure 9) corresponding to ten sizes ranging from 1 μm to 10 μm, staggered from μm to μm, instead of the two curves I and II of figure 1.

The envelope of these curves having the appearance of a hyperbola, the same series of curves is presented in FIG. 10 but by plotting the inverses α ^"1 (expressed in radians ^{" 1} ) of the angles α. The envelope becomes a line segment corresponding to Z little different from 1.36, where Z = 2π _x R _x / λ was defined above. We see that each size shines the most in a particular range of angles. This interval is defined by the value and the expression of Z which we have given above. It is therefore in the zone of the focal plane 10, zone corresponding to this interval, that it is necessary to place a ring to have the best illumination for this size. This is where discrimination is best.

We thus obtain a law of "cutting" the focal plane.

By way of example, an assembly of 11 rings intended to receive light from particles of sizes ranging from 0.84 μm to 9 μm is given by table I below, for diffracted light having a wavelength of 0.5 μm. the optimal angular cutting of the rings is made according to the law: α

1.36 = 2πR -. λ

This law optimizes the discrimination of drop sizes.

Table I

Now consider the determination of the preferred number of rings. Figure 1 also shows that a single measurement of the illumination E, made for example at α = 0.06 radians, does not make it possible to know whether the family of drops has 1 μm or 10 μm in diameter, nor to know the number of points. Using the measurement of the illumination E ₀ of the drops, we would have two solutions for this number: 127000 drops of 1 μm or 1270 drops of 10 μm.

A measurement of the opacity indicating 1% absorption would ensure that the diffracting family is formed of 10 μm drops. A second measurement at another angle would have confirmed this.

In general, it takes M + l measurements (angles, opacity, illumination of the drops) to have the number of diffracting drops in each size among M sizes.

So preferably use M + l rings. This number of rings should not be increased too much because, in a device according to the invention, each ring collects only a fraction approximately equal to 1 / M of all the light received by the means of concentration and separation d 'such a device. In the invention, all of the diffracted light is collected at optimum angles corresponding to the sizes sought. This eliminates the effect of interference peaks, an effect which hampered the analysis of the measurements in the prior art. It also provides the maximum sensitivity which is used either to measure lower concentrations of particles or to maximize the speed of data acquisition.

The present invention is not limited to the measurement of opaque drop sizes, the diameters of which range from 0.1 μm to 100 μm.

It applies to the measurement of the sizes of all kinds of particles of substantially spherical shapes, these particles being transparent or translucent or opaque and having sizes belonging to the interval ranging from 0.1 μm to 1 mm.

One can for example use the invention for the granulometry of powders of materials such as plaster, cement and iron ore. A powder of this kind is then illuminated, placed opposite a device conforming to the invention, on a support which is transparent to the light used and which is kept fixed or moved in front of the device.

Admittedly, GB 2044445A (Coulter Electronics) discloses systems for measuring the energy and direction of a light flux coming from particles, in order to identify the characteristics of these particles. These systems essentially comprise concentration-separation devices with non-plane mirrors, in which the concentration means are not separated from the separation means.

Such devices allow a correct measurement of the particle sizes only for the particles which are located exactly at the focus of the mirrors. For the other particles, the geometric aberrations due to the mirrors prevent correct measurements.

The present invention overcomes this drawback through the use of optical concentration means, comprising a focusing optic having a plane entry face and an aspherical exit face, and optical separation means, distinct from the optical concentration means. . A good image is thus obtained on an optical field of the order of 1 °.

Claims

1. Device for measuring the sizes of substantially spherical particles, this device being characterized in that it comprises: - a light source (4) capable of providing a light beam (6) intended to illuminate the particles (12), these beam diffracting particles,

- optical means for concentration and separation (46, 49-51; 46, 52-54; 46, 58-60-62) provided to receive the light thus diffracted and capable of separating this diffracted light into a plurality of annular zones concentric and to concentrate the parts of the diffracted light, which correspond respectively to these annular zones, at a plurality of points (24, 26, 28) different from each other, and

- photodetection means (30, 32, 34) provided for detecting the light intensities corresponding respectively to these points, the particle sizes being determined as a function of these light intensities, a device in which the optical means for concentration and separation comprise:

optical concentration means (46) capable of concentrating the diffracted light, the optical concentration means comprising a focusing optic having a planar input face and an aspherical output face, and

- optical separation means comprising a plurality of annular portions (49-51, 52-54, 58-

60-62) of optical deflection means, these portions annulars being provided to intercept the light thus concentrated and to deflect the light thus intercepted in respective directions different from each other.

2. Device according to claim 1, wherein the optical deflection means are prisms (48, 50).

3. Device according to claim 2, in which the angle (α, β) of each prism is less than 20 °.

4. Device according to claim 1, wherein the optical deflection means are light reflection means.

5. Device according to claim 1, wherein the optical deflection means are diffraction gratings.

6. Device according to any one of claims 1 to 5, further comprising means

(84-86, 88) preventing the diffraction of light at the interfaces of the annular portions which comprise the optical means of concentration and separation.

7. Device according to any one of claims 1 to 6, in which the number of concentric annular zones is equal to M + 1 where M is the number considered of different sizes of the particles (12).

8. Device according to any one of claims 1 to 7, in which the focusing optic is a lens having a first planar face and second aspherical face of curvature. corresponding to a minimum coma, pierced along its axis with a blind hole opening into the first face of the lens, with a polished wall, and of a depth such that, when a light is sent towards the second face, the hole is crossed by the light rays reflected successively on the first face and the second face of the lens.

9. Device according to any one of claims 1 to 7, in which the focusing optics is a dioptric system capable of focusing at its focal point a substantially parallel monochromatic light beam and of a type interposing only two successive dioptres on the path of the rays of the beam, this diopter system comprising a central lens having a first planar face and a second aspherical face of profile revolution corresponding to a minimum coma, pierced along its axis with a hole opening in the first face of the lens, with a polished wall, and of sufficient depth so that, when a light is sent towards the second face, the hole is crossed by the lumlinous rays reflected successively by the first and second faces of the central lens, and an annular lens surrounding the central lens, also having a first planar face and a second aspherical face, projecting from the second me face of the central lens of a length such that the light rays successively reflected on the first face and the second face of the annular lens have a pseudo-focusing outside the lenses.