Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume or surface-area of porous materials
  • G01N15/02—Investigating particle size or size distribution
  • G01N15/0205—Investigating particle size or size distribution by optical means
  • G01N15/0211—Investigating a scatter or diffraction pattern
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/47—Scattering, i.e. diffuse reflection
  • G01N2021/4704—Angular selective
  • G01N2021/4711—Multiangle measurement
  • G01N2021/4716—Using a ring of sensors, or a combination of diaphragm and sensors; Annular sensor

Definitions

• the present invention relates to a device for measuring, by diffraction, the sizes of substantially spherical particles.
• a commercially available device is already known, intended to measure, by diffraction, the sizes of opaque drops whose diameters range from 0.1 ⁇ m to 100 ⁇ m.
• a set of these drops is illuminated by means of a laser.
• the drops infinitely diffract the light emitted by the laser.
• the light thus diffracted admits the optical axis of the device as the axis of symmetry of revolution.
• the number of drops in the volume illuminated by the laser must be limited.
• N of drops of 1 ⁇ m 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 ⁇ m, 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 ⁇ m and suppose that the laser provides a light power of 5W to illuminate the drops.
• 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 ⁇ near the optical axis of the device up to approximately 50 n for an angle. diffraction equal to 0.5 radians.
• the drops are distributed randomly in the area illuminated by the laser.
• 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 ⁇ m to 1000 ⁇ m.
• 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.
• 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
• - 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.
• these optical deflection means are prisms.
• the angle of each prism is small, less than 20 °, to avoid geometric aberrations.
• these optical deflection means are light reflection means.
• these optical deflection means are diffraction gratings ("diffraction gratings").
• 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.
• 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.
• 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.
• 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.
• 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
• FIG. 2 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 ⁇ m to 100 ⁇ m.
• 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
• 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.
• annular zones 18 and 20 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 .
• photodetection means of the lights respectively concentrated at points 24, 26 and 28.
• 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.
• 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.
• the concentration and the separation take place at the same time.
• 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.
• 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.
• 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.
• 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.
• an optic glass-planar lens with an optical index of 1.618 is used as the optic.
• 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.
• 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.
• 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.
• 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.
• angles of the prisms are small, less than 20 °, so as not to cause aberrations.
• angles ⁇ and ⁇ of the prisms 48 and 50 are respectively 10 ° and 5 °.
• 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.
• 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.
• 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.
• 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.
• the zones of the resin are successively exposed photosensitive corresponding to these networks so as to obtain the holograms.
• 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,
• 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
• 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.
• 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.
• the photosensitive resin layer is formed on the flat face of the optics 46.
• 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.
• 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.
• a network for example an array of photodiodes or a CCD type detector.
• 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.
• 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.
• each chamfer thus formed is made opaque, for example by depositing a layer of black paint thereon.
• FIG. 8 shows the internal edge 80 and the external edge 82 of this ring 40.
• FIG. 8 shows the chamfers 84 and 86 respectively formed on these edges 80 and 82 and the layer of black paint 88 formed on each chamfer.
• the procedure is the same in the case where annular portions of prisms are used (FIG. 4A).
• areas absorbing light are preferably formed between the exposed areas. To do this, we put a ring of black paint on it.
• a measurement of the opacity indicating 1% absorption would ensure that the diffracting family is formed of 10 ⁇ m drops. A second measurement at another angle would have confirmed this.
• 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.
• 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 ⁇ m to 100 ⁇ m.
• 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.
• 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.
• 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 °.

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• 1999
  • 1999-11-29 FR FR9914992A patent/FR2801671B1/fr not_active Expired - Fee Related
• 2000
  • 2000-11-28 EP EP00985341A patent/EP1240496A1/de not_active Withdrawn
  • 2000-11-28 WO PCT/FR2000/003318 patent/WO2001040766A1/fr not_active Application Discontinuation
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