JP2004271340A - Instrument for measuring particle size distribution - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide the so-called diffraction/scattering type measuring instrument for measuring a particle size distribution sharply reduced in cost as compared with a conventional one and prevented from the lowering of measuring precision though made compact. <P>SOLUTION: The instrument for measuring particle size distribution is equipped with a light source 2 for emitting fundamental light L displaying a small diameter direction HD and a large diameter direction HV on its cross section, the condensing lens 6 arranged on the optical axis of the fundamental light L and interposed between the light source 2 and a group of particles C to be measured and a photodetector 4 for detecting the intensity and anglular distribution of diffracted/scattered light L'. The diameter D6 of the lens 6 is made larger than the small diameter dimension DS of the cross section of the fundamental light and smaller than the large diameter dimension DL thereof in a lens-installated region and the light detecting surface 4a of the photodetector 4 is set to a position avoiding the vicinity of the axial line AV of the large diameter direction in the cross section of the fundamental light. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a so-called diffraction / scattering type particle size distribution measuring apparatus for measuring a particle size distribution based on an intensity angle distribution of diffracted / scattered light generated by irradiating a dispersion target particle group with basic light. is there.
[0002]
[Prior art]
As shown in Patent Document 1, this type of diffraction / scattering type particle size distribution measuring device irradiates a laser beam onto a measurement target particle dispersed in a cell via a condenser lens and generates scattered light there. Is detected by a photodetector, and the particle size distribution of the particles to be measured is determined from the scattering pattern based on MIE scattering theory or the like.
[0003]
Conventionally, a He-Ne laser has been used as a light source of the laser light. However, since the He-Ne laser is relatively expensive, an inexpensive semiconductor laser has recently been used.
[0004]
[Patent Document 1]
JP-A-9-257683
[Problems to be solved by the invention]
However, in general, the divergence angle of a laser beam emitted from a semiconductor laser greatly differs between a direction parallel to the polarization plane and a direction perpendicular to the plane of polarization, and a small-diameter direction and a large-diameter direction perpendicular to the laser beam appear on the cross section. If a lens is designed in accordance with its large-diameter dimension so that all light is incident on the lens, the lens becomes large, expensive and bulky. Of course, if the lens is placed at a position close to the semiconductor laser, the diameter of the lens can be reduced.
[0006]
On the other hand, if only the lens is made smaller without changing the distance between the laser and the lens, a photodetector is installed because the basic light that can not enter the lens is diffracted at the periphery of the lens and affected by spherical aberration Interference fringes are generated at the focal plane of the laser beam. When the light enters the photodetector, the background value of the scattered light intensity signal output from the photodetector is increased, and the measurement of the weak scattered light becomes difficult.
[0007]
Therefore, the present invention uses a low-priced light source such as a semiconductor laser as a light source and uses a compact condensing lens, thereby making it possible to significantly reduce the cost and size as compared with the conventional one, while reducing the measurement accuracy. It is a main object of the present invention to provide a so-called diffraction / scattering type particle size distribution measuring apparatus which does not cause the measurement.
[0008]
[Means for Solving the Problems]
That is, the particle size distribution measuring apparatus according to the present invention measures the particle size distribution of the measurement target particle group based on the intensity angle distribution of diffraction / scattered light generated by irradiating the dispersed measurement target particle group with the basic light. A light source that emits basic light whose cross-section has spread in the small-diameter direction and the large-diameter direction due to the reason that the spread angle differs depending on the polarization direction, and the light source and the measurement device that are arranged on the optical axis of the basic light. A condensing lens interposed between the target particle groups, and a photodetector for detecting an intensity angular distribution of the diffracted / scattered light, and the diameter of the condensing lens is set to a value corresponding to the fundamental light at a portion where the condensing lens is installed. While the cross-section is larger than the dimension along the first direction, which is the smaller-diameter direction, and smaller than the dimension along the second direction, which is the larger-diameter direction, the light-receiving surface of the photodetector is the same as that in the basic light cross section. Along the second direction Characterized in that it is set at a position avoiding the cord near.
[0009]
In such a case, since the lens diameter is smaller than the second dimension in the large diameter direction of the basic light section at the installation position, it is possible to promote compactness and cost reduction. On the other hand, in this case, since the basic light is larger than the lens in the second direction, interference fringes and the like may occur on the focal plane along the second direction. Since the position is set so as to avoid the vicinity of the center line of the basic light section along the second direction, the light receiving surface of the photodetector does not overlap the area where the interference fringes or the like occur, and the interference fringes may be caused. This makes it possible to accurately measure even weakly scattered light by suppressing an increase in background value. In this specification, “light” includes ultraviolet light, infrared light, and the like in addition to visible light. The lens diameter is the diameter of a portion that effectively acts as a lens. The diameter of the basic light section is, for example, a diameter of a portion having a light intensity up to a predetermined ratio in comparison with the light intensity at the center, and a portion having a light intensity higher than a predetermined value that can be effectively used for particle size distribution measurement. Is the diameter of
[0010]
As a specific embodiment, a light-receiving surface of the photodetector may have a fan shape that expands right and left around a center line of the basic light along the first direction.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to FIGS. 1, 2, and 3. FIG.
[0012]
In the particle size distribution measuring apparatus 1 according to the present embodiment, the scattering pattern (angular distribution of the scattered light intensity) of the scattered light L ′ generated when the measurement target particles C are irradiated with light is determined by the particle size according to MIE scattering theory or the like. Utilizing this, the particle size distribution is measured by detecting the scattering pattern.
[0013]
Specifically, as shown schematically in FIG. 1, a laser beam L as a basic light emitted from a semiconductor laser 2 as a light source is applied to a group of particles C to be measured dispersed in a cell 3. The intensity of the diffracted or / and scattered light (hereinafter referred to as scattered light) L ′ generated there is detected by the plurality of photodetectors 4 and based on the values of the scattered light intensity signals output from the respective photodetectors 4. The particle size distribution is calculated by the information processing device 5. Between the semiconductor laser 2 and the cell 3, there is provided a circular condenser lens 6 for refracting the laser light L output from the semiconductor laser 2 in a direction to converge. The light receiving surface 4a of the photodetector 4 is set on the focal plane of the photodetector 4.
[0014]
Each part will be described in detail. The semiconductor laser 2 outputs, for example, a laser beam L having a wavelength of 650 nm in the present embodiment. Thus, the spread angle of the laser light L differs between the direction parallel to the polarization plane and the direction perpendicular to the plane of polarization. In this example, the angle is 30 ° in the direction perpendicular to the polarization plane (hereinafter referred to as vertical polarization direction VD) and 10 ° in the direction parallel to the polarization plane (hereinafter referred to as parallel polarization direction HD) at full angle at half maximum. Therefore, the cross-sectional dimension of the laser light L is large until the condensing lens 6 is reached, where the vertical polarization direction VD, which is the second direction, is a large-diameter direction, and the parallel polarization direction HD, which is the first direction, is large. It becomes smaller in the small diameter direction.
[0015]
The cell 3 is a transparent container made of glass or the like, and is arranged on a sample circulation path in which the sample is circulated by, for example, a pump or the like, so that the internal sample always flows and the particles C to be measured are dispersed. It is a flow type. Needless to say, the particle C to be measured may be dispersed by another method such as a batch method.
[0016]
The light detector 4 is, for example, a photodiode, and has its light receiving surface 4a set to a focal plane orthogonal to the optical axis LC of the laser light L and located at the focal point of the laser light L as described above. As shown in FIG. 2, the light receiving surface 4a has a fan shape centered on the optical axis LC of the laser light L, and the light receiving surfaces 4a of the plurality of detectors 4 are arranged concentrically to form an array. are doing. Since the scattering angle of the scattered light L ′ generated at the small-diameter particles increases, the light detector 4 at the center receives the scattered light L ′ from the large-diameter particles. L ′ is received. The light receiving surface 4a on the optical axis LC is for measuring the intensity of the transmitted laser light L.
[0017]
As shown in FIG. 1, the information processing device 5 receives the scattered light intensity signals output from the respective photodetectors 4, calculates the intensity angle distribution of the scattered light L ′ based on the values, and calculates the particle size. It measures the distribution. More specifically, the signal receiving unit 51 receives an analog scattered light intensity signal output from each photodetector 4 and performs processing such as impedance conversion and AD conversion. A calculation unit 52 is provided for calculating a particle size distribution and the like by taking in a scattered light intensity signal from an input / output interface and calculating according to a predetermined program based on MIE scattering theory and the like stored in a memory.
[0018]
In this embodiment, as shown in FIGS. 3 and 4, the diameter D6 of the condenser lens 6 is set to a dimension (small diameter) along the parallel polarization direction HD in the cross section of the laser beam L at the portion where the condenser lens 6 is installed. It is configured to be sufficiently larger than the dimension (DS) and smaller than the dimension (large-diameter dimension) DL along the vertical polarization direction VD.
Here, the cross-sectional dimension DS or DL of the laser light is a dimension in a range where the light intensity I (x) as viewed in the cross section of the laser light L is 1 / e ＾ 2 of the light intensity I (0) on the optical axis. Shall be referred to.
For example, if the laser light has a Gaussian distribution, if the distance from the optical axis is x,
I (x) = I (0) exp (−2x ＾ 2 / w0 ＾ 2)
Therefore, the dimension in the range where x = w0 is satisfied.
"To make the diameter D6 of the condenser lens 6 sufficiently larger than DS" means to set the radius D6 / 2 of the condenser lens to three to four times w0. In this manner, the diameter of the condenser lens 6 can be set to be the smallest diameter in a direction parallel to the laser beam L on the focal plane in a direction HD that is hardly affected by diffraction or the like. .
[0019]
On the other hand, the light receiving surface 4a of the photodetector 4 is formed into a fan shape that spreads symmetrically about the center line AH along the parallel polarization direction HD of the laser beam cross section at that portion, and the vertical polarization of the laser beam cross section is obtained. It is configured to avoid the vicinity of the center line AV along the direction VD.
[0020]
Therefore, in such a case, since the lens diameter D6 is smaller than the large diameter dimension DL of the laser beam cross section, the size can be reduced and the cost can be promoted.
[0021]
On the other hand, as a result of this, due to diffraction and spherical aberration occurring at the peripheral edge of the lens in the vertical polarization direction VD, the interference fringes only along the vertical polarization direction axis AV at the focal plane as shown in FIGS. Intense light that becomes background such as IF is generated. However, in the present embodiment, as shown in FIG. 2, the light receiving surface 4a of the photodetector 4 has a fan shape that spreads symmetrically to the left and right with the parallel polarization direction axis AH as the center line, and the area P where the interference fringes IF occur is formed. Since the light receiving surface 4a of the photodetector 4 is not overlapped with the light source 4, an increase in the background value of the scattered light intensity signal due to the reception of the interference fringes IF can be suppressed, and highly accurate scattered light can be obtained. Strength measurement becomes possible.
[0022]
Further, in the present embodiment, as shown in FIG. 2, the light receiving surfaces 4a of the photodetector 4 are provided on both sides with the large-diameter axis AV as a boundary. The scattered light L 'increases and the measurement accuracy improves.
[0023]
Note that the present invention is not limited to the above embodiment. For example, a plurality of condenser lenses may be combined, or a resin such as plastic may be used. Of course, it is not limited to a spherical lens, but may be an aspheric lens. Also, in order to remove stray light, the light may be once collected and passed through a pinhole.
[0024]
Further, the light receiving surface of the photodetector may be provided only on one side with respect to the center line of the laser beam cross section, or side scattered light or back scattered light is detected as described in JP-A-2000-146814. Needless to say, a light detector may be further provided. Also, the fan shape of the light receiving surface of each photodetector does not need to be all opened at the same angle as in the above-described embodiment. The fan angles of the surfaces may be different from each other. In addition, the light receiving surfaces of the photodetectors facing each other with the laser beam cross-section center line as a boundary may be provided at positions where the gaps between the photodetectors complement each other. In this way, a larger number of scattered lights having different distances from the optical axis center can be detected, and particularly, the measurement resolution of large-diameter particles can be improved.
[0025]
Of course, the present invention can be applied not only to laser light whose divergence angle differs depending on the polarization direction, but also to laser light having a different shape cross section whose cross section has a large diameter direction and a small diameter direction, and has the same effect. It can be played.
[0026]
In addition, the present invention is not limited to the illustrated example, and various changes can be made without departing from the gist of the present invention.
[0027]
【The invention's effect】
As described in detail above, according to the present invention, since the lens diameter is smaller than the second dimension, which is the large-diameter dimension of the basic light section at the installation position, the compactness and the cost reduction are promoted. it can. On the other hand, in this case, since the basic light is larger than the lens in the second direction, interference fringes and the like may occur on the focal plane along the second direction. The light receiving surface of the photodetector does not overlap with the area where the interference fringes and the like occur because the positions are set so as to avoid the vicinity of the center line of the cross section of the basic light along the second direction. And it is possible to accurately measure even weakly scattered light.
[Brief description of the drawings]
FIG. 1 is an overall schematic diagram showing a basic structure of a particle size distribution measuring device according to an embodiment of the present invention.
FIG. 2 is a partial front view showing a part of the photodetector in the embodiment.
FIG. 3 is an explanatory diagram illustrating a difference in a spread angle of a laser beam in the embodiment.
FIG. 4 is an explanatory diagram illustrating a difference in a spread angle of a laser beam in the embodiment.
FIG. 5 is an enlarged view in which the vicinity of a photodetector in FIG. 3 is enlarged.
FIG. 6 is an enlarged view in which the vicinity of the photodetector in FIG. 4 is enlarged.
FIG. 7 is an exemplary cross-sectional view showing a cross-sectional shape of the laser light at a focal plane according to the first embodiment;
[Explanation of symbols]
2. Light source (semiconductor laser)
4 Photodetector 4a Light receiving surface 6 Condensing lens C Particle to be measured L Basic light (laser light)
L ': Diffraction / scattered light HD: First direction (parallel polarization direction)
VD: Second direction (vertical polarization direction)
LC: Optical axis DS: Small diameter dimension (dimension along the first direction)
DL: Large diameter dimension (dimension along the second direction)
AH: center line along the first direction AV: center line along the second direction

Claims (2)

The particle size distribution of the measurement target particle group is measured based on the intensity angle distribution of the diffracted / scattered light generated by irradiating the dispersed measurement target particle group with the basic light, and the spread angle differs depending on the polarization direction. For example, a light source that emits basic light whose cross-section has a small-diameter direction and a large-diameter direction spread on the cross section, and a condensing lens that is arranged on the optical axis of the basic light and is interposed between the light source and the particle group to be measured. A light detector for detecting an intensity angle distribution of the diffracted / scattered light, and the diameter of the condensing lens is set along a first direction which is a small-diameter direction of a basic light cross-section at a portion where the condensing lens is installed. While being larger than the dimension and smaller than the dimension along the second direction, which is the large-diameter direction, the light receiving surface of the photodetector was avoided near the center line along the second direction in the basic light section. Set to position Particle size distribution measuring apparatus according to claim. The particle size distribution measuring device according to claim 1, wherein the light receiving surface of the photodetector has a fan shape that expands left and right about a center line of the basic light along a first direction.

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