Particle size measuring apparatus

An optical system for irradiation comprising a light source, a lens, an optical fiber, a rectangular waveguide, an objective lens and a prism and an optical system for receiving light comprising an objective lens, an aperture and an optical fiber are arranged with their optical axes intersecting one another at a point P in a measuring volume. In the optical system for irradiation, the light emitted from the optical fiber and having an intensity distribution expressed by a normal distribution curve is changed to a light having a uniform intensity distribution and a rectangular cross section, which is irradiated through the prism to the point p in the measuring volume. A light scattered at an angle of 90.degree. by a particle flowing through the measuring volume is guided through the optical system for receiving light to a photodetector which converts the scattered light to an electric signal (current) called a scattered light pulse. An arithmetic unit calculates the particle size from the height of this scatterd light pulse and the flow speed from the pulse width.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a particle size measuring apparatus for optically 
measuring the particle size and/or particle size distribution of 
particles. 
2. Description of the Related Art 
Among the prior art methods, there is a method measuring light intensity 
scattered by one particle to be measured. This method is usually called 
the Coulter counter. A prior particle size measuring apparatus embodying 
this method is described in A. Ederhof, and G. Bibelius, "Determination of 
Droplet Sizes and Wetness Fraction in Two-phase-flows Using a 
Light-scattering Technic", Institution of Mechanical Engineering Journal 
of Mechanical Engineering Science, 1976, pages 21 to 27. Similar 
apparatuses have been disclosed in Japanese Patent Disclosure No. 
78-13625, 81-58638 and 81-145330. 
FIG. 1 shows the particle size measuring apparatus in the above paper. The 
apparatus includes an optical system for irradiation comprising light 
source 1, optical fiber 3, aperture 15, and objective lens 5 and an 
optical system for receiving light comprising objective lens 7, prism 16, 
aperture 8, and optical fiber 9. The two optical systems are arranged in 
such a manner that the angle between the optical axes is 90.degree. at the 
point P. The light emitted from light source 1 and transmitted through 
optical fiber 3 is irradiated to aperture 15. The image of aperture 15 is 
formed at the point P by means of objective lens 5. The light scattered at 
an angle of 90.degree. by a particle existing at the point P is collected 
by objective lens 7 and turned at an angle of 90.degree. by prism 16. The 
objective lens 7 forms an image at aperture 8 at the point P. 
The scattered light through aperture 8 is guided by optical fiber 9 to 
photomultiplier 10. The photomultiplier 10 converts the scattered light to 
the electric current signal. The electric signal thus produced is fed to 
waveform analyzer 17 where the electric signal proportional to scattered 
light is inverted to the particle size (diameter). The oscilloscope 18 
monitors the electric signal. 
As shown schematically in FIG. 2, the point P is, in fact, a cube with some 
dimensions which are defined by the images of apertures 15 and 8 and 
constitutes the measuring volume. 
When a particle flows through this measuring volume in the direction 
perpendicular to both the two optical axes of objective lenses 5 and 7, 
the light scattered by the particle at an angle of 90.degree. is received 
by the optical system for receiving light and is guided into 
photomultiplier 10 and is converted into an electric signal. 
This electric signal is in a form as shown in FIG. 3. The pulse width 
corresponds to the flow speed of the particle and the pulse height to the 
particle size (diameter). To put more specific, the pulse width increases 
as the the particle flowing speed decreases and the pulse height becomes 
higher as the particle size increases. Hereafter, this pulse is referred 
to as the scattered light pulse. 
The intensity of the scattered light at an angle of 90.degree. by a 
particle of a known size can be calculated by Mie's theory or Fraunhofer's 
diffraction theory. Therefore, if the relationship between particle size 
and scattered light intensity is obtained by calculation beforehand, it is 
possible to determine particle sizes from the scattered light pulse height 
measured by waveform analyzer 17. 
In the above prior apparatus, however, the intensity distribution of the 
irradiated light is in the form of normal distribution since light is 
irradiated through optical fiber 3 to the measuring volume. Thus, it is 
difficult to illuminate the measuring volume uniformly. That is to say, 
the light intensity is higher toward the center of the measuring volume 
and it decreases toward the peripheral area. 
Hence, this prior apparatus has a drawback that the scattered light pulse 
height varies whether the particle flows the central area or the 
peripheral area of the measuring volume. The drawback results in errors in 
particle size measurements. 
To avoid this problem, it is necessary to guide particles to be measured 
via a very thin pipe and finely control the flowing position of the 
particles so that the particles flow an area (central area) in the 
measuring volume where the intensity of the projected light is relatively 
uniform. However, this control itself is no easy matter and if this 
control is implemented, it is impossible to use the whole area of the 
measuring volume, and the number of particles that can be measured per 
unit time decreases notably. Consequently, it takes a long time to obtain 
particle size. And, to obtain a particle size distribution is virtually 
impossible. 
As described above, with particle size measuring apparatuses of the prior 
art, measurement errors occur depending on the flowing position of the 
particles in the measuring field of view. If the flowing position is 
limited to the central area to solve this problem, long time has to be 
taken for measurement. 
SUMMARY OF THE INVENTION 
The object of this invention is to provide a particle size measuring 
apparatus capable of solving the above-mentioned problem that a long time 
is taken in measurement if the flowing position of particles is limited to 
reduce measurement errors that occur depending on the flowing position in 
the measuring volume and also capable of measuring the size of many 
particles with high accuracy and in a short time. 
According to this invention, a particle size measuring apparatus comprises 
a light source for emitting a light; a waveguide, provided between the 
light source and a measuring volume through which a particle is flowed, 
for converting the light emitted from the light source to a light having a 
uniform intensity distribution and for irradiating a converted light to 
the measuring volume; an optical system for receiving a light scattered at 
a predetermined angle by the particle and for producing a scattered light 
pulse for each particle; and an arithmetic unit for obtaining the size of 
the particle based on the scattered light pulse height. 
With a particle size measuring apparatus according to this invention, the 
light irradiated into the measuring volume is in a uniform distribution. 
There is not much change in the level of the scattered light even if there 
are changes in the flowing position of particles in the measuring volume, 
thus assuring high accuracy for particle size measurement. 
Further, it is not necessary to limit to a narrow extent the flowing 
position of the particle and it is possible to feed a large number of 
particles through the measuring value for a short time by using a thick 
pipe and supply particles at a high flow speed. As a result, data of 
particle size distribution can be obtained in a short time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A particle size measuring apparatus as a preferred embodiment of this 
invention will now be described with reference to the accompanying 
drawings. 
Referring to FIG. 4, light source 41 is a laser source, a xenon lamp, a 
halogen lamp or an LED, for example. The light from this light source 41 
is incident to optical fiber 43 through lens 42. Optical fiber 43 is used 
to guide the light from light source 41 to the neighborhood of a measuring 
position. 
The light emitted from optical fiber 43 is incident to rectangular 
waveguide 44. 
The intensity distribution of the light emitted from optical fiber 43 is a 
normal distribution. The light intensity is high at the center (at the 
axis of the optical fiber) and decreases toward the peripheral area. 
Rectangular waveguide 44 serves to shape the image irradiated from 
rectangular waveguide 44 into a rectangular form. The light emitted from 
rectangular waveguide 44 is guided through objective lens 45 and prism 46 
to the point P at the measuring position. In this case, objective lens 45 
and prism 46 are arranged so that the image of the outlet cross section of 
rectangular waveguide 44 can be formed at the point P. The optical system 
for irradiation is formed of light source 41, lens 42, optical fiber 43, 
rectangular waveguide 44, objective lens 45 and prism 46. 
On the other hand, the image at the point P is incident through objective 
lens 47 and aperture 48 to optical fiber 49 by the receiving optical 
system for receiving light, which is so arranged that its optical axis 
intersects perpendicularly to the optical axis of the optical system for 
irradiation at the point P. In this case, objective lens 47 is arranged so 
that the image of aperture 48 is formed at the point P. Under the above 
arrangement, a measuring volume defined by the image of the outlet cross 
section of rectangular waveguide 44 and the image of aperture 48 is formed 
at the point P as schematically shown in FIG. 5. Aperture 48 may be in any 
form, but if the flow speed of the particle is to be measured, the cross 
section of rectangular waveguide 44 must be in a rectangular or square 
form. If particle size only is measured, the cross section of waveguide 44 
need not be in a rectangular or square form, but may be in a circular 
form. 
The scattered light in the 90.degree. direction by the particles in this 
measuring volume is transmitted through objective lens 47, aperture 48 and 
optical fiber 49 into photodetector 50 such as a photomultiplier whereby 
the incident scattered light is converted to an electric signal (current) 
called a scattered light pulse as shown in FIG. 3. 
This scattered light pulse is subjected to current-to-voltage conversion in 
amplifier 51 and amplified therein. The amplified voltage pulse is input 
to waveform memory device 52. Waveform memory device 52 converts the 
analog input scattered light pulse to digital data and stores it in a 
digital memory as a shape of a waveform. The waveforms of scattered light 
pulses stored in the memory are read by arithmetic unit 53. 
The intensity of the scattered light due to a particle with a known 
particle size can be calculated using Mie's theory or Fraunhofer's 
diffraction theory. 
FIG. 6 shows the relationship between the size (diameter) of particles 
being measured that flow through the measuring volume and the pulse 
height. Using this relationship, the scattered light pulse height is 
converted to the particle size. 
On the basis of the relationship between the size of particles being 
measured and the scattered light pulse height, arithmetic unit 53 obtains 
the particle size and speed (width of the scattered light pulse) of many 
particles. Arithmetic unit 53 obtains particle size distribution and speed 
distribution from data of particle size and speed obtained and displays 
the results on display unit 54. 
Rectangular waveguide 44 is formed of a rectangular rod of transparent 
glass, for example. As shown in FIG. 7, light emitted from optical fiber 
43 is changed into light with a rectangular sectional form by utilizing 
total reflection at the glass surfaces and the intensity distribution is 
made uniform as indicated by the characteristic line a. The characteristic 
line b indicates the intensity distribution (normal distribution) when 
rectangular waveguide 44 is not used. To be more specific, when the light 
is incident to rectangular waveguide 44 formed of transparent glass, the 
light incident into the central portion of the end face of rectangular 
waveguide 44 reaches the central portion of the measuring volume. However, 
the light obliquely incident into waveguide 44, is totally reflected and 
is substantially subjected to a Fourier-transform. 
The intensity of the irradiated light in the measuring volume is 
distributed uniformly as shown in FIG. 7. The longer the length of 
rectangular waveguide 44, the more uniform the intensity distribution 
becomes. However, if the irradiation loss is great at objective lens 45, 
it is difficult to obtain a uniform intensity distribution since the 
Fourier transformed components of higher order are lost. Therefore, it is 
desirable to use objective lens 45 of a large aperture. 
Then, the scattered light pulse height can be made at almost the same level 
both when the particle flows the central portion of the measuring volume 
and when it flows the peripheral area. This improves the accuracy of 
particle size measurement based on the scattered light pulse height. In 
addition, since there are fewer variations in the width of the scattered 
light pulse according to the position where particles flow, the accuracy 
is improved in particle speed measurement on the basis of pulse width. As 
this apparatus can be used effectively for measurement in the whole area 
of the measuring volume, it is possible to flow a large number of 
particles per unit time and measure a large number of particles in a short 
time, thereby obtaining particle size distribution and speed distribution 
quickly. 
When a laser source is used for light source 41 of the optical system for 
irradiation, there is a problem of intensity irregularity due to speckles. 
This problem can be mitigated by inserting light-scattering member 61 such 
as a ground glass plate, for example, at the incident end face of 
rectangular waveguide 44 as shown in FIG. 7. The light-scattering member 
may be inserted at any position of the emitting optical system. Instead of 
providing a ground glass plate, the end face of rectangular waveguide 44 
may be finished as a ground glass. 
This invention is not limited to the preferred embodiment described above 
but may be embodied in various forms. For example, with regard to 
rectangular waveguide 44, it is possible to form rectangular rod-like 
cavity 62 in the center of waveguide 44 as shown in FIG. 8 and attach 
mirrors 63 or light-reflecting film on the four internal surfaces of this 
cavity 62. 
Though the configuration of FIG. 4 is such that the light scattered at an 
angle of 90.degree. is used for measurement, the scattering angle is not 
limited to 90.degree. . If the scattering angle is decreased, the 
scattered light pulse height can be increased and the signal-to-noise 
ratio in measurement can be improved. Conversely, if the scattering angle 
is increased to be greater than 90.degree. to measure backward scattered 
light, the measuring system can be reduced in size. In short, the optimal 
scattering angle has only to be selected according to the purpose and 
conditions such as the arranged position of a measuring apparatus. 
In the above embodiment, the particle size measuring apparatus is so 
arranged as to measure the flow speed as well as the size of particles. 
Needless to say, however, this invention can be applied to equipment for 
measuring particle size only. When particle size only is measured, a pulse 
height analyzer may be used in place of waveform memory device 52 and 
arithmetic unit 53. 
Another modification of a rectangular waveguide according to the present 
invention will be now described. 
FIG. 9 is a perspective view showing this modification. In the embodiment 
shown in FIG. 4, rectangular waveguide 44 is arranged separate from 
optical fiber 43. On the other hand, in the modification shown in FIG. 9, 
rectangular waveguide 70 is arranged so as to be in contact with optical 
fiber 74, thus improving the precision in making them coaxial with each 
other. Rectangular waveguide 70 is comprised of four rectangular prisms 
70a, 70b, 70c, and 70d, all made of synthetic quartz and having 
rectangular cross sections and rectangular prism 72 made of synthetic 
quartz and having an approximately 0.2 mm square cross section. These 
prisms 70a-70dare connected by an epoxy type bonding material so that 
waveguide 70 as formed has, in its center portion, a prism-shaped channel 
extending therethrough having an approximately 0.2 mm square cross 
section. Rectangular prism 72 is inserted into this channel and is 
connected to prisms 70a-70d by an epoxy type bonding material. The 
boundary between prism 72 and prisms 70a-70d which is the bonding material 
acts as a total reflecting surface if the reflective indexes of the 
bounding material and prism 72 are set so as to satisfy the condition of 
total reflection. In this modification, the reflective index n.sub.a of 
the bonding material is 1.394 and that n.sub.g of prism 72 is 1.45. The 
numerical aperture (N.A.) of the rectangular waveguide is sin .theta. = 
.sqroot. n.sub.g .sup.2 -n.sub.a .sup.2 = 0.4 . The N.A. of optical fiber 
74 is a little smaller than that of waveguide 70. Therefore, while a light 
is being transmitted through channel 72, the light is rendered so as to 
have a uniform intensity distribution. 
Further, it is possible to omit using the bonding material between prism 72 
and prisms 70a-70d if the diameter of prism 72 is exactly the same as that 
of channel formed in the center portion of prisms 70a-70d. In this case, 
the boundary between prism 72 and prisms 70a-70d acts as a total 
reflecting surface if the reflective indexes of the prisms 70a-70d and 
prism 72 are set so as to satisfy the condition of total reflection. 
The front end of optical fiber 74 is connected to the rear end of 
rectangular waveguide 70. More accurately, the covering overlapping the 
front end portion of optical fiber 74 is removed, thus unwrapping body 74a 
of the optical fiber. Body 74a is arranged to be clamped by two prisms 76a 
and 76b, and those prisms are bonded together with a bonding material. The 
front ends of prisms 76a and 76b are bonded together with the rear end of 
waveguide 70 by using a bonding material of lens-bond type or of 
ultraviolet-hardening type. The front end surfaces of prisms 76a and 76b 
have been subjected to the optical polishing process. As described above, 
waveguide 70 and optical fiber 74 can be stably combined with each other. 
As a result, optical fiber 74 and waveguide 70 can be kept coaxial with 
each other. FIG. 10 is a side view of this modification. 
Thus, according to the modification of the present invention, the following 
advantages can be attained: 
1. The waveguide 70 has light guide prism 72 with a small cross section and 
a measuring volume is formed by enlarging the cross section of the light 
beam emitted from prism 72 by an optical system 45. This enables the 
distance between the the optical system 45 and the measuring volume to be 
increased, as a result of which the probe is free from the restriction of 
its location. 
2. For the reason that the size of the lens 45 is subjected to restriction, 
it is preferable to reduce the N.A. of the waveguide 70 as much as 
possible. If the N.A. is large, the light radiated from the optical fiber 
74 will be scattered, resulting in that accurate image-formation cannot be 
performed, and that the measuring volume cannot be formed so as to be 
exactly square. In this modification, the value of the N.A. is controlled 
by controlling the refractive index n.sub.g of the bonding material. 
As set forth above, according to this invention, it is possible to make 
uniform the intensity distribution of the irradiated light in the 
measuring volume by providing a waveguide in the optical system for 
irradiation, thereby substantially increasing the measuring accuracy of 
particle size and obtaining a particle size distribution curve in a short 
time.