METHOD AND DEVICE FOR DETERMINING THE SIZE OF A TRANSPARENT PARTICLE

Abstract

A method is described for determining the size of a transparent particle (2), wherein the particle (2) is illuminated with light from a light source (6), wherein using a radiation detector (7) a time-resolved intensity curve of light from the light source (6) scattered on the particle (2) is measured at a preselectable scattering angle .sub.s, wherein characteristic scattered light peaks are determined in the intensity curve, and wherein the size of the particle (2) is determined on the basis of the time difference between two scattered light peaks, characterized in that, with the help of two radiation detectors (7) or light sources (6), a first and a second time-resolved intensity curve of scattered light, scattered on the particle (2) in the forward direction, are measured; a transmission peak (12) and a reflection peak (11) are determined from the first intensity curve and from the second intensity curve; a first time difference between the transmission peaks (12) is determined, and a second time difference between the reflection peaks (11) is determined; a characteristic variable is determined from the ratio of the first time difference and the second time difference; and a size determination is performed for the particles (2) for which the characteristic variable corresponds to a preselectable value. (FIG. 3)

Claims

1. A method for determining the size of a transparent particle, wherein the particle is illuminated with light from a light source, wherein a time-resolved intensity curve of light from the light source scattered on the particle is measured at a preselectable scattering angle .sub.s using a radiation detector, wherein characteristic scattered light peaks are determined in the intensity curve, and wherein the size of the particle is determined on the basis of the time difference between two scattered light peaks, wherein either (i) a first and a second time-resolved intensity curve of light from the light source, scattered on the particle in the forward direction, is measured using two radiation detectors arranged on both sides of an optical axis of the light source, spaced a distance apart in the direction of the particle trajectory, or (ii) the particle is illuminated with two light sources spaced a distance apart from one another in the direction of the particle trajectory and arranged on both sides of an optical axis of the radiation detector, and the time-resolved intensity curve of light scattered in the forward direction, measured with the radiation detector, is broken down into a first intensity curve, caused by the first light source, and a second intensity curve, caused by the second light source; a transmission peak and a reflection peak are each determined from the first intensity curve and from the second intensity curve; a first time difference between two different transmission peaks and/or reflection peaks and a second time difference, which is different from the first time difference, between two different transmission peaks and/or reflection peaks, are determined; a characteristic variable is determined from the ratio of the first time difference and the second time difference; and a size determination is performed Preliminary Amendment only for the particles for which the characteristic variable corresponds to a preselectable value.

2. The method according to claim 1, wherein either the two radiation detectors are arranged symmetrically on both sides of the optical axis of the light source and a first and a second time-resolved intensity curve of scattered light of the light source scattered on the particle in the forward direction is measured, or the two light sources are arranged symmetrically on both sides of an optical axis of the radiation detector, and the particle is illuminated by the two light sources arranged symmetrically and a distance apart in the direction of the particle trajectory.

3. The method according to claim 1, wherein the first time difference between the transmission peak of the first intensity curve and the transmission peak of the second intensity curve and the second time difference between the reflection peak of the first intensity curve and the reflection peak of the second intensity curve are determined.

4. The method according to claim 1, wherein the scattering angle .sub.s or the scattering angle .sub.s.sup.(1) and .sub.s.sup.(2) are predetermined, so that the characteristic variable =t.sub.00/t.sub.11 is between 0.5 and 2.5.

5. The method according to claim 1, wherein one of several predetermined refractive indices m is assigned to the particle on the basis of the characteristic variable .

6. The method according to claim 1, that wherein a spatial intensity distribution of the light source along the optical axis is determined and is compared with an intensity distribution of the reflection peak and/or of the transmission peak overtime.

7. The method according to claim 6, wherein a size determination is performed only for those particles for which the reflection peak and/or the transmission peak has an intensity distribution over time that correlates with the spatial intensity distribution of the light source.

8. The method according to claim 6, wherein the velocity v of the particle is determined from the width of the intensity distribution of the reflection peak over time and/or from the width of the transmission peak.

9. A device for determining the size of a particle using a light source having a radiation detector for light from the light source scattered by the particle and using an analysis unit that can be connected to the radiation detector for transmission of data, wherein two radiation detectors are arranged in the forward direction on both sides of an optical axis of the light source spaced a distance apart in the direction of the particle trajectory or two light sources are arranged in the forward direction on both sides of an optical axis of the radiation detector spaced a distance apart in the direction of the particle trajectory, wherein the light source or the light sources are situated on a first side of a measurement volume and the radiation detector or the radiation detectors are situated on opposite sides of the measurement volume.

10. The device according to claim 9, wherein the two radiation detectors are either arranged symmetrically on both sides of an optical axis of the light source at a scattering angle s which is the same by amount in the forward direction or at a distance apart from one another in the direction of the particle trajectory, or the two light sources are arranged symmetrically on both sides of an optical axis of the radiation detector at a scattering angle s which is the same by amount in the forward direction with a distance between them in the direction of the particle trajectory.

11. The device according to claim 9, wherein the light source emits light that is not coherent.

12. The device according to claim 11, wherein the light source has an LED.

13. The device according to claim 9, wherein the light source creates a light curtain.

14. The method according to claim 4, wherein the scattering angle .sub.s or the scattering angle .sub.s.sup.(1) and .sub.s.sup.(2) are predetermined, so that the characteristic variable =t.sub.00/t.sub.11 is approximately 1.5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Exemplary embodiments which are illustrated in the drawings are explained in greater detail below. They show:

[0040] FIG. 1 shows a schematic diagram of a particle illuminated by a light source and the curves of a few beams that have been drawn in the figures, occurring for a predetermined scattering angle .sub.s,

[0041] FIG. 2 shows a schematic relationship between the spatial intensity distribution of a beam of light of the light source striking the particle and an intensity distribution of the measured scattered light over time, which correlates with same,

[0042] FIG. 3 shows a schematic diagram of a device for determining the size of a particle,

[0043] FIG. 4 shows the schematically diagrammed intensity curves of light scattered by a particle in the scattering angle .sub.s over time, the light being measured by a first radiation detector and a second radiation detector in the forward scattering direction,

[0044] FIG. 5 shows a schematic diagram of various values of the characteristic variable as a function of various materials and/or refractive indices m of the particles and

[0045] FIG. 6 shows a schematic diagram of a device for determining the size of a particle like that in FIG. 3 but showing an asymmetrical arrangement of the two radiation detectors instead.

DESCRIPTION OF VARIOUS AND PREFERRED EMBODIMENTS

[0046] FIG. 1 shows schematically the relevant beams for determining the particle size for the method according to the invention in a scattering process in a scattering angle .sub.s. Of a light source (not shown in FIG. 1), a beam of light 1 strikes a particle 2 which is moving through the beam of light 1 crossing through the beam of light 1. The beam of light 1 is reflected from the outside on the interface 3 of a particle 2 to the surrounding medium and transmitted through the particle 2 on emerging from the particle 2 through refraction on entrance and exit from the particle 2. FIG. 1 shows the solid-line beams for the reflection and for the transmission that occur at a predetermined scattering angle .sub.s and can be detected. A reflection beam 4 is reflected at the interface 3. A transmission beam 5 is refracted into the interior of the particle 2 and is refracted again on emerging from the particle 2.

[0047] The respective angle of incidence .sub.i of the solid-line beams which generate corresponding intensity peaks in a time-resolved intensity curve correlates with the intersection with the interface 3 of the particle 2. For an assumed ideal spherical shape of the particle 2, the angle of incidence .sub.i may be determined as a function of the scattering angle .sub.s used for the measurement and the refractive index m of particle 2 with the help of geometric considerations and/or with the help of ray tracing programs in practice.

[0048] Because of the different paths and transit times that can be determined in advance at a given scattering angle .sub.s for the reflection beam 4 as well as for the transmission beam 5, the individual beams create chronologically spaced peaks which can be detected with a detector (not shown). Since the time difference between the individual peaks depends on the particle size, among other things, the particle size can be determined starting from a time-resolved intensity curve which was detected with the detector.

[0049] FIG. 2 shows only schematically the relationship between a spatial intensity distribution of the incident beam of light 1 and the intensity curve of the scattered light detected at scattering angle .sub.s over time. An intensity distribution of the incident beam of light 1 which is essentially Gaussian leads to a curve of the measured intensity of the scattered light plotted as a function of time, which is also approximately Gaussian. Such an intensity peak can be measured for all the solid line beams described above.

[0050] The beam of light 1 striking the particle 2 is imaged in the detector due to the particle 2 crossing the beam of light 1, and this can be described by a mathematical transformation. The width b of the spatial intensity distribution of the incident beam of light 1 corresponds to the width a of the time-resolved peak of the scattered light. The particle velocity v is obtained from the quotient of the spatial width b and the time difference which corresponds to the width a:

v=b/.

[0051] The width b and the width can be determined, for example, based on a half-width determination of the respective peaks. The spatial intensity distribution of the incident beam of light 1 should therefore be determined in advance with the greatest possible precision.

[0052] A device for performing the method according to the invention, diagrammed as an example in FIG. 3, requires only a few inexpensive components. A light source 6 and two photodetectors 7 must be arranged and equipped relative to one another, so that the light, which is scattered by a particle 2 passing by and is generated by the light source 6, can be detected in both photodetectors 7 which are arranged at the same scattering angle .sub.s relative to an optical axis 8 of the light source 6 and are directed at a corresponding measurement volume 9, the light source 6 is arranged on a first side of the measurement volume 9 and the two photodetectors 7 are arranged on the second side which is opposite the first side.

[0053] Since no interference properties need be utilized for determination of the particle size d, the light source 6 may be any sufficiently bright source of light that can be focused in a suitable manner. The light source 6 need not emit coherent light so that it is also possible to use LEDs, for example. If the sizes d of the particle 2 are to be determined with different trajectories, then the light source 6 may also be designed as a light curtain or the like. An analysis unit 10 is connected to the photodetector 7 for transmission of data and is suitable for analyzing a time-resolved intensity distribution measured with the photodetectors 7. The analysis unit 10 optionally has a suitable memory device for the measured values.

[0054] FIG. 4 shows schematically the two time-resolved intensity curves of the light scattered on particle 2 and measured by the two photodetectors 7 at the scattering angle .sub.s. This is plotted as a function of the time t in s, representing the intensity of the electric measurement signal S generated by a detector. Each intensity curve has a reflection peak 11 and a transmission peak 12, definitely separated from the former. Any peaks generated by scattering of a higher order do not have any mentionable intensity and are therefore negligible.

[0055] The time differences t.sub.00 and t.sub.11 can be determined as the difference in the respective maximums of the reflection peaks 11 and the transmission peaks 12. The two time differences t.sub.00 and t.sub.11 are derived according to

[00002]$.Math..Math.t_{11}\left(d,v,{}_s,m\right)=\frac{d}{v}.Math.\sin \left({}_i^{p=1}.Math.\left({}_S,m\right)\right)$$.Math..Math.t_{00}\left(d,v,{}_S\right)=\frac{d}{v}.Math.\cos \left(\frac{{}_S}{2}\right)$

from the particle properties, i.e., the particle size d, particle velocity v and refractive index m, as well as from the scattering angle .sub.s, which is predetermined by the measurement apparatus.

[0056] The time differences t.sub.00 and t.sub.11 each depend on the size d and the velocity v of the particle 2. However, a characteristic variable , which is determined as the quotient of the two time differences t.sub.00 and t.sub.11 according to the following equation:

[00003]$\frac{.Math..Math.t_{00}}{.Math..Math.t_{11}}=\frac{\frac{d}{v}.Math.\cos \left(\frac{{}_s}{2}\right)}{\frac{d}{v}.Math.\sin \left({}_i^{p=1}\left({}_s,m\right)\right)}=\frac{\cos \left(\frac{{}_s}{2}\right)}{\sin \left({}_i^{p=1}\left({}_s,m\right)\right)}:=\left({}_s,m\right)$

is independent of the particle size d and the velocity v and depends only on the scattering angle .sub.s and the relative refractive index m. The scattering angle .sub.s can be predetermined by the equipment design of the measurement apparatus and/or by the arrangement and equipment of a detector relative to the light source.

[0057] The relative refractive index m can also be determined in advance from known particles 2 in a known medium. The angle of incidence .sub.i.sup.p=1 is a geometric variable, which depends only on the scattering angle .sub.s and the relative refractive index m under the assumption of an ideal spherical shape of the particle, and can be determined in advance. Thus a table of values may also be calculated in advance for the characteristic variable as a function of the parameters, and a value and/or a value range to which the characteristic variable from the measured intensity distribution must conform in order for the respective intensity distribution to be taken into account and used for the determination of particle size.

[0058] If the measured intensity distribution should yield a substantially different characteristic variable , this must regularly be attributed to the fact that the individual peaks 11 and 12 cannot be assigned to a single particle 2 but instead occur due to superpositioning of multiple scattering effects on different particles 2, for example, or the respective particle 2 does not have an approximately spherical shape and therefore the geometric boundary conditions assumed for the path distances and transit times of the solid line beams 4 and 5 are not correct.

[0059] Instead of or in addition to the ratio of the time differences t.sub.00 and t.sub.11, it is also possible to determine the time difference t.sub.01 of the two peaks 11 and 12 in relation to one another in a measured intensity distribution and to use these differences in the respective ratio of the time differences t.sub.00 and t.sub.11 for the calculation of the characteristic variable , where the following relationships hold:

[00004]$\frac{.Math..Math.t_{01}}{.Math..Math.t_{11}}=\frac{\frac{d}{v}.Math.\sin \left({}_i^{p=1}\left({}_s,m\right)\right)+\frac{d}{v}.Math.\cos \left(\frac{{}_s}{2}\right)}{\frac{d}{v}.Math.\sin \left({}_i^{p=1}\left({}_s,m\right)\right)}=1+\frac{\cos \left(\frac{{}_s}{2}\right)}{\sin \left({}_i^{p=1}\left({}_s,m\right)\right)}:=1+\left({}_s,m\right)$$\mathrm{and}$$\frac{.Math..Math.t_{01}}{.Math..Math.t_{11}}=\frac{\frac{d}{v}.Math.\sin \left({}_i^{p=1}\left({}_s,m\right)\right)+\frac{d}{v}.Math.\cos \left(\frac{{}_s}{2}\right)}{\frac{d}{v}.Math.\cos \left(\frac{{}_s}{2}\right)}=1+\frac{\sin \left({}_i^{p=1}\left({}_s,m\right)\right)}{\cos \left(\frac{{}_s}{2}\right)}:=1+\frac{1}{\left({}_s,m\right)}$

with each of these equations, the value of the characteristic variable can be determined independently of the respective other relationships.

[0060] In addition, it is possible to perform two or three different calculations for the characteristic variable and to compare the respective values obtained. If the values determined for the characteristic variable do not match, the intensity distributions affected by this should not be used for an analysis because differences in the characteristic variable also indicate that the individual peaks 11 and 12 cannot be assigned to a single particle 2.

[0061] In FIG. 5 the theoretically determined values for the characteristic variable are plotted as a function of the scattering angle .sub.s in degrees for various refractive indices between m=1.1 and m=1.7 in increments of 0.1 each. For the analysis of the measured values, a value of 1.5 is advantageous for the characteristic variable . As a result, for a measurement of the size of water droplets in air with a refractive index m=1.33, for example, a scattering angle .sub.s of approximately 21 is especially advantageous and should be taken into account and optionally preset for the structural design of a measurement apparatus.

[0062] For the purpose of illustration, FIG. 6 shows that an asymmetrical arrangement of the two photodetectors 7 relative to the optical axis 8 of the light source 6 is also possible. Consequently, the two photodetectors 7 each have a scattering angle .sub.s.sup.(1) and/or .sub.s.sup.(2) relative to the optical axis 8.

[0063] The characteristic variable is thus a function of the two scattering angles .sub.s.sup.(1) and .sub.s.sup.(2) as well as the refractive index m according to the following equation:

[00005]$\left({}_S^{\left(1\right)},{}_S^{\left(2\right)},m\right)=\frac{.Math..Math.t_{00}}{.Math..Math.t_{11}}=\frac{\frac{d/2}{v}\left[\cos \left(\frac{{}_S^{\left(1\right)}}{2}\right)+\cos \left(\frac{{}_S^{\left(2\right)}}{2}\right)\right]}{\frac{d/2}{v}\left[\sin \left({}_{i,\left(1\right)}^{p=1}\left({}_s^{\left(1\right)},m\right)\right)+\sin \left({}_{i,\left(1\right)}^{p=1}\left({}_s^{\left(2\right)},m\right)\right)\right]}=\frac{\cos \left(\frac{{}_S^{\left(1\right)}}{2}\right)+\cos \left(\frac{{}_S^{\left(2\right)}}{2}\right)}{\sin \left({}_{i,\left(1\right)}^{p=1}\left({}_S^{\left(1\right)},m\right)\right)+\sin \left({}_{i,\left(1\right)}^{p=1}\left({}_S^{\left(2\right)},m\right)\right)}.$

[0064] Corresponding relationships can also be formulated and calculated for the relationships of other time differences to one another as a function of the two scattering angles .sub.s.sup.(1) and .sub.s.sup.(2) as well as the refractive index m, as already mentioned above such as, for example, t.sub.01/t.sub.00 or t.sub.01/t.sub.11.