Method and optical means for determining dimensional characteristics of the particle distribution in a collection of particles

• US 4,037,965 A
• Filed: 03/23/1976
• Issued: 07/26/1977
• Est. Priority Date: 03/23/1976
• Status: Expired due to Term

• Current Assignee: Leeds & Northrup Co.
• Original Assignee: Leeds & Northrup Co.
• Inventors: Weiss, Edward Leonhardt
• Primary Examiner(s): Corbin, John K.
• Assistant Examiner(s): Koren, Matthew W.
• Application Number:
  US05/669,788
• Time in Patent Office: 490 Days
• Field of Search: 356/71, 356/102, 356/103, 350/162 SF
• US Class Current: 356/336
• CPC Class Codes: G01N 15/0211 Investigating a scatter or diffraction pattern
• CPC Class Codes: G01N 2021/4716 Using a ring of sensors, or a combination of diaphragm and sensors; Annular sensor

Abstract

The far-field diffraction pattern produced by illuminating a collection of particles with a coherent light source is spatially modulated by a filter mask having a first aperture. The aperture is shaped to pass a total light flux proportional to a particular moment of the particle distribution over a certain particle size range. Light flux is passed by another aperture at greater or lesser radii and adjacent the first aperture. That flux is effectively subtracted from the flux passed by the first aperture as by subtracting the resulting signals produced by a photo detector at different times in response to the flux passed by the filter. The difference signal provides a measure of the particular moment within the same limits of accuracy but over a wider or narrower particle size range, respectively.

Claims

1. The method for measuring a dimensional characteristic of the particles of a collection in a certain size range comprising the steps of:

passing a beam of light through the collection of particles,

detecting the low angle diffracted light flux from said particles over a first range of diffraction angles such that the flux detected represents said dimensional characteristic over said certain size range,

detecting the low angle diffracted light flux from said particles over a second range of diffraction angles such that the flux detected represents said dimensional characteristic over a lesser portion of said certain size range, and

subtracting the flux detected over said second range from the flux detected over said first range to obtain as a result of said subtraction a measure of said dimensional characteristic within the same limit of error over a different portion of said certain size range than would be obtained by only detecting the flux over said first range.

2. The method of claim 1 in which said different portion is a wider portion.

3. The method of claim 1 in which said different portion is a narrower portion.

4. Apparatus for measuring a dimensional characteristic of the particles of a collection in a certain size range comprising;

means for passing a beam of light through the collection of particles,

means for detecting the low angle diffracted light flux from said particles over a first range of diffraction angles such that the flux detected represents said dimensional characteristic over said certain size range,

means for detecting the low angle diffracted light flux from said particles over a second range of diffraction angle such that the flux detected represents said dimensional characteristic over a lesser portion of said certain size range, and

means for subtracting the flux detected over said second range from the flux detected over said first range to obtain as a result of said subtraction a measure of said dimensional characteristic within the same limit of error over a different portion of said certain size range than would be obtained by only detecting the flux over said first range.

5. Apparatus as set forth in claim 4 in which the diffracted light flux from the particles over said first range of diffraction angles includes all particles in said certain size range, and

the diffracted light flux from the particles over said second range of diffraction angles includes particle of a limited size range within said certain size range so that the result of said comparison is a measure of the dimensional characteristics in said certain size range.

6. The method for improving the measurement of the moments of the particle distribution by number for a collection of particles comprising the steps of,

passing a light beam through said collection,

measuring a portion of the low-angle, forward-scattered light flux from the particles,

modulating the response of said measurement as a function of the angle of diffraction of the flux being measured so that the response of the measurement to the flux diffracted by each particle in said light beam is such that the response of the measurement to the low-angle, forward-scattered light flux measured is proportional to the desired moment, said modulating function having a range of values such that through the span of diffraction angles over which said response is modulated the function has a positive value over one range of angles and a negative value over another range of angles.

7. Apparatus for improving the measurement in a collection of particles of the moments of the particle distribution by number comprising,

means for passing a light beam through said collection,

means operable to measure the total of the low-angle, forward-scattered light flux from the particles,

means for modulating the response of said measurement as a function of the angle of diffraction of the flux being measured so that the response to the flux diffracted by each particle in said light beam is such that the total flux measured is proportional to the desired moment, said modulating function having a range of values such that through the span of diffraction angles over which said response is modulated the function has a positive value for a part of the span including the smallest angle and a negative value for a part of the span adjacent the part of the span including the smallest angle.

8. A method for calculating in a collection of particles the moments of the particle distribution by number comprising the steps of

passing a light beam through said collection

spatially modulating the low-angle, forward-scattered light flux from the beam, said spatial modulation being effected by

a first filter area including the area just outside said beam with the transmission of said first filter area varying with the radius from the beam center as required to produce a total flux transmission through said filter area proportional to said moment within certain error limits and over a certain particle size range, and

a second filter area adjacent said first area to produce a total transmitted flux through said second area which when subtracted from that transmitted by said first area will produce a result in effective total flux which is proportional to said moment within the said error limits over a greater range of particle sizes, and

subtracting the value of total flux transmitted through said second filter area from the total flux transmitted through said first area.

9. Apparatus for improving the measurement in a collection of particles of the moments of the particle distribution by number comprising;

means for passing a light beam through said collection,

means operable to measure the total of the low-angle, forward-scattered light flux from the particles,

first means for modifying said measurement as a function of the angle of diffraction of the flux being measured so that response to the flux diffracted by each particle of said collection is such that the total of the flux measured approximates the desired moment within predetermined limits over a certain particle size range, and

second means for modifying said measurement in sense opposite that of said first modifying means as a function of the angle of diffraction of the flux being measured so that the resulting modified measurement is proportional to said moment within said limits over a greater particle size range.

10. Apparatus as set forth in claim 9 in which

said first means modifies said measurement so as to make the total flux measured substantially proportional to the third moment with the addition of flux transmission at the inner edge of the mask to provide excessive compensation for flux lost due to the inner limits of the mask, and

said second means modifies the measurement as a function of the angle of diffraction of the flux over areas adjacent in radial position to the compensating area of the first means to thereby compensate for the flux lost at the inner mask limit.

11. The method for improving the measurement in a collection of particles of the fourth moment of the particle distribution by numbers comprising the steps of

passing a light beam through said collection

measuring the light flux falling on an annulus in the far-field diffraction pattern just outside the periphery of the light beam

measuring the light flux falling on an annulus adjacent said first mentioned annulus, and

subtracting said measurement of the flux falling on the last mentioned annulus from the measurement of the flux falling on the first mentioned annulus.

12. A method for measuring a value proportional to the average of the fourth power of the diameter of the individual particles making up a collection through which a beam of light has been transmitted comprising the steps of

measuring a portion of the total diffracted flux intercepting the smallest usable annular area in the Fraunhofer plane outside the incident light beam

measuring a portion of the total diffracted flux intercepting an annular area adjacent said smallest area, and

subtracting a multiple of said last measurement from a multiple of said first measurement to produce said average of the fourth power of the diameter of the individual particles making up said collection.