Optical double-slit particle measuring system

• US 4,441,816 A
• Filed: 03/25/1982
• Issued: 04/10/1984
• Est. Priority Date: 03/25/1982
• Status: Expired due to Term

• Current Assignee: United States Department of Energy
• Original Assignee: United States Department of Energy
• Inventors: Hencken, Kenneth R., Wang, James C. F., Tichenor, Daniel A.
• Primary Examiner(s): Sikes, William L.
• Assistant Examiner(s): Koren, Matthew W.
• Application Number:
  US06/361,954
• Time in Patent Office: 747 Days
• Field of Search: 356/335, 356/336, 356/384, 356/385, 356/386, 250/237 R, 250/237 G, 250/222.2
• US Class Current: 356/335
• CPC Class Codes: G01N 15/0205 by optical means

Abstract

A method for in situ measurement of particle size is described. The size information is obtained by scanning an image of the particle across a double-slit mask and observing the transmitted light. This method is useful when the particle size of primary interest is 3 μm and larger. The technique is well suited to applications in which the particles are non-spherical and have unknown refractive index. It is particularly well suited to high temperature environments in which the particle incandescence provides the light source.

Claims

1. A method for measuring the size of particles in a certain size range moving along a path comprising the steps of:

(a) emitting a beam of light from a given particle in a particle-containing medium within said path;

(b) positioning a double-slit mask at the image plane of an optical focusing element located between said path and double-slit mask, the double-slit mask having a first slit of a width equal to or larger than the largest particle diameter of interest and a second slit spaced therefrom of a width equal to or smaller than the diameter of the smallest particle of interest;

(c) positioning a photodetector means at a location behind said double-slit mask;

(d) collecting with said optical-focusing element all of the light reflected from a given particle to form a particle image thereof and causing said image to scan periodically across the slits of the double-slit mask;

(e) measuring the light intensity transmitted through the slits of the double-slit mask by the photodetector means, and wherein the distance between said first and second slits is sufficient to cause said means to generate a double-pulsed output signal, first pulse being related to the amount of light transmitted by said particle image through said first slit and a second pulse being related to the amount of light passing through said second slit; and

(f) determining a particle diameter from the relationship of the peak values of the respective pulses of the double-pulsed output signal.

3. A method as recited in claims 1 or 2, in which the particle diameter is determined from the formula:

##EQU6## wherein;

d=particle diameter

W₂ =width of said second slit

I₂ =peak intensity of said second pulse derived from light transmitted through slit of W₂

I₁ =peak intensity of said first pulse derived from light transmitted through said first slit.

2. A method for measuring the size of particles in a certain size range moving along a path comprising the steps of:

(a) emitting a first beam of light from a given particle in a particle-containing medium within said path;

(b) projecting a second beam of light having a wavelength other than that of said first beam of light from said given particle;

(c) positioning a double-slit mask at the image plane of an optical focusing system including a spectral filter capable of reflecting the wavelength of the first beam and of transmitting the wavelength of the second beam, said optical focusing system being located between said path and said double-slit mask, the double-slit mask having a first slit of a width equal to or larger than the largest particle diameter of interest, and a second slit spaced therefrom of a width equal to or smaller than the diameter of the smallest particle of interest;

(d) positioning a first photodetector means at a location behind said double-slit mask and positioning a second photodetector means at a location behind said spectral filter mask;

(e) collecting with said optical focusing element all of the light reflected from said given particle to form a particle image thereof and causing, by means of the spectral filter, the particle image of the first beam to scan across the slits of the double-slit mask and the particle image of the second beam to be detected by the second photodetector means;

(f) measuring the light of the first beam transmitted through the slits of the double-slit mask by the first photodetector means, and wherein the distance between said first and second slits is sufficient to cause said means to generate a double-pulsed output signal, a first pulse being related to the amount of light transmitted by said particle image through said first slit and a second pulse being related to the amount of light passing through said second slit.

4. A system for measuring the size of the particles in a certain size range moving along a path comprising:

(a) optical focusing means located between said path for forming an image and an image plane of a given particle moving along said path;

(b) a double-slit mask of opaque material located at the image plane of the optical focusing means, said double-slit mask having a first slit of a width equal to or larger than the largest particle of interest and a second slit spaced therefrom of a width equal to or smaller than the diameter of the smallest particle of interest;

and

(c) a photodetector means disposed behind the double-slit mask and wherein the distance between said first and second slits is sufficient to cause said means to generate a double-pulsed output signal, a first pulse being related to the amount of light transmitted by said particle image through said first slit and a second pulse being related to the amount of light passing through said second slit.

5. A system as recited in claim 4, including a means for projecting a beam of light onto said given particle in the path of the particles of a certain size range.

6. A system as recited in claim 5, in which the optical focusing means is disposed at an angle of 90°

with respect to the means for projecting a beam of light onto the path of the particles of a certain size range.

7. A system as recited in claim 4, including a means for displaying the signals generated by the photodetector means.

8. A system as recited in claim 7, in which said means for displaying signal is an oscilloscope.

9. A system for measuring the size of particles in a certain size range moving along a path comprising:

(a) laser means disposed so as to project a beam of light onto a given particle in the path of particles of a certain size range;

(b) optical focusing means forming an image in an image plane of said given particle;

(c) spectral filter means for transmitting light of the wavelength of the laser beam and reflecting light of other wavelengths;

(d) a double-slit mask located at the image plane of the optical focusing element and means in the reflecting path of the spectral filter means, said double-slit mask having first a first slit of a width equal to or larger than the largest particle of interest and a second split spaced therefrom of a width equal to or smaller than the diameter of the smallest particle of interest;

(e) first photodetector means disposed behind the double-slit mask and wherein the distance between said first and second slits is sufficient to cause said means to generate a double-pulsed signal from the light transmitted through the slits of the double-slit mask, a first pulse being related to the amount of light transmitted by said particle image through said first slit and second pulse being related to the amount of light passing through said second slit;

(f) second photodetector means for generating a signal from the laser beam transmitted through the spectral filter;

(g) coincidence circuit means for generating a coincidence signal when the signal from said first and second photodetectors are generated simultaneously; and

(h) circuit means for comparing the amplitude of the first and second pulses in response to said coincidence signal.

10. A system as recited in claim 9, including a means for projecting a beam of light of a wavelength other than the laser beam into the path of particles of a certain size range.

11. A system as recited in claim 10, including a means for displaying the signals generated by the first photodetector means when a coincident signal is generated by said coincident circuit means.