Improved particle sensor and method for assaying a particle

• US 5,467,189 A
• Filed: 01/12/1995
• Issued: 11/14/1995
• Est. Priority Date: 01/22/1993
• Status: Expired due to Term

• Current Assignee: Venturedyne Ltd (Carnes Company, Inc.)
• Original Assignee: Venturedyne Ltd (Carnes Company, Inc.)
• Inventors: Chandler, David L., Kreikebaum, Gerhard
• Primary Examiner(s): Pham, Hoa Q.
• Application Number:
  US08/384,601
• Time in Patent Office: 306 Days
• Field of Search: 356/335-343, 356/73, 356/301, 356/39, 356/317, 356/318, 356/244, 356/246, 250/574, 250/575, 250/222.2
• US Class Current: 356/336
• CPC Class Codes: G01N 15/0205 by optical means
• CPC Class Codes: G01N 2021/6469 Cavity, e.g. ellipsoid
• CPC Class Codes: G01N 21/53 within a flowing fluid, e.g. smoke

Abstract

The disclosure involves a particle sensor having a mirror cavity unobstructed by masks and the like. A light detector is at the mirror secondary focal point and well outside the mirror cavity. A variation includes a beam splitter and a secondary light detector to improve detection of larger particles. A second embodiment includes a pair of elliptical mirrors offset along the light beam. Light reflected by the second mirror represents only changes in laser power and light scattered by gas molecules. The resulting signal is subtracted from that produced by the first mirror to obtain a relatively "clean" signal useful to assay very small particles.

Claims

1. In a particle sensor containing gas molecules and including (a) a first elliptical mirror with a cavity, a first major axis and primary and secondary focal points along such first major axis, (b) a particle-illuminating beam of light having a wavelength and extending along a beam axis intersecting the primary focal point, and (c) an inlet for introducing airborne particles into a sensor view volume, the improvement wherein:

a light detector is at the secondary focal point;

the region between the primary focal point and the light detector is substantially unobstructured;

the primary focal point of the first mirror is coincident with the view volume;

the sensor includes a second mirror having a second major axis offset along the beam axis and spaced from the first major axis, the second mirror reflecting light scattered substantially only by gas molecules; and

the beam axis and the first major axis define an angle greater than 0°

therebetween;

whereby particles having a maximum dimension substantially less than the wavelength are detected.

2. The sensor of claim 1 wherein the beam axis and the first major axis define and included angle not greater than 90°

.

3. The sensor of claim 1 wherein the beam of light is used to assay particles, particles are introduced along the path, and a path and the beam axis defined an included angle of less than 90°

.

4. The sensor of claim 1 including a light trap and wherein the light beam has a wavelength and the light trap comprises a plate absorbing unwanted, non-scattered light of that wavelength.

5. The sensor of claim 4, wherein the plate is outside the cavity and is angularly oriented with respect to the beam axis.

6. The sensor of claim 5 wherein the plate comprises a light bandpass filtered having a nominal bandpass wavelength range and the wavelength of the beam of light is outside the nominal bandpass wavelength.

7. The sensor of claim 1 including a light trap and wherein the light beam has a wavelength and the light trap comprises a primary plate absorbing non-scattered light of that wavelength and further comprises a secondary plate absorbing unwanted light reflected from the primary plate.

8. The sensor of claim 1 wherein the second mirror has a primary focal point displaced along the beam axis.

9. The sensor of claim 8 wherein the second major axis of the second mirror is substantially parallel to the first major axis of the first mirror.

10. The sensor of claim 1 wherein light is scattered in the view volume by airborne particles and by gas molecules and wherein:

the light detector is a first detector receiving light scattered by an airborne particle and by gas molecules and providing a first detector signal;

and the sensor also includes;

a second detector receiving light scattered substantially only by gas molecules and reflected by the second mirror, the second detector providing a second detector signal; and

,

the first detector signal and the second detector signal are processed in a subtraction circuit providing an output signal representing substantially only light scattered by an airborne particle.

11. The sensor of claim 10 wherein:

the first mirror has a major axis;

the first detector is coincident with the major axis of the first mirror;

the second mirror has a major axis; and

,

the second detector is coincident with the major axis of the second mirror.

12. An improved particle sensor comprising:

a light beam extending along a beam axis;

a particle flow path intersecting the beam axis and defining a view volume therewith;

a first apparatus collecting light scattered by an airborne particle and by gas molecules; and

,

a second apparatus collecting light scattered substantially simultaneously and substantially only by gas molecules.

13. The sensor of claim 12 including first and second detectors and wherein:

the first apparatus includes a mirror;

light scattered by an airborne particle and by gas molecules and reflected by the mirror impinges on the first detector providing a first output signal;

light scattered substantially only by gas molecules and collected by the second apparatus impinges on the second detector providing a second output signal; and

,

the sensor includes a circuit subtracting the signals.

14. The sensor of claim 13 wherein the first output signal and the second output signal each include components resulting from (a) shot noise, (b) random detector noise and (c) changes in laser power ad the circuit substrates from the first output signal that component of the second output signal resulting from changes in laser power.

15. The particle sensor of claim 12 wherein:

the second apparatus is free of mirrors and comprises an imaging system having at least one lens.

16. The particle sensor of claim 15 wherein the second apparatus includes a detector having a central portion and an opaque mask preventing scattered light from striking the central portion.

17. The particle sensor of claim 12 wherein the second apparatus comprise an imaging system having at least one lens far collecting light scattered by an airborne particle and by gas molecules.

18. The particle sensor of claim 17 wherein the second apparatus includes a detector and an opaque mask substantially preventing light scattered by an airborne particle from striking the detector.

19. A method for assaying a particle illuminated by a light beam and including the steps of:

detecting a first quantum of light scattered by a particle and by gas molecules;

detecting a second quantum of light scattered substantially simultaneously and substantially only by gas molecules.

20. The method of claim 19 wherein:

the step of detecting the first quantum of light includes reflecting the first quantum of light from a first mirror;

the step of detecting the second quantum of light includes reflecting the second quantum of light from a second mirror.

21. The method of claim 20 wherein:

the first quantum of light is scattered at a first locus;

the second quantum of light is scattered at a second locus;

each mirror has a focal point;

the focal point of the first mirror is at the first locus; and

,

the focal point of the second mirror is at the second locus.

22. The method of claim 20 further including the step of:

generating a first signal representing the first quantum of light;

generating a second signal representing the second quantum of light; and

subtracting the signals.

23. The method of claim 22 wherein the step of generating a first signal includes:

providing an elliptical first mirror having a primary and a secondary focal point, the first mirror reflecting scattered light to a first detector at the secondary focal point of the first mirror;

and the step of generating a second signal includes;

providing an elliptical second mirror having a primary and a secondary focal point, the second mirror reflecting scattered light to a second detector at the secondary focal point of the second mirror.

24. The method of claim 19 wherein the second quantum of light is detected by imaging it toward a detector.

25. The method of claim 24 wherein the detector includes a central portion and the step of detecting a second quantum of light includes preventing scattered light from being received at the central portion

26. An improved particle sensor comprising:

a light beam extending along a beam axis;

a particle flow path intersecting the beam axis and defining a view volume therewith;

a first apparatus including a light-reflecting mirror having a focal point substantially coincident with the view volume and collecting light scattered by an airborne particle and by gas molecules; and

,

a second apparatus comprising (a) an imaging system having at least one lens collecting light scattered by an airborne particle and by gas molecules, and (b) a detector having an opaque mask preventing light scattered by an airborne particle from striking the detector, the detector thereby detecting light scattered substantially only by gas molecules.

27. A method for assaying a particle illuminated by a light beam and including the steps of:

detecting a first quantum of light scattered at a first locus by a particle and by gas molecules;

reflecting the first quantum of light from a first mirror having a focal point at the first locus;

detecting a second quantum of light scattered at a second locus substantially only by gas molecules;

reflecting the second quantum of light from a second mirror having a focal point at the second locus.

28. The method of claim 27 including the steps of:

generating a first signal representing a first quantum of light;

generating a second signal representing the second quantum of light; and

subtracting the signals,

and wherein the step of generating a first signal includes;

providing an elliptical first mirror having a primary and a secondary focal point, the first mirror reflecting scattered light to a first detector at the secondary focal point of the first mirror;

and the step of generating a second signal includes;

providing an elliptical second mirror having a primary and a secondary focal point, the second mirror reflecting scattered light to a second detector at the secondary focal point of the second mirror.