Method and apparatus for measuring the light scattering properties of small particles

The measurement of the scattering properties of very small particles by electro-optical means generally requires the use of an intense, though highly spatially inhomogeneous, light source such as a laser. Many instruments require, therefore, that the intersection of the particle stream with the illumination source be precisely regulated so that the flux incident on the particle be known accurately. A method and apparatus are described by which means the absolute intensity of the light incident on the particle need not be known. A special structure and measurement process are described by which means small particles are differentiated from larger particles grazing the illumination beam.

PRIOR RELATED PATENT 
The present invention is directed to a method and apparatus of considerable 
utility in the characterization of small particles by measuring their 
light scattering properties. In particular, this invention permits the use 
of spatially inhomogeneous light beam sources such as produced by lasers 
to achieve these measurements. 
Expressly incorporated by reference herein is the following co-pending 
Patent Application: 
U.S. patent application Ser. No. 390,980, 
Title: Process and Apparatus for Identifying or Characterizing Small 
Particles, 
Inventor: Philip J. Wyatt and Gregory M. Quist, 
Date of Filing: June 22, 1982. 
DEFINITIONS 
The term "light" shall means electromagnetic radiation. 
The term "size parameter" shall mean .rho., where .rho.=2.pi.a/.lambda. and 
a is the means particle radius and .lambda. is the wavelength of the 
incident electromagnetic radiation in the medium in which the particles 
are measured. 
The term "very small particle" shall mean any particle whose size parameter 
is less than one. 
The term "small particle" shall mean any particle whose size parameter is 
less than six. 
The term "large particle" shall mean a particle whose size parameter is 
greater than six. 
The term "beam" shall mean light propagating in a parallel or nearly 
parallel direction. 
The term "beam diameter" of an incident light source, with a Gaussian 
intensity profile, such as a laser, shall refer to the diameter of the 
beam measured between the points at which the intensity has fallen to 
1/e.sup.2 the intensity at the center of the beam. 
The term "scattering efficiency" of a particle shall mean the ratio of its 
total scattering cross section to its geometrical cross section. 
The term "forward scattering direction" shall mean all rays, i.e. directed 
line segments, propagating at an angle less than 90 degrees with respect 
to the direction of the incident beam. 
The term "backward scattering direction" shall mean all rays, i.e. directed 
line segments, propagating at an angle greater than 90 degrees with 
respect to the direction of the incident beam. 
The term "impact parameter" shall mean the distance of closest approach of 
the particle from the center of the light beam. 
For plane polarized light, the plane perpendicular to the direction of the 
wave's electric field is called the V-plane and said plane polarized light 
is vertically polarized with respect to said perpendicular plane. The 
corresponding H-plane is perpendicular to the V-plane and contains the 
wave's incident electric field. 
The particles that we will be discussing primarily throughout this 
specification are particles between 10 nm and 1000 nm and the illumination 
sources are usually lasers operating in the visible portion of the 
electromagnetic spectrum. As will be evident to those skilled in the art 
of light scattering techniques, this restriction is unnecessary and, 
indeed, our invention could equally well apply to particles of sizes 
outside of this range and lasers producing radiation that is not visible. 
SUMMARY OF THE INVENTION 
A particle passing through a laser beam scatters light as a spherical 
outgoing wave. When the particle's size parameter approaches zero, the 
ratio of the light scattered in the forward direction to light scattered 
in the backward direction approaches unity. At the same time, the 
scattering efficiency rapidly approaches zero. The power density of a 
typical laser beam falls rapidly from the center of the beam in a Gaussian 
manner with the 1/e.sup.2 diameter typically from 0.3 to 1.0 mm. Thus the 
total amount of light scattered by a particle depends not only on its size 
and composition but also on its impact parameter with respect to the beam 
center. The accurate characterization of the particle by means of the 
light it scatters depends critically, for virtually all applications, on 
being able to differentiate between scattering signatures of a very small 
particle and the corresponding signature of a larger particle whose impact 
parameter is too large to yield meaningful scattering data for the 
available detection system. The present invention allows this distinction 
by collecting light over a relatively large solid angle in the forward 
scattering direction and a similarly large solid angle in the backward 
scattering direction. By measuring the ratio of these two quantities and 
measuring the scattered intensities in a few other directions with respect 
to the direction of the incident light source, the distinction between 
small and large particles is established with a high degree of certainty.

BACKGROUND 
The measurement and counting of aerosol and, to a lesser extent, hydrosol 
particles is an extremely important requirement for many types of 
industrial and health-oriented activities. Many critical manufacturing 
functions require the use of so-called clean rooms with the classification 
of such cleanliness based primarily upon the size and number of the 
largest particles present. In recent years, for example, the fabrication 
of very large scale integrated circuits, VLSI circuits, has required that 
the particle sizes present in areas where such devices are fabricated be 
less than 100 nm. The monitoring of work areas where asbestos insulation 
is being removed or where similar dangerous asbestos fibers are present 
requires, according to various Federal standards, that such fibers be 
monitored for both their presence and size. The performance of surgical 
procedures in operating rooms requires that the atmospheric environment be 
clear of particulates and especially bacteria. Indeed, even the 
manufacture and test of the high efficiency filters requires that they be 
checked by means of a particle sizing device. Many other types of 
biological and research endeavors could not be performed without the high 
efficiency filtering of the atmospheric environment and the continued 
monitoring of this environment for the presence of unwanted particles. 
There are other needs to monitor particles including the need to detect 
the presence of dangerous particles such as insecticides, bacteria, oxides 
of heavy metals, etc. 
Because of the aforementioned requirements for the accurate classification 
and, especially, the sizing of very small particles, many types of 
instruments have been developed and marketed based on rapid 
electro-optical techniques. These systems generally require an 
illumination source such as collimated white light or a monochromatic 
light source such as a laser. Not only do lasers provide an exceptionally 
high light flux incident on particles passing through their beams, but 
they also play a major role in the spatial isolation of the particles 
because of their generally narrow beam cross section. A He-Cd laser, for 
example, as manufactured by Linconix, Inc., has a beam diameter of less 
than 0.3 mm. 
Most traditional laser structures produce beams of the order of 1 mm. The 
term "beam diameter," of course, refers to the 1/e.sup.2 diameter, since 
the intensity profile of a TEM.sub.oo mode laser is Gaussian. This 
non-uniformity results in significant problems with "conventional" 
laser-based particle sizers since, for such instruments, the systems 
measure the total amount of light scattered by each single particle as it 
passes through the beam. The total amount of scattered light collected and 
measured is then associated with an average particle size. This 
assumption, that the total scattering cross section is proportional to the 
geometrical cross section, is erroneous, and is discussed, for example, in 
the article by Cooke and Kerker in Applied Optics, Vol. 13, page 272 
(1974), or in Kerker's textbook The Scattering of Light and Other 
Electromagnetic Radiation. Nevertheless, all measurements of the deduced 
total scattering cross section, or any other relative scattering quantity, 
do require a knowledge of the intensity of the incident illumination. 
Obviously, the accurate placement of the particle precisely at the beam 
center is critical in the deduction of its "perceived" size. 
The strip map technique, discussed in the corresponding patent application 
referenced above, does not require that each particle measured be exposed 
to an identical incident flux. Indeed, all subsequent characterizations 
(size, shape, refractive index, etc.) may be derived by considering 
various light scattering ratios or fractional differences between detected 
signals at different angular locations with respect to the direction of 
the incident illumination. Consider detector means distributed over the 
surface of a sphere with the light source illuminating a single particle 
at the center of the sphere. As long as each significant detector receives 
a sufficiently large scattered flux as the particle passes through the 
beam, the absolute position of the particle within the beam is not 
important. But if the particle to be analysed just grazes the beam, say at 
a 1/e.sup.6 distance, then the incident beam intensity on the particle may 
be too small to yield a meaningful signal at some detector locations. From 
the strip map input parameter requirements, such signals would result in 
meaningless values and, subsequently, erroneous particle classification. 
In order to establish that a larger (300 to 1000 nm) particle has grazed 
the beam and hence resulted in some measured intensities being 
significantly in error and that the particles, accordingly, cannot be 
classified, requires a new measurement technique. Very small particles 
(&lt;300 nm), even if they strike the beam at its most intense region, will 
also scatter negligible light into most (or all) detectors. Thus, these 
particles too must be identified, distinguished from larger grazing 
particles, and classified, if possible. This invention describes a unique 
means to cope with the problems associated with the characterization of 
small particles, as well as the handling of larger, grazing particles. 
DETAILS OF THE INVENTION 
In all the foregoing discussions and those that follow, reference to 
particle size is made in specific units of length, i.e. nanometers. These 
units are to be interpreted in terms of the dimensionless size parameter, 
.rho., by assuming that the incident radiation is in the visible part of 
the electromagnetic spectrum, i.e. around 500 nm. For this case, a 
particle of diameter 100 nm would have a .rho.-value of about 0.6. 
The accurate characterization and identification of a particle by light 
scattering means with spatially inhomogeneous light beams requires that 
particles at least as small as 100 nm by suitably detected and 
differentiated from their larger particle counterparts. By being able to 
classify such particles, the associated measuring instrument must be able 
to cull their larger grazing counterparts, as shall soon be seen. FIG. 1 
presents the variation of scattered light intensity with angle for 
vertically polarized incident light at a wavelength of 514 nm for 
particles of radius 50 nm and 4 different refractive indices. All curves 
have been normalized at .degree.. In terms of the strip map identification 
procedures, the different refractive indices could not be distinguished: 
only their size could be derived. In examining similar patterns for 
particles up to about 200 nm, we have confirmed this same type of 
"degeneracy." FIG. 2, for example, presents a similar plot for particles 
of radius 100 nm. However, with increasing size (irrespective of 
refractive index) to this 200 nm limit, we have noted that the forward 
(20.degree.) to backward (160.degree.) scattered intensity ratio increases 
to about 5 for the very largest particles in this size range. For these 
very small particles, the angular resolution of the measurement is not 
important: we could equally well use the integrated scattered intensities 
from, say, 20.degree. to 30.degree. divided by the integrated intensities 
from 150.degree. to 160.degree. and obtain an equivalently monotonic size 
response for this ratio. Since, furthermore, for the small particles and 
plane polarized incident light, the azimuthal variation of scattered 
intensity has a simple cos.sup.2 .phi. form, we can also integrate over 
all scattering angles .phi. and still maintain a monotonic variation of 
this front/back ratio with size. Naturally, we are talking here about 
relatively regular small particles whose size and structure have no major 
effect on the azimuthal scattered intensity. 
The reason that we choose to integrate the scattered intensities over such 
large solid angles is that these smaller particles do not scatter much 
light: at 100 nm their scattering efficiencies are often only 1%, i.e. the 
scattering cross section is 1% of the geometrical cross section. By 
concerning ourselves initially with the problem of detecting and sizing 
smaller particles, we have discovered a means, to be described presently, 
to accomplish this while discarding data from larger, grazing particles, 
which also produce low light level signals. 
The scattering chamber of the preferred embodiment of our invention is a 
spherical chamber. A hemisphere of this chamber is shown in FIG. 3. It has 
been designed to collect light from the two large solid angle front/back 
regions discussed above. These regions are hereafter called wedge ports or 
"wedges," i.e. wedges milled out of the sphere in the 
20.degree.-30.degree. and 150.degree.-160.degree. direction, respectively. 
The scattered light passing through these wedge ports is collected 
subsequently into two corresponding detectors. 
FIG. 3 presents a hemispherical section of the preferred embodiment of the 
invention. The scattering chamber is assembled from two such hemispherical 
sections attached by means of a flange 2. One hemisphere contains bolt 
holes 3 in its flange and the opposite hemisphere has threaded holes. 
Along the Z-axis 4 the light or laser source passes and the aerosol 
particles are introduced through a port 5 lying on the Y-axis 6. A 
corresponding port, not shown, on the opposite hemisphere contains an 
aerosol exhaust port. Between the two hemispheres lies an O-ring to help 
make the final structure air tight. The flanges lie in the X-Z plane. The 
X-axis is shown at 10. Also shown in FIG. 3 are sets of small apertures 7 
used to hold small photodetectors or optical fibers. In the preferred 
embodiment of the invention, optical fibers are fused to gradient 
refractive index lenses, such as the SELFOC lenses manufactured by the 
Japan Glass Works. These lenses are then inserted into the apertures 7 to 
provide light collection means at different angular positions on the 
surface of the sphere with respect to the direction of the incident light 
beam 4. The other end of the optical fiber is attached to its own distinct 
photometric detector such as a photomultiplier of the type R647HA 
manufactured by Hamamatsu Corporation. Alternatively, the fibers from all 
apertures may be combined and fused to the optical fiber faceplate 
photocathode of a multianode microchannel plate array tube such as 
manufactured by the Litton Electron Tube Division of Litton Industries. 
Light from a given optical fiber would then produce a signal at a single 
anode of the aforementioned structure. Many other types of optical 
collection and amplification means will be immediately evident to those of 
ordinary skill in the art of light scattering measurements and are 
included here by general reference. Note that in the preferred embodiment 
of this invention, the various light collecting apertures will be placed 
along arcs on great circles. If these great circles be spaced at 
45.degree. with respect to one another, then the incident light along the 
Z-axis should be vertically polarized with respect to one of these great 
circles, say great circle 8. This same beam would then be horizontally 
polarized with respect to the great circle shown at 90.degree. to it at 9. 
The two remaining great circles of the preferred embodiment would each lie 
at 45.degree. with respect to the circles 8 and 9. 
FIG. 4 shows the relation between the Cartesian axes 10, 6, and 4 of FIG. 3 
and the polar angles .theta. at 11 and .phi. at 12. The distance to a 
particular point on the spherical surface is .rho. at 13. 
The special wedge ports, 1, of FIG. 3 are shown in further detail in FIG. 
5. They are cut symmetrically into each hemispherical structure. A bundle 
of optical fibers is fused and placed into each port to depth control 
means 14. The light incident upon a bundle lying in such a wedge port is 
further collimated by channel 15 and stop 16. 
FIG. 6 shows a cross sectional view through one of the great circles 
containing the detector aperture means 7. Details of the apertures of the 
preferred embodiment are indicated at polar angles 11 from 10.degree. 
through 90.degree.. Not shown in such detail are the apertures for the 
angles between 90.degree. and 170.degree.. The wedge port structure for 
the optical bundles 1 is shown in cross section with bundle stop 14 and 
collimating elements 15 and 16. The laser or other light source would be 
attached at 17. Note that the aerosol inlet port 6 of FIG. 3 does not lie 
in any of the preferred great circles containing detector means 7. 
In the preferred embodiment of this invention, the two optical fiber 
bundles would subtend polar angles 25.degree..+-..degree. and 
155.degree..+-..degree., front and back respectively. The two forward 
bundles, one from the top hemisphere and one from the lower hemisphere, 
would be combined and fused to the photocathode faceplace of a single 
layer photomultiplier such as the R980 manufactured by Hamamatsu. The two 
corresponding rear bundles, centered on .theta.=155.degree., would be 
similarly combined and fused to a separate photomultiplier photocathode. 
In the preferred embodiment of this invention, the spherical chamber would 
have a radius of about 41 mm which would correspond to the optimal 
collection efficiency of an OPCL-10A SELFOC optical collimator. The 
diameter of this SELFOC lens is 1.8 mm yielding a solid angle .DELTA. 
.OMEGA. subtended at the lens by a scattering particle at the chamber 
center of about 0.00012 steradian. Each combined optical fiber wedge 
structure subtends approximately 0.033 steradian. For very small particles 
scattering flux isotropically, the wedges would collect almost 300 times 
more scattered flux, making the detection of very small particles 
practical even for laser sources of modest power. Note that the inner 
diameter of the chamber of the preferred embodiment is only about 1.5" and 
the outer diameter is a little over 3". This small size will yield the 
maximum scattered flux entering each SELFOC lens consistent with its 
narrow field of view, as has been previously discussed. It also permits 
the optical fiber bundles to be well collimated and reduces the spurious 
reflections within the chamber. Finally, this small size results in good 
air seals and, thereby, helps maintain laminar flow of the aerosol stream. 
The means by which data from larger particles that just graze the beam may 
be recognized and then discarded on the basis of the fiber bundle 
containing wedge ports 1 is quite straightforward. For all sets of data 
collected, we must examine the intensity ratio of the front wedge port 
collected light to the rear wedge port collected light. For the larger 
particles, this ratio will be very large, often as great as 100:1. If the 
calculated ratio exceeds, say, 10 then the data would be discarded as long 
as the front wedge port detector did not approach saturation and most of 
the individual detector means 7 collected no signals. A larger particle 
passing through the center of the beam, or near it, will produce a signal 
that often could saturate the forward wedge port detector. In this event, 
the data from the other detectors would be kept and processed to 
characterize the particle by the strip map technique or other means. 
Naturally, there are various situations whereby the scattered intensity 
around 25.degree. might be very small for a large, regular or irregular, 
particle and the resulting ratio not so easy to recognize. We believe that 
it will be extremely rare to obtain a small ratio without saturating 
either wedge port detector and thereby misclassify, a large grazing 
particle as a small particle. A large particle producing an anomalous 
wedge scattering ratio will also produce signals at most of the remaining 
detector locations. With a modest illumination source, none of these would 
detect any signal at all from a very small particle event. Thus, the wedge 
port scheme will permit the accurate classification of many small 
particles while preventing the inadvertent misclassification of virtually 
all larger particles that only graze the beam. 
FIG. 7 shows more details of a scattering chamber hemisphere viewed along 
the Z-axis. The four detector planes, lying in great circles, are oriented 
at 22.5.degree., 67.5.degree., 112.5.degree. and 157.5.degree. with 
respect to the X-Y plane, and are shown as 18, 19, 20, and 21, 
respectively, in FIG. 7. In the preferred embodiment, the electric field 
of the incident plane polarized laser beam would lie in the 112.5.degree. 
plane, 20. Thus, the conventional V-plane, is the 22.5.degree. plane 18 
and the H-plane lies at 90.degree. to it, i.e. at 112.5.degree., 20. The 
two remaining planes, 19 and 21, lie at 45.degree., respectively, to the 
V- and H-planes. This geometry of the preferred embodiment permits 
measurement at all angles, and particularly at 90.degree., in all planes 
without obstruction from the aerosol handling system or the flange plane. 
Also shown in the figure is a typical wedge port 1, various apertures to 
hold the SELFOC optical collimators 7, the X-axis 10, the Y-axis 6, and 
the Z-axis 4. The laser enters through the Z-axis 4 and the aerosol 
through the Y-axis 6. 
While there has hereinbefore been presented what is at present considered 
to be the preferred embodiment or process, it will be apparent to those of 
ordinary skill in the art that many modifications and variations may be 
made therefrom without departing from the true spirit and scope of the 
invention. All such variations and modifications, therefore, are 
considered to be a part of the invention.