Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2002-07-17
2004-09-21
Luu, Thanh X. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S222200, C356S336000
Reexamination Certificate
active
06794671
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to methods and apparatus for optical sensing, including counting and sizing of individual particles of varying size in a fluid suspension, and more particularly, to such methods and apparatus which yield higher sensitivity and coincidence concentration than can be realized by optical sensors of conventional design.
2. Description of Related Art
It is useful to review the principles underlying the traditional method of optical particle counting, hereinafter referred to as single-particle optical sensing (SPOS). Sensors that are used to implement SPOS are based on the physical technique of light extinction (LE) or light scattering (LS), or some combination of the two. The optical design of a traditional SPOS sensor based on the LE technique is shown schematically in
FIG. 1. A
fluid, consisting of a gas or liquid, in which particles of various sizes are suspended, is caused to flow through a physical flow channel
10
, typically of rectangular cross section. Two of the opposing parallel surfaces
12
and
14
defining the flow channel are opaque, while the remaining two opposing parallel surfaces
16
and
18
perpendicular to the opaque pair are transparent, comprising the “front” and “back” windows of the flow cell
10
. A beam of light
20
of appropriate shape enters front window
16
of flow cell
10
, passes through the flowing fluid and particles, exits flow cell
10
through back window
18
and impinges on a relatively distant light-extinction detector D
LE
.
The width of front and back windows
16
and
18
along the direction defined by the x-axis is defined as “a” (FIG.
1
). The depth of flow cell
10
, along the direction defined by the y-axis, parallel to the axis of the incident light beam, is defined as “b.” Suspended particles of interest are caused to pass through flow cell
10
along the direction defined by the z-axis (from top to bottom in
FIG. 1
) at a steady, appropriate rate of flow, F, expressed in units of milliliters (ml) per second, or minute.
The optical sensing zone
22
(“OSZ”), or “view volume,” of the sensor is the thin region of space defined by the four internal surfaces of flow channel
10
and the ribbon-like beam of light that traverses channel
10
. The resulting shape of the OSZ resembles a thin, approximately rectangular slab (having concave upper and lower surfaces, as described below), with a minimum thickness defined as 2w, oriented normal to the longitudinal axis of flow cell
10
(FIG.
1
). Source of illumination
24
is typically a laser diode, having either an elliptical- or circular-shaped beam, with a gaussian intensity profile along each of two mutually orthogonal axes and a maximum intensity at the center of the beam. Two optical elements are typically required to create the desired shape of the incident light beam that, together with the front and back windows of the flow channel
10
, defines the OSZ. The first optical element is usually a lens
26
, used to focus the starting collimated beam at the center (x-y plane) of flow cell
10
. The focused beam “waist,” or width, 2w, is proportional to the focal length of the lens and inversely proportional to the width of the starting collimated beam, defined by its 1/e
2
intensity values. The focused beam width, 2w, also depends on the orientation of the beam, if its cross section is not circular.
The second optical element is typically a cylindrical lens
28
, used to “defocus,” and thereby widen, the light beam in one direction—i.e. along the x-axis. In effect, cylindrical lens
28
converts what otherwise would be a uniformly focused beam (of elliptical or circular cross section) impinging on the flow cell, into a focused “line-source” that intersects flow channel
10
parallel to the x-axis. The focal length and location of cylindrical lens
28
are chosen so that the resulting beam width (defined by its 1/e
2
intensity points) along the x-axis at the center of the flow cell is much larger than the width, a, of the flow channel
10
. As a result, front window
16
of the sensor captures only the top portion of the gaussian beam, where the intensity is nearly uniform. Substantial uniformity of the incident intensity across the width (x-axis) of the flow channel
10
is essential in order to achieve optimal sensor resolution. The intensity profile along the z-axis of the resulting ribbon-like light beam is also gaussian, being brightest at the center of the OSZ and falling to 1/e
2
at its “upper” and “lower” edges/faces, where the distance between these intensity points defines the thickness, 2w, of the OSZ.
The shape of the OSZ
22
deviates from that of an idealized, rectangular slab shape suggested in FIG.
1
. Rather, the cross-sectional shape of the OSZ in the y-z plane resembles a bow tie, or hourglass, owing to the fact that the incident light beam is focused along the y-axis. However, assuming that the optical design of the sensor has been optimized, the focal length of the focusing lens will be much larger than the depth, b, of the flow cell. Therefore, the “depth of field” of the focused beam—defined as the distance between the two points along the y-axis at which the beam thickness expands to 2×2w—will be significantly larger than the depth, b, of the flow cell. Consequently, the variation in light intensity will be minimal along the y-axis.
The ribbon-like light beam passes through the fluid-particle suspension and impinges on a suitable light detector D
LE
(typically a silicon photodiode). In the absence of a particle in the OSZ, detector D
LE
receives the maximum illumination. A particle that passes through the OSZ momentarily “blocks” a small fraction of the incident light impinging on detector D
LE
, causing a momentary decrease in the photocurrent output of detector D
LE
and the corresponding voltage “V
LE
” produced by suitable signal-conditioning means. The resulting signal consists of a negative-going pulse
30
of height &Dgr;V
LE
, superimposed on a d.c. “baseline” level
32
of relatively large magnitude, V
0
, shown schematically in FIG.
2
. Obviously, the larger the particle, the larger the pulse height, &Dgr;V
LE
, both in absolute magnitude and as a fraction of V
0
.
The detector signal, V
LE
, is processed by an electronic circuit
34
, which effectively removes the baseline voltage, V
0
, typically either by subtracting a fixed d.c. voltage from V
LE
or by “a.c. coupling,” using an appropriate high-pass filter. This action allows for capture of the desired negative-going pulses of various heights, &Dgr;V
LE
. The resulting signal pulses are then “conditioned” further, typically including inversion and amplification. Each pulse is digitized using a fast, high-resolution analog-to-digital (A/D) converter, allowing its height to be determined with relatively high accuracy. A calibration table is generated, using a series of “standard” particles (typically polystyrene latex spheres) of known diameter, d, spanning the desired size range. This set of discrete values of &Dgr;V
LE
vs d is stored in computer memory and typically displayed as log &Dgr;V
LE
vs log d, with a continuous curve connecting the points. The set of measured pulse heights, &Dgr;V
LE
, are easily converted to a set of particle diameters, d, by interpolation of the calibration table values.
In principle, there are several physical mechanisms that can contribute to the light extinction effect. These include refraction, reflection, diffraction, scattering and absorbance. The mechanisms of refraction and reflection dominate the LE effect for particles significantly larger than the wavelength of the incident light, typically 0.6-0.9 micrometers (&mgr;m). In the case of refraction, the light rays incident on a particle are deflected toward or away from the axis of the beam, depending on whether the refractive index of the particle is larger or smaller, respectively, than the refractive index of the surrounding fluid. Provided the two refractive indices differ sufficiently and the (small) detector e
Nicoli David F.
Toumbas Paul
Field Milton M.
Luu Thanh X.
Particle Sizing Systems, Inc.
Sohn Seung C.
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