Patent Description:
This invention was made with government support under Contract No. NNX14CC65P awarded by National Aeronautics and Space Administration (NASA). The government has certain rights in the invention.

The invention relates to particle imaging. More particularly, the invention relates to imaging of particles using convergent light.

A very wide range of industrial processes use liquid droplets and solid particles of irregular shapes and sizes. Grinding powders, medical inhalers, and spray painting are just a few such examples. Industrial processes including coatings produced by thermal and other sprays typically involve determinations of particle parameters -- for example, particle size, shape, velocity, and position in space. The area of aircraft icing involves supercooled water droplets in the presence of ice crystals and ice particles (spherical frozen droplets). Existing techniques cannot accurately and reliably measure the size of these particles. Furthermore, existing techniques cannot separate the liquid droplets from the ice particles and ice crystals.

Existing particle imaging techniques include incorporating bright-field imaging using arc flash lamps, pulsed lasers, and pulsed LEDs for illumination. These techniques typically use charge-coupled device ("CCD") cameras or Complementary Metal Oxide Semiconductor ("CMOS") cameras to record the shadow images of the particles. These techniques typically use collimated or nearly collimated light with diffusers to illuminate the particle field. In these techniques, however, the out of focus particles under relatively dense particle field conditions typically produce shadows that complicate the detection and measurement of the in focus particle shadow images. In addition, larger particles in the light beam path can extinguish or obscure the light beam which causes a loss of the smaller particle image at the sample volume. Such losses of images result in an unacceptable bias in the sampling statistics.

Typically, the lasers used for the particle imaging techniques are edge emitting laser diodes. The edge emitting laser diodes are generally made up of cleaved bars diced from the wafers. As a result of the high index of refraction between air and the semiconductor material, the cleaved bars facets act as mirrors. For the edge emitting laser diodes, light oscillates parallel to the active layers and escapes sideways resulting in an elliptical laser beam profile.

Unfortunately, the lasers used for the existing particle imaging techniques produce visible diffraction rings around features and speckles. The diffraction rings and the speckles of laser radiation are detrimental to line-of-sight microscopy. Speckles and diffraction of laser radiation degrade an image quality and the background light intensity distribution that becomes noisy and nonuniform.

Methods and apparatuses to provide multi-beam imaging of particles are described. In a first aspect, a method to provide multi-beam imaging of particles is set out in claim <NUM>.

In a second aspect, an apparatus using the method to provide multi-beam imaging of particles in accordance with the first aspect is set out in claim <NUM>.

In a third aspect, a non-transitory machine-readable medium comprising the apparatus of the second aspect is set out in claim <NUM>.

Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below.

The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements, in which:.

Methods and apparatuses to provide multi-beam imaging of particles using a plurality of vertical cavity surface emitting lasers are described. It should be noted that term "particle" is referred herein to a droplet, a bubble, or any other object. The particle can have a spherical shape, a deformed sphere shape, or any other shape. The particle can comprise a liquid, a solid material, a bubble, or any combination thereof.

In the invention, an apparatus to image particles includes a plurality of vertical cavity surface emitting lasers (VCSELs) to generate a plurality of light beams propagating through a particle field. The plurality of the VCSELs converge with each other to form a measurement volume within the particle field. The plurality of the VCSELs is configured to provide uniformity in a background illumination of the measurement volume. An imaging optics is coupled to at least one of the plurality of VCSELs. A digital camera is coupled to the imaging optics to obtain a shadow image of a particle passing through the measurement volume at a focal plane of the digital camera. A processor is coupled to the digital camera. The VCSEL light sources are used for multi-beam imaging of particles to provide high quality illumination that is uniform and free of laser speckle. VCSELs have lower manufacturing costs and are highly reliable as compared to edge emitting laser diodes. Currently, VCSELs are only used in data communication, local-area networks and face recognition systems.

Typically, the laser diodes used as an illumination source for imaging techniques have laser-specific characteristics, e.g., diffraction and speckle that degrade image quality. The use of an array of VCSELs for particle multi-beam imaging advantageously decreases diffraction and speckle and increases uniformity of the background illumination of the particle measurement volume comparing to existing techniques.

<FIG> shows a schematic of one embodiment of an apparatus <NUM> to provide multi-beam imaging using a plurality of vertical cavity surface emitting laser sources (VCSELs). Apparatus <NUM> comprises a plurality of VCSEL sources, e.g. a VCSEL source <NUM> and a VCSEL source <NUM>, to generate a plurality of illuminating light beams -- e.g., a VCSEL source <NUM> is configured to generate an illuminating light beam <NUM> and a VCSEL source <NUM> is configured to generate an illuminating light beam <NUM>-- that converge with each other to form a measurement volume <NUM> within a particle field <NUM> to provide uniformity in illumination of a background <NUM> on an image plane <NUM> of an imaging system that is associated with the measurement volume <NUM> and to remove out of focus shadows in image plane <NUM>. As shown in <FIG>, the illumination light intensity produced by the converging light beams of the VCSEL sources <NUM>, <NUM> at different locations on the background <NUM> is similar, so that the light intensity distribution across the entire background <NUM> is uniform. In the invention, the illumination of the background <NUM> is substantially free of speckles and diffraction patterns, as described in further detail below. Imaging optics are coupled to at least one of the plurality of vertical cavity surface emitting lasers, as described in further detail below. A digital camera is coupled to the imaging optics to obtain a shadow image of a particle passing through the measurement volume <NUM> at a focal plane <NUM> of the digital camera, as described in further detail below. A processor is coupled to the digital camera, as described in further detail below.

Each of the plurality of converging light beams <NUM> and <NUM> is generated by at least two VCSELs. At least some of the VCSELs generating the plurality of converging light beams <NUM> and <NUM> are a part of a vertical cavity surface emitting laser array, as described in further detail below. An array of VCSELs is configured to generate the plurality of converging light beams <NUM> and <NUM>. For one embodiment, the array of VCSELs that is configured to generate at least one of the converging light beams <NUM> and <NUM> includes at least six VCSELs. For one embodiment, at least one of the light beams <NUM> and <NUM> is generated by the array of VCSELs that are arranged in a circular pattern, as described in further detail below.

As shown in <FIG>, illuminating light beam <NUM> propagates on an optical path along a direction <NUM> and illuminating light beam <NUM> propagates on an optical path along a direction <NUM> through a particle field <NUM>. The optical path of beam <NUM> is different from the optical path of beam <NUM>. For one embodiment, illuminating light beams <NUM> and <NUM> have the same wavelength. For one embodiment, the wavelength of the illuminating light beams <NUM> and <NUM> is about <NUM>. For another embodiment, illuminating light beams <NUM> and <NUM> have different wavelengths. For one embodiment, at least one of the illuminating light beams <NUM> and <NUM> generated by VCSELs is a pulsed beam. For one embodiment, at least one of the pulsed illuminating light beams <NUM> and <NUM> generated by the VCSELs for multi-beam imaging of particles has a fast rise time on the order of <NUM> picoseconds (ps), as described in further detail below. For one embodiment, the beams are pulsed in unison to "freeze" the particle motion, as described in further detail below.

For an embodiment, at least two illuminating beams crossing at a common point to form the measurement volume are used to image particles. For more specific embodiment, a number of illuminating light beams converging at a common point to form the measurement volume to image the particles is in an approximate range from <NUM> to <NUM>.

For one embodiment, the VCSEL sources for multi-beam imaging of particles are configured to have a very short coherence length that is similar to that of the light emitting diodes (LEDs) that minimizes laser speckle. For one embodiment, the VCSEL arrays for the multi-beam imaging of particles have an output power in an approximate range from about <NUM> milliwatt (mW) to <NUM> kilowatt (kW). For one embodiment, the VCSEL arrays for the multi-beam imaging of particles have an output power of about 10W.

For one embodiment, each of the plurality of light beams <NUM> and <NUM> produced by the VCSELs has a full beam divergence angle that is not greater than <NUM> degrees. For one embodiment, each of the plurality of light beams <NUM> and <NUM> produced by the VCSELs has a power that is at least <NUM> milliwatt (mW). For one embodiment, the VCSELs of the multi-beam particle imaging system output about <NUM> W under continuous operation, and over <NUM> W peak power under pulsed operation.

For one embodiment, VCSEL array chips are configured to replace traditional LEDs used for as illuminators with silicon CCD or CMOS cameras. An advantage of the VCSELs over the LEDs is that the divergence angle of the VCSEL beam is substantially smaller than the divergence angle of the LED beam. For one embodiment, the full divergence angle of the VCSEL beam is only about <NUM> degrees. For one embodiment, the full divergence angle of the VCSEL beam is about <NUM> degrees which is ideal for long range applications such as for the modular hyperspectral imaging (HSI).

For one embodiment, the speckle of the one or more VCSEL sources used for multi-beam imaging of particles is less than <NUM>% to increase uniformity of background illumination comparing to conventional systems. In the invention, the background illumination provided by the multiple light beams generated by the VCSELs for multi-beam imaging of particles is substantially free of speckles and diffraction patterns.

The VCSELs are more eye-safe than edge emitting lasers for multi-beam imaging of particles, especially if used with diffusors to further homogenize the beams. An added advantage is that the VCSEL devices are much more powerful than edge emitting lasers and have intense collimated light beams. Although the VCSEL devices produce very high intensity light beams, the character of the light beams is more along the lines of the LED than a laser. For one embodiment, the VCSEL source used for multi-beam imaging of particles is advantageously arranged in a simple silica integrated circuit (IC) chip-like configuration that provides ease of mounting and compactness.

The VCSEL devices have a number of advantages over edge emitting laser diodes including their ability to operate at relatively high temperatures so cooling systems are not required. For aircraft icing applications, the VCSEL may be in an environment close to the probe heaters so the temperature insensitivity is important. The VCSELs can deliver very high power per unit area reaching approximately <NUM> W/cm<NUM>. The VCSELs emit a circular beam which can be designed to have a Gaussian intensity distribution. This simplifies the optics needed for transforming the beam to a near top hat intensity distribution using, for example, diffractive optical elements (DOEs). VCSEL lasers are more reliable than edge emitting laser diodes. Typical failures are predicted to be <NUM> billion device hours (estimated Mean Time To Failure (MTTF) is about <NUM> years). In terms of pricing, VCSELs are approaching the price of LEDs. VCSELs can be processed easily into monolithic 2D arrays when higher power is required.

In the invention, the illuminating light beams for convergent multi-beam illumination of particles are produced by a plurality of vertical cavity surface emitting lasers (VCSELs).

As shown in <FIG>, the particle field <NUM> comprises particles, such as a particle <NUM> and <NUM>. Illuminating light beam <NUM> and illuminating light beam <NUM> converge with each other to form measurement volume <NUM>. As shown in <FIG>, measurement volume <NUM> is a region where light beams <NUM> and <NUM> overlap with each other. The beam <NUM> is at an angle <NUM> to an optical axis <NUM>. The beam <NUM> is at an angle <NUM> to optical axis <NUM>. For an embodiment, if one of the angles <NUM> and <NUM> is a <NUM> degree angle, the other one of the angles <NUM> and <NUM> can be any angle other than <NUM> degree. For one embodiment, the beam intersection angle is determined by an f-number (f#) of the imaging system. Generally, the f# is defined as a lens focal length divided by the lens diameter. For another embodiment, each beam is detected by a separate lens and the image transferred to a common image plane so that larger beam intersection angles can be used.

As shown in <FIG>, illuminating light beams <NUM> and <NUM> intersect at an angle <NUM>. For an embodiment, angle <NUM> is a sum of angles <NUM> and <NUM>. Particle <NUM> passes a portion of the measurement volume <NUM> at a focal plane <NUM> of an imaging system that includes a digital camera and produces an individual shadow <NUM> along direction <NUM> of beam <NUM> and an individual shadow <NUM> along direction <NUM> of beam <NUM>. Particle <NUM> passes a portion of the measurement volume <NUM> at a distance <NUM> away from focal plane <NUM> and produces an individual shadow <NUM> along direction <NUM> of beam <NUM> and an individual shadow <NUM> along direction <NUM> of beam <NUM>.

A cross-sectional view <NUM> of a measurement volume <NUM> along an axis A-A' perpendicular to direction <NUM> comprises individual shadows <NUM> and <NUM>. A cross-sectional view <NUM> of a measurement volume <NUM> along an axis B-B' perpendicular to direction <NUM> comprises individual shadows <NUM> and <NUM>, as shown in <FIG>. A shadow image <NUM> of particle <NUM> is formed on image plane <NUM> of an imaging system. Shadow image <NUM> is formed as a superposition of individual shadows <NUM> and <NUM>. As shown in <FIG>, shadow image <NUM> is substantially different from a background <NUM> produced by beams <NUM> and <NUM>. An individual shadow image <NUM> is formed from individual shadow <NUM> and an individual shadow image <NUM> is formed from individual shadow <NUM> on image plane <NUM>. The individual shadow images <NUM> and <NUM> of particle <NUM> do not overlap and are separated from each other and from shadow image <NUM> on image plane <NUM>. For one embodiment, a contrast between the shadow image <NUM> and the background <NUM> is greater than a contrast between the shadow image <NUM> of particle <NUM> outside the focal plane <NUM> and the background <NUM>. For one embodiment, the illumination intensities of the individual shadow images <NUM> and <NUM> of particle <NUM> are substantially the same as that of the background <NUM>.

For one embodiment, the illumination intensity of the background <NUM> is monitored, and an intensity of at least one of the plurality of VCSELs is adjusted based on the monitored background illumination to obtain the shadow image <NUM>. For one embodiment, a contrast between the shadow image <NUM> and the background <NUM> is measured. For one embodiment, it is determined if the measured contrast is greater than a predetermined contrast. If the measured contrast is greater than the predetermined contrast, the shadow image <NUM> is detected from the background <NUM>. If the contrast is not greater than the predetermined contrast, the shadow image <NUM> is not detected from the background <NUM>. The particle <NUM> is identified based on the detected shadow image <NUM>. The size of the identified particle <NUM> is determined from the shadow image <NUM>, as described in further detail below.

For an embodiment, a displacement distance of each of the individual shadow images <NUM> and <NUM> from the shadow image <NUM> indicates that shadows of particles outside the focal plane are displaced in the image plane at the camera and will not form deep shadows. The displacement distance in the image plane can be used to estimate the distance of the particles from the focal plane in the sample volume. This information can be used to determine if the particle is in sufficient focus to be sized accurately. For example, a displacement distance <NUM> between individual shadow images <NUM> and <NUM> is determined. For example, a displacement distance <NUM> between individual shadow image <NUM> and shadow image <NUM> on image plane <NUM> is determined. For example, a displacement distance <NUM> between individual shadow image <NUM> and shadow image <NUM> on image plane <NUM> is determined. For an embodiment, the focus of shadow image <NUM> is evaluated based on the distances <NUM> and <NUM> to determine a type of the particle.

For one embodiment, the type of the particle comprises a particle state -- e.g., a liquid, a solid, a bubble in liquid or solid, or any combination thereof. For another embodiment, the type of the particle represents a particle shape, e.g., a spherical, oval, a multi-sided shape-e.g., triangular, rectangular, square, diamond, rhombus, other multi-sided shape, -- or any other particle shape. For an embodiment, the particle information is determined based on the type of the particle. For one embodiment, the particle information comprises a particle velocity, a particle size, or any other particle information. For an embodiment, the size of the particle is at least <NUM> microns ("µm"). For another embodiment, the size of the particle is less than <NUM> microns ("µm"). For yet another embodiment, the size of the particle is in an approximate range from about <NUM> to about <NUM>. For an embodiment, the plurality of illuminating light beams are synchronized with a digital camera, as described in further detail below.

<FIG> shows a schematic of an apparatus <NUM> to image particles using VCSELs according to another embodiment. Apparatus <NUM> comprises a plurality of VCSELs to generate a plurality of illuminating light beams--e.g., VCSEL sources <NUM>, <NUM>, <NUM>, <NUM> and <NUM> to generate illuminating light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>-- that converge with each other to form a measurement volume <NUM> within a particle field to provide uniformity in illumination of a background <NUM> on an image plane <NUM> of an imaging system that is associated with measurement volume <NUM> and to remove out of focus shadows in image plane <NUM>. As shown in <FIG>, the illumination light intensity produced by the converging light beams of the VCSEL sources at every location of the background <NUM> is similar, so that the light intensity distribution across the entire background <NUM> is uniform. In the invention, the illumination of the background <NUM> is substantially free of speckles and diffraction patterns, as described in further detail below. An imaging optics is coupled to at least one of the plurality of vertical cavity surface emitting lasers, as described in further detail below. A digital camera is coupled to the imaging optics to obtain a shadow image of a particle passing through the measurement volume <NUM> at a focal plane <NUM> of the digital camera, as described in further detail below. A processor is coupled to the digital camera, as described in further detail below.

In the invention, each of the plurality of converging light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> is generated by at least two VCSELs. At least some of the VCSELs generating the plurality of converging light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> are a part of a vertical cavity surface emitting laser array, as described in further detail below. For one embodiment, an array of VCSELs is configured to generate at least one of the plurality of converging light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>. For one embodiment, the array of VCSELs that is configured to generate at least one of the plurality of converging light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> includes at least six VCSELs. For one embodiment, at least one of the light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> is generated by the array of VCSELs that are arranged in a circular pattern, as described in further detail below.

As shown in <FIG>, each of the illuminating light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> propagates on a respective optical path along a respective direction through the particle field comprising particles, such as particles <NUM>, <NUM>, <NUM> and <NUM>. For example, illuminating light beam <NUM> propagates on an optical path along a direction <NUM> and illuminating light beam <NUM> propagates on an optical path along a direction <NUM>. For an embodiment the optical paths of the illuminating light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> are different. For an embodiment, the wavelengths of the illuminating light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> are similar. For one embodiment, the wavelength of the illuminating light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> is about <NUM>. For another embodiment, the wavelengths of the illuminating light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> are different. For an embodiment, at least one of the illuminating light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> is a pulsed beam. For one embodiment, at least one of the pulsed illuminating light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> generated by the VCSELs for multi-beam imaging of particles has a fast rise time on the order of <NUM> picoseconds (ps), as described in further detail below. For one embodiment, the beams are pulsed in unison to "freeze" the particle motion, as described in further detail below. For one embodiment, particles <NUM> and <NUM> represent particles <NUM> and <NUM>.

As shown in <FIG>, illuminating light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> converge to form measurement volume <NUM> at focal plane <NUM> of an imaging system. Measurement volume <NUM> is a region where all illuminating light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> overlap, as shown in <FIG>. The illuminating light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> intersect with each other at multiple angles. Each of the illuminating light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> is at a respective angle to an optical axis <NUM>. As shown in <FIG>, the beam <NUM> is at an angle <NUM> to an optical axis <NUM>. The beam <NUM> is at an angle <NUM> to optical axis <NUM>. For an embodiment, if one of the illuminating light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> is at a <NUM> degree angle to optical axis <NUM>, other ones of the illuminating light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> can be at any angle other than <NUM> degree to optical axis <NUM>.

As shown in <FIG>, illuminating light beams <NUM> and <NUM> intersect at an angle <NUM>. For an embodiment, angle <NUM> is a sum of angles <NUM> and <NUM>. Particle <NUM> passing a portion of the measurement volume <NUM> at focal plane <NUM> produces a plurality of individual shadows from each of the illuminating light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>, such as an individual shadow <NUM> from beam <NUM> and an individual shadow <NUM> from beam <NUM>. Particle <NUM> passing a portion of the measurement volume <NUM> at a distance <NUM> away from focal plane <NUM> produces a plurality of individual shadows from each of the illuminating light beams <NUM>, <NUM>, <NUM>, <NUM> and <NUM>, such as an individual shadow <NUM> from beam <NUM> and an individual shadow <NUM> from beam <NUM>. A cross-sectional view <NUM> of a measurement volume <NUM> along an axis A-A' perpendicular to direction <NUM> comprises individual shadows <NUM> and <NUM>. A cross-sectional view <NUM> of a measurement volume <NUM> along an axis B-B' perpendicular to direction <NUM> comprises individual shadows <NUM> and <NUM>, as shown in <FIG>. A shadow image <NUM> of particle <NUM> is formed on an image plane <NUM> of an imaging system. Shadow image <NUM> is formed as a superposition of individual shadows -- e.g., such as <NUM> and <NUM>-- from each of the illuminating light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>.

As shown in <FIG>, shadow image <NUM> is substantially different from a background <NUM> produced by the illuminating light beams <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>. Individual shadow images are formed from individual shadows on image plane <NUM>, such as individual shadow images <NUM> and <NUM>. For example, individual shadow image <NUM> is formed from individual shadow <NUM> and individual shadow image <NUM> is formed from individual shadow <NUM>. Each of the individual shadow images is positioned at a distance <NUM> from shadow image <NUM> on image plane <NUM>. The individual shadow images of particle <NUM> do not overlap and are separated from each other by a distance on image plane <NUM>. For one embodiment, a contrast between the shadow image <NUM> and the background <NUM> is greater than a contrast between each of the individual shadow images <NUM> and <NUM> and the background <NUM>. The illumination intensities of the individual shadow images <NUM> and <NUM> are substantially the same as that of background <NUM>.

For an embodiment, the illuminating light beams are synchronized with the digital camera, as described in further detail below. In the invention, one or more intersection angles of the illuminating light beams are adjusted to set the measurement volume <NUM> at the focal plane <NUM> of the imaging system and to remove the shadow images of the particles outside the measurement volume <NUM>. For an embodiment, a quantity of the illuminating light beams is adjusted to remove the shadow images of the particles outside the measurement volume <NUM>.

<FIG> is a view <NUM> showing one embodiment of a system <NUM> to provide multi-beam imaging using a plurality of VCSELs. A system <NUM> comprises a transmitter system <NUM> and a receiver system <NUM>. Transmitter system <NUM> includes one or more VCSEL sources -- e.g., a VCSEL source <NUM> and a VCSEL source <NUM>-- generating a plurality of illuminating light beams -- e.g., VCSEL source <NUM> is configured to generate an illuminating light beam <NUM> and VCSEL source <NUM> is configured to generate an illuminating light beam <NUM> propagating on multiple optical paths through a particle field comprising particles -- e.g., particles <NUM>, <NUM>, and <NUM> moving along a direction <NUM>. In the invention, each of the VCSEL sources includes an array of VCSELs. The illuminating light beams generated by the VCSEL sources converge to form a measurement volume at a focal plane <NUM> of an imaging system to provide uniformity in a background illumination on an image plane <NUM> of the imaging system and to remove out of focus shadows in the image plane, as described above. The illuminating light beams of <FIG> are represented by the illuminating light beams of <FIG> and <FIG>.

For an embodiment, transmitter system <NUM> comprises a light source coupled to the one or more VCSEL sources to generate a triggering light beam (not shown). For an embodiment, the triggering light beam is sent to generate a plurality of illuminating light beams, if the particle is detected in the measurement volume, as described in <CIT>.

Referring back to <FIG>, receiver system <NUM> comprises an imaging optics coupled to at least one of the plurality of VCSEL sources and one or more digital cameras coupled to the imaging optics to provide a shadow image of a particle <NUM> passing through the measurement volume at focal plane <NUM>, as described above. For one embodiment, multiple digital cameras--e.g., digital cameras <NUM>, <NUM> and <NUM> -- are used adjust a dynamic range of the particles. For one embodiment, the size dynamic range of the particles is about <NUM>:<NUM>.

As shown in <FIG>, the imaging optics of the receiver system <NUM> comprises one or more receiver lenses <NUM> to receive the individual shadows of the particle <NUM> from each of the illuminating light beams, as described above. One or more image transfer lenses <NUM> transfer individual shadow images of the particle <NUM> to a beam splitter <NUM>. Beam splitter <NUM> splits the illuminating light beams comprising the individual shadow images of the particle <NUM> into a portion <NUM> and a portion <NUM>. Portion <NUM> is sent to one or more focusing lenses <NUM> to form a shadow image of the particle <NUM> on an image plane of a digital camera <NUM>. Portion <NUM> is sent to one or more focusing lenses <NUM> to form a shadow image of the particle <NUM> on an image plane of a digital camera <NUM>. The imaging system can optionally comprise a beam splitter <NUM> to split the illuminating light beams comprising the individual shadow images of the particle <NUM> into a portion <NUM> and a portion <NUM>. In this case, portion <NUM> is sent to beam splitter <NUM> to form a shadow image of the particle <NUM> on corresponding image planes of digital cameras <NUM> and <NUM>. Portion <NUM> is sent to one or more focusing lenses <NUM> to form a shadow image of the particle <NUM> on an image plane of a digital camera <NUM>. For one embodiment, the multiple camera approach is used to allow different magnifications to enable sizing particles with high resolution over a wide size range.

A processing system <NUM> is coupled to the receiver system <NUM>. Processing system <NUM> comprises a processor <NUM>, a memory <NUM>, and a display <NUM> to display shadow images of the particles passing through the measurement volume. For one embodiment, the processor <NUM> is coupled to one or more digital cameras to monitor the background illumination and to adjust an intensity of at least one of the plurality of VCSELs based on the monitored background illumination to obtain the shadow image of the particle.

For one embodiment, the processor <NUM> is coupled to one or more digital cameras to measure a contrast between the shadow image and the background and to determine if the measured contrast is greater than a predetermined contrast. If the measured contrast is greater than the predetermined contrast, the processor <NUM> is configured to detect the shadow image from the background. If the contrast is not greater than the predetermined contrast, the processor <NUM> is configured to not detect the shadow image from the background. The processor <NUM> is configured to identify the particle based on the detected shadow image. The processor <NUM> is configured to determine the size of the identified particle from the shadow image, as described in further detail below.

For another embodiment, the processor <NUM> is configured to detect the shadow image, to evaluate a depth of field of the particle, a focus of the particle, or both based on the shadow image to determine a type of the particle based on the evaluation, and to determine particle information based on the type of the particle, as described in further detail below.

For one embodiment, the size range of the imaging system is extended to cover a dynamic size range of from <NUM> to <NUM>. This can be accomplished with high resolution and very efficient image capture and transfer to the image processing computer using different magnifications and separate CMOS cameras.

As described in <FIG>, <FIG> and <FIG>, the multi-beam illumination system includes multiple light sources that are aligned to converge to a common measurement (probe) volume. For one embodiment, the multiple light sources are pulsed simultaneously to pulse durations below <NUM> nanoseconds (ns), therefore freezing the motion of the particles, even at the small scales resolved by the microscope imaging system. This approach provides an intense relatively uniform illumination in the sample volume, while minimizing laser speckle and diffraction, by merging the phases of the different laser wave fronts. Producing a more homogeneous background is critical in achieving high resolution and high quality images. An advantage of the multi-beam illumination comes from the different optical paths that the converging laser beams traverse to the measurement volume. If a large particle crosses one beam outside of the sample volume, the shadow produced by this particle does not significantly affect the background illumination because the other beams are not coincidently affected by this particle. Only particles that pass within the depth-of-field of the imaging system at the probe volume which is coincident with where the beams cross and overlap form sharp deep shadows. Particles passing the probe volume outside the focal plane do not produce deep shadows because the shadow appears in only one beam at any instant and location. Thus, even for relatively dense sprays and under conditions with very large drops (SLD conditions), the background noise due to out of focus images is minimized.

<FIG> is a view <NUM> illustrating a multi-beam illumination system according to one embodiment. A plurality of illuminating light beams -- e.g., illuminating light beams <NUM>, <NUM><NUM>, <NUM>, and <NUM> -- propagate on respective optical paths along respective directions through a particle field comprising particles, such as particles <NUM>, <NUM>, and <NUM>. Particles <NUM>, <NUM>, and <NUM> represent particles described above with respect to <FIG>. The illuminating light beams <NUM>, <NUM><NUM>, <NUM>, and <NUM> represent the illuminating light beams described with respect to <FIG>. The illuminating light beams converge to form a measurement volume at a focal plane of an imaging system. The focal plane of the imaging system propagates along an axis A-A'. The measurement volume is a region where all of the illuminating light beams overlap, as shown in <FIG>.

Particle <NUM> passing a portion of the measurement volume at the focal plane of the imaging system produces a plurality of individual shadows from each of the illuminating light beams that overlap to form a shadow image <NUM> of the particle <NUM> on the image plane of the imaging system. A cross-sectional view <NUM> of the measurement volume along the axis A-A' perpendicular to an optical axis <NUM> comprises the shadow image <NUM>. Particle <NUM> passing illuminating light beam <NUM> before the focal plane A-A' produces an individual shadow <NUM> from the illuminating light beam <NUM>. Particle <NUM> does not produce shadows from other illuminating light beams, such as illuminating light beams <NUM> and <NUM>. A cross-sectional view <NUM> of the illuminating light beams along an axis B-B' perpendicular to optical axis <NUM> comprises individual shadow <NUM> caused by beam <NUM>. Particle <NUM> passing illuminating light beams <NUM> and <NUM> after the focal plane A-A' produces an individual shadow <NUM> from the illuminating light beams <NUM> and <NUM>. Particle <NUM> does not produce individual shadows from other illuminating light beams, such as illuminating light beams <NUM> and <NUM>. A cross-sectional view <NUM> of the illuminating light beams along an axis C-C' perpendicular to optical axis <NUM> comprises shadow <NUM> that is a superposition of the individual shadows caused by beams <NUM> and <NUM>.

A graph <NUM> illustrates one embodiment of light intensity <NUM> versus a distance on a CCD array <NUM>. The light intensity <NUM> varies from peak intensity to a full shadow, as shown in graph <NUM>. The peak intensity corresponds to a background condition when there is no particle that crosses at least one of the beams. As shown in graph <NUM>, light intensity of the shadow image <NUM> of the particle <NUM> in a region <NUM> of the CCD array is substantially lower than the light intensity in a region <NUM> and a region <NUM> of the CCD array. That is, the shadow image of particle <NUM> is substantially different from the background peak light intensity and from the light intensity of the individual shadow images <NUM> and <NUM>.

As shown in <FIG>, the illuminating light beams converge at the measurement volume to emulate white light illumination. For one embodiment, color CCDs (RGB) are used to extract color information on the shadow image. Color information can provide additional dimensional information on the particle shadows. Delays between various color illuminations can also be used to measure velocity (analogous to double pulsing a single laser). For one embodiment, double pulse imaging is implemented to provide shadow particle image velocimetry (PIV) images to obtain droplet and spray structure size and velocity. The duration of the one or more illumination pulses is naturally limited by the resolution of the imaging system and the speed of the target particle. For one embodiment, for a microscopic imaging, maximum pulse duration of about <NUM> ns is used (<NUM> blur at <NUM>/s). For example, if the particle is moving at <NUM>/s, it will move <NUM> in <NUM> ns. This will blur the edges by <NUM> microns. For smaller particles, shorter pulse duration is used to minimize blur. For one embodiment, parameters of the illuminating light beams, such as an intersection angle, a number of beams, and wavelengths are optimized to prevent the individual shadow images of the particles outside the focal plane from being formed.

<FIG> is a view <NUM> illustrating a schematic layout of a vertical cavity surface emitting laser (VCSEL) source of a multi-beam particle imaging system according to one embodiment. Generally, a VCSEL source includes a plurality of semiconductor layers that are grown on top of each other on a substrate, as known to one of ordinary skill in the art of VCSEL device manufacturing. As shown in <FIG>, the VCSEL source includes an active quantum well (QW) layer <NUM> on a diffraction Bragg grating (DBR) layer <NUM> on a substrate <NUM> on a metal contact layer <NUM>. A DBR layer <NUM> is deposited on an oxide aperture layer <NUM> on active QW layer <NUM>. A metal contact layer <NUM> is deposited on DBR layer <NUM>. An insulating layer <NUM> is deposited on portions of DBR layer <NUM>, DBR layer <NUM> and metal contact layer <NUM>. A metal pad <NUM> is deposited on insulating layer <NUM> to connect to metal contact layer <NUM>. Generally, in the lasing medium of the VCSEL, the light oscillates perpendicular to the layers and escapes through the top (or bottom) of the device. As shown in <FIG>, a light <NUM> oscillates perpendicular to the layers <NUM>, <NUM>, <NUM>, <NUM> and <NUM> and outputs through the top of the device structure. This results in a nearly top hat circular beam profile <NUM>, as shown in <FIG>.

<FIG> is a view <NUM> showing an example of the light beam profile <NUM> generated by the VCSEL source according to one embodiment. As shown in <FIG>, the light beam profile <NUM> is a substantially circularly symmetric beam profile. For one embodiment, the beam divergence angle of the VCSEL source is approximately <NUM> degrees (<NUM>/e<NUM>). For one embodiment, the light beam produced by the VCSEL source is collimated for transmission over the distances larger than the transmission distances of the LEDs. The beam profile <NUM> is a quasi-top hat profile which is close to ideal for particle imaging applications.

<FIG> is a view <NUM> showing examples of multi-beam illumination images of particles that pass the measurement volume at different distances from the focal plane of the imaging system. As shown in <FIG>, the shadow images of the particle formed by the each of the illuminating beams move away from each other as the particle moves away from the focal plane of the imaging system. An image <NUM> shows shadow images of the particles passing the measurement volume at a focal plane of the imaging system. As shown in image <NUM>, the individual shadow images of the particle formed by each of the illuminating beams fully overlap to form a shadow image <NUM> that is substantially separate from the background. An image <NUM> shows the shadow images of the particles passing the measurement volume at about <NUM> away from the focal plane of the imaging system. As shown in image <NUM>, the individual shadow images of the particle from each of the illuminating beams overlap only partially (about <NUM>%) resulting in a shadow image <NUM> that is more blurred and less separate from the background than image <NUM>. An image <NUM> shows the shadow images of the particles passing the measurement volume at about <NUM> away from the focal plane of the imaging system. As shown in image <NUM>, the shadow images of the particle from each of the illuminating beams overlap only about <NUM>% resulting in a shadow image <NUM> that is more blurred and less separate from the background than image <NUM>.

Capturing high-resolution images at large distances is challenging, but the difficulty increases when dense particle fields obscure the optical path between the light source, the probe volume and the collection optics. The multi-beam illumination approach relies on different optical paths from the light sources to overcome punctual shadowing by individual particles along the beam path.

Existing particle imaging systems suffer from lack of uniformity in the background illumination. The coherent and highly monochromatic laser sources used in the illumination system produce speckle and diffraction patterns. The speckle and diffraction patterns prevent the image processing algorithm from performing under optimal conditions, thus affecting the results. <FIG> is a view <NUM> illustrating a two-dimensional illumination profile <NUM> of a VCSEL array <NUM> and a two-dimensional illumination profile <NUM> of a LED system <NUM> according to one embodiment. The beam divergence angle of the VCSEL arrays is approximately <NUM> degrees. For one embodiment, the VCSEL array for the multi-beam particle imaging includes a plurality of VCSELs coupled to an integrated optics that shapes the beam to narrow divergence distributions, so that the beam generated by the VCSEL array can be collimated for transmission over a distance that is larger than the transmission distance of the LEDs. For one embodiment, VCSEL array <NUM> represents one of the VCSEL arrays described above. As shown in <FIG>, the illumination distribution represented by the two-dimensional profile <NUM> has substantially narrow angle compared to that of two-dimensional illumination profile <NUM> that makes the VCSEL advantageous when it comes to collecting and remotely directing the light onto a scene for example. LEDs are typically equipped with a polymethyl-methacrylate (PMMA) or glass dome, shaping the light from quasi-Lambertian from the die emitter to a wide somewhat Gaussian distribution. The VCSEL devices on the other hand present a light emission profile close to a top-hat shape, with a possible dip on the axis. Proper optical implementation of the VCSEL allows efficient light transmission. For one embodiment, the VCSEL arrays for the multi-beam imaging of particles are configured to have a very short coherence length that is similar to that of the LED system that minimizes laser speckle. For one embodiment, the VCSEL arrays for the multi-beam imaging of particles are configured to have speckle of less than <NUM>% that makes the VCSELs ideal for imaging systems to provide the uniformity of the illumination. For one embodiment, the VCSEL sources the VCSEL arrays for the multi-beam imaging of particles are pulsed at very high rates and have very fast rise times, generally below a nanosecond, that make the VCSEL sources ideal for pulsed laser particle imaging applications. For one embodiment, the VCSEL array for the multi-beam imaging system produces <NUM> mW output power at the <NUM> wavelength, and includes about <NUM> lasers.

For one embodiment, the multi-beam system to image particles includes an electronic driver and an optical system that implements the VCSEL technology. The pulsed characteristics of VCSEL are known to be fast, as these devices are widely used for radio-transmission. For one embodiment, the VCSEL arrays used for the multi-beam imaging of particles are configured to output the power to generate the flux necessary to illuminate the scene with enough intensity for the imaging system to properly respond. For one embodiment the electronic driver of the multi-beam imaging system is capable of producing short pulses of high current (up to <NUM> A), and voltage from 0V to <NUM> V to drive the VCSEL array provide high-speed performance and short pulse characteristics of the illuminating light beams.

<FIG> is a view <NUM> showing measured pulse profiles of the VCSEL source of the multi-beam imaging system according to one embodiment. As shown in <FIG>, a measured pulse profile <NUM> of the VCSEL source is generated at driving pulse duration <NUM> ns, a measured pulse profile <NUM> of the VCSEL source is generated at driving pulse duration <NUM> ns and a measured pulse profile <NUM> of the VCSEL source is generated at driving pulse duration <NUM> ns.

For one embodiment, the ramps of the profiles overlap over the corresponding "active" period of the VCSEL. The slew rate of the VCSEL for the first <NUM> % of the rise is very fast, and then slows down until a plateau. As shown in <FIG>, the pulse signal rises in level with pulse duration. An explanation could be that the higher thermal load is achieved at longer pulse duration, thus driving the VCSEL spectrum to shorter wavelengths, where the photodetector is more sensitive.

<FIG> is a view <NUM> of a table <NUM> illustrating characteristic properties of the VCSEL system regarding switching performance according to one embodiment. The table <NUM> shows that the measured pulse duration is closer to the set duration, with only <NUM> ns difference, across all durations tested from <NUM> to <NUM> ns. This reveals that the switching time of the VCSEL is short, on the order of nanoseconds, despite the rise time being close to <NUM> ns. The fall time may be more representative of the optical switching performance, with a transient achieved in less than <NUM> ns. A close look at the signals in <FIG> shows that the slew rate of the driver is really fast up to approximately <NUM> % of the signal amplitude, after what the slew rate decrease, affecting the <NUM>-<NUM> % rise time figures. As shown in <FIG>, the slew rate drops after about <NUM> % of the signal amplitude that may be due to load capacitance and driver interaction.

<FIG> is a view <NUM> showing a measured spectrum <NUM> of the VCSEL array of the multi-beam imaging system according to one embodiment. As shown in <FIG>, the VCSEL array outputs highly monochromatic light comparing to the LED illumination.

As shown in <FIG>, the output wavelength of the VCSEL array is centered at <NUM>. The VCSEL array outputs a beam that has a spectral spread close to <NUM> full-width at half-maximum. Such a narrow spectral range may be problematic for line-of-sight extinction, as it could produce diffraction patterns around edges, such as interface between liquid or frozen drops and the surrounding ambient gases. Typically, the wavelength of the illumination determines the diffraction-limited resolution. A longer wavelength may be detrimental to optical resolution in microscope systems. Narrow-divergence, nearly-collimated light generated by the VCSEL array presents an advantage over the LED illumination for remote illumination of an area with high energy density and for beam shaping.

<FIG> is a photograph <NUM> of the emitting area of the VCSEL array <NUM> of the multi-beam imaging system according to one embodiment. The photograph <NUM> is acquired with a digital microscope. As shown in <FIG>, the emitting area of the VCSEL array <NUM> is approximately <NUM> in diameter. The VCSEL array <NUM> includes individual VCSELs, such as a VCSEL <NUM>. As shown in <FIG>, each VCSEL of the array has a hexagonal shape. As shown in <FIG>, the VCSELs of the array are arranged in a circular pattern to generate a single beam. For one embodiment, the VCSEL array is equipped with a lens, such that the beam divergence angle is reduced from a typical VCSEL beam divergence angle of <NUM> degrees to approximately <NUM> degrees to increase uniformity in the background illumination to obtain the shadow image of the particle.

For one embodiment, the multi-beam particle imaging system including one or more VCSEL arrays to generate multiple illuminating light beams improves image quality substantially by removing speckle from the background, and by reducing or eliminating diffraction patterns. For one embodiment, the multi-beam particle imaging system including one or more VCSEL arrays to generate multiple illuminating light beams is arranged in a line-of-sight configuration for bright field optical microscopic applications.

<FIG> is a view <NUM> showing a resolution chart <NUM> captured by a long distance microscope when a scene is illuminated by a LED source and a resolution chart <NUM> captured by the long distance microscope when the scene is illuminated by a VCSEL source according to one embodiment. Both LED and VCSEL illumination sources are pulsed, and the camera is synchronized with the light pulses.

As shown in <FIG>, the resolution chart <NUM> represents more uniform and speckle free light intensity distribution than the resolution chart <NUM>. As shown in <FIG>, the resolution chart <NUM> has a near top-hat profile of the light intensity distribution comparing to resolution chart <NUM>. As shown in <FIG>, the resolution chart <NUM> represents higher resolution than that of the resolution chart <NUM>.

<FIG> is a schematic <NUM> illustrating another embodiment of a multi-beam imaging system that includes VCSEL sources to generate multiple illuminating light beams. As shown in <FIG>, the multi-beam imaging system includes a VCSEL source <NUM> and a VCSEL source <NUM>. VCSEL source <NUM> generates an illuminating light beam <NUM> and VCSEL source <NUM> generates an illuminating light beam <NUM> that intersect with each other and propagate through a lens <NUM>. Lens <NUM> has a focal distance <NUM> and a focal distance <NUM>. For one embodiment, each of the VCSEL sources <NUM> and <NUM> represents one of the VCSEL sources described above. Particles <NUM> and <NUM> pass the measurement volume formed by overlapping illuminating light beams <NUM> and <NUM> at planes <NUM> and <NUM> respectively. Illuminating light beams forming the shadows of the particles -- such as shadows <NUM> and <NUM> -- are admitted into the lens <NUM> and the digital camera --e.g., CMOS, CCD array, or other digital camera.

Initially, plane <NUM> is a focal plane of the lens <NUM>. Particle <NUM> at the focal plane of the lens <NUM> produces a single focused shadow image <NUM> on an image plane <NUM>. Particles away from the focal plane, such as particle <NUM> produce two out of focus shadow images at the image plane. After the camera is re-focused to plane <NUM>, the individual shadows of particle <NUM> collapse onto a single shadow <NUM> at an image plane <NUM> whereas the shadow at image plane <NUM> will separate into an individual shadow <NUM> and an individual shadow <NUM>.

The depth of field and the circle of confusion are generally set by the requirements of the imaging system. The depth of field problem, however, is one of the most serious sources of measurement error when using imaging systems to measure particle size distributions. Generally, the depth of field refers to a range over which an optical instrument produces a sharp image of an object. Particles detected outside of the depth of field of the receiver optics cause a significant increase in the measurement error because the sizes of the unfocused particle images appear to be different from the true values.

The camera monitors the attenuation or shadow pattern produced by individual particles by detecting and identifying them from the bright background generated by the illumination. Proper and adequate illumination is an important parameter that needs to be adjusted carefully to obtain high quality imaging data. That is why it is paramount to be able to measure the background intensity on the CMOS camera and adjust the light intensity accordingly to capture high-quality particle images.

<FIG> is a view <NUM> showing an image of a background <NUM> illuminated by converging beams generated by a VCSEL system of the multi-beam imaging apparatus according to one embodiment. As shown in <FIG>, the illumination light intensity produced by the converging light beams of the VCSEL sources at different locations of the background <NUM>--- such as a location <NUM> and a location <NUM>--- is similar, so that the distribution of the light intensity across the entire background <NUM> is uniform. The smooth and uniform background illumination contributes to the proper operation of the image processing algorithm, which relies on the intensity contrast between the background and particles to identify a feature and quantify the feature size.

<FIG> is a view <NUM> showing an image of monodisperse droplets <NUM> generated using VCSEL illumination of the multi-beam imaging apparatus according to one embodiment. As shown in <FIG>, the background lighting is uniform and shows no speckle or diffraction noise.

<FIG> is a view <NUM> showing an image of monodisperse droplets <NUM> generated using VCSEL illumination of the multi-beam imaging apparatus according to another embodiment. This image was acquired with a lower background illumination than that of the image in <FIG>. As shown in <FIG>, the background lighting on the image <NUM> is the uniform gray lighting with no speckle or diffraction noise.

<FIG> is a view <NUM> showing a dense spray image <NUM> generated using VCSEL illumination of the multi-beam imaging apparatus according to one embodiment. Dense spray image <NUM> shows the increased contracts between in focus particles (dark shadows) and the out-of-focus particles (gray background) comparing to the images generated using conventional light sources, as shown in <FIG>.

<FIG> is a view <NUM> showing an image of a dilute particle field <NUM> illuminated using the multi-beam diode lasers according to one embodiment. In this image, one can observe the signature of speckle on the background that has been smoothened because of the multi-beam approach. The background light intensity distribution is not uniform, and local dispersion is fairly high that stresses the image processing methods and reduces the quality of the particle characterization results.

<FIG> is a view <NUM> showing an image of a dense particle field <NUM> illuminated using the multi-beam diode lasers according to one embodiment. As shown in <FIG>, the background is much more chaotic than the background shown in <FIG>, with larger disparity in intensity, even at the local level. This high level of scatter in the background illumination intensity comes from the multiple diffraction patterns produced by particles interfering with the beams, while they are out of the focal plane of the imaging system. The speckle and diffraction increase errors for the software in identifying and measuring the particles images.

<FIG> is a view showing another embodiment of a system <NUM> to image particles. System <NUM> comprises a transmitter system <NUM>. Transmitter system <NUM> comprises one or more light sources -- e.g., a VCSEL array <NUM> and a VCSEL array <NUM> -- generating a plurality of illuminating light beams, such as light beams <NUM> and <NUM>. Transmitter system <NUM> includes a light source <NUM> to generate a triggering light beam <NUM>. The transmitter system <NUM> comprises a synchronization module <NUM> to synchronize the illuminating light beams with a digital camera system <NUM>. The illuminating light beams are configured to propagate on multiple optical paths through a particle field comprising particles -- e.g., particles <NUM> and <NUM>. The illuminating light beams are configured to converge to form a measurement volume <NUM> at a focal plane of the imaging system. The illuminating light beams of <FIG> are represented by the illuminating light beams of <FIG>, <FIG> and <FIG>. The illuminating light sources of the transmitter system <NUM> comprise VCSELs and the plurality of illuminating light beams are generated by one or more VCSEL arrays.

For an embodiment, the triggering light beam <NUM> generated by the triggering light source propagates through a center of the measurement volume <NUM>. For an embodiment, the triggering light source comprises a laser, an LED, or both.

A receiver system comprises an imaging optics -- e.g., one or more lenses <NUM> -- and a digital camera system <NUM> comprising one or more digital cameras to provide a shadow image of the particle passing through the measurement volume at a focal plane, as described above. For one embodiment, the wavelength of the triggering beam is different from the wavelength of the illuminating light beams. When a particle passes through the measurement volume, the triggering light beam <NUM> is deflected from the particle onto photodetector system <NUM> that outputs a trigger signal <NUM> indicating the presence of the particle in the measurement volume <NUM> to a logic circuitry <NUM> to drive one or more laser sources. Logic circuitry <NUM> outputs a trigger signal <NUM> to drive one or more light sources of the transmitter system <NUM> to generate converging illuminating light beams. Logic circuitry <NUM> outputs a trigger signal <NUM> to trigger digital camera system <NUM>. Multiple VCSEL lasers are used to simultaneously illuminate a particle field from multiple directions. A trigger laser and photodetector are used to detect the presence of particles in the measurement volume. This information is used to pulse the multiple illumination beams. The laser beams are combined by a receiver lens which creates a frozen shadow (e.g., bright-field image) of the particles on the CMOS sensor. The use of multi-angle illumination significantly reduces measurement errors due to depth-of-field variations that are a problem for conventional instruments.

The imaging system can optionally comprise one or more beam splitters (not shown) to split the illuminating light beams onto multiple digital cameras, as described above with respect to <FIG>. An image acquisition and processing system <NUM> is coupled to the digital camera system <NUM>. Processing system <NUM> comprises a processor <NUM>, a memory <NUM>, and a display <NUM> to perform the methods, as described herein. For an embodiment, processor <NUM> is configured to perform particle analysis that involves identifying particles that are in-focus, calculating various shape parameters, and classifying particles. For an embodiment, processor <NUM> is configured to differentiate between liquid drops and ice crystals.

<FIG> shows a flow chart of a method <NUM> to provide multi-beam imaging of particles according to one embodiment. At operation <NUM> a plurality of light beams that converge with each other to form a measurement volume within a particle field are generated using a plurality of vertical cavity surface emitting lasers (VCSELs) to provide uniformity in a background illumination of the measurement volume, as described above. At operation <NUM> a shadow image of a particle passing through the measurement volume at a focal plane of a digital camera is obtained. At operation <NUM> a contrast between the obtained shadow image and the background illumination is determined. At operation <NUM> the particle is identified based on the contrast. At operation <NUM> the size of the identified particle is determined based on the shadow image, as described above.

<FIG> shows a flow chart of a method <NUM> to provide multi-beam imaging of particles according to another embodiment. At operation <NUM> a plurality of light beams that converge with each other to form a measurement volume within a particle field are generated using a plurality of VCSELs to provide uniformity in a background illumination of the measurement volume, as described above. At operation <NUM> the uniformity of background illumination is monitored. For example, it is determined if a difference in the background illumination at different locations of the background area on the image is greater than a predetermined threshold. At operation <NUM> an output beam of at least one of the VCSELs is adjusted based on monitoring. For example, one or more parameters (e.g., output light intensity, angle, pulse, a wavelength, or any combination thereof) of at least one of the VCSELs is adjusted to increase the uniformity of the background illumination, if the difference in the background illumination at different locations of the background area on the image is greater than the predetermined threshold. At operation <NUM> a shadow image of the particle passing through the measurement volume at a focal plane of a digital camera is determined based on the adjusted background.

<FIG> illustrates one embodiment of a system to image particles. A system <NUM> comprises one or more mounting fixtures, such as a mounting fixture <NUM> and a mounting fixture <NUM>. Mounting fixture <NUM> holds a portion of a transmitter sub-system comprising a plurality of VCSEL sources, such as a VCSEL array <NUM> and a VCSEL array <NUM>. Mounting fixture <NUM> holds a portion of the transmitter sub-system comprising a plurality of VCSEL sources, such as a VCSEL array <NUM> and a VCSEL array <NUM>. Each of the VCSEL sources comprises one or more focusing lenses coupled to an output of the VCSEL array to form an illuminating laser beam. A plurality of illuminating laser beams, such as an illuminating laser beam <NUM> and an illuminating laser beam <NUM> are generated by the VCSEL sources that converge with each other to form a measurement volume within a particle field to provide uniformity in a background illumination associated with the measurement volume, as described above. For an embodiment, system <NUM> comprises a receiver module (not shown) comprising an imaging optics to provide a shadow image of a particle passing through the measurement volume at a focal plane of the first digital camera, as described above.

<FIG> illustrates one embodiment of a system to image particles. As shown on <FIG>, system <NUM> includes a transmitter <NUM> including a plurality of VCSELs to generate illuminating light beams converging to form a measurement volume to illuminate particles <NUM>, as described above. As shown in <FIG>, a receiver <NUM> is coupled to receive the shadows of particles <NUM>, as described above. As shown in <FIG>, receiver <NUM> is coupled to a signal processor <NUM>. As shown in <FIG>, a subsystem <NUM> comprising a central processing unit ("CPU"), a subsystem <NUM> comprising a graphics processing unit ("GPU"), that may be coupled with a display device, one or more subsystems <NUM> comprising one or more I/O controllers coupled to one or more I/O devices, a memory <NUM> (comprising a volatile RAM, a ROM and a non-volatile memory (e.g., flash memory or a hard drive), or any combination thereof), and a signal processor <NUM> comprising a microcontroller are coupled to a bus <NUM>. At least one of a subsystem <NUM> and a signal processor <NUM> are configured to perform methods as described above. Memory <NUM> may be used to store data that when accessed by the data processing system, cause the data processing system to perform one or more methods to provide multi-beam imaging of particles, as described above.

Claim 1:
A method to provide multi-beam imaging of particles, comprising:
generating a plurality of light beams comprising a first light beam (<NUM>) and a second light beam (<NUM>) that converge with each other to form a measurement volume (<NUM>) within a particle field using a plurality of vertical cavity surface emitting laser arrays (<NUM>) comprising a first vertical cavity surface emitting array (<NUM>, <NUM>) and a second vertical cavity surface emitting array (<NUM>, <NUM>), wherein the first vertical cavity surface emitting array comprises a first plurality of vertical cavity surface emitting lasers (<NUM>) that generate the first light beam and the second vertical cavity surface emitting array comprises a second plurality of vertical cavity surface emitting lasers (<NUM>) that generate the second light beam that converges with the first light beam at an intersection angle (<NUM>) to provide uniformity in a background illumination (<NUM>) of the measurement volume on an image plane (<NUM>) of an imaging system;
monitoring the uniformity of the background illumination of the measurement volume on the image plane (<NUM>) of the imaging system (<NUM>);
adjusting an angle of at least one of the first plurality of vertical cavity surface emitting lasers of the first vertical cavity surface emitting array based on the monitoring (<NUM>) to increase the uniformity of the background illumination of the measurement volume on the image plane (<NUM>) of the imaging system; and
obtaining, by a digital camera (<NUM>), a first shadow image (<NUM>) of a first particle (<NUM>) passing through the measurement volume at a focal plane (<NUM>) of the digital camera (<NUM>) based on the adjusting (<NUM>), wherein the intersection angle (<NUM>) of the first light beam and the second light beam is adjusted to remove shadow images of other particles outside the measurement volume (<NUM>).