Methods and apparatuses for contact-free holographic imaging of aerosol particles

Methods and apparatuses provide holographic contact-free imaging of aerosol particles in an efficient manner. One apparatus for holographic imaging of an aerosol particle may include: a delivery device configured to deliver the particle into a region; a light source for outputting a first beam of light and a second beam of light, wherein the first beam travels into the region producing a first light wave which is un-scattered by the particle and a second light wave that is scattered by the particle, and the second beam does not travel into the region; a beam splitter for combining the second beam with the scattered light of the first beam into combined interference light; an image sensor for sensing an interference pattern created by the combined interference light; and an image processor configured to generate an image of the aerosol particle based on the sensed interference pattern.

FIELD OF INVENTION

Embodiments of the present invention generally relate to particle imaging and, more particularly, to methods and apparatuses for contact-free holographic imaging of aerosol particles.

BACKGROUND OF THE INVENTION

The in situ characterization of small aerosol particles is a persistent objective in many applied contexts. Examples include the determination of atmospheric aerosol composition for climate modeling and the detection of biological or chemical weapons agents for defense applications. Various measurements and calculations of single and multiple-particle scattering patterns are known. The overall goal of such work is to infer information relating to the particles' physical form, such as size and shape, by analyzing the angular structure of these patterns. Unfortunately, a fundamental limitation of this approach is the absence of an unambiguous quantitative relationship between a pattern and the corresponding particle properties, i.e., the so-called inverse problem. Consequently, the inference of these properties from the patterns has proved to be very difficult in practice, except for the simplest of cases. Ideally, one would prefer to image the particles directly, thus eliminating the complexity and ambiguity associated with interpretation of the scattering patterns.

However, the typical particle size range of interest for many applications is roughly 0.1-10 μm. Because of the small size, direct images are possible in only part of this range and only with high numerical-aperture (NA) optics and small focal volumes. Such imaging typically requires the collection and immobilization of particle samples, and thus, is not a practical technique for particle characterization in applications requiring high sample through-put or images of the particles in their undisturbed form, i.e., in situ images.

Holography is an alternative technique that combines useful elements of both conventional imaging and scattering. Fundamentally, this is a two-step process: first, an object is illuminated with coherent light, and then the intensity pattern resulting from the interference of this light with that scattered by the particle is recorded. The resulting pattern constitutes the hologram, from which an image of the object is reconstructed. Traditionally, holograms are recorded with photographic film due to the film's high resolution. Such a high resolution medium is required to capture the finer features of the interference pattern. The subsequent chemical development of the film is costly and time consuming which greatly limits the practical utility of the technique.

BRIEF SUMMARY OF THE INVENTION

Methods and apparatuses for holographic contact-free imaging of aerosol particles in an efficient manner are described herein according to embodiments of the present invention.

According to one embodiment, an apparatus for holographic imaging of an aerosol particle may include: a delivery device configured to deliver the particle into a region; a light source for outputting a first beam of light and a second beam of light, wherein the first beam travels into the region producing a first light wave which is un-scattered by the particle and a second light wave that is scattered by the particle, and the second beam does not travel into the region; a beam splitter for combining the second beam with the scattered light of the first beam into combined interference light; an image sensor for sensing an interference pattern created by the combined interference light; and an image processor configured to generate an image of the aerosol particle based on the sensed interference pattern.

According to another embodiment, a method for holographic imaging of an aerosol particle may include: delivering the particle into a region; outputting a first beam of light and a second beam of light, wherein the first beam travels into the region producing a first light wave which is un-scattered by the particle and a second light wave that is scattered by the particle, and the second beam does not travel into the region; separating the scattered light from the un-scattered light of the first beam; combining the second beam with the scattered light of the first beam into combined interference light; sensing the interference pattern of the combined interference light; and generating an image of the aerosol particle based on the sensed interference pattern.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention comprise forming a holographic image of particles using a light source to generate a wave which scatters when interacting with the aerosol particle and an un-scattered reference wave and recording the interference pattern between the scattering wave and the reference wave on an image sensor and using a computer-aided algorithm to generate an image from the interference pattern.

Aerosol particles of interest may include, for example, environmental hazards (e.g., asbestos, smog, smoke, etc.), chemicals, toxins, biological contaminants or spores (e.g.,E. colior anthrax), chemical or biological warfare agents, hydrosols (i.e., particles in water), other airborne contaminants (e.g., dust, pollen, or other pollutants), and the like. Particles may be already present in the air or originate in water, dirt, or other substances which may then become airborne.

The particle characterization techniques disclosed herein extend capabilities of elastic light scattering by using holography to form images of one or more particles, rather than their scattering patterns in embodiments. A particle's far-field scattering pattern interferes across the surface of an image sensor along with a reference wave having a portion of the light incident on the particle. For example, the scattered and reference wave may overlap in an in-line configuration embodiment and be separated, for instance, in an off-axis configuration, in other embodiments.

The resulting intensity distribution recorded by the image sensor is the particle's hologram. A three-dimensional image of the particle can then be generated computationally from the hologram. Further computational analysis of the resulting image can be performed to satisfy the requirements of the intended application, such as, for example, detection of particle size, shape, and/or other characteristics thereof. One advantage of these techniques is that there may be no further need to interpret or invert a complicated scattering pattern, as information can be retrieved directly from the image. Holography permits the reconstruction of phase, magnitude, or both, of the scattered wave pattern, for instance.

Using digital media, rather than traditional photographic film, holograms can be recorded rapidly in succession as the particles flow through the apparatus. This makes for a high “through-put” detection technique. Reconstructing images of a particle from its hologram digitally enables application of any number of image-analysis techniques intended to characterize the particle. For example, the digitally reconstructed particle-images can be automatically correlated with a library of simulated or measured images. This may enable a rapid detection/identification system.

FIG. 1is a functional block diagram of an inline apparatus for holographic imaging of aerosol particles in accordance with embodiments of the present invention. Apparatus100includes an optical device102, a delivery device104, a trigger device106, an image sensor108, and an image processor or controller110. The optical device102comprises a laser light source114and a focusing lens116. The delivery device104delivers particles into a region105. In one embodiment, the particles are extracted from the air in the surrounding environment. In other embodiments, the particles are captured elsewhere and delivered to the delivery device for imaging. The trigger device106is actively detecting whether a particle has entered scattering volume region105. When the trigger device106detects, as discussed below, that a particle has entered the region105, a signal is sent to the optical device102, indicating that the laser source114should be enabled. The optical device102may further enable the laser source114to pulse a laser into focusing lens116. The focusing lens116creates a focused light wave that is directed at the region105where aerosol particles are detected, so that the particles scatter the focused light waves. In addition, the focusing lens116supplies light that is not scattered by the particles, and this light is incident on the sensor108. The combination of the un-scattered and scattered light creates an interference pattern which is sensed by the sensor108. The processor110executes a computer-implemented algorithm on the interference pattern, which constitutes the hologram, and generates an image112of the detected particles. In an embodiment, the computer-aided algorithm is a Fourier Transform and the Fourier transform is implemented computationally as a Fast Fourier Transform (FFT).

FIG. 2is a schematic of an implementation200of apparatus100ofFIG. 1in accordance with embodiments of the present invention. Implementation200comprises a laser202, a series of lenses and optics,204,206,208,210,212,214and216, an image sensor218(such as a CCD, shown), two photomultiplier tubes (PMT)220and228, each with line filters222and230, irises224and232, lenses226and234, and two diode lasers236and240, each with convex lenses238and242. In an embodiment, the laser202may be a70ns pulsed Nd:YAG laser (e.g., Spectra Physics Lasers, Inc., model Y70-532Q), frequency doubled to 532 nm. In an embodiment, implementation200is an “inline” configuration where all of the component parts202-218are configured in a line on a single plane. In other embodiments, the laser202may generate and output light that is ultraviolet (UV), visible, infrared (IR), or the like.

The optical device102inFIG. 1may be implemented with the laser202and the optics204-216. The laser202outputs a light wave203into a polarizer204. The polarizer204polarizes the light203. In an embodiment, the polarizer is a Glan-Thompson polarizer, though the present invention does not restrict the type of polarizer used. The light from the polarizer204is directed towards a convex lens206for focusing onto a pinhole208, which, in an embodiment, the convex lens206has a focal length of 75 mm and the pinhole208has a 50 μm diameter. The primary lobe of the pinhole diffraction pattern produced by pinhole208illuminates a second pinhole210with a diameter of 25 μm. These pinholes “clean” the beam203improving its spatial coherence and enhancing the quality of the hologram. All but the primary lobe of this second pinhole pattern is blocked by iris212where convex lens214then collimates the beam, which is brought to a focus by convex lens216at a point approximately 2 mm from the aerosol nozzle outlet244. In an embodiment, the convex lens214may have a focal length of 300 mm and the convex lens216has a focal length of 30 mm.

The aerosol nozzle outlet244and the aerosol nozzle inlet246may form the delivery device104inFIG. 1. In this embodiment, a nozzle and suction tube are used to direct aerosol particles into the scattering volume area (region), however the present invention does not limit the delivery mechanism.FIG. 2(a) shows, in more detail, one embodiment of the region. In other embodiments, the delivery device204includes a free-flowing channel of aerosol or particles entrained in a microfluidic channel or flow.

The optical trigger device106inFIG. 1may be implemented using the photomultiplier tubes220and228along with the diode laser beams236and240. In an embodiment, the PMTs220,228are a Hamamatsu Corp. Model No. H6780-02 and the diode laser beam236is a 635 nm wavelength laser beam. The beam from the laser236is focused using the convex lens238into the region where the particles217are delivered by delivery device244.

The focused laser beam scatters from particles217in the region. The scattered waves are focused using a convex lens234and pass through an iris232, a 635 nm line-filter230and then passes to photomultiplier tube228if the wavelength of the focused light is 635 nm. The diode laser240generates a 670 nm beam which is focused using convex lens242into the region where particles217are delivered. The focused laser beam scatters from particles217in the region. The scattered waves are focused using a convex lens226and pass through an iris224, a 670 nm line-filter222and then to photomultiplier tube220if the wavelength of the focused light is 670 nm. The laser202may be pulsed only when both photomultipliers220and228detect light for their particular wavelength, at which point it is established that there is particle217in the region. The controller247uses a processor239to execute a FFT module250to generate the particle's image254from the hologram sensed by sensor218. For instance, the sensor218may be spaced apart from the particle stream by a distance of approximately 8 cm in some cases. In an embodiment, the controller247identifies the particle by matching its holographic image with images stored in database252stored in memory of the controller247. The database252may store a digital library of simulated or measured images. The resolution of the image produced by the controller247for this embodiment of the inline system is on the order of 5 μm, but may readily be improved to less than 5 μm by using a shorter wavelength laser202and a sensor218with smaller pixel dimensions.

FIG. 2(b) illustrates another embodiment of implementation200using fiber optics instead of fixed laser202. Here, a main laser260may be a triggerable, pulsed fiber laser (e.g., 400 nm range). Light from this pulsed fiber laser is conveyed to the “region” by an optical fiber262and an output lens264(such as an aspherical lens combination). The same focusing to form a virtual point source is performed and the particle is illuminated similarly as in implementation200. The diode lasers270,271for the PMT trigger system can also convey light to the region by optical fiber272,273which is focused by to output lens274,275. The PMTs280,281receive their light through optical fibers282,283. The coupling lens284,285near the region are guarded by interference line filters286,287that serve the same purpose as in the trigger system inFIG. 2. The incorporation of fiber optics can simplify the optical design, by making the device more compact and lighter. Using optical fibers may also eliminate the delicate alignment requirements of the optical elements. And, they can also make the system more durable and less vulnerable to mechanical shock.

FIG. 2(c) illustrates yet another embodiment of implementation200of apparatus100ofFIG. 1. The region may be defined by a particle trap225, such as a spherical-void electrodynamic levitator (SVEL), which the beam from the laser202enters through a window thereof, and the scattered beam exits through a window thereof. For example, the particle trap225may have a spherical void (e.g., 25 mm dia.) used to confine particles and may include small holes (e.g., 6.3 mm dia.) to allow for the introduction of particles and optical access to the trapping region. An adjustable voltage may be applied to the particle trap to control particle flow and trapping for particle detection. Particles confined in the particle trap225are illuminated by light diverting from the collimated/focused beam of light, which effectively acts as a virtual point source producing a spherical wave. This diverging illumination wave continues to expand as it reaches the image sensor218along with the scattered light from the particle217. The resulting interference pattern between these waves across the sensor is the hologram. By using a short-focal length lens to form a virtual source near the particle, the light illuminating the particle can be more intense than it would be if only a pinhole is used for illumination. This results in a relative amplification of the particle's scattered wave at the sensor, which may enhance the interference structure of the hologram leading to improved particle-image quality. If the virtual source is formed near the particle, there can be much more flexibility in “working distance” between the lens214and the aerosol particle stream. The working distance is based on the focal length of the lens214, and may enable the working distance to be readily changed by changing the focal length of the lens214. By contrast, in conventional detection apparatuses there is usually a pinhole or a microscope objective which must be spaced very close (e.g., <1 mm) to the sample. This spacing is inherent to the conventional apparatuses and thus is not readily changeable.

FIG. 2(d) illustrates one embodiment of the PMT sensing units ofFIG. 2in more detail. Here, the lens226(234) and the iris224(232) are used to spatially filter the scattered trigger light from the particles217. If a particle217ais in the desired location (e.g., the intersection of the trigger beams or the “region”) then its light makes it to the PMT. If there is a stray particle217baway from the desired location, its light227is blocked by the iris224(232). The light passing through the iris224(232) passes through line filter222(230) onto the PMT220(228). The various elements may be housed in a lens tube223to prevent stray light from being detected. This arrangement may enhance the sensitivity of the trigger system to respond only to a particle in the trigger beam-intersection, especially when many particles are present in and around the region at the same time.

FIG. 2(e) illustrate one embodiment of the trigger system electronics for the optical trigger device106inFIG. 1. The output signals from the photomultipliers220and228may be amplified using amplifiers291,292, respectively. In some embodiments, amplifiers291,292may be ORTEC Model 750 amplifiers. The output from amplifiers291,292is then input to a quad analyzer293(e.g., an ORTEC Model 850) which in turn outputs two signals to processor294,295(e.g., SRS analog processor). Output from the processors294,295is input to logic unit296(e.g., an ORTEC Model CO4020 Logic Unit) which may be configured to perform the function of an AND gate circuit and output a signal to the laser202. The signal may be a TTL signal, for instance.

In some embodiments, one or more of the NPBS204, the pin hole208, and the convex lens216illustrated inFIG. 2may be omitted from implementation200. This is because the polarizer and/or dual pin holes may only be needed, in some instances, if the light source (laser) has poor beam quality. In addition, other optical elements may be removed where they are considered redundant or superfluous.

Further using a pulsed light source may also permit investigation of particle systems in motion. Other elements in various embodiments may be the same or similar as those in implementation200shown inFIG. 2.

The light scattering pattern from an aerosol particle is generally a function of its size, shape, composition and/or surface structure. The light scattering pattern signal may be used to characterize the particle. In particular, the angular structure of a particle's scattering pattern may be related to the particle's overall shape through a Fourier-transform relationship. For example, information relating to the largest length of the particle is contained in the small angle region of the scattering pattern. Likewise, small length scale features of the particle, such as surface roughness, are generally contained in larger angle regions of the pattern. Consequently, holographic imaging of the particle's overall shape dictates that the hologram is formed from a portion of the particle's scattering pattern that includes the small forward scattering angles. However, because of the small size of the particles, it may be difficult to separate this small forward angle region of the pattern from the much more intense unscattered illumination beam. This problem may be overcome or ameliorated using a spatial filtering technique in which the unscattered light is removed from the pattern.

A digital hologram or interference image may include gray-scale image data which corresponds to the intensity distribution of interference of the incident and scattered waves across the image sensor. A nested ring appearance, which may be visible at some portions of the holographic image for a point-like particle, is due to the intersection of the spherical wave structure of the scattered wave with the planar incident wave.

To obtain images of a particle from the digital hologram, one or more pixels of the hologram may be regarded as a point electric-dipole with polarization proportional to the gray-value of the pixel. If a pixel in the hologram is black, indicating no light on that portion of the image sensor, then the polarization may be assumed to be zero. Conversely, for a white pixel value, the Maxwell volume integral equation may be used to calculate the electric field resulting from the radiation of the collection of dipoles corresponding to the hologram pixel. The magnitude and/or phase of this radiation, or reconstruction field is then calculated in a plane parallel to the hologram, but separated from it by a distance equal to the separation between the particle and the image sensor. This results in a two-dimensional computer-generated image of the particle in this place as given by the distribution of reconstruction-field magnitude. The phase, the magnitude, or both, of this field can be used to generate a three-dimensional rending of the particle.

One embodiment for generating an image of the sample aerosol particle according to the sensed interference pattern will now be described. Let the source of the reference wave, laser202, be located at a distance “I” from the particle217and the sensor218at a distance d. Provided that kl and kd are large enough to satisfy far-field conditions described in M. Mishchenko, L. Travis, and A. Lacis, “Multiple scattering of light by particles: radiative transfer and coherent backscattering,” Cambridge: Cambridge University Press, 2006, pp. 74-78, herein incorporated by reference in its entirety.

The reference and scattered waves will be transverse and spherical at the sensor218and can be represented entirely by their scattering amplitudes as described by M. Berg and G. Videen, “Digital holographic imaging of aerosol particles in flight,” Journal of Quantitative Spectroscopy & Radiative Transfer 112 (2011) pp. 1776-1783, herein incorporated by reference in its entirety, as follows:

respectively. Then, the intensity of the total wave across the sensor's face is:

Iholo⁡(r)=c⁢⁢ɛ0r2⁢E1ref⁡(r^)+E1sca⁡(r^)2,(2)
where c and ε0are the vacuum speed of light and electric permittivity of free space, respectively. Expanding Eq. (2) gives:

where the asterisk denotes complex conjugation. The quantity

C⁢⁢ɛ0r2⁢E1ref⁡(r^)2=Iref
in Eq. (3) is the intensity across the sensor218when no particle is present, and hence can be considered a known quantity (reference) measured before the introduction of an aerosol sample. Subtracting the reference intensity from Eq. (3) and dividing the remaining terms by the reference gives:

Often, the intensity of the reference wave at the sensor218is much greater than that of the scattered wave. This is especially true in this work where the objects being illuminated are small particles, as opposed to the macroscopic sized objects involved in many other applications. This means that the term

Icon⁡(r)≃[E1ref⁡(r^)]*⁢E1sca⁡(r^)+[E1sca⁡(r^)]*⁢E1ref⁡(r^)E1ref⁡(r^)2(5)
This intensity pattern, which is the difference between two measurements, with and without the particle217present, is known as a contrast hologram. The key characteristic of Iconis its linear dependence on the amplitude of the particle's scattered wave. This means that the phase of the scattered wave over the detector is encoded in the measurement. Consequently, the interference pattern can be used to reconstruct unambiguously an image of the particle that closely resembles that obtained from conventional microscopy. The contrast hologram is envisioned as a transmission diffraction-grating illuminated by a normally incident plane wave, i.e., a reconstruction wave. In an embodiment, the Fresnel-Kirchhoff approximation is then used to describe the light diffracted from this grating in a parallel plane separated by a distance z from the grating along the z-axis. If z corresponds to the distance between the particle and sensor218during the hologram measurement (z=d) the resulting diffraction pattern in this so-called reconstruction plane yields an image254of the particle217. The image254may be essentially equivalent to a conventional microscope image of the particle217.

One advantage of using the Fresnel-Kirchhoff approximation to generate the reconstructed particle image is that the approximation's mathematical form is essentially a discrete Fourier transform of the sensor218pixel values constituting Icon. This enables the use of the fast Fourier transform (FFT) in the calculation, thus substantially reducing the computation time required to render the particle image. In practice, d may not be known to sufficient accuracy to be able to reconstruct an image from a single application of the reconstruction routine. This inaccuracy is due to the variation in particle positions in the aerosol stream as they enter the region105. Consequently, the image-reconstruction stage includes a focusing-like procedure: First an initial image is reconstructed using an estimate of d based on the experimental layout. Then, the reconstruction plane is scanned along the z-axis in small steps until the reconstructed image comes into focus.

In other embodiments, the reconstruction of the holographic image may use a computational procedure using a Kirchoff-Helmholtz formula, such as Equations 2 and 3 of U.S. Pat. No. 6,411,406 entitled “Holographic Microscope and Method of Hologram Reconstruction,” herein incorporated by reference in its entirety.

By using the disclosed holographic techniques, multiple particles may be in the scattering region at a time. In addition, the holographic techniques do not require particles to be in each other's far-field zones. Thus, if multiple particles are present when the hologram is recorded, they can be independently imaged digitally from the hologram by varying the distance from the hologram to the reconstruction plane. Accordingly, it is possible to digitally target or “focus-in” on one particle or another.

Holographic images that can be used to characterize the morphology of particles may be on the order of the wavelength of the illuminating light, which may be between 4 and 7 μm in size and larger for visible light, and resolutions may be obtained approximately 1/10 of this wavelength, for example, in some implementations.

FIG. 3is a functional block diagram of another apparatus300for holographic imaging of aerosol particles in accordance with at least one embodiment of the present invention. Apparatus300comprises an optical device302, a delivery device304, a trigger device306, a sensor308and a controller310. The optical device302generates a particle-illuminating wave312and a reference wave314. The particle-illuminating wave312is routed through the region316. The delivery device304delivers particles from the air in the surrounding environment into region316, and the particle-illuminating wave312scatters when it hits the particles in the region316. The reference wave314from the optical device302is not sent through the region316, thus it is un-scattered by particles in the region316. The combination of the particle-illuminating wave312with the reference wave314results in interference of the two waves producing an interference pattern, i.e., the hologram, which is captured by sensor308. The sensor308couples the sensed hologram to controller310. In an embodiment, the controller310performs a Fast Fourier transform on the hologram to generate an image318of the particles.

FIG. 4is a functional implementation400of the apparatus300in accordance with one of the embodiments of the present invention. Apparatus400includes an optical device402, a trigger system404, a sensor406and a processor408. A delivery device is not shown in this figure, but may be similar to delivery device244and246shown inFIGS. 2 and 2(a). In an embodiment, the optical device402comprises a light source412(e.g., a 70 ns pulsed Nd:YAG laser, e.g., Spectra Physics Lasers, Inc., model Y70-532Q, frequency doubled to 532 nm), a non-polarizing cube beam splitter (NPBS)414, an angled mirror416a concave expanding lens418, a convex collimating lens420, a convex collimating lens422, a mirror424with a small through-hole425, a second NPBS426, a convex focusing lens428, iris429, and a convex collimating lens430. When the length of the pulse is very short, the motion of the particle during this time may be assumed to be negligible such that the particle may be considered “frozen” in space.

The detecting device404comprises two diode lasers432and436, respectively emitting 635 nm and 670 nm light in an embodiment. The beams emitted from the diode lasers432and436intersect where a particle401will be delivered and scatter the respective wavelength light to be detected by photomultipliers434and438. The detecting device404may further include additional elements (not shown) similar to those shown inFIG. 2, such as convex lens238and242for focusing the laser beams emitted from the diode lasers432,434onto the particle401, convex lens226,234for focusing the scattered light from the particle401toward the PMTs434,438, and filters222,230for selectively filtering a particular wavelength of light that is received by the PMTs434,438. When the PMTs434and438detect a particle401, the optical device402is enabled and the laser light source pulses laser beam403.

The NPBS414splits the laser beam403into two beams413and417. Beam413interacts with particle401causing a first scattering pattern, in addition to non-scattered light, to emerge at lens422. The lens422serves two purposes. First, the lens422is placed at a distance of one focal length from the scattering volume where particle401is located, such that the scattered light is collimated when it leaves lens422. Second, the un-scattered illuminating beam is brought to a focus at the back focal plane of lens422. Since it is desired to remove this portion of un-scattered light, the mirror424reflects the scattered light and the un-scattered illuminated focused beam passes through through-hole425of the mirror424. In some embodiments, the mirror424can be configured to separate the scattered and unscattered light down to very small angles i.e., 0.1 degree from the forward direction. The scattered light is then reflected to NPBS426. It will be appreciated that other optical elements might also be used to separate the scattered light from the unscattered light. This separation also removes the D.C. or so-called “zero order” region in the hologram, which is the high-intensity region due to the unscattered light.

The second split wave417from NPBS414serves as a reference beam and is deflected towards the mirror416, which may be placed at an angle (e.g., a 45 degree angle) in an embodiment, further deflecting the beam. In some embodiments, a neutral density (ND) filter415may be provided to maximize the contrast of the interference fringes by making the field intensity of the reference beam417approximately the same as that of the scattered light field. The lens418expands the beam417and the convex lens420collimates the light from lens418onto the NPBS426. The NPBS426then combines the reference beam417with the particle scattered wave reflected from mirror424into combined interference light. The NPBS426may be placed on a rotation stage, introducing a small offset angle between the reference wave and the collimated scattered wave (not shown). This introduces a small angular separation between the so-called “twin images” that occur during the image reconstruction stage. Convex lenses428and430image the interference pattern of the reference wave and the collimated wave through the iris429onto the sensor406. The lens pair428,230and the iris429in front of the sensor406are used to suppress stray light from dust and the like that collects on the optical surfaces between the region and the sensor406. In an embodiment, the sensor406is a Charged Coupled Device (CCD) camera, but in other embodiments, the sensor406may be any sensor capable of capturing an image such as an Intensified Charged Coupled Device (ICCD) camera or Complementary Metal-Oxide-Semiconductor (CMOS) camera, and the like. In an embodiment, a Finger Lakes Instrumentation, LLC Model ML8300 CCD having a pixel size of approximately 5.4 μm may be used. The sensor406outputs the interference pattern to the computer system408, containing a processor440and memory442with a FFT444and a database446. The processor executes the FFT and generates an image410of the particles. The resolution of the holographic image410produced by the system408for a dual-beam system is, in this embodiment, on the order of about 1 micron.

FIG. 4(a) illustrates another embodiment of implementation400ofFIG. 4partially implemented with fiber optics. Here, a main laser460may be a triggerable, pulsed fiber laser (e.g., 400 nm range). The neutral density filter415inFIG. 4may be replaced by the variable coupler462that can be used to change the proportion of fiber-laser-light from the pulsed fiber laser460that is split between the two optical fibers463,464that go off to collimators465,466for conveying beams413′,417′. The collimators465,466may include aspherical lenses, in some instances. The same focusing to form a virtual point source is performed and the particle is illuminated as in implementation400. The diode lasers470,471for the PMT trigger system can also convey light to the region by optical fiber472,473which is focused by to output lens474,475. The PMTs480,481now receive their light through optical fibers482,483. The coupling lens484,485near the region are guarded by interference line filters487,488that serve the same purpose as in the trigger system inFIG. 4.

FIG. 4(b) illustrates one embodiment of mirror424in more detail. The through-hole mirror is configured to separate small angle scattered light and remove any direct current (DC) terms in the holographic image. The through-hole425can be formed by drilling an angled hole with respect to the mirror-surface normal direction, for example. Here, the angled hole is shown at 45 degrees.

Other elements of implementation400may be the same or similar as those of implementations200in some embodiments, such as, for example, optical elements/systems for focusing or collimating light, trigger system electronics and a particle trap.

By using two separate beams, it may be further possible to modify the intensity of the reference beam, the incident beam, or both, to increase the contrast of the fringes on the sensor406. And, with this configuration, the particle's forward scattered light can be collected over an angular range of about 0.1 to 20 degrees in the polar angle and 0 to 360 in the azimuthal angle. The interference pattern recorded by the sensor406may be a digital off-axis hologram. From this hologram, an image, e.g., a three-dimensional image of the particle, can be reconstructed computationally. Holograms can be recorded for one or more particles in the aerosol sample as they flow through the scattering volume substantially in real-time. The scattered field remains stable with the particle located at different positions within the scattering volume.

Limitations on the apparatus may be the read-out (or refresh rate) of the image sensor, the size of the image sensor, and/or the processing speed of the image processor for digital reconstruction. Features in the interference pattern at the image sensor plane that are finer than the pixel size may be averaged out across that pixel, although, this may result is some image loss, in some instances. The finite size of the image sensor can also limit the resolution with the resolution of the reconstructed image being diffracted limited by the array pixels. Assuming a pixel size of 5.4 micrometers and an image sensor array size of 3000×3000 pixels, reconstruction resolution of image features with length scaled less than one micron may not be feasible in some instances.

In addition to pixel size, there may also be a restriction on the maximum particle size that may be feasible for imaging. This limitation may originate from the requirement that the collimated scatting wave and the incident reference wave intersect at the image sensor plane at an angle large enough to separate the particle image from the so-called “zero order” term. This term represents a region in the reconstruction plane around the forward direction where the autocorrelation of the particle's image forms than its image alone. For example, one typical upper size-limit for 532 nm illuminating light may be about 20 micrometers. This may not be a critical limitation for all monitoring situations, since particles of interest tend to be smaller than 10 micrometers and larger particles tend to fall out of the atmosphere more rapidly.

FIG. 5is a flow diagram of a function500for holographic imaging of aerosol particles in accordance with embodiments of the present invention. The method500is an implementation of execution of the FFT module250executed by the processor249in memory248. The method500begins at step502and proceeds to step504. At step504, the interference pattern of the scattered light wave and the un-scattered wave, i.e., the hologram, is received from the sensor218. The processor246then performs the Fast-Fourier transform on the interference pattern at step506. The method then outputs an image254of the particles217at step508. The method ends at step510.

FIG. 6is an illustration of the holograms and microscope images of a cluster of ragweed pollen particles. Image600is a hologram for the ragweed pollen particle produced by sensor218according to an embodiment of the present invention. Image602is a further-filtered hologram Iconproduced by the sensor218and described above. The method500is then applied to image602to produce an image604of the ragweed pollen particles. Rings surrounding the images may be out of focus images which are artifacts of the in-line implementation200. This may also be the case for implementation400. Although, if a small angle separation is deliberately interposed between the scattered and the reference wave images (for instance, using a rotating stage, as discussed above) then unwanted artifacts may be separated from the real image, such as during reconstruction.

Finally, image606is a depiction of a conventional microscope image of the same cluster of ragweed pollen particles, for comparative purposes.

By comparing these images, one can see that the holographic apparatus successfully produces an accurate image of the pollen cluster, with sufficient resolution to discern individual pollen particles and even a faint signature of the single-particle surface roughness seen in the microscope images. This corresponds to a resolution roughly between 8-10 μm, although a more rigorous resolution analysis is not performed. Referring to the measured and contrast holograms shown in this figure, it is apparent how subtraction of the incident beam across the CCD, i.e., Irefremoves noise due to imperfections in the incident beam profile. This has the consequence of producing a “cleaner” contrast hologram, which subsequently improves the particle image. The holographic and microscope images of the cluster may differ slightly in overall size and detailed structural form. Although it is clearly the same cluster in (c) and (d), the differences are likely due to shifting of the cluster on the microscope slide during transfer from the apparatus to the microscope.FIG. 7is a block diagram of an embodiment of a computer system700in accordance with one or more aspects of the present invention. The computer system700may be used to implement a portion of any one of the apparatuses100,200,300and400for holographic imaging of aerosol particles. The computer system700includes a processor702, various support circuits704, and memory706. The processor702may include one or more microprocessors known in the art. The support circuits704for the processor702include conventional cache, power supplies, clock circuits, data registers, I/O devices710, and the like. An input/output (I/O) interface708may be directly coupled to the memory706or coupled through the supporting circuits704. The I/O interface708may also be configured for communication with input devices and/or output devices710, such as, network devices, various storage devices, mouse, keyboard, display, and the like.

The memory706, or computer readable medium, stores non-transient processor-executable instructions and/or data that may be executed by and/or used by the processor702. These processor-executable instructions may comprise firmware, software, and the like, or some combination thereof. Modules having processor-executable instructions that are stored in the memory706comprise controller module718and an interference pattern database716. The controller module718comprises a delivery module720, a trigger module722and a sensor module724which comprises a Fast Fourier Transform module714and a filter module712. The delivery module720controls the delivery device304to deliver aerosol particles into region316. The controller718detects when the trigger module722senses a particle in region316and enables optical device302. The controller118then enables sensor308to sense the scattered and un-scattered wave interference pattern. The sensor module724executes the filter module to perform background subtraction on the sensed image to generate Iconand executes the FFT module714to produce an image from Icon. The system700then identifies the hologram and/or the image produced with stored images in the database716. The memory706may include one or more of the following random access memory, read only memory, magneto-resistive read/write memory, optical read/write memory, cache memory, magnetic read/write memory, and the like, as well as signal-bearing media, excluding non-transitory signals such as carrier waves and the like.

FIG. 8is a flow diagram of a function800for holographic imaging of aerosol particles in accordance with embodiments of the present invention. The method800is an implementation of execution of the controller718executed by the processor702in memory706. The method800begins at step802and proceeds to step804, where the particles are delivered into a region by the delivery module720. At step806, the trigger module enables the optical device302to generate a particle-illuminating wave and a reference wave (unscattered). When the particle-illuminating wave interacts with aerosol particles, a scattered wave is generated. At step808, the sensor module724senses the interference pattern between the scattered wave and the un-scattered wave, where the interference pattern is the hologram. At step810, the sensor output is processed by filter module712, to background subtract the sensed hologram. At step812, the FFT module714is executed to generate an image of the particles318. The method ends at step814.

The apparatuses and methods disclosed herein may further be used in conjunction with other technologies, such as fluorescence, Raman spectroscopy, Laser-induced breakdown spectroscopy (LIBS) in a detector to additionally characterize particles.

Various elements, devices, modules and circuits are described above in associated with their respective functions. These elements, devices, modules and circuits are considered means for performing their respective functions as described herein.