Methods and apparatuses for contact-free holographic imaging of aerosol particles
Inventors
Berg, Matthew J. • Videen, Gorden
Assignees
United States Department of the Army
Publication Number
US-8830476-B2
Publication Date
2014-09-09
Expiration Date
2032-03-19
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Abstract
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.
Core Innovation
The invention provides methods and apparatuses for contact-free holographic imaging of aerosol particles in an efficient manner. One embodiment includes delivering aerosol particles into a region where a light source outputs a first beam that interacts with the particle, producing an un-scattered light wave and a scattered light wave, and a second beam that does not travel into the region. A beam splitter then combines the second beam with the scattered light to create an interference pattern sensed by an image sensor. An image processor reconstructs images of the aerosol particles from this interference pattern.
The problem addressed arises from the difficulty in characterizing small aerosol particles in situ, particularly ranging from 0.1 to 10 micrometers. Conventional techniques analyze scattering patterns to infer particle properties like size and shape, but a fundamental limitation is the ambiguous inverse relationship between scattering patterns and particle features. Direct imaging methods, while preferable, are impractical because they require particle collection and immobilization, low throughput, and are limited by the small particle size and high numerical aperture optics requirements.
Holography offers a solution by recording interference patterns between coherent light and light scattered by particles, allowing computational reconstruction of particle images without interpretation of scattering patterns. Traditional holography, using photographic film, is limited by cost and processing time. This invention replaces photographic film with digital sensors and computational reconstruction methods, using either inline or off-axis optical configurations and efficient digital processing like Fourier transforms to provide fast, high-throughput, three-dimensional imaging of aerosol particles in situ, including their size, shape, and morphology.
Claims Coverage
The patent discloses multiple inventive features centered on holographic aerosol particle imaging using a delivery and trapping mechanism, dual-beam light sources, optical separation of scattered and unscattered light, and computational image reconstruction.
Particle trapping and controlled delivery
A delivery device that generates an airflow to deliver aerosol particles to and from a region together with a particle trap that holds particles substantially motionless in the region and can subsequently release them into the airflow.
Dual-beam light source configuration
A light source outputs a first beam that traverses the region producing an un-scattered light wave and a scattered light wave from the trapped particle, and a second beam that does not enter the region; the second beam is combined with the scattered light to form interference light.
Optical system for focusing and separating light
Use of beam splitters, mirrors with through-holes, and lenses arranged to separate scattered light from un-scattered light, and to combine the second beam with scattered light; including features like rotation stages for angular offset, filtering, and focusing optics for the first beam, second beam, and combined interference light.
Optical trigger device using crossed diode lasers and photomultiplier tubes
A trigger system comprising two diode laser beams of differing wavelengths intersecting in the region and two photomultiplier tubes detecting simultaneously scattered light; laser pulses are triggered only when both detectors confirm particle presence.
Use of pulsed laser light source and digital image sensing
A pulsed laser light source aligned with an image sensor (e.g., CCD or CMOS camera) capturing interference patterns generated from combined scattered and reference beams.
Image processor executing fast Fourier transform for image reconstruction
An image processor configured to perform a Fast-Fourier transform on the interference pattern to generate particle images, with further capabilities to identify particles via digital image databases and construct three-dimensional renderings using phase and magnitude data.
Modification of intensity of reference beam for contrast optimization
Inclusion of filters or couplers to adjust the field intensity of the second beam to match approximately the intensity of the scattered light, enhancing interference contrast in holograms.
Methods for holographic imaging including particle trapping, beam output, optical separation, interference sensing, and image generation
Processes for generating airflow, trapping particles, outputting dual beams where one beam produces scattered and un-scattered waves, separating scattered light, combining beams into interference light, sensing the interference pattern, and computationally generating images including three-dimensional renderings.
Trapping multiple particles and generating individual images
The method also covers holding multiple particles simultaneously in the region and generating individual holographic images of each particle.
The claims cover a comprehensive system combining airflow delivery and particle trapping, dual-beam optical setups for holographic imaging with light separation and interference pattern detection, pulsed laser sources, optical triggering for particle detection, digital sensing and computational reconstruction via Fourier transforms, and methods for both single and multiple particle imaging in an efficient, contact-free manner.
Stated Advantages
High throughput detection by rapid successive recording of holograms using digital media instead of slower photographic film.
Direct imaging of aerosol particles eliminates complex and ambiguous inversion of scattering patterns.
Capability to reconstruct three-dimensional images from holograms, permitting detailed particle morphology characterization.
Flexibility in working distance between optics and particle, enabled by virtual point source illumination and non-contact operation.
Use of fiber optics in certain embodiments increases compactness, durability, and simplifies alignment.
The dual-beam configuration permits adjustment of reference beam intensity to optimize interference fringe contrast.
Ability to separately image multiple particles in the scattering region digitally by focusing computationally on different reconstruction planes.
Eliminates need for particle immobilization, enabling in situ imaging of undisturbed aerosol particles.
Documented Applications
Characterization of environmental hazards like asbestos, smog, and smoke.
Detection and identification of biological contaminants and spores such as E. coli and anthrax.
Detection of chemical or biological warfare agents for defense applications.
Monitoring of chemical toxins and other airborne pollutants like dust and pollen.
Potential integration with fluorescence, Raman spectroscopy, and laser-induced breakdown spectroscopy for enhanced particle characterization.
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