Contact-free holographic imaging of aerosol particles from mobile platforms
Inventors
Berg, Matthew J. • Kemppinen, Osku P. • VIDEEN, GORDEN W.
Assignees
Kansas State University • United States Department of the Army
Publication Number
US-11353392-B2
Publication Date
2022-06-07
Expiration Date
2040-09-21
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Abstract
A device includes a filter that enhances a beam profile of a received pulsed laser; a first optical element to direct the pulsed laser as a reference wave towards an optical sensor; an open cavity positioned between the first optical element and the optical sensor. The open cavity receives an aerosol particle, which enters the open cavity from any direction. The reference wave illuminates the aerosol particle. An illuminated particle generates and directs an object wave towards the optical sensor. A pixel array is connected to the optical sensor. The pixel array receives the reference wave and the object wave. The optical sensor creates a contrast hologram comprising an interference pattern of the illuminated particle. A processor creates an image of the illuminated particle based on the contrast hologram.
Core Innovation
The invention provides a device and an opto-electric system for contact-free, digital holographic imaging of aerosol particles in situ using a pulsed laser and an optical sensor to capture interference patterns produced by illumination of aerosol particles. The device includes a filter to enhance the beam profile of a received pulsed laser, optical elements to direct the pulsed laser as a reference wave, an open cavity where aerosol particles enter and are illuminated, and a pixel array connected to an optical sensor that creates a contrast hologram representing the interference pattern of the illuminated particles. A processor reconstructs images of the particles from the contrast hologram.
The system further includes a pair of laser beams—a trigger beam and a hologram beam—that are coaxial and shaped to detect particles entering the sensing region. The trigger beam's scattered light is detected by a photomultiplier tube to generate a beam trigger signal, which causes a delayed pulse of the hologram beam to illuminate the particle and enables digital hologram capture. The design supports real-time or post-measurement image reconstruction and allows for a large sensing volume where particles may enter from any direction. The device may be housed in a portable structure configured for transportation by an aerospace vehicle such as a UAV.
The problem addressed by the invention stems from the significant uncertainty in climate forcing due to aerosol particles, especially coarse mode aerosols (CMAs) >1 μm in size, whose morphology, size distribution, and optical properties remain poorly characterized. Conventional aerosol characterization methods are limited, particularly in situ and for non-spherical particles, impairing accurate climate model inputs. The lack of rapid, contact-free, precise aerosol particle imaging limits understanding of aerosol impacts on climate, human health, and atmospheric processes. The invention solves the need for accurate, in situ, digital holographic imaging of aerosol particles, enabling characterization of size, shape, and distribution using a compact, portable, and mobile device suitable for use on drones or other flying platforms.
Claims Coverage
The patent discloses two main independent claims, each defining devices and a method for holographic imaging of aerosol particles using a pulsed laser, optical sensing, and image reconstruction. These claims cover the structure, laser beam shaping and directing, particle illumination and detection, signal triggering, and image processing features.
Device with enhanced pulsed laser beam and open cavity for aerosol particle holography
A device includes a filter enhancing a pulsed laser's beam profile; a first optical element to direct the pulsed laser as a reference wave towards an optical sensor; optics shaping a second laser into a collimated hollow annular trigger beam coaxial with the reference wave; an open cavity between the optical element and sensor receiving aerosol particles; the reference wave illuminating particles that generate object waves; a pixel array receiving both waves and producing a contrast hologram from their interference; and a processor that creates particle images in response to trigger beam interaction.
Opto-electric system with paired laser sources and compartments for triggered holographic imaging
An opto-electric system comprises: an optics compartment shaping, combining, redirecting paired laser beams; a sensing compartment with paired laser sources generating trigger and hologram beams, the latter coaxial with and within the trigger beam, illuminating particles in a sensing region causing an object wave; an optical sensor producing a contrast hologram from interference; an electronics compartment with a signal generator producing a beam trigger signal upon particle illumination, a control system delaying the hologram beam pulse accordingly, a memory device storing holograms, and a processor creating images from holograms.
Method of generating a contrast hologram and imaging aerosol particles
A method comprising receiving a pulsed laser; directing it as a reference wave to an optical sensor; receiving aerosol particles in a sensing cavity; illuminating particles by the reference wave; generating object waves from illuminated particles; directing object waves to the sensor; generating a contrast hologram from interference patterns; selectively delaying the pulsed laser to position particles relative to the reference wave; and creating particle images from the contrast hologram.
The claims collectively cover a system utilizing coordinated pulsed and continuous laser beams, optical elements to shape and direct these beams coaxially through an open sensing cavity, detection and triggering means to capture interference patterns from aerosol particles, and processing steps to reconstruct detailed images from holographic data, all integrated in a portable, aerospace-compatible housing.
Stated Advantages
Enables contact-free, rapid imaging of aerosol particles in situ, resolving particles nominally larger than ten micrometers in size.
Allows imaging of multiple particles entering the sensor volume simultaneously from any direction.
Provides 3D imaging capability by reconstructing focused images at various depth planes from a single hologram.
Portable and lightweight design suitable for mounting on commercial-grade UAVs and other aerospace vehicles.
Digital holography avoids the need for pre-set focal planes and allows image reconstruction post-measurement.
Improves hologram fringe contrast by background subtraction techniques enhancing image quality.
The contrived trigger system enables selective imaging of particles approximately 10 micrometers and larger while also capturing smaller particles co-located in the beam.
Documented Applications
Field research for in situ characterization of coarse-mode aerosols including airborne mineral dust and primary biological aerosol particles.
Deployment on unmanned aerial vehicles, drones, helicopters, airplanes, balloons, blimps, aerostats, and other flying platforms for atmospheric aerosol imaging.
Measurement in environments such as near pollinating spruce trees to analyze biological aerosols and road dust fields to study mineral dust aerosols.
Climate change studies requiring morphological and distribution data of aerosols to reduce uncertainties in radiative forcing.
Tracking and real-time monitoring of natural and manmade aerosols including smoke, volcanic ash, pollutants, and biological agents.
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