22 research outputs found
In-situ measurement and characterization of cloud particles using digital in-line holography
No full textSatellite measurement validations, climate models, atmospheric radiative transfer models and cloud models, all depend on accurate measurements of cloud particle size distributions, number densities, spatial distributions, and other parameters relevant to cloud microphysical processes. And many airborne instruments designed to measure size distributions and concentrations of cloud particles have large uncertainties in measuring number densities and size distributions of small ice crystals. HOLODEC (Holographic Detector for Clouds) is a new instrument that does not have many of these uncertainties and makes possible measurements that other probes have never made. The advantages of HOLODEC are inherent to the holographic method.
In this dissertation, I describe HOLODEC, its in-situ measurements of cloud particles, and the results of its test flights. I present a hologram reconstruction algorithm that has a sample spacing that does not vary with reconstruction distance. This reconstruction algorithm accurately reconstructs the field to all distances inside a typical holographic measurement volume as proven by comparison with analytical solutions to the Huygens-Fresnel diffraction integral. It is fast to compute, and has diffraction limited resolution. Further, described herein is an algorithm that can find the position along the optical axis of small particles as well as large complex-shaped particles. I explain an implementation of these algorithms that is an efficient, robust, automated program that allows us to process holograms on a computer cluster in a reasonable time.
I show size distributions and number densities of cloud particles, and show that they are within the uncertainty of independent measurements made with another measurement method. The feasibility of another cloud particle instrument that has advantages over new standard instruments is proven. These advantages include a unique ability to detect shattered particles using three-dimensional positions, and a sample volume size that does not vary with particle size or airspeed. It also is able to yield two-dimensional particle profiles using the same measurements
The feasibility of a galaxy infrared slitless prism survey
No full textWe examine the issues surrounding an IR slitless prism survey for galaxies with highly redshifted Hα to see if some of the next generation of IR instruments should have the capability to conduct such surveys. We model the galaxy and star surface density as a function of J, H, and K magnitudes to estimate field crowding with depth. Using data from the Subaru 8.2 m telescope and Infrared Camera and Spectrograph detector as a benchmark, we estimate the number of detectable galaxies as a function of telescope aperture size, object magnitude, and line strength. We find that while crowding need not be a major problem, it is still difficult for even a large telescope to obtain a useful emission-line galaxy sample in the J, H, or K bands primarily because of the high IR background and the faintness of the galaxies with Hα in these bands
Persephone's paradox : the author's journey into the underworld
No full textIn the most fundamental of Freud’s discoveries, it has been argued, there exists a large part of the psyche which is not under the direct conscious control of the individual. In referring to this as ‘the unconscious’, Freud generated a paradox: how can we know of the existence of the unknowable?(Fowler 1981: 193). This un/knowable unconscious underpins the fugal narrative in its many variations. Varied perhaps, but structural similarities also unite narratives such as these. ‘Underworld narratives’ rely on a fugal descent into the underworld, including the underworld of the unconscious, which forms the structure of these narratives in terms of both their ongoing framework and signified content. The underworld into which the protagonist descends may be of a personal, social or cultural nature, or any combination of these. This paper examines two novels which emphatically represent this type of narrative structure. Most saliently, Thomas Mann’s Death in Venice, first published in 1911, and also Susanna Moore’s contemporary novel, In the Cut (1995), which offers a similar theme of fatality and surrender. Both of these novels have as their main character an author whose self-repression leads to a fugal, self-destructive projection of their desire onto a ‘ perfect’ object for the narrator’s unconscious purposes. This paper is based on research for my recently completed PhD thesis, ‘The Writer’s Fugue: authorship, subjectivity and the self’, which applies the multivalent concept of ‘fugue’ to the creative writing process. As Deleuze argued in his work on Proust, a work of art is analogous to a machine, because it is essentially productive of certain truths (Deleuze 2000: 146). Mann’s and Moore’s murderous fugal narratives explore analogous truths about the psychological state of writing and ‘being’ a writer, both of which involve a necessary level of repression which can be represented analogously as a form of ‘death’
Practical methods for automated reconstruction and characterization of particles in digital in-line holograms
No full textHologram reconstruction algorithms often undersample the phase in propagation kernels for typical parameters of holographic optical setups. Given in this paper is an algorithm that addresses this phase undersampling in reconstructing digital in-line holograms of particles for these typical parameters. This algorithm has a lateral sample spacing constant in reconstruction distance, has a diffraction limited resolution, and can be implemented with computational speeds comparable to the fastest of other reconstruction algorithms. This algorithm is shown to be accurate by testing with analytical solutions to the Huygens-Fresnel propagation integral. A low-pass filter can be applied to enforce a uniform minimum particle size detection limit throughout a sample volume, allowing this method to be useful in measuring particle size distributions and number densities. Tens of thousands of holograms of cloud ice particles are digitally reconstructed using the algorithm discussed. Positions of ice particles in the size range of 20 νm-1.5 mm are obtained using an algorithm that accurately finds the position of large and small particles along the optical axis. The digital reconstruction and particle characterization algorithms are implemented in an automated fashion with no user intervention on a computer cluster. Strategies for efficient algorithm implementation on a computer cluster are discussed. © 2009 IOP Publishing Ltd
Recommended from our members
Holographic Measurement of Drop-on-Demand Drops in Flight
No full textThe analysis of images of ink drops in flight can provide information about jet straightness, drop velocity and volume.
However trade-offs between field of view, optical and digital resolution and other factors such as depth of field and optical
distortion, limit the accuracy and amount of information available from a single image. In-line, digital holograms of drops in flight
can capture information from fields of view at least as large as the area of the digital sensor. Using mathematical reconstruction
techniques particularly suited to sparse, small objects of regular geometry the accuracy of measurement can potentially be submicrometer on drop position and diameter.
This paper describes our experimental apparatus, hologram reconstruction techniques and the results of experiments on
imaging drops. We also discuss techniques to improve the accuracy of the technique in the direction of the optical axis
Application of holography and automated image processing for laboratory experiments on mass and fall speed of small cloud ice crystals
No full textAn ice cloud chamber was developed at the Johannes Gutenberg University of Mainz for generating several thousand data points for mass and sedimentation velocity measurements of ice crystals with sizes less than 150 µm. Ice nucleation was initiated from a cloud of supercooled droplets by local cooling using a liquid nitrogen cold finger. Three-dimensional tracks of ice crystals falling through the slightly supersaturated environment were obtained from the reconstruction of sequential holographic images, automated detection of the crystals in the hologram reconstructions, and particle tracking. Through collection of the crystals and investigation under a microscope before and after melting, crystal mass was determined as a function of size. The experimentally obtained mass versus diameter (m(D)) power law relationship resulted in lower masses for small ice crystals than from commonly adopted parameterizations. Thus, they did not support the currently accepted extrapolation of relationships measured for larger crystal sizes. The relationship between Best (X) and Reynolds (Re) numbers for columnar crystals was found to be X=15.3 Re1.2, which is in general agreement with literature parameterizations
Airborne digital holographie system for cloud particle measurements
No full textAn in-line holographic system for in situ detection of atmospheric cloud particles [Holographic Detector for Clouds (HOLODEC)] has been developed and flown on the National Center for Atmospheric Research C-130 research aircraft. Clear holograms are obtained in daylight conditions at typical aircraft speeds of 100 m s-1. The instrument is fully digital and is interfaced to a control and data-acquisition system in the aircraft via optical fiber. It is operable at temperatures of less than -30 °C and at typical cloud humidities. Preliminary data from the experiment show its utility for studies of the three-dimensional spatial distribution of cloud particles and ice crystal shapes. © 2004 Optical Society of America
HOLIMO II: a digital holographic instrument for ground-based in situ observations of microphysical properties of mixed-phase clouds
No full textISSN:1867-1381ISSN:1867-8548ISSN:1867-854
Impact of surface and near-surface processes on ice crystal concentrations measured at mountain-top research stations
No full textAbstract. In-situ cloud observations at mountain-top research stations regularly measure ice crystal number concentrations (ICNCs) orders of magnitudes higher than expected from measurements of ice nucleating particle (INP) concentrations. Thus, several studies suggest that mountain-top in-situ measurements are influenced by surface processes, e.g. blowing snow, hoar frost or riming on snow covered trees, rocks and the snow surface. A strong impact on the observed ICNCs on mountain-top stations by surface processes may limit the relevance of such measurements and possibly affects the development of orographic clouds. This study assesses the impact of surface processes on in-situ cloud observations at the Sonnblick Observatory in the Hohen Tauern Region, Austria. Vertical profiles of ICNCs above a snow covered surface were observed up to a height of 10 m. The ICNC decreases at least by a factor of two at 10 m, if the ICNC at the surface is larger than 100 L−1. This decrease can be up to one order of magnitude during in-cloud conditions and reached its maximum of more than two orders of magnitudes when the station was not in cloud. For one case study, the ICNC for regular and irregular ice crystals showed a similar relative decrease with height, which cannot be explained by the above mentioned surface processes. Therefore, two near-surface processes are proposed to enrich ICNCs and explain these finding. Either sedimenting ice crystals are captured in a turbulent layer above the surface or the ICNC is enhanced in a convergence zone, because the cloud is forced over a mountain. These two processes would also have an impact on ICNCs measured at mountain-top stations if the surrounding surface is not snow covered. Conclusively, this study strongly suggests that ICNCs measured at mountain-top stations are not representative for the properties of a cloud further away from the surface.
</jats:p
Lagrangian particle tracking in three dimensions via single-camera in-line digital holography
No full textLagrangian particle trajectories are measured in three spatial dimensions with a single camera using the method of digital in-line holography. Lagrangian trajectories of 60-120 μm diameter droplets in turbulent air obtained with data from one camera compare favorably with tracks obtained from a simultaneous dual-camera data set, the latter having high spatial resolution in all three dimensions. Using the single-camera system, particle motion along the optical axis is successfully tracked, allowing for long, continuous 3D tracks, but the depth resolution based on standard reconstruction methods is not sufficient to obtain accurate acceleration measurements for that component. Lagrangian velocity distributions for all three spatial components agree within reasonable sampling uncertainties and Lagrangian acceleration distributions agree for the two lateral components. An equivalent single-camera, imaging-based 2D tracking system would be challenged by the particle densities tested, but the holographic configuration allows for 3D tracking in the dilute limit. The method also allows particle size, shape and orientation to be measured along the trajectory. Lagrangian measurements of particle size provide a direct measure of particle size uncertainty under realistic conditions sampled from the entire measurement volume. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
