In-line Holographic Imaging Research Articles

Accurate holographic imaging of colloidal particle pairs by Rayleigh-Sommerfeld reconstruction

In-line holographic optical imaging has the unique capability of high speed imaging in three dimensions at rates limited only by the imaging rate of the camera used. In this technique the 3D data is recorded on the detector in a form of a hologram generated by diffraction between the scattered and unscattered light passing through the sample. For dilute samples of single particles or a small cluster of particles, this technique was shown to result in particle tracking with spatial positioning accuracy of a few nanometers. For dense suspension only approximate reconstruction were achieved with systematic axial positioning errors. We propose a scheme to extend accurate holographic microscopy to dense suspensions, by calibrating the Rayleigh-Sommerfeld reconstruction algorithm against Lorentz-Mie scattering theory. We perform this calibration both numerically and experimentally and define the parameter space in which accurate imaging is achieved, and in which numerical calibration holds. We demonstrate the validity of our approach by imaging two attached particles and measuring the distance between their centers with 36 nm accuracy. A difference of 50 nm in particle diameter is easily measured.

A robust cell counting approach based on a normalized 2D cross-correlation scheme for in-line holographic images

To achieve the important aims of identifying and marking disease progression, cell counting is crucial for various biological and medical procedures, especially in a Point-Of-Care (POC) setting. In contrast to the conventional manual method of counting cells, a software-based approach provides improved reliability, faster speeds, and greater ease of use. We present a novel software-based approach to count in-line holographic cell images using the calculation of a normalized 2D cross-correlation. This enables fast, computationally-efficient pattern matching between a set of cell library images and the test image. Our evaluation results show that the proposed system is capable of quickly counting cells whilst reliably and accurately following human counting capability. Our novel approach is 5760 times faster than manual counting and provides at least 68% improved accuracy compared to other image processing algorithms.

Implementation of Step-Frequency Continuous-Wave Scheme in Millimeter-Wave Inline Holography for Interferences Elimination

A millimeter-wave (MMW) inline holographic imag ing method based on step-frequency continuous-wave (SFCW) scheme is presented in this letter. By analyzing and decomposing the received SFCW power in range domain, it is found that the background and twin-image interferences can be well separated from the image-related term, and therefore can be removed with appropriate range windows. Moreover, a corresponding mathematical algorithm is developed to reconstruct the image from the inline hologram. To validate the effectiveness of the presented approach, a 200-GHz prototype system is designed and fabricated. Experimental results demonstrate the good performance of the imaging system and its potential application in high-resolution target detection.

Lens-free computational imaging of capillary morphogenesis within three-dimensional substrates

Endothelial cells cultured in three-dimensional (3-D) extracellular matrices spontaneously form microvessels in response to soluble and matrix-bound factors. Such cultures are common for the study of angiogenesis and may find widespread use in drug discovery. Vascular networks are imaged over weeks to measure the distribution of vessel morphogenic parameters. Measurements require micron-scale spatial resolution, which for light microscopy comes at the cost of limited field-of-view (FOV) and shallow depth-of-focus (DOF). Small FOVs and DOFs necessitate lateral and axial mechanical scanning, thus limiting imaging throughput. We present a lens-free holographic on-chip microscopy technique to rapidly image microvessels within a Petri dish over a large volume without any mechanical scanning. This on-chip method uses partially coherent illumination and a CMOS sensor to record in-line holographic images of the sample. For digital reconstruction of the measured holograms, we implement a multiheight phase recovery method to obtain phase images of capillary morphogenesis over a large FOV (24 mm2) with ≈ 1.5 μm spatial resolution. On average, measured capillary length in our method was within approximately 2% of lengths measured using a 10 × microscope objective. These results suggest lens-free on-chip imaging is a useful toolset for high-throughput monitoring and quantitative analysis of microvascular 3-D networks.

Challenges in using GPUs for the reconstruction of digital hologram images

In-line holographic imaging is used for small particulates, such as cloud or spray droplets, marine plankton, and alluvial sediments, and enables a true 3D object field to be recorded at high resolution over a considerable depth. To reconstruct a digital hologram a 2D FFT must be calculated for every depth slice desired in the replayed image volume. A typical in-line hologram of ∼ 100 micrometre-sized particles over a depth of a few hundred millimetres will require O(1000) 2D FFT operations to be performed on an hologram of typically a few million pixels. In previous work we have reported on our experiences with reconstruction on a computational grid. In this paper we discuss the technical challenges in making efficient use of the NVIDIA Tesla and Fermi GPU systems and show how our reconstruction code was optimised for near real-time video slice reconstruction with holograms as large as 4K by 4K pixels. We also consider the implications for grid and cloud computing approaches to hologram replay, and the extent to which a GPU can replace these approaches, when the important step of locating focussed objects within a reconstructed volume is included.

Liquid velocity distribution in slug flow in a microchannel

The liquid velocity distributions in two-phase slug flows in a nearly rectangular microchannel etched on a microfluidic chip were investigated using a three-dimensional tracking method for submicron fluorescent particles seeded in the working liquid (water). The Taylor bubbles generated from dissolved air in the water through heating the micro-fluidic chip to 35–55°C had low velocities, so they had the very small Capillary and Reynolds numbers. The change in the Taylor bubble shape with the flow conditions such as the bubble velocity and temperature was negligible in the present study. The shapes of the fully developed Taylor bubbles were determined by analyzing the interference fringes in in-line holographic images of the microchannel section containing the bubble to estimate the bubble cross-sectional area. The three-dimensional liquid velocity distribution for the two-phase slug flow was obtained using a least-squares fit of the measured tracer particle velocities to an analytic velocity distribution for Poiseuille flow in a rectangular channel to determine the mean liquid velocity and the liquid flow rate in the slug. The liquid velocity was normalized using the measured instantaneous bubble velocity to remove the influence of the slug and the bubble velocity fluctuations on the liquid velocity distribution. The results show that the mean liquid velocity through the microchannel corners is 2.3 times the bubble velocity, which is in agreement with previous observations of the maximum liquid velocity in the corners of 3.5–3.8 times the bubble velocity.

Two-dimensional plasma grating by non-collinear femtosecond filament interaction in air

We experimentally demonstrated that two-dimensional (2D) plasma gratings could be generated in air by nonlinear interaction of three femtosecond filaments. The intensity interference of non-collinearly overlapped filaments was self-projected along a relatively long distance and accompanied with a wavelength-scale periodic change of the refractive index in the encircling air due to periodic plasma density modulation. The 2D plasma gratings supported 2D diffraction of the generated third-harmonic pulses. By using in-line time-resolved holographic imaging and time-delayed diffraction, the 2D plasma gratings were evidenced to last a few tens picoseconds after the excitation pulses.

Design of an in-line, digital holographic imaging system for airborne measurement of clouds

We discuss the design and performance of an airborne (underwing) in-line digital holographic imaging system developed for characterizing atmospheric cloud water droplets and ice particles in situ. The airborne environment constrained the design space to the simple optical layout that in-line non-beam-splitting holography affords. The desired measurement required the largest possible sample volume in which the smallest desired particle size (∼5 μm) could still be resolved, and consequently the magnification requirement was driven by the pixel size of the camera and this particle size. The resulting design was a seven-element, double-telecentric, high-precision optical imaging system used to relay and magnify a hologram onto a CCD surface. The system was designed to preserve performance and high resolution over a wide temperature range. Details of the optical design and construction are given. Experimental results demonstrate that the system is capable of recording holograms that can be reconstructed with resolution of better than 6.5 μm within a 15 cm(3) sample volume.

High-speed microjet particles measurement using in-line pulsed holography

Transient in-line holographic imaging system is optimized for recording the holograms of detonation loaded microjet particles with a velocity higher than 5 km/s. Then an adaptive multithreshold image segmentation method is developed to improve the measurement precision of particle size and number. The measured size of ejected particles is from several microns to over ten microns, and the processing errors of particle number and size are less than 5% and 15%, respectively. The statistical results also show that the size, number, and velocity of microjet particles vary depending on the surface conditions of the Al metal debris. Compared with a uniform metal sample, the nonuniform metal sample with cone-shaped hole exhibits more ejected particles, larger particle size, and higher velocity.

Quantitative measurement of size and three-dimensional position of fast-moving bubbles in air-water mixture flows using digital holography

We present a digital in-line holographic imaging system for measuring the size and three-dimensional position of fast-moving bubbles in air-water mixture flows. The captured holograms are numerically processed by performing a two-dimensional projection followed by local depth estimation to quickly and efficiently obtain the size and position information of multiple bubbles simultaneously. Statistical analysis on measured bubble size distributions shows that they follow lognormal or gamma distributions.

3-D Visualization and Identification of Biological Microorganisms Using Partially Temporal Incoherent Light In-Line Computational Holographic Imaging

We present a new method for three-dimensional (3-D) visualization and identification of biological microorganisms using partially temporal incoherent light in-line (PTILI) computational holographic imaging and multivariate statistical methods. For 3-D data acquisition of biological microorganisms, the band-pass filtered white light is used to illuminate a biological sample. The transversely and longitudinally diffracted pattern of the biological sample is magnified by microscope objective (MO) and is optically recorded with an image sensor array interfaced with a computer. Three-dimensional reconstruction of the biological sample from the diffraction pattern is accomplished by using computational Fresnel propagation method. Principal components analysis and nonparametric inference algorithms are applied to the 3-D complex amplitude biological sample for identification purposes. Experiments indicate that the proposed system can be useful for identifying biological microorganisms. To the best of our knowledge, this is the first report on using PTILI computational holographic microscopy for identification of biological microorganisms.

Preliminary study of a phase-contrast cone-beam computed tomography system: the edge-enhancement effect in the tomographic reconstruction of in-line holographic images

A phase-contrast cone-beam computed tomography (PC CBCT) system is proposed for small-animal imaging that incorporates the in-line holography technique into CBCT system. Theoretical analysis shows that the in-line holographic image can be approximately transformed into line integrals of an object function composed of an attenuation term and a phase term. The Fresnel diffraction theory is applied to generate in-line holographic images along a circular orbit, and the Feldkamp-Davis-Kress algorithm is applied to reconstruct the object function. The proposed system was investigated using a numerical phantom, and the reconstruction was evaluated using the edge-enhancement factor and the relative reconstruction error. The reconstruction results show that all the structures in the numerical phantom are bounded with enhanced edges with negligible artifacts. These enhanced edges make the reconstruction visually sharper and clearer. The results show that while the relative reconstruction errors are very close to that of the conventional CBCT reconstruction, having a small cone angle, weak attenuation, small focal spot size, and high-resolution detector are preferred for a greater edge-enhancement effect.

Off-axis holographic tomography for diffracting scalar wavefields

Inverse scattering theories are available that permit tomographic reconstruction of the complex-valued refractive index distribution of weakly scattering objects from knowledge of intensity measurements. These imaging methods are valuable in applications that involve high-frequency optical wavefields, in which direct wavefield phase measurements can be difficult experimentally. The so-called in-line holographic imaging geometry has been well-studied, in which two measurements of the forward scattered wavefield intensity are acquired on distinct parallel detector planes that are perpendicular to the direction of the illuminating wavefields. In this work, based on the principles of intensity diffraction tomography, a reconstruction theory for off-axis holographic tomography with diffracting scalar wavefields is developed and investigated. A distinct feature of the method is that, at each tomographic view angle, the object is illuminated by use of two plane waves that propagate in different directions. The intensities of the forward scattered wavefields are measured on a single detector behind the object. This permits the direction of the probing wavefield to be varied for acquisition of the necessary measurement data, rather than the detector placement as required by conventional in-line holographic methods. The developed image reconstruction method is validated and investigated by use of computer-simulation studies.

Nonlinear hot-image formation of an intense laser beam in media with gain and loss

An analytical treatment and results of numerical calculations are presented for the hot-image formation in media with gain and loss. A new approximately functional expression of the hot-image intensity is derived, showing that it is several times larger than the background light intensity, indicating a great damage threat to high-power laser systems. The image appears in a plane located downstream from the nonlinear medium at a distance equal to that from the scatterer to the medium. Finally, it is validated that the hot-image formation is akin to real-time, in-line and volume-phase Fraunhofer holographic imaging. The predictions of the theory are found to be in good agreement with simulations carried out for ns-pulse propagation.

Nonlinear hot-image formation of an intense laser beam in media with gain and loss

An analytical treatment and results of numerical calculations are presented for the hot-image formation in media with gain and loss. A new approximately functional expression of the hot-image intensity is derived, showing that it is several times larger than the background light intensity, indicating a great damage threat to high-power laser systems. The image appears in a plane located downstream from the nonlinear medium at a distance equal to that from the scatterer to the medium. Finally, it is validated that the hot-image formation is akin to real-time, in-line and volume-phase Fraunhofer holographic imaging. The predictions of the theory are found to be in good agreement with simulations carried out for ns-pulse propagation.

Quantitative diffraction-enhanced x-ray imaging of weak objects

Theoretical aspects of quantitative diffraction-enhanced imaging of weak objects are considered using the Fourier optics approach. The amplitude and phase transfer functions are introduced by analogy with the well-known case of in-line (holographic) imaging. The inverse problem of the reconstruction of the phase and amplitude of the incident wave from recorded images is solved in the case of non-absorbing objects and objects consisting of a single material and in the general case of objects with uncorrelated refraction and absorption characteristics. A comparison is given between the solutions to the inverse problem obtained using the new formalism and the geometric-optics approximation.

Theoretical analysis of hot image effect from phase scatterer

The nonlinear hot image effect from phase scatterer in a high-power laser system has been investigated theoretically and numerically. According to the Fresnel-Kirchhoff diffraction integral and nonlinear paraxial wave equation, we derive the functional relationship of the intensity of hot image and its location. It is shown that the image intensity is likely several times larger than that from amplitude scatterer, and the location is situated in the equidistant downstream plane of the scatterer in an optical component located upstream from a nonlinear element. Good agreements are shown between the theoretical prediction and the computer codes developed for ns-pulse propagation simulations. The results indicate that the formation of hot image is akin to the real-time, in-line and volume-phase holographic imaging.

Speckle noise arising from coherently illuminated polydisperse and continuous size distributions of discs

An existing model of the speckle noise affecting images in far-field in-line holographic imaging is extended to distributions of disc-shaped particles which are no longer of uniform diameter. In this model the signal-to-noise ratio in replayed images from ideal in-line holograms in the Fraunhofer regime is limited by the speckle field arising from the overlapping Airy patterns. The original study was motivated by holographic particle image velocimetry and assumed that all the particles were of uniform diameter. We extend this model to cover polydisperse, power-law, exponential and Gaussian distribution functions and include an explicit limit on our maximum particle diameter to ensure that the hologram is recorded in the far-field of all the particles.

Resolution characteristics of holographic particle image velocimetry

The resolution characteristics of in-line holographic imaging systems for application to velocity field measure- ment in turbulent and other vortical flows has been investigated experimentally. It is shown that the diameter of seed particles strongly influences the maximum size of the test volume that can be used as well as the spatial resolution of the measurement. Velocity measurements in a vortex ring flow using a two-axis holographic recording system are reported. The spatial resolution of this system is approximately 1 mm. OLOGRAPHIC imaging of flows seeded with small particles can be a useful technique for velocity field mea- surement in turbulent and other vortical flows. Several investi- gators have reported velocity measurements using holographic techniques.1'2 Two basic approaches have been proposed. In most cases in-line doubly exposed holograms of the seeded flow are obtained. The velocity is determined from the dis- placement of the particles on the reconstructed images. This approach is what we call holographic particle image velocime- try (HPIV) because it is a natural extension of planar particle image velocimetry.3 A second approach4'5 is to obtain a tempo- ral sequence of holograms. The velocity is determined by tracking individual particles as a function of time. The main advantage of HPIV compared to particle tracking techniques is that the particle displacement is determined by local analysis of the reconstructed images which results in reduced image processing requirements. Turbulent flow measurement requires a large spatial dy- namic range. This requirement becomes more severe as the Reynolds number is increased. A large spatial dynamic range requires imaging of very small particles in a large test volume. Research on in-line holographic imaging systems for particle size measurement has shown a limit to the test volume depth of 50-80 far-field distances.6'7 (The far-field distance is defined here as d2/ where d is the diameter of the particle and X the wavelength of the laser used to record the hologram.) The depth of field of the reconstructed particle images is of the order of one far-field distance.8 The depth of field of the particle image is a good measure of the spatial resolution of the velocity measurement. It follows that the resolution character- istics of the holographic imaging system can severely limit the spatial dynamic range of HPIV. In this paper we examine experimentally the resolution char- acteristics of in-line holographic imaging systems. A two-axis holographic recording system designed for high resolution ve- locity field measurements is described. Finally, velocity mea- surements in a vortex ring flow using this HPIV system are reported. The dynamic range requirements of typical HPIV recording systems dictate that the hologram be positioned at a large distance from the particles. In this case the light intensity distribution on the hologram plane is given by Fraunhofer theory9 _ , Trt/2 . 1=1- —— sin 2\z . Air2 |~ /T£/A/Tcfr) —— sin —— } \2Ji —— / —— 2\z \Xz/ |_ \\z/l XzJ '<Kdr

A technique for projection x-ray lithography using computer-generated holograms

X-ray projection lithography has recently been explored as a method for the manufacture of &amp;lt;200 nm linewidth integrated circuits. A method is described whereby projected lithographic images can be formed with x rays by means of a transmission hologram. The form of the hologram is computed by an algorithm that eliminates the unwanted signals normally present as systematic errors in in-line holographic images. Example calculations are shown in which holograms are constructed to generate images of a given test pattern. The holographic images are shown to have excellent fidelity with respect to both resolution and contrast. Such an approach to projection x-ray lithography requires an x-ray beam with very little coherence and is thus compatible with high wafer throughput schemes; in addition, image fidelity remains high even when moderately small contaminant particles block the light from small regions of the hologram. Fabrication limits for the single optic involved are compatible with current technological limits. The approach is applicable over a large range of soft-x-ray wavelengths, indicating the feasibility of a 100 nm resolution system with 6 μm depth of field using a carbon mask system operating at 5 nm, or other materials at shorter wavelengths.