Showing papers on "Point spread function published in 2006"

Depth from diffracted rotation.

Abstract: The accuracy of depth estimation based on defocus effects has been essentially limited by the depth of field of the imaging system. We show that depth estimation can be improved significantly relative to classical methods by exploiting three-dimensional diffraction effects. We formulate the problem by using information theory analysis and present, to the best of our knowledge, a new paradigm for depth estimation based on spatially rotating point-spread functions (PSFs). Such PSFs are fundamentally more sensitive to defocus thanks to their first-order axial variation. Our system acquires a frame by using a rotating PSF and jointly processes it with an image acquired by using a standard PSF to recover depth information. Analytical, numerical, and experimental evidence suggest that the approach is suitable for applications such as microscopy and machine vision.

Extended depth of field imaging for high speed object analysis

Abstract: A high speed, high-resolution flow imaging system is modified to achieve extended depth of field imaging. An optical distortion element is introduced into the flow imaging system. Light from an object, such as a cell, is distorted by the distortion element, such that a point spread function (PSF) of the imaging system is invariant across an extended depth of field. The distorted light is spectrally dispersed, and the dispersed light is used to simultaneously generate a plurality of images. The images are detected, and image processing is used to enhance the detected images by compensating for the distortion, to achieve extended depth of field images of the object. The post image processing preferably involves de-convolution, and requires knowledge of the PSF of the imaging system, as modified by the optical distortion element.

Improved in vivo photoacoustic microscopy based on a virtual-detector concept

Abstract: Recently an in vivo high-resolution backward-mode photoacoustic microscope was developed that shows potential for applications in dermatology and related cancer research. However, the limited depth of focus of the large-numerical-aperture (NA) ultrasonic lens employed in this system causes the image quality to deteriorate significantly in the out-of-focus region. To solve this problem, we devised and explored, for the first time to our knowledge, a virtual-detector-based synthetic-aperture focusing technique, combined with coherence weighting, for photoacoustic microscopy with such a large-NA transducer. Images of phantoms show that the proposed technique improves the −6 dB−6 dB lateral resolution from 49–379 to 46–53

Breaking the resolution limit in light microscopy

Abstract: Fluorescent imaging microscopy has been an essential tool for biologists over many years, especially after the discovery of the green fluorescent protein and the possibility of tagging virtually every protein with it. In recent years dramatic enhancement of the level of detail at which a fluorescing structure of interest can be imaged have been achieved. We review classical and new developments in high-resolution microscopy, and describe how these methods have been used in biological research. Classical methods include widefield and confocal microscopy whereas novel approaches range from linear methods such as 4Pi, I(5) and structured illumination microscopy to non-linear schemes such as stimulated emission depletion and saturated structured illumination. Localization based approaches (e.g. PALM and STORM), near-field methods and total internal refraction microscopy are also discussed. As the terms 'resolution', 'sensitivity', 'sampling' and 'precision' are sometimes confused, we explain their clear distinction. Key concepts such as the point spread function and the Abbe limit, which are necessary for an in depth understanding of the presented methods, are described without requiring extensive mathematical training.

Annular-ring CMUT arrays for forward-looking IVUS: transducer characterization and imaging

Abstract: In this study, a 64-element, 1.15-mm diameter annular-ring capacitive micromachined ultrasonic transducer (CMUT) array was characterized and used for forward-looking intravascular ultrasound (IVUS) imaging tests. The array was manufactured using low-temperature processes suitable for CMOS electronics integration on a single chip. The measured radiation pattern of a 43 /spl times/ 140-/spl mu/m array element depicts a 40/spl deg/ view angle for forward-looking imaging around a 15-MHz center frequency in agreement with theoretical models. Pulse-echo measurements show a -10-dB fractional bandwidth of 104% around 17 MHz for wire targets 2.5 mm away from the array in vegetable oil. For imaging and SNR measurements, RF A-scan data sets from various targets were collected using an interconnect scheme forming a 32-element array configuration. An experimental point spread function was obtained and compared with simulated and theoretical array responses, showing good agreement. Therefore, this study demonstrates that annular-ring CMUT arrays fabricated with CMOS-compatible processes are capable of forward-looking IVUS imaging, and the developed modeling tools can be used to design improved IVUS imaging arrays.

Optical frequency domain imaging with a rapidly swept laser in the 815-870 nm range

Abstract: Optical frequency domain imaging (OFDI) in the 800-nm biological imaging window is demonstrated by using a novel wavelength-swept laser source. The laser output is tuned continuously from 815 to 870 nm at a 43.2-kHz repetition rate with 7-mW average power. Axial resolution of 10-µm in biological tissue and peak sensitivity of 96 dB are achieved. In vivo imaging of Xenopus laevis is demonstrated with an acquisition speed of 84 frames per second (512 axial lines per frame). This new imaging technique may prove useful in comprehensive retinal screening for medical diagnosis and contrast-agent-based imaging for biological investigations.

Programmable vector point-spread function engineering

Abstract: We use two nematic liquid crystal spatial light modulators (SLM’s) to control the vector point spread function (VPSF) of a 1.3 numerical aperture (NA) microscope objective. This is achieved by controlling the polarization and relative phase of the electric field in the objective’s pupil. We measure the resulting VPSF’s for several different pupil field polarization states. By using single fluorescent molecules as local field probes, we are able to map out the focal field distributions and polarization purity of the synthesized fields. We report the achieved field purity and address the experimental issues that currently limit it.

Accurate Astrometry and Photometry of Saturated and Coronagraphic Point Spread Functions

Abstract: For ground-based adaptive optics point source imaging, differential atmospheric refraction and flexure introduce a small drift of the point spread function (PSF) with time, and seeing and sky transmission variations modify the PSF flux. These effects need to be corrected to properly combine the images and obtain optimal signal-to-noise ratios, accurate relative astrometry and photometry of detected companions as well as precise detection limits. Usually, one can easily correct for these effects by using the PSF core, but this is impossible when high dynamic range observing techniques are used, like coronagraphy with a non-transmissive occulting mask, or if the stellar PSF core is saturated. We present a new technique that can solve these issues by using off-axis satellite PSFs produced by a periodic amplitude or phase mask conjugated to a pupil plane. It will be shown that these satellite PSFs track precisely the PSF position, its Strehl ratio and its intensity and can thus be used to register and to flux normalize the PSF. This approach can be easily implemented in existing adaptive optics instruments and should be considered for future extreme adaptive optics coronagraph instruments and in high-contrast imaging space observatories.

Elimination of depth degeneracy in optical frequency-domain imaging through polarization-based optical demodulation

Abstract: A novel optical frequency-domain imaging system is demonstrated that employs a passive optical demodulation circuit and a chirped digital acquisition clock derived from a voltage-controlled oscillator. The demodulation circuit allows the separation of signals from positive and negative depths to better than 50 dB, thereby eliminating depth degeneracy and doubling the imaging depth range. Our system design is compatible with dual-balanced and polarization-diverse detection, important techniques in the practical biomedical application of optical frequency-domain imaging.

Radial Hilbert transform with Laguerre-Gaussian spatial filters

Abstract: We analyze the point spread function (PSF) of the image processing system for radial Hilbert transform and propose a novel spiral phase filter, called the Laguerre-Gaussian spatial filter (LGSF). Theoretical analysis and real experiments show that the LGSF possesses some advantages in comparison with the conventional spiral phase plate (SPP). For example, the PSF of the imaging system with a LGSF presents smaller suboscillations than that with the conventional SPP, which allows us to realize a radial Hilbert transform for achieving a high contrast edge enhancement with high resolution.

Non-iterative numerical method for laterally superresolving Fourier domain optical coherence tomography

Abstract: A numerical deconvolution method to cancel lateral defocus in Fourier domain optical coherence tomography (FD-OCT) is presented. This method uses a depth-dependent lateral point spread function and some approximations to design a deconvolution filter for the cancellation of lateral defocus. Improved lateral resolutions are theoretically estimated; consequently, the effect of lateral superresolution in this method is derived. The superresolution is experimentally confirmed by a razor blade test, and an intuitive physical interpretation of this effect is presented. The razor blade test also confirms that this method enhances the signal-to-noise ratio of OCT. This method is applied to OCT images of medical samples, in vivo human anterior eye segments, and exhibits its potential to cancel the defocusing of practical OCT images. The validity and restrictions involved in each approximation employed to design the deconvolution filter are discussed. A chromatic and a two-dimensional extensions of this method are also described.

Points, Pixels, and Gray Levels: Digitizing Image Data

Abstract: Microscopical images are now almost always recorded digitally. To accomplish this, the flux of photons that forms the final image must be divided into small geometrical subunits called pixels. The light intensity in each pixel will be stored as a single number. Changing the objective magnification, the zoom magnification on your confocal control panel, or choosing another coupling tube magnification for your charge-coupled device (CCD) camera changes the size of the area on the object that is represented by one pixel. If you can arrange matters so that the smallest feature recorded in your image data is at least 4 to 5 pixels wide in each direction, then all is well.

Three-dimensional micro-PTV using deconvolution microscopy

Abstract: A three-dimensional micro-particle tracking velocimetry (micro-PTV) scheme is presented using a single camera with deconvolution microscopy. This method devises tracking of the line-of-sight (z) flow vectors by correlating the diffraction pattern ring size variations with the defocusing distances of small particle locations. The working principle is based on optical serial sectioning microscopy, or equivalently deconvolution microscopy, that records images of an infinitesimally small particle, and generates a point-spread function of the three-dimensional diffraction patterns. A new image-processing algorithm has also been developed to digitally identify the center locations and measure the radii of the diffraction rings, which allows simultaneous tracking of all three-vector components. The developed PTV technique uses a 40×, 0.75 NA dry objective lens with 500-nm fluorescent seeding particles of SG=1.05, and successfully measures the fully three-dimensional fields flowing over a spherical obstacle snuggly fitted inside a 100 μm × 100 μm micro-channel. The volumetric measurement resolution of the present system is equivalent to a 5.16 μm × 5.16 μm × 5.16 μm cube, and the overall measurement uncertainty for single-point velocity vector detection is estimated to ±7.58%.

Lensless coherent imaging by phase retrieval with an illumination pattern constraint

Abstract: It is often possible to reduce the requirements on an imaging system by placing greater demands either on an illumination system or on post-detection processing of the data collected by the system. An extreme example of this is a system with no receiver optics whatsoever. By illuminating an object or scene with coherent light having a shaped illumination pattern, the receiver can be a simple detector array with no imaging optics, detecting the speckle intensity pattern reflected from the object; an image of the object can be reconstructed by a phase retrieval algorithm.

Analysis of a collinear holographic storage system: introduction of pixel spread function

Abstract: Image formation in a collinear holographic storage system was analyzed. The wavefront from each pixel of a spatial light modulator was regarded as a plane wave in the recording medium, and its wave vector was determined by the position of the pixel. The hologram in the recording medium was treated as the summation of all gratings written by all combinations of two plane waves. The image of a data page was formed by diffraction of the reference waves by all gratings. The results of the simulation showed good agreement with experiment. We introduced the pixel spread function to describe the image formation characteristics. Analysis of the pixel spread function reveals that a radial-line pixel pattern for reference waves gave a sharper image than other reference pixel patterns. It is also shown that a random phase modulation applied to each reference pixel improved the image formation.

Identification of parameters and restoration of motion blurred images

Abstract: This paper proposes a technique for restoring the motion blurred images. Restoration of blurred images is very important problem in tracking and identification of criminals, where image of a human face or number plate of a running vehicle taken in hit and run situation gets blurred due to relative motion between the imaging system and object/face. For restoration of motion blurred images, knowledge of the point spread function (PSF) is very important. The motion blur PSF is characterized by two parameters, namely blur direction and blur length. This paper also presents a method to identify the parameters of the PSF from the blurred and noisy images using the log spectrum of the blurred images. These parameters are used to restore the images. The experimental results demonstrate that the image can successfully be restored from the image with substantial amount of natural and artificial blur.

3D particle tracking on a two-photon microscope.

Abstract: A 3D single-particle-tracking (SPT) system was developed based on two-photon excitation fluorescence microscopy that can track the motion of particles in three dimensions over a range of 100 mum and with a bandwidth up to 30 Hz. We have implemented two different techniques employing feedback control. The first technique scans a small volume around a particle to build up a volumetric image that is then used to determine the particle's position. The second technique scans only a single plane but utilizes optical aberrations that have been introduced into the optical system that break the axial symmetry of the point spread function and serve as an indicator of the particle's axial position. We verified the performance of the instrument by tracking particles in well-characterized models systems. We then studied the 3D viscoelastic mechanical response of 293 kidney cells using the techniques. Force was applied to the cells, by using a magnetic manipulator, onto the paramagnetic spheres attached to the cell via cellular integrin receptors. The deformation of the cytoskeleton was monitored by following the motion of nearby attached fluorescent polystyrene spheres. We showed that planar stress produces strain in all three dimensions, demonstrating that the 3D motion of the cell is required to fully model cellular mechanical responses.

Comparison of Widefield/Deconvolution and Confocal Microscopy for Three-Dimensional Imaging

Abstract: The biggest limitation inherent in optical microscopy is its lateral spatial resolution, which is determined by the wavelength of the light used and the numerical aperture (NA) of the objective lens. Another important limitation is the resolution in the direction of the optical axis, conventionally called z, which is related to the depth of field. The presence of a finite aperture gives rise to undesirable and rather complicated characteristics in the image. In essence, the depth of field depends on the size of structure or spatial frequency being imaged.

All-optical axial super resolving imaging using a low-frequency binary-phase mask.

Abstract: In this paper we present a new approach for obtaining all-optical axial super-resolving imaging by using a non-diffractive binary phase mask inserted at the entrance pupil of an imaging lens. The designed element is tested numerically and experimentally on various practical testing benches and eventually is inserted into the lens of a cellular phone camera.

IRACproc: a software suite for processing and analyzing Spitzer/IRAC data

Abstract: We have developed a post-Basic Calibrated Data pipeline processing software suite called "IRACproc". This package facilitates the co-addition of dithered or mapped Spitzer /IRAC data to make them ready for further analysis with application to a wide variety of IRAC observing programs. In acting as a wrapper for the Spitzer Science Center's MOPEX software, IRACproc improves the rejection of cosmic rays and other transients in the co-added data. In addition, IRACproc performs (optional) Point Spread Function (PSF) fitting, subtraction, and masking of saturated stars. The under/critically sampled IRAC PSFs are subject to large variations in shape between successive frames as a result of sub-pixel shifts from dithering or telescope jitter. IRACproc improves cosmic ray and other transient rejection by using spatial derivative images to map the locations and structure of astronomical sources. By protecting sources with a metric that accounts for these large variations in the PSFs, our technique maintains the structure and photometric reliability of the PSF, while at the same time removing transients at the lowest level. High Dynamic Range PSFs for each IRAC band were obtained by combining an unsaturated core, derived from stars in the IRAC PSF calibration project, with the wings of a number of bright stars. These PSFs have dynamic ranges of ~10 7 and cover the entire IRAC field of view. PSF subtraction can drastically reduce the light from a bright star outside the saturated region. For a bright star near the array center it is possible to detect faint sources as close as ~15-20" that would otherwise be lost in the glare. In addition, PSF fitting has been shown to provide photometry accurate to 1-2% for over-exposed stars.

Tutorial on Practical Confocal Microscopy and Use of the Confocal Test Specimen

Abstract: The other chapters in this book give the reader an in-depth description of every important aspect of biological confocal microscopy that we could think of. This chapter is to provide the novice user of this instrument with a basic understanding of the practical information needed to use it effectively. Because the computer interfaces of the various commercial instruments vary greatly, this chapter will stress the important features of microscopical optics and the basics of sampling that are common to all instruments.

Modeling ultrasound imaging as a linear, shift-variant system

Abstract: Wo solve the equation that governs acoustic wave propagation in an inhomogeneous medium to show that the radio-frequency (RF) ultrasound signal can he expressed as the result of filtering the scatterer field with a point-spread function. We extend the analysis to make the link between the RF ultrasound signal and the representation of ultrasound scatterers as vectors with small magnitude and random phase in the complex plane. Others have previously performed parts of this analysis. The contribution of the present paper is to provide a single, coherent treatment emphasizing the assumptions that have to be made and the physical consequences of the models derived. This leads to insights into the interaction of monopole and dipole scattering, useful techniques for simulating and analyzing speckle statistics in the complex plane and a new expression for the normalized covariance of the analytic RF ultrasound signal in terms of the complex envelope of the point-spread function

Evaluating the lateral resolution of the adaptive optics scanning laser ophthalmoscope

Abstract: We present a statistical assessment of the lateral resolution of the adaptive optics scanning laser ophthalmoscope AOSLO .W e adopt a 2-D Gaussian function to approximate the AOSLO point spread function PSF, which is dominated by the residual wavefront aberration and characterized by the Strehl ratio. Thus, we derive the lateral resolution in the presence of residual wave aberrations, which is inversely proportional to square root of the Strehl ratio. The mod- eling, while not sufficient in describing the fine structure of the real PSF, demonstrates good conformance to the lateral cross section of the real PSF. With this model, the lateral resolution of our current AOSLO was computed to be 1.65 to 2.33 m, which agreed well with the actual result. We also reveal the relationships among the lateral reso- lution and other three measures of the AOSLO imaging property in- cluding the Strehl ratio, the PSF, and the root mean square rms of wavefront aberration. © 2006 Society of Photo-Optical Instrumentation Engineers. DOI: 10.1117/1.2166434

Resolution of coherent and incoherent imaging systems reconsidered - Classical criteria and a statistical alternative

Abstract: The resolution of coherent and incoherent imaging systems is usually evaluated in terms of classical resolution criteria, such as Rayleigh’s. Based on these criteria, incoherent imaging is generally concluded to be ‘better’ than coherent imaging. However, this paper reveals some misconceptions in the application of the classical criteria, which may lead to wrong conclusions. Furthermore, it is shown that classical resolution criteria are no longer appropriate if images are interpreted quantitatively instead of qualitatively. Then one needs an alternative criterion to compare coherent and incoherent imaging systems objectively. Such a criterion, which relates resolution to statistical measurement precision, is proposed in this paper. It is applied in the field of electron microscopy, where the question whether coherent high resolution transmission electron microscopy (HRTEM) or incoherent annular dark field scanning transmission electron microscopy (ADF STEM) is preferable has been an issue of considerable debate.

The anisoplanatic point-spread function in adaptive optics

Abstract: The effects of anisoplanatism on the adaptive optics point‐spread function are investigated. A model is derived that combines observations of the guide star with an analytic formulation of anisoplanatism in order to generate predictions for the adaptive optics point‐spread function at arbitrary locations within the field of view. The analytic formulation captures the dependencies of anisoplanatism on aperture diameter, observing wavelength, angular offset, zenith angle, and turbulence profile. The predictions of this model are compared to narrowband 2.12 and 1.65 μm images of a 21" binary (m_v = 7.3, 7.6) acquired with the Palomar adaptive optics system on the 5 m Hale Telescope. Contemporaneous measurements of the turbulence profile made with a DIMM/MASS (differential image motion monitor/multiaperture scintillation sensor) unit are used together with images of the primary to predict the point‐spread function of the binary companion. Predicted companion Strehl ratios are shown to match measurements to within a few percent, whereas predictions based on the isoplanatic angle approximation are highly discrepant. The predicted companion point‐spread functions are shown to agree with observations to 10%. These predictions are used to measure the differential photometry between binary members to an accuracy of 1 part in 10^3, and the differential astrometry to an accuracy of 1 mas. Errors in the differential astrometry are shown to be dominated by differential atmospheric tilt jitter. These results are compared to other techniques that have been employed for photometry, astrometry, and high‐contrast imaging.

Experimental validation of a simple model capable of predicting the phase contrast imaging capabilities of any x-ray imaging system.

Abstract: Phase contrast (PC) imaging is one of the most exciting emerging x-ray imaging techniques, with the potential of removing some of the main limitations of conventional radiology. After extensive experimentation carried out particularly at synchrotron radiation (SR) facilities, the scientific community agrees that it is now time to translate these ideas towards the first clinical implementations. In this framework, a complete model, based on Fresnel/Kirchoff diffraction integrals, was devised. This model accounts for source dimensions, beam spectrum and divergence and detector point spread function (PSF), and can thus be applied to any x-ray imaging system. In particular, by accepting in input the above parameters along with the ones describing the sample, the model can be used to optimize the geometry of the set-up, i.e. to assess the source-to-sample and sample-to-detector distances which maximize feature detection. The model was evaluated by acquiring a range of images of different samples with a laboratory source, and a good agreement was found between simulated and experimental data in all cases. In order to maximize the generality of the results, all acquisitions were carried out using a polychromatic source and an energy-resolving detector; in this way, a range of monochromatic images could be obtained as well as polychromatic images, which can be created by integrating different parts of the acquired spectra. One of the most notable results obtained is that in many practical cases polychromatic PC imaging can provide the same image quality as its monochromatic counterpart. This is an important step in the wider application of PC using conventional sources.

Statistical and Information-Theoretic Analysis of Resolution in Imaging

Abstract: In this paper, some detection-theoretic, estimation-theoretic, and information-theoretic methods are investigated to analyze the problem of determining resolution limits in imaging systems. The canonical problem of interest is formulated based on a model of the blurred image of two closely spaced point sources of unknown brightness. To quantify a measure of resolution in statistical terms, the following question is addressed: "What is the minimum detectable separation between two point sources at a given signal-to-noise ratio (SNR), and for prespecified probabilities of detection and false alarm (Pd and Pf )?". Furthermore, asymptotic performance analysis for the estimation of the unknown parameters is carried out using the Crameacuter-Rao bound. Although similar approaches to this problem (for one-dimensional (1-D) and oversampled signals) have been presented in the past, the analyzes presented in this paper are carried out for the general two-dimensional (2-D) model and general sampling scheme. In particular the case of under-Nyquist (aliased) images is studied. Furthermore, the Kullback-Liebler distance is derived to further confirm the earlier results and to establish a link between the detection-theoretic approach and Fisher information. To study the effects of variation in point spread function (PSF) and model mismatch, a perturbation analysis of the detection problem is presented as well

Objective assessment of image quality. IV. Application to adaptive optics.

Abstract: The methodology of objective assessment, which defines image quality in terms of the performance of specific observers on specific tasks of interest, is extended to temporal sequences of images with random point spread functions and applied to adaptive imaging in astronomy. The tasks considered include both detection and estimation, and the observers are the optimal linear discriminant (Hotelling observer) and the optimal linear estimator (Wiener). A general theory of first- and second-order spatiotemporal statistics in adaptive optics is developed. It is shown that the covariance matrix can be rigorously decomposed into three terms representing the effect of measurement noise, random point spread function, and random nature of the astronomical scene. Figures of merit are developed, and computational methods are discussed.

Motion Blur Identification Based on Differently Exposed Images

Abstract: In this paper we introduce a new method of motion blur identification that relies on the availability of two, differently exposed, image shots of the same scene. The proposed approach exploits the difference in the degradation models of the two images in order to identify the point spread function (PSF) corresponding to the motion blur, that may affect the longer exposed image shot. The algorithm is demonstrated through a series of experiments that reveal its ability to identify the motion blur PSF even in the presence of heavy degradations of the two observed images.