Showing papers on "Point spread function published in 1993"

Orthogonal-plane Fluorescence Optical Sectioning: Three-Dimensional Imaging of Macroscopic Biological Specimens

Abstract: An imaging technique called orthogonal-plane fluorescence optical sectioning (OPFOS) was developed to image the internal architecture of the cochlea. Expressions for the three-dimensional point spread function and the axial and lateral resolution are derived. Methodologies for tissue preparation and for construction, alignment, calibration and characterization of an OPFOS apparatus are presented. The instrument described produced focused, high-resolution images of optical sections of an intact, excised guinea-pig cochlea. The lateral and axial resolutions of the images were 10 and 26 microns, respectively, within a 1.5-mm field of view.

Image recovery from data acquired with a charge-coupled-device camera

Abstract: A model for data acquired with the use of a charge-coupled-device camera is given and is then used for developing a new iterative method for restoring intensities of objects observed with such a camera. The model includes the effects of point spread, photoconversion noise, readout noise, nonuniform flat-field response, nonuniform spectral response, and extraneous charge carriers resulting from bias, dark current, and both internal and external background radiation. An iterative algorithm is identified that produces a sequence of estimates converging toward a constrained maximum-likelihood estimate of the intensity distribution of an imaged object. An example is given for restoring images from data acquired with the use of the Hubble Space Telescope.

Hubble Space Telescope characterized by using phase-retrieval algorithms

Abstract: We describe several results characterizing the Hubble Space Telescope from measured point spread functions by using phase-retrieval algorithms. The Cramer-Rao lower bounds show that point spread functions taken well out of focus result in smaller errors when aberrations are estimated and that, for those images, photon noise is not a limiting factor. Reconstruction experiments with both simulated and real data show that the calculation of wave-front propagation by the retrieval algorithms must be performed with a multiple-plane propagation rather than a simple fast Fourier transform to ensure the high accuracy required. Pupil reconstruction was performed and indicates a misalignment of the optical axis of a camera relay telescope relative to the main telescope. After we accounted for measured spherical aberration in the relay telescope, our estimate of the conic constant of the primary mirror of the HST was - 1.0144.

Restoration of Astronomical Images by Iterative Blind Deconvolution

Abstract: We have developed a modified version of the Iterative Blind Deconvolution (IBD) algorithm of Lane, applicable to different types of astronomical data. Besides using positivity, convolution, and support constraints, we have also applied band-limit, multiple image, and Fourier modulus constraints. By using all the available image constraining information, we are able to successfully recover object and point spread function information from noisy data. The algorithm's performance under controlled conditions using simulated data for a binary source object, a compact multiple quasi-point source object, and an extended object with low contrast are demonstrated

Object shape dependent PSF model for SPECT imaging

Abstract: An analytical expression for the point spread function (PSF) and the line spread function (LSF) of a parallel hole gamma camera is presented, for homogeneous media and for photons having mainly Compton interactions in the object. The PSF of scattered photons is described by convolving a zeroth-order modified Bessel function of the second kind with the unscattered PSF, which is approximated by a Gaussian. The complete PSF (scatter plus nonscatter) depends on the source distance (z) and depth (d) of the source in the object. For convex-shaped emitting objects, the dependence of the PSF on the object contour can be incorporated by a simple correction. Thus, a complete mathematical model of the imaging of an activity distribution in a homogeneous medium is obtained. The model has been evaluated for /sup 99m/Technetium line sources by using a LEAP collimator at various energy windows ( Delta E). It is shown that the model is valid to a high accuracy at Delta E=15%, for a large range of values of z and d. >

Blur identification by residual spectral matching

Abstract: The estimation of the point spread function (PSF) for blur identification, often a necessary first step in the restoration of real images, method is presented. The PSF estimate is chosen from a collection of candidate PSFs, which may be constructed using a parametric model or from experimental measurements. The PSF estimate is selected to provide the best match between the restoration residual power spectrum and its expected value, derived under the assumption that the candidate PSF is equal to the true PSF. Several distance measures were studied to determine which one provides the best match. The a priori knowledge required is the noise variance and the original image spectrum. The estimation of these statistics is discussed, and the sensitivity of the method to the estimates is examined analytically and by simulations. The method successfully identified blurs in both synthetically and optically blurred images. >

Imaging by a high aperture optical system

Abstract: Three-dimensional imaging in telecentric systems of non-unity magnification is considered using a scalar approximation. It is found that the imaging performance can be very different according to whether the image is formed directly in the image space or by scanning in the object space. It is shown that this result does not violate the principle of reciprocity. The point spread function is investigated for systems obeying, amongst others, either the sine or Herschel conditions. The results have great importance for scanning systems, in particular confocal microscopes, and explain some anomalies in the literature. A clarification of the principle of equivalence is given.

Infrared imaging of the host galaxies of radio-loud and radio-quiet quasars

Abstract: We present initial results from the first deep near-infrared imaging study of the host galaxies of an extensive sample of low-redshift (z<0.4) radio-loud and radio-quiet quasars. To minimize selection effects, the radio-loud and radio-quiet subsamples have been selected to be indistinguishable in terms of their distributions on the V-z plane, and classification as radio-loud or radio-quiet is defined in terms of 5-GHz luminosity. We discuss the problems associated with accurate determination of the infrared point spread function (PSF) and our adopted solution of an a posteriori selection from a library of observationally determined PSFs. Images reaching surface brightness levels of μ K =22 mag arcsec −2 of the quasar hosts, before and after subtraction of the nuclear component, are then presented

Three-dimensional image reconstruction by digital tomo-synthesis using inverse filtering

Abstract: Conventional X-ray tomosynthesis with film can provide a sagittal slice image with a single scan. This technique has the advantage of enabling reconstruction of a sagittal slice which is difficult to obtain from the X-ray CT system. However, only an image on the focal plane is obtained by a single scan. Furthermore, the image is degraded by superimpositions of the structures outside of the focal plane. A new three-dimensional image reconstruction method is proposed. This method utilizes a three-dimensional convolution process with an inverse filter function which is derived analytically by the point spread function of the projection and backprojection geometry. A digital tomosynthesis system has also been constructed for the purpose of evaluating the proposed method. This system was used in phantom experiments and clinical evaluations, and it was confirmed that the proposed method was able to reconstruct a better three-dimensional image with less artifacts from outside of the focused slice. >

Simulation of fan-beam-type optical computed-tomography imaging of strongly scattering and weakly absorbing media.

Abstract: Numerical simulations of the transmission of a light impulse through strongly scattering and weakly absorbing slab media and of fan-beam-type optical computed-tomography imaging for cylindrical media are presented. A hybrid calculation scheme of scattering by the Monte Carlo method is employed to obtain the temporal variation of transmittance of the light impulse through the media. A set of projection data is provided by temporally extrapolating the difference in the optical density between the absorbing object and the nonabsorbing reference to the shortest time of flight. For the case of identical scattering between the object and reference, the reconstructed image of the difference in the absorption coefficient has better accuracy and spatial resolution than those images by the time-gating method.

Image data coding and compression system utilizing controlled blurring

Abstract: Data representing the image of an original picture having high resolution are first acquired. The data are then processed in accordance with a blurring function, which can be a randomly distributed point spread function. Processing can be carried out either in object space or in spatial-frequency space. In object space a multi-dimensional convolution is required. In the spatial frequency domain, processing is performed by a pointwise product of data representing the Fourier transform of the original image and the Fourier transform of the blurring function. The resulting blurred image is thereby coded such that the original scene is no longer discernible, but can be recovered provided the blurring function is known. Additionally, this blurred image can be shown to contain the detail of the original but can be stored or transmitted as data which are compressed by compression ratios which can range from 2 to 100. The compressed data can be restored to provide an image with detail like the original by inverse filtering using data representing the Fourier transform of the blurring function.

Imaging through the atmosphere: practical instrumentation-based theory and verification of aerosol modulation transfer function

Abstract: Recent experimental measurements of overall atmospheric modulation transfer function (MTF) indicate a significant difference between overall atmospheric and turbulence MTF’s except often at midday, when turbulence is strong. We suggest a physical explanation for these results that essentially relates to a practical instrumentation-based atmospheric aerosol MTF that is a modification of the classical aerosol MTF theory. Based on radiative transfer theory, this practical approach takes into account the effect of finite field of view, finite dynamic sensitivity, and finite spatial bandwidth of every existing imaging system. These generally limit the scattering angles of received light to values far less than the diffraction limit for aerosols, thereby decreasing blur radius and increasing spatial frequency bandwidth. This can explain the broadening of the aerosol MTF from that theoretically expected. We discuss the asymptote value that the measured aerosol MTF approaches at high spatial frequencies, which is significantly higher than the theoretical prediction of turbid medium transmittance. An important conclusion that we derive is that the aerosol MTF is often the dominant part in the actual overall atmospheric MTF. In addition, there seems to be an inescapable trade-off between image resolution and image irradiance. The system designer must choose between imaging of faint and bright objects at the expense of image quality or imaging of either faint or bright objects with improved image quality. The concepts here are basic to all long-range imaging through the atmosphere.

Simultaneous Acquisition of Spectral and Spatial Intensity Distribution

Abstract: A method for recording and reconstructing spectral and spatial intensity distributions of sources is described. Three-dimensional information (two for spatial and one for wavelength) of the sources is simultaneously recorded as their projected forms by an imaging system including a crossed phase grating. A modified multiplicative algebraic technique is developed to reconstruct the original distribution. In experiments we obtained nine projection patterns with equal brightness of a color slide illuminated by a tungsten lamp. The reconstructed distribution shows good agreement with that measured by interference filters.

On the accuracy of PSF representation in image restoration

Abstract: Point spread function (PSF) models derived from physical optics provide a more accurate representation of real blurs than simpler models based on geometrical optics. However, the physical PSF models do not always result in a significantly better restoration, due to the coarse sampling of the recording device and insufficiently high signal-to-noise ratio (SNR) levels. Low recording resolutions result in aliasing errors in the PSF and suboptimal restorations. A high-resolution representation of the PSF where aliasing errors are minimized is used to obtain improved restorations. The SNR is the parameter which ultimately limits the restoration quality and determines the need for an accurate PSF model. As a rule of thumb, the geometrical PSF can be used in place of the physical PSF without significant loss in restoration quality when the SNR is less than 30 dB. >

Three-Dimensional Digital Confocal Raman Microscopy

Abstract: We describe an iterative image restoration technique which functions as digital confocal microscopy for Raman images. We deconvolute the lateral and axial components of the microscope point spread function from a series of optical sections, to generate a stack of well-resolved Raman images which describe the three-dimensional topology of a sample. The technique provides an alternative to confocal microscopy for three-dimensional microscopic Raman imaging.

A general bayesian image reconstruction algorithm with entropy prior. preliminary application to hst data

Abstract: This paper describes a general Bayesian iterative algorithm with entropy prior for image reconstruction. It solves the cases of both pure Poisson data and Poisson data with Gaussian readout noise. The algorithm maintains positivity of the solution; it includes case-specific prior information (default map) and flatfield corrections; it removes background and can be acclerated to be faster than the Richardson-Lucy algorithm. In order to determine the hyperparameter that balances the entropy and likelihood terms in the Bayesian approach, we have used a likeliehood cross-validation technique. Cross-validation is more robust than other methods because it is less demanding in terms of the knowledge of exact data characteristics and of the point spread function. We have used the algorithm to reconstruct successfully images obtained in different space and ground based imaging situations. It has been possible to recover most of the original intended capabilities of the Hubble Space Telescope Wide Field and Planetary Camera and Faint Object Camera from images obtained in their present state. Semi-real situations for the future Wide Field Planetary Camera 2 show that even after the repair of the spherical aberration problem, image reconstruction can play a key role in improving the resolution of the cameras, well beyond the design of the Hubble instruments. We also show that ground based images can be reconstructed successfully with the algorithm. A technique which consists of dividing the CCD observations into two frames, with one half the exposure time each, emerges as a recommended procedure for the utilization of the described algorithms. We have compared our technique with two commonly used reconstruction algorithms: the Richardson-Lucy and the Cambridge Maximum Entropy algorithms.

Real-time two-dimensional holographic imaging by using an electron-beam-addressed spatial light modulator

Abstract: A real-time electron-beam-addressed spatial light modulator-based holographic imaging system has recently been proposed. We present results of two-dimensional holographic imaging and further demonstrate that the proposed system can be readily adaptable to automation by using digital computers for robust holographic imaging. Specifically, by nonlinear processing of a digitally stored hologram, fringe contrast enhancement has been achieved.

Effect of system point spread function, apparent size, and detector instantaneous field of view on the infrared image contrast of small objects

Abstract: Prediction of the performance of IR sensor systems in the detection and classification of features in an image requires estimation of the sensor signals produced by a given feature (source) and the background in the image. This process includes estimation of the effects that the detector, the scanning system (if used), and the optical system have on the performance of the system in a specified environment. The analysis of sensor performance takes into account the emission characteristics of the source and background and often assumes an extended source. For some tasks, the apparent size of a source, as well as the emission characteristics, must be taken into account in determining the scene contrast produced by the source. This requirement to account for the apparent size of a source arises as a result of the reduction in image contrast of a source of small apparent size due to optical blurring caused by the formation of the image. An analysis of the effect of apparent source size, the detector instantaneous field of view, and optical blurring on the image contrast produced by imaging IR sensor systems is presented.

A technique of scatter-glare correction using a digital filtration.

Abstract: A scatter‐glare correction technique for x‐ray images acquired with an antiscatter grid was developed In the technique, the scatter‐glare image was estimated from exposure conditions and subtracted from the acquired image The basic procedure in the estimation of the scatter‐glare image is convolution filtering; however, the novel aspects of the technique are as follows: (1) To estimate the scatter‐glare intensity, a formula that does not include the term of object thickness was used With this formula, the correction can be performed, even for nonuniform phantoms; (2) To estimate the scatter‐glare distribution, the experimental scatter‐glare point spread function (PSF) was directly used as a convolution kernel Although the shape of the PSF changed slightly for water thicknesses of 5–25 cm, we applied the PSF measured at a water thickness of 15 cm to the correction experiments For the stepped water phantom (10–20 cm), scatter‐glare estimation produced an average error of 10%, with respect to the lead bar data Furthermore, the improvement of image quality and quantitative accuracy resulting from the correction was examined

A bar code location algorithm using a two-dimensional approach

Abstract: Proposes a method for locating bar code blocks on parcels. One of the main difficulties involved is the combination of the very poor quality and the potential high complexity of the gray-level images. The image resolution has been chosen to be as low as possible in order to enlarge the field of view for a given number of CCD elements. Furthermore the focus of the camera has previously needed to be kept constant in spite of the variation in height of the objects resulting in the presence of high convolution distortion such that the width of the point spread function can be much larger than the width of a single stripe. The approach relies on the extraction of areas having a high density of mono-oriented gradient. This bar code finder algorithm works correctly for all images of a base provided by the French postal services. >

X-ray evidence for particulate contamination on the AXAF VETA-1 mirrors

Abstract: X-ray testing of the AXAF outer mirror pairs, in the VETA-1 configuration, reveals a point spread function (PSF) with unexpectedly large wings at low energies. Although the angular dependence in the wings of the PSF is close to that expected for diffractive scattering from surface roughness, the energy dependence differs substantially. Analyses of the observed X-ray PSF, images near ring focus, and single-quadrant images at conjugate focus suggest that the excess scattering observed at low X-ray energies results from diffractive scattering by relatively small grains (as small as a few tenths micrometer in radius). We develop a simple model for the contribution of scattering by particulates to the PSF. Merging this model with that for scattering by surface roughness, we fit the combined model to the observed energy-dependent PSF, in order to estimate parameters and associated uncertainties characterizing the grain-size distribution and the surface-roughness power spectral density. In particular, we find that the fractional coverage of the mirrors by particulates is approximately 1 x 10 exp -4 (for grain radii between 0.1 and 10 microns), and that the rms surface-roughness is approximately 0.7 nm (for spatial frequencies between 1/mm and 1000/mm).

Condenser optics, partial coherence, and imaging for soft-x-ray projection lithography

Abstract: A condenser system couples the radiation source to an imaging system, controlling the uniformity and partial coherence at the object, which ultimately affects the characteristics of the aerial image. A soft-x-ray projection lithography system based on a ring-field imaging system and a laser-produced plasma x-ray source places considerable constraints on the design of a condenser system. Two designs are proposed, critical illumination and Kohler illumination, each of which requires three mirrors and scanning for covering the entire ring field with the required uniformity and partial coherence. Images based on Hopkins' formulation of partially coherent imaging are simulated.

Optoelectronic morphological image processor.

Abstract: A morphological optoelectronic image processor based on the threshold decomposition concept is described and demonstrated. Binary slices of a gray-scale input image are optically convolved with a binary structuring element of arbitrary size and shape in a noncoherent convolver. The slices are displayed on a liquid-crystal spatial light modulator of 320 × 264 pixels. The kernels are implemented as modifications of the system impulse response. The processor’s convolution patterns are recorded with a CCD camera and fed into a PC by a frame grabber. Subsequent elementary morphological operations are looped. Examples of processing an input image of 256 × 256 pixels and 16 gray levels with kernels of arbitrary size are presented.

Restoration of differently blurred versions of an image with measurement errors in the PSF's

Abstract: Restoration of an object from T observations is considered. Each image is distorted by a different deterministic blur and additive noise. The point spread function (PSF) for each observation is unknown; however, a noisy measurement of it is available. Taking the errors in measurements of the PSFs into consideration, the maximum-likelihood and Wiener filters are derived. It is shown that these filters give better results when the regression filter and the conventional Wiener filter, i.e., the one which ignores the presence of the noise in the PSFs. The consistency and the ill-conditioning characteristics of the filters are discussed. Regularized forms for these filers are obtained. >

Superresolving masks for incoherent scanning microscopy

Abstract: An approach to achieve superresolution in confocal scanning microscopy by using the singular-system approach to the inverse problem was recently proposed. It consists of using a specially designed mask that performs the task of data inversion by means of all-optical processing. We discuss an approximate binary form of such a mask that permits its practical manufacture for use in incoherent confocal microscopy. The performance characteristics of such an approximate mask are compared with those of an exact mask and with those of a conventional confocal scanning microscope. Although the resolution of the approximate microscope is slightly inferior to the exact one, they are both still advances over the conventional confocal one (the improvement of resolution being 65% and 70%, respectively).

Improved estimation of fraction images using partial image restoration

Abstract: It is noted that mixed pixels are unavoidable in remote sensing imagery, but signature mixing models can be effective in improving the usefulness of the image data. Unmixing error caused by sensor system point spread function (PSF) degradation is studied, and the application of image restoration before spectral unmixing to improve the unmixing results is investigated. It is shown that the characteristics of the problem makes the design of a restoration filter differ from that for visual enhancement. Simulation results show that, depending on the scene complexity, unmixing error can be reduced by up to 72% using the proposed restoration scheme. It is concluded that image restoration is useful in increasing the accuracy of spectral unmixing. >

Superresolving scanning optical microscopy using holographic optical processing

Abstract: Two novel superrresolving scanning microscopes, one of which uses coherent imaging and the other incoherent imaging, are described. The optical arrangement used in the coherent microscope is similar to that in a scanning confocal microscope with the detector pinhole replaced by a special holographic mask, a Fourier lens, and a pinhole. The incoherent design uses two intensity-transmittance masks, two integrating detectors, and an electronic subtractor. The design of the microscopes is based on the results of singular-system theory, and the mask forms are calculated by means of this analysis. These arrangements obviate the need for an array of detectors to implement singular-system processing, and in the coherent case direct phase measurement is no longer required. Experimental results are presented that demonstrate a significant resolution improvement for a one-dimensional low-numerical-aperture coherent microscope.