Showing papers on "Point spread function published in 1995"

Proposed method for molecular optical imaging

Abstract: We can resolve multiple discrete features within a focal region of m spatial dimensions by first isolating each on the basis of n ≥ 1 unique optical characteristics and then measuring their relative spatial coordinates. The minimum acceptable separation between features depends on the point-spread function in the (m + n)-dimensional space formed by the spatial coordinates and the optical parameters, whereas the absolute spatial resolution is determined by the accuracy to which the coordinates can be measured. Estimates of each suggest that near-field fluorescence excitation microscopy/spectroscopy with molecular sensitivity and spatial resolution is possible.

Computed-tomography imaging spectrometer: experimental calibration and reconstruction results

Abstract: A temporally and spatially nonscanning imaging spectrometer is described in terms of computedtomography concepts, specifically the central-slice theorem. A sequence of three transmission sinusoidalphase gratings rotated in 60° increments achieves dispersion in multiple directions and into multiple orders. The dispersed images of the system's field stop are interpreted as two-dimensional projections of a three-dimensional (x, y, λ) object cube. Because of the size of the finite focal-plane array, this imaging spectrometer is an example of a limited-view-angle tomographic system. The imaging spectrometer's point spread function is measured experimentally as a function of wavelength and position in the field of view. Reconstruction of the object cube is then achieved through the maximum-likelihood, expectation-maximization algorithm under the assumption of a Poisson likelihood law. Experimental results indicate that the instrument performs well in the case of broadband and narrow-band emitters.

Focused image recovery from two defocused images recorded with different camera settings

Abstract: Two new methods are presented for recovering the focused image of an object from only two blurred images recorded with different camera parameter settings. The camera parameters include lens position, focal length, and aperture diameter. First a blur parameter 0 is estimated using one of our two recently proposed depth-from-defocus methods. Then one of the two blurred images is deconvolved to recover the focused image. The first method is based on a recently proposed spatial domain convolutioddeconvolution transform. This method requires only the knowledge of ~7 of the camera’s point spread function (PSF). It does not require information about the actual form of the camera’s PSF. The second method, in contrast to the first, requires full knowledge of the form of the PSF. As part of the second method, we present a calibration procedure for estimating the camera’s PSF for different values of the blur parameter 0. In the second method, the focused image is obtained through deconvolution in the Fourier domain using the Wiener filter. For both methods, results of experiments on actual defocused images recorded by a CCD camera are given. The first method requires much less computation than the second method. The first method gives satisfactory results for up to medium levels of blur and the second method gives good results for up to relatively high levels of blur.

Monte Carlo simulation of the atmospheric point-spread function with an application to correction for the adjacency effect.

Abstract: Monte Carlo techniques are used to simulate atmospheric point-spread functions (PSF’s) that are appropriate for the viewing geometries typical of the Airborne Visible–Infrared Imaging Spectrometer (AVIRIS). A model sensor is located at an altitude of 20 km and views a Lambertian surface through a horizontally homogeneous and vertically stratified atmosphere. Simulations show the effects on the PSF of variation of the aerosol phase function, the aerosol optical thickness, the sensor viewing angle, and the wavelength. An algorithm that uses the PSF to correct high-contrast images for adjacency effects is developed and applied to an AVIRIS image of Big Pine Key in the Florida Keys. A method to approximate the atmospheric PSF’s without the need to resort to a Monte Carlo simulation is described. Correction of the AVIRIS image through the use of the approximated PSF is consistent with a previous correction. Error analysis is difficult and scene dependent; however, the correction algorithm is shown to be capable of indicating regions of high-contrast images in which conventional estimates of surface-leaving radiance are likely to be unreliable due to adjacency effects.

Comparison of three-dimensional imaging properties between two-photon and single-photon fluorescence microscopy

Abstract: Summary The imaging performance in single-photon (l-p) and two­ photon (2-p) fluorescence microscopy is described. Both confocal and conventional systems are compared in terms of.the three-dimensional (3-D) point spread function and the 3-D optical transfer function. Images of fluorescent sharp edges and layers are modelled •. giving resolution in transverse and axial directions. A comparison of the imaging properties is also given for a 4Pi confocal system. Confocal 2-p 4Pi fluorescence microscopy gives the best axial resolution in the sense that its 3-D optical transfer function has the strongest response along the axial direction.

Comparison of Wide-Field/Deconvolution and Confocal Microscopy for 3D Imaging

Abstract: Optical microscopy has always been a central technique to biological research, but in recent years its importance has vastly increased, mainly because of the introduction of epifluorescence imaging, which gives very sensitive detection, coupled with a multitude of highly specific fluorescent probes. Furthermore, it is often possible to obtain useful images noninvasively at light levels that are not damaging to living cells. Microinjection and other cell-loading methods can therefore be used in combination with fluorescence microscopy to analyze and modify subcellular structure and function in living cells.

Performances of endoscopic holography with a multicore optical fiber

Abstract: A holographic setup that involves the use of a multicore optical fiber as an in situ recording medium has been developed. The hologram is transmitted to a CCD camera for electronic processing, and the image is reconstructed numerically, providing more flexibility to the holographic process. The performances of this imaging system have been evaluated in terms of the resolution limit and robustness relative to noise. The experimental cutoff frequency has been measured experimentally over a range of observation distances (4–10 mm) and presents a very good agreement with the predictions made by simulation. The system features a resolution of 5-μm objects for a 4-mm observation distance. The different sources of noise have been analyzed, and their influence on resolution has been proved to be nonrelevant.

Fast SPECT simulation including object shape dependent scatter

Abstract: A fast simulator of SPECT projection data taking into account attenuation, distance dependent detector response, and scatter has been developed, based on an analytical point spread function model. The parameters of the scatter response are obtained from a single line source measurement with a triangular phantom. The simulator is able to include effects of object curvature on the scatter response to a high accuracy. The simulator has been evaluated for homogeneous media by measurements of /sup 99m/Tc point sources placed at different locations in a water-filled cylinder at energy windows of 15% and 20%. The asymmetrical shapes of measured projections of point sources are In excellent agreement with simulations for both energy windows. Scatter-to-primary ratio (SPR) calculations of point sources at different positions in a cylindrical phantom differ not more than a few percent from measurements. The simulator uses just a few megabytes of memory for storing the tables representing the forward model; furthermore, simulation of 60 SPECT projections from a three-dimensional digital brain phantom with 6-mm cubic voxels takes only ten minutes on a standard workstation. Therefore, the simulator could serve as a projector in iterative true 3-D SPECT reconstruction. >

Experimental examination of the quantitative imaging properties of optical diffraction tomography

Abstract: Optical diffraction tomography (ODT) is examined experimentally by reconstruction of a cross section of a transparent, cylindrical object with known geometry and refractive index. The object is immersed in three liquids of different refractive index to produce varying degrees of weak scattering. In each case the reconstructed size, shape, and refractive index are in good agreement with the known characteristics. This shows that quantitative images of transparent, cylindrical objects can be obtained by ODT. The images obtained from experimental data were also shown to be in good agreement with those obtained from computer-simulated data, thus verifying our computer simulations.

Blind deconvolution of fluorescence micrographs by maximum-likelihood estimation

Abstract: We report some recent algorithmic refinements and the resulting simulated and real image reconstructions of fluorescence micrographs by using a blind-deconvolution algorithm based on maximum-likelihood estimation. Blind-deconvolution methods encompass those that do not require either calibrated or theoretical predetermination of the point-spread function (PSF). Instead, a blind deconvolution reconstructs the PSF concurrently with deblurring of the image data. Two-dimensional computer simulations give some definitive evidence of the integrity of the reconstructions of both the fluorescence concentration and the PSF. A reconstructed image and a reconstructed PSF from a two-dimensional fluorescent data set show that the blind version of the algorithm produces images that are comparable with those previously produced by a precursory nonblind version of the algorithm. They furthermore show a remarkable similarity, albeit not perfectly identical, with a PSF measurement taken for the same data set, provided by Agard and colleagues. A reconstructed image of a three-dimensional confocal data set shows a substantial axial smear removal. There is currently an existing trade-off in using the blind deconvolution in that it converges at a slightly slower rate than the nonblind approach. Future research, of course, will address this present limitation.

Monte Carlo modeling of underwater-image formation: validity of the linear and small-angle approximations

Abstract: A Monte Carlo model has been used to compute a set of point-spread functions (PSF's) and modulation transfer functions (MTF's) that determine underwater-image quality in a range of different environments. The results have been used to analyze the range of application under which a linear-approximation theory holds. Conclusions of the study are that the linear-approximation theory seems to hold quite well over a broad range of applications. The ramifications of the Wells small-angle-scattering theory that predicts the PSF from a knowledge of the volume-scattering function (VSF) are also considered.Discrepancies are noted between a predicted and a computationally obtained MTF; these discrepancies increase with range. Therefore, the results of the simulations indicate that the small-angle-scattering theory is more valid at a limited number of attenuation lengths. The results of the simulations indicate that the theory is valid to approximately three attenuation lengths.

The three-dimensional point spread functions of a microscope objective in image and object space.

Abstract: SUMMARY The three-dimensional point spread function (3-D PSF) of an optical system in image space is distinguished from the 3-D PSF in object space and the relation between the two 3-D PSFs is derived. By using this relation one 3-D PSF can be easily obtained from the other. The 3-D PSFs are given in a single integral expression, which can be computed numerically. The results of this study can be used in 3-D image processing for microscopy and have been applied to the analysis of the diffusion of fluorescent molecules in a 3-D porous medium.

Faint Object Camera imaging and spectroscopy of NGC 4151

Abstract: We describe ultraviolet and optical imaging and spectroscopy within the central few arcseconds of the Seyfert galaxy NGC 4151, obtained with the Faint Object Camera on the Hubble Space Telescope. A narrowband image including (O III) lambda(5007) shows a bright nucleus centered on a complex biconical structure having apparent opening angle approximately 65 deg and axis at a position angle along 65 deg-245 deg; images in bands including Lyman-alpha and C IV lambda(1550) and in the optical continuum near 5500 A, show only the bright nucleus. In an off-nuclear optical long-slit spectrum we find a high and a low radial velocity component within the narrow emission lines. We identify the low-velocity component with the bright, extended, knotty structure within the cones, and the high-velocity component with more confined diffuse emission. Also present are strong continuum emission and broad Balmer emission line components, which we attribute to the extended point spread function arising from the intense nuclear emission. Adopting the geometry pointed out by Pedlar et al. (1993) to explain the observed misalignment of the radio jets and the main optical structure we model an ionizing radiation bicone, originating within a galactic disk, with apex at the active nucleus and axis centered on the extended radio jets. We confirm that through density bounding the gross spatial structure of the emission line region can be reproduced with a wide opening angle that includes the line of sight, consistent with the presence of a simple opaque torus allowing direct view of the nucleus. In particular, our modelling reproduces the observed decrease in position angle with distance from the nucleus, progressing initially from the direction of the extended radio jet, through our optical structure, and on to the extended narrow-line region. We explore the kinematics of the narrow-line low- and high-velocity components on the basis of our spectroscopy and adopted model structure.

Effect of the specimen refractive index on the imaging of a confocal fluorescence microscope employing high aperture oil immersion lenses

Abstract: The effect of the specimen refractive index on the resolution, image brightness and axial scaling in confocal fluorescence microscopy is theoretically investigated for ten of the most popular fluorophores. The calculations are based on a vectorial theory based on Fermat's principle. We show that the axial scaling of the image does not depend on the fluorophore used. Axial scaling factors are derived for objects embedded in glycerol and water. For each fluorophore, the maximum intensity and the full-width-half-maxima of the confocal point spread function are given as a function of the focusing depth. The full-width-half-maxima allow for an assessment of the axial and lateral resolution when deeper regions of the specimen are to be investigated.

The point spread function of the soft X-ray telescope aboard Yohkoh

Abstract: The point spread function of the SXT telescope aboard Yohkoh has been measured in flight configuration in three different X-ray lines at White Sands Missile Range. We have fitted these data with an elliptical generalization of the Moffat function. Our fitting method consists of chi squared minimization in Fourier space, especially designed for matching of sharply peaked functions. We find excellent fits with a reduced chi squared of order unity or less for single exposure point spread functions over most of the CCD. Near the edges of the CCD the fits are less accurate due to vignetting. From fitting results with summation of multiple exposures we find a systematic error in the fitting function of the order of 3% near the peak of the point spread function, which is close to the photon noise for typical SXT images in orbit. We find that the full width to half maximum and fitting parameters vary significantly with CCD location. However, we also find that point spread functions measured at the same location are consistent to one another within the limit determined by photon noise. A 'best' analytical fit to the PSF as function of position on the CCD is derived for use in SXT image enhancemnent routines. As an aside result we have found that SXT can determine the location of point sources to about a quarter of a 2.54 arc sec pixel.

Microwave near-field imaging with open-ended waveguide—Comparison with other techniques of nondestructive testing

Abstract: The results of an experimental study of microwave imaging with an openended waveguide at 30 GHz for NDE of dielectric materials are presented. Using a hybrid measurement method, the near-field point spread function is investigated. Improvement of sensitivity and resolution via deconvolution also is discussed. A comparison of results obtained with more prominent NDE methods as ultrasonics, X-rays, and thermal waves demonstrates the performance of the technique.

Phase transfer and point-spread function of the human eye determined by a new asymmetric double-pass method.

Abstract: A recent study has shown that the double-pass method provides a good estimate of the ocular modulation transfer function (MTF) but that it does not yield the phase transfer function (PTF) [ J. Opt. Soc. Am. A12, 195 ( 1995)]. Therefore, one cannot recover the true retinal point-spread function (PSF). We present a modification of the double-pass method to overcome this problem. The key is to break the symmetry between the two passes. By using an unexpanded Gaussian input beam, we produce a diffraction-limited PSF for the first pass. Then, by using a large exit pupil, we get an aberrated PSF for the second pass. The double-pass aerial image is the cross correlation of both PSF’s, so that the Fourier transform of such an aerial image directly provides the true retinal PTF, up to the cutoff frequency of the effective (small), diffraction-limited entrance pupil. The resulting double-pass aerial image is a blurred version of the true retinal PSF. Thus it shows the effect not only of even symmetric aberrations but also of odd and irregular aberrations such as coma. We have explored two different ways to retrieve the true retinal PSF: (a) deblurring of the aerial image and (b) PSF reconstruction combining PTF data with conventional double-pass MTF. We present promising initial results with both artificial and real eyes.

Analysis of the CLEAN algorithm and implications for superresolution

Abstract: The capability of the CLEAN algorithm, which is able to develop image information corresponding to spatial frequencies for which the imaging system’s optical transfer function (OTF) is equal to zero, is shown to be dependent on the limited size of the object being imaged. It is found that this capability is available without a severe signal-to-noise-ratio penalty only for the recovery of a spatial frequency that is sufficiently close to some other spatial frequency for which the OTF is not equal to zero. As used here the term “sufficiently close” means that the magnitude of the separation of the spatial frequencies is less than one half of the inverse of the size of the object being imaged. This represents a limitation of CLEAN’s capability deriving from object size. It is suggested that this capability can be thought of in terms of superresolution, with the same limitation in regard to object size.

High fidelity optical system for electronic imaging

Abstract: An optical subsystem (14) for use in an electronic imaging system (10) utilizes a spatial filter (20) having a concentric optical path profile for removing unwanted structures in the optically formed image. In a first embodiment, the filter (20) includes an axisymmetric conical surface (24) formed on an optical substrate (22) to provide a light path modifying profile designed to achieve a desired circular point spread function in an image plane (16). Other embodiments include an elliptical/conical profile, and an axisymmetric, concentric ring profile. The filter profile may be single-point diamond turned into a flat filter substrate, or can be combined/integrated with the profile of an existing optical system element, such as lens (18). The present invention further provides a method for removing unwanted artificial image structures by generating an imaging system point spread function which is a hollow, closed path.

Two- and multiphoton detection as an imaging mode and means of increasing the resolution in far-field light microscopy: A study based on photon-optics

Abstract: A photon-optics interpretation of the image formation in a scanning single or two-photon excitation fluorescence microscope is given. This interpretation predicts the possibility of two-photon or multiphoton imaging modes based on simultaneous detection of two or more photons. We point out that by simultaneous detection of n photons stemming from the same point, the detection point-spread-function is raised to the nth power. In a two-photon detection microscope, pairs of photons rather than single photons are taken as the signal. We discuss the fundamental requirements of two-photon detection microscopy and the potential and limitations of this imaging mode. Two-photon detection leads to a reduction of the spatial extent of the detection point spread function and therefore to an increase of resolution, but also to a reduction of the total signal. The theoretically predicted narrowing of the point spread function as well as the reduction of the detected signal is confirmed in an experiment simulating the two-photon detection situation. In addition, we show a precise recording of the point-spread function of a 1.4 numerical aperture oil immersion lens at a wave length of 750 nm and the corresponding point-spread-function of a two-photon detection imaging mode.

In vivo imaging of the skin in the 100 MHz region using the synthetic aperture concept

Abstract: A major design problem concerning medical high frequency broadband imaging systems is caused by the strong attenuation of the tissue, which limits the maximum depth of penetration and the achieveable signal-to-noise-ratio (SNR). To address this problem, strongly focused transducers with a high energy density in a narrow focal region are utilized. To achieve images of high quality and resolution over a large depth range despite the short depth of field of those devices, the Synthetic Aperture Focusing Technique (SAFT) is employed. For SAFT-processing, the focus area is considered as a virtual source of approximately spherical waves. This hypothesis is supported by a simulation of the point-spread-function of a focused transducer, which is based on the Rayleigh-Sommerfeld-integral in the time domain.

Implementation of a modified Richardson-Lucy method for image restoration on a massively parallel computer to compensate for space-variant point spread of a charge-coupled-device camera

Abstract: Several imaging devices are characterized by a space-variant point-spread function (PSF), such as the wide-field/planetary camera of the Hubble Space Telescope (HST). Several techniques for image recovery that use data from such imagers approximate the space-variant PSF by a space-invariant PSF. A modified Richardson-Lucy method is implemented that accommodates the space-variant PSF of the HST as well as corrections for background counts, nonuniform flat field, and readout noise. The implementation runs on the DEC mppl2000 Sx/Model 200 massively parallel computer. Restorations of simulated HST images are obtained with a space-variant PSF and, for comparison, with a space-invariant approximation. Results of these processing methods are compared, and it is found that a residual artifact appears in restorations when a space-invariant PSF is used owing to the mismatch of the PSF kernel used in the restoration and the space-variant one underlying the image acquired with the telescope. This residual artifact is effectively eliminated when the processing is based on the space-variant PSF.

High-resolution restoration of images distorted by the atmosphere, based on an average atmospheric modulation transfer function

Abstract: A new method of real-time high-resolution imaging through the atmosphere is presented. This technique is based on knowledge of average atmospheric modulation transfer function (MTF) at the time the image is received. Atmospheric effects are modeled by a noisy spatial frequency filter including an average component described by the average atmospheric MTF and a noisy component modeled by the atmospheric point spread function's power spectral density. The noisy component represents random changes in the atmospheric MTF. Analytical results are accompanied by experimental image restoration examples, indicating significant image quality improvement based on knowledge of average atmospheric MTF, which includes both turbulence and aerosol MTF components. This method can be used to help overcome the jitter characteristics of turbulence, and is capable of yielding real-time image restoration with resolution limited essentially only by the hardware itself. Turbulence blur, aerosol blur, and contrast degradation are all corrected simultaneously, unlike adaptive optics, which corrects for turbulence only.

Method for the restoration of images disturbed by the atmosphere

Abstract: A method of real-time high resolution imaging through the atmosphere is presented. This technique is based on knowledge of average atmospheric Modulation Transfer Function (MTF) at the time the image is received. Atmospheric effects are characterized by a noise spatial frequency filter including an average component described by the average atmospheric Modulation Transfer Function, and a noisy component modeled by the atmospheric Point Spread Function's power spectral density. The noisy component represents random changes in atmospheric MTF. The new method of image restoration results in significant image quality improvement based upon knowledge of average atmospheric MTF which includes both turbulence and aerosol MTF components. This method can be used to help overcome the jitter characteristics of turbulence, and is capable of yielding real-time image restoration with resolution limited essentially only by the hardware itself. Turbulence blur, aerosol blur, and contrast degradation are all corrected simultaneously in real time.

On the limitations of the confocal scanning optical microscope as a profilometer

Abstract: We present a numerical study of the performance of a one-dimensional Confocal Scanning Optical Microscope (CSOM) when used as a profiler of highly reflecting surfaces. The study is conducted by means of a computer simulation that describes the interaction between the incident beam and the sample in a fairly rigorous manner. The degradation of the depth discrimination as a function of the local surface slope is illustrated with examples, and a criterion for the maximum allowable slope is proposed. We also show that the instrument has difficulties in profiling surfaces that have steps or sharp edges and it is argued that, in general, problems are encountered when the surface profile cannot be considered locally flat in the scale of the Point Spread Function (PSF) of the optical system. This implies that the lateral resolution of the instrument is far worse as a profilometer than when used as an imaging system, and that the unresolved features introduce spurious details on the estimated profile.

Characterization of high-numerical-aperture lenses by spatial autocorrelation of the focal field.

Abstract: We demonstrate a new method for the characterization of high-numerical-aperture lenses based on the f luorescent detection of the spatial autocorrelation of the focal field. Numerical calculations without any fitting parameters, based on scalar diffraction theory, are in excellent agreement with the experimental results. The novel method provides lens characterization parameters similar to those obtained with pointspread function measurements in real time, which may be expected to be suitable for high-numerical-aperture lens testing and alignment procedures. © 1995 Optical Society of America The imaging properties of a high-numerical-aperture (high-NA) lens are generally given in terms of the lens’s optical transfer function or, equivalently if the imaging process is linear in the detected quantity, by the Fourier transform of the optical transfer function, the point-spread function (PSF). These functions express the ability of the lens to transfer spatial frequencies. Measurement techniques for the PSF of high-NA lenses include, among others, three-dimensional imaging— and subsequent analysis—of highly scattering, or f luorescing, spheres that are much smaller than the typical width of the PSF 1‐4 either in full field or by scanning the sphere or the detector. Alternatively one can image the edge of a mirror in ref lection, although some care must be taken in comparing ref lection with f luorescence measurements. 4 The major drawback of these techniques is that they are slow and consequently require long-term nanometer stability of the optical setup. Also, the sample preparation imposes certain constraints on these methods. For optimization of high-NA lens systems, industrial testing of objectives, and three-dimensional image restoration techniques 5 a real-time technique would be useful for lens characterization. In this Letter we demonstrate a new technique, the point-spread autocorrelation function (PSAF) technique, which provides a straightforward, accurate, and versatile alternative for real-time characterization of high-NA lenses. First we introduce the new functional dependence in general terms, after which we demonstrate the capabilities of the technique, using theoretical modeling and experimental verification. In the novel PSAF technique we introduce a new function, GsDr, td, for the characterization of optical lens properties based on the measurement of the autocorrelation of the focal field of a (high-NA) lens:

Restoration of images from the scanning-tunneling microscope.

Abstract: During the acquisition of an image from any probe microscope instrument, various noise sources cause distortion in the observed image. It is often the case that impulsive disturbances cause bright groups of pixels to replace the actual image data in these locations. Furthermore, the images from a probe microscope show some amount of blurring caused both by the instrument function and the material properties. In almost all image-processing applications it is important to remove any impulsive distortion that may be present before deblurring can be attempted. We give a technique for detecting these impulses and reconstructing the image. This technique is superior to the standard global application of median filters for the case considered. The reconstruction is limited only to the affected regions and therefore results in a much sharper and more meaningful image. With the assumption of Gaussian blur it is then possible to propose several different deblurring methodologies. We present a novel Wiener-filter deblurring implementation and compare it to both maximum-entropy and Richardson–Lucy deblurring.