Showing papers on "Point spread function published in 1997"

Estimation of the adaptive optics long-exposure point-spread function using control loop data

Abstract: Astronomical images obtained with adaptive optics systems can be enhanced by using image restoration techniques. However, this usually requires an accurate knowledge of the system point-spread function (PSF) which is variable in time. We present a method to estimate the PSF related to each image, using data from the adaptive optics control computer, namely, the wave-front sensor measurements and the commands to the deformable mirror, accumulated in synchronization with the acquisition. This method requires no extra observing time and has been successfully tested on PUEO, the Canada–France–Hawaii Telescope adaptive optics system. With this system, accurate PSF estimations could be achieved for guide stars of magnitude 13 or brighter.

Measurement of the point spread function in MRI using constant time imaging

Abstract: The point spread function is a fundamental property of magnetic resonance imaging methods that affects image quality and spatial resolution. The point spread function is difficult to measure precisely in magnetic resonance even with the use of carefully designed phantoms, and it is difficult to calculate this function for complex sequences such as echo-planar imaging. This report describes a method that measures the point spread function with high spatial resolution at each pixel in samples of uniform intensity distribution. This method uses additional phase encoding gradients before the echo-planar acquisition that are constant in length but vary in amplitude. The additional gradients are applied to image the contents within each individual voxel. This method has been used to measure the point spread function for echo-planar imaging to demonstrate the effects of limited k-space sampling and transverse relaxation, as well as the effects of object motion. By considering the displacement of the point spread function, local distortions due to susceptibility and chemical shift effects have been quantified and corrected. The method allows rapid assessment of the point spread function in echo-planar imaging, in vivo, and may also be applied to other rapid imaging sequences that can be modified to include these additional phase encoding gradients.

Effects of specimen refractive index on confocal imaging

Abstract: Summary The aberrations introduced when focusing within a specimen with a refractive index equal to that of water using an oil-immersion objective are investigated theoretically. The peak intensity in the confocal point spread function drops by a factor of two for focusing less than 10 mm into the specimen. The effects on scaling of dimensions in the resulting images are discussed. The image exhibits an axial stretching by a factor of about 1.12.

Computer Phantoms for Simulating Ultrasound B-Mode and CFM Images

Abstract: Programs capable of simulating ultrasound images have recently been developed. This opens the possibility of evaluating transducers and focusing schemes not only from their point spread function, but also from an imaging point of view. The calculation of the ultrasound field is based on linear acoustics using the Tupholme- Stepanishen method for calculating the spatial impulse response. Any transducer can be simulated by splitting the aperture into rectangular or triangular sub-apertures, and the calculation can include any transducer excitation and apodization. The acoustic settings can be controlled in the entire image through dynamic apodization and focusing. The transmit and receive apertures can be defined independently of each other. Frequency dependent attenuation can also be included in the simulation.

Spectrally weighted twisted projection imaging: reducing T2 signal attenuation effects in fast three-dimensional sodium imaging.

Abstract: A scheme for the reduction of T2 signal attenuation effects in three-dimensional twisted projection imaging is presented. By purposely reducing the sample density at the high spatial frequencies, a considerable reduction in readout time is achieved. The reduction in readout time leads to decreased T2 signal attenuation which translates into improved signal-to-noise ratio (SNR). The SNR improvement is achieved without decreasing the image's resolution since the point spread function depends on the sample weighting as well as the T2 attenuation. The results indicate that SNR improvements of up to 40% can be achieved using the proposed scheme.

The orthogonal transfer ccd

Abstract: We have designed and built a new type of CCD that we call an orthogonal transfer CCD (OTCCD), which permits parallel clocking horizontally as well as vertically. The device has been used successfully to remove image motion caused by atmospheric turbulence at rates up to 100 Hz, and promises to be a better, cheaper way to carry out image motion correction for imaging than by using fast tip/tilt mirrors. We report on the device characteristics, and find that the large number of transfers needed to track image motion does not significantly degrade the image either because of charge transfer inefficiency or because of charge traps. For example, after 100 sec of tracking at 100 Hz approximately 3% of the charge would diffuse into a skirt around the point spread function. Four nights of data at the Michigan-Dartmouth-MIT (MDM) 2.4-m telescope also indicate that the atmosphere is surprisingly benign, in terms of both the speed and coherence angle of image motion. Image motion compensation improved image sharpness by about 0.5'' in quadrature with no degradation over a field of at least 3 arcminutes.

A probability description of the Yule-Nielsen effect, I : Tone reproduction and image quality in the graphic arts

Abstract: The Yule-Nielsen effect, also called optical dot gain, has often been modeled based on convolutions between halftone dot patterns and a point spread function, PSF, characteristic of the paper. The form of the PSF is generally assumed or measured empirically. An alternative approach to modeling the Yule-Nielsen effect employs a probability function P p , which describes the fraction of reflected light emerging between halftone dots and under dots. The probability model is shown to fit experimental data on the Yule-Nielsen effect quite well. Moreover, the model can be implemented with simple algebraic expressions rather than the convolution or Fourier calculations required for PSF models. In addition, the quantitative relationship between P p and PSF is demonstrated.

Photon-noise-limited operation of intensified CCD cameras

Abstract: Intensified CCD cameras are increasingly being used in quantitative applications, which requires not only a greater understanding of their operation but also more detailed modeling to predict their performance more accurately. We have developed a model based on photon-noise-limited operation that incorporates the effects of the point spread function of the intensifier on signal-to-noise ratio. These effects are absent in other models, which renders them inadequate to model the camera performance properly. Calculations of noise-equivalent irradiance with our model are shown to be in good agreement with experimental results presented for two Xybion intensified cameras, Models GEN-III IMC and NIR DCIC intensified cameras.

Imaging very-low-contrast objects in breastlike scattering media with a time-resolved method.

Abstract: An investigation was performed of the effectiveness of a time-resolved method for imaging very-low-contrast features embedded in highly scattering media. Experiments employed slabs of breastlike material into which were inserted small cylindrical objects having either a scattering or an absorption coefficient of 4, 2, 1.5, and 1.1 times greater than the surrounding medium. An attempt was made to quantify the degree of contrast produced by each object. The results indicate that time-gating is far more effective at enhancing the contrast of the scattering inhomogeneities than of the absorbing inhomogeneities. This observation is shown to agree with a diffusion-based model, which also predicts that time-gating can decrease the contrast of absorbing inhomogeneities unless very short time-gates can be employed.

Acoustical imaging system

Abstract: An acousto-optic imaging system (18) for imaging an object including a source of acustic energy which illuminates the object; an acousto-optic imaging plate (10) which receives scattered acoustic energy from the object, said scattered acoustic energy (37) forming an acoustic image at the acousto-optic imaging plate (10), a source of optical energy (22) which produces optical energy whose characteristics are modified by changes in the acousto-optic imaging plate (10) caused by the received acoustic energy (37); and an optical imager (32) which forms a nonholographic image of modified optical energy. The source of acoustic energy is pulsed such that the intensity of the time integral of light detected by the optical imager (32) is an image having an spatially varying intensity. The intensity at a point in the optical image is related to the distance from a reference position of a corresponding point in the object.

Measuring MTF of Paper by Sinusoidal Test Pattern Projection

Abstract: Image quality of hardcopy is significantly influenced by paper characteristics. Light scattering phenomena in paper produce optical dot gain, which has a large influence on the tone reproduction characteristics of halftone images. Light scattering phenomena in paper can be represented by the modulation transfer function (MTF) of paper. However, little has been reported on techniques for measuring the MTF of paper. In this study, a new technique for measuring MTF of paper is proposed. We have developed a modified microdensitometer to project a sinusoidal test pattern on sample paper. The reflection density distribution of the projected sinusoidal test pattern was measured by the microdensitometer, and the MTFs of various types of papers obtained. The point spread function (PSF) of paper was calculated by the Fourier transform of the measured MTF. The PSF could be expressed by an exponential function. The obtained PSFs were applied to a model to predict reflection density of the halftone image. This result showed that the predicted density is approximately the same as the measured density in the halftone image and the estimated optical dot gain is well correlated to the measured optical dot gain.

Enhanced compressibility tomography

Abstract: We are interested here in the inversion of data leading to high quality imaging of physical parameters. We deal with ultrasonic diffraction tomography and first show how far field diffraction measurements of an incident plane wave give access to the spatial Fourier transform of a composite object. In the case of an acoustic model (soft tissues), this object is characterized by two parameters, e.g., the compressibility and density, each being affected by its own point spread function. The development of quantitative imaging proceeds from the separation of each parameter contribution. This can be done by measuring the scattered field over an arc for several transmitter positions around the object. This allows us, under specified conditions, to reconstruct either the compressibility, or the velocity, or the impedance maps. We have focused on compressibility imaging for which we propose a novel algorithm based on a redundant reconstruction procedure. We present tomograms of biological phantoms obtained with our experimental set-up.

Optical dot gain in a halftone print : Tone reproduction and image quality in the graphic arts

Abstract: The scattering of light within paper can affect the tone characteristics of a printed halftone image. A halftone image is formed by variation in the average reflectance, which is determined by the size of the ink dots. Photon migration within the paper from noninked to inked regions tends to increase the photon absorption and thus decrease the halftone reflectance-the dots are effectively larger than their physical size. This effect is known as optical dot gain or as the Yule-Nielson effect. The degree of optical dot gain depends on the distance that the photons migrate within the paper, which in turn depends on the paper's scattering and absorption characteristics, and on the thickness of the paper. We develop a theory that expresses the halftone reflectance in terms of the halftone microstructure-the screen period, dot size, dot shape, and ink transmission-and the effects due to the paper. The paper effects are represented in the theory by a point spread function, which is a conditional probability density that characterizes the photon migration within the paper, and by the paper's reflectance. We construct a model of photon transport within the paper by solving the transport equation using a diffusion approximation, from which we derive a point spread function. We interpret the expanded Murray-Davies model of halftone reflectance in terms of the theory developed here by giving a probabilistic interpretation to optical dot gain. We show that optical dot gain can be related to a single numerical parameter. Using the diffusion point spread function, we show how this parameter is related to the physical quantities that characterize the paper.

Paraxial theory of planar integrated systems

Abstract: We discuss the paraxial approximation for optical systems with an oblique orientation of the optical axis. Our investigation provides modifications necessary to adapt the parabolic approximation of conventional optics to planar integrated free-space optics. Our results are applied to a planar imaging system. For a single-lens system we discuss a design for arbitrary magnification factors. In addition, we calculate the space–bandwidth product of the imaging system, considering geometrical constraints of the concept of planar integration. This provides an upper bound for the number of spatial data channels that can be realized with planar optics.

Optical transfer functions of 4Pi confocal microscopes: theory and experiment.

Abstract: We measure the point-spread function in the two main configurations of 4Pi confocal microscopy as well as in the traditional confocal arrangement and derive the optical transfer functions from the experimental data. The optical transfer functions are in good agreement with their theoretical counterparts. We find a 3.5- to 5-fold increased axial bandwidth of the 4Pi confocal microscope and hence confirm the enhanced spatial-frequency content of 4Pi images.

Modeling the Yule-Nielsen effect on color halftones

Abstract: The Neugebauer approach to modeling color cmy halftones generally has to be modified to correct for the Yule-Nielsen light scattering effect. The most common modification involves the Yule-Nielsen n factor. A less common, but more fundamentally correct modification of the Neugebauer model involves a convolution of the halftone geometry with the point spread function, PSF, of the paper. The probability model described in the current report is less complex than the PSF convolution approach but is still much less empirical than the Yule-Nielsen n model. The probability model assumes the Neugebauer equations are correct and that the Yule-Nielsen effect manifests itself in a variation in the XYZ tristimulus values of the eight Neugebauer primary colors as a function of the amounts of c, m, and y printed. The model describes these color shifts as a function of physical parameters of the ink and paper that can be measured independently. The model is based on the assumption that scattering and absorption probabilities are independent, that the inks obey Beer-Lambert optics, and that ink dots are printed randomly with perfect hold-out. probabilities are the model is most easily tested by measuring the shift in the color of the paper between the halftone dots, and experimental microcolorimetry is presented to verify the model.

Holography imaging apparatus holography display apparatus holography imaging method and holography display method

Abstract: A diaphragm having an aperture, the size of which is equal to or smaller than λf/p (λ is the wavelength, p is the imaging resolution, and f is the focal length of an imaging optical system) is arranged on the focal plane of the object space of the imaging optical system, and object light via the aperture is imaged by the imaging optical system. The object light via the imaging optical system and reference light are brought to interference to form interference fringes, and an image of the interference fringes is sensed. Using the hologram sensed in this manner, the image of the object to be sensed is reconstructed by adopting an imaging optical system equivalent to that upon imaging, and setting the positional relationship between the formed hologram and the imaging optical system in correspondence with that between the imaging optical system and the imaging surface upon imaging. In this way, a holography imaging apparatus which can vary the field of view and can sense a high-quality hologram using an imaging device having a relatively low spatial resolution is realized, and a holography display apparatus which can display an image by eliminating distortion with respect to the original image from a hologram sensed by the holography imaging apparatus of this invention is realized.

Laser Guide Star for 3.6m and 8m telescopes: Performances and astrophysical implications

Abstract: We have constructed an analytical model to simulate the behavior of an adaptive optics system coupled with a sodium laser guide star. The code is applied to a 3.6-m and 8m class telescopes. The results are given in terms of Strehl ratio and full width at half maximum of the point spread function. Two atmospheric models are used, one representing good atmospheric conditions (20 per cent of the time), the other median conditions. Sky coverage is computed for natural guide star and laser guide star systems, with two different methods. The first one is a statistical approach, using stellar densities, to compute the probability to find a nearby reference. The second is a cross-correlation of a science object catalogue and the USNO catalogue. Results are given in terms of percentage of the sky that can be accessed with given performances, and in terms of number of science object that can be observed, with Strehls greater than 0.2 and 0.1 in K and J bands.

Three‐dimensional microscopy in thick biological samples: A fresh approach for adjusting focus and correcting spherical aberration

Abstract: A modified optical system for the light microscope has been devised in order to remotely shift the focal plane and to manipulate the point spread function for any given objective lens. An adjustable telescope system is inserted into the microscope tube so as to move the intermediate image position, thus achieving two goals of fundamental importance for the three-dimensional imaging of biological samples. First, it allows the focus to be rapidly varied without actually moving the objective lens. This permits high throughput three-dimensional microscopy of living specimens. Secondly, it makes possible the compensation of objective lens spherical aberration. This distortion is especially significant when high numerical aperture objectives are utilized to image deep into thick specimens.

Diagnosing unknown aberrations in an adaptive optics system by use of phase diversity.

Abstract: We outline a novel method for estimating a fixed aberration that is in the image path but not in the wave-front-sensor (WFS) path of an adaptive optics (AO) imaging system. We accomplish this through a nontraditional application of the Gonsalves [Proc. SPIE 207, 32 (1997)] least-squares phase-diversity technique, using an ensemble of images and WFS data. The diversity phases required for this technique are provided by the temporal differences in WFS residual phase measurements for different members of the ensemble. We demonstrate the technique by using actual observations from an operational AO system exhibiting such an aberration. An estimate of this aberration was obtained by the proposed algorithm that agrees reasonably well with the observed point-spread function.

The Orthogonal Transfer CCD

Abstract: We have designed and built a new type of CCD that we call an orthogonal transfer CCD (OTCCD), which permits parallel clocking horizontally as well as vertically. The device has been used successfully to remove image motion caused by atmospheric turbulence at rates up to 100 Hz, and promises to be a better, cheaper way to carry out image motion correction for imaging than by using fast tip/tilt mirrors. We report on the device characteristics, and find that the large number of transfers needed to track image motion does not significantly degrade the image either because of charge transfer inefficiency or because of charge traps. For example, after 100 sec of tracking at 100 Hz approximately 3% of the charge would diffuse into a skirt around the point spread function. Four nights of data at the Michigan-Dartmouth-MIT (MDM) 2.4-m telescope also indicate that the atmosphere is surprisingly benign, in terms of both the speed and coherence angle of image motion. Image motion compensation improved image sharpness by about 0.5 arcsec in quadrature with no degradation over a field of at least 3 arcminutes.

Digital method for measuring the focus error

Abstract: We show how the focus longitudinal error z can be deduced from the observed image. This deduction requires some digital image processing. Once the focus error z is known, one may compensate for it either by shifting the lens or by deblurring the image digitally. We have studied systems with a circular or with a rectangular aperture. In the former case, the defocus causes some dark rings in the frequency spectrum of the image. With a rectangular aperture, the defocus creates a set of straight, orthogonal dark lines. These dark zones provide a quantitative diagnosis of the focus error. Some experimental evidence supports our concept.

Restoration of three-dimensional quasi-binary images from confocal microscopy and its application to dendritic trees

Abstract: For the analysis of learning processes and the underlying changes of the shape of excitatory synapses (spines), 3-D volume samples of selected dendritic segments are scanned by a confocal laser scanning microscope. For a more detailed analysis, such as the classification of spine types, binary images of higher resolution are required. Simple threshold methods have disadvantages for small structures because the microscope point spread function (PSF) causes a darkening and a spread. The direction-dependent PSF leads to shape errors. To reconstruct structures and edge positions with a resolution smaller than one voxel a parametric model for the dendrite and the spines is created. In our application we use the known tree-like structure of the nerve cell as a- priori information. To create the model, simple geometrical elements (cylinders with hemispheres at the ends) are connected. The model can be adapted for size and position in sub-pixel domain. To estimate the quadratic error between the microscope image and the model, the model is sampled with the same resolution as the microscope image and convolved by the microscope PSF. During an iterative process the parameters of the model are optimized. In contrast to other pixel-based methods. the number of variable parameters is much slower. The influence of small deviations in the microscope image (caused by the inhomogeneous biological materials) is reduced.

Application of image restoration methods for confocal fluorescence microscopy

Abstract: The analysis of the three-dimensional structure of tissue, cells and cellular constituents play a major role in biomedical research. Three-dimensional images, acquired by confocal fluorescence microscopes play a key role in this analysis. However, the imaging properties of these microscopes give rise to diffraction-induced blurring phenomena. These distortions hamper subsequent quantitative analysis. Therefore, restoration algorithms that invert these distortions will improve these analyses. We have tested the performances of the Richardson-Lucy, and the ICTM algorithm in a simulation experiment and found a strong dependency of their performances on the signal-to-noise ratio of the image. We propose a pre-filtering to reduce the noise in the image without hampering the object. We have applied a Gaussian filter and a median filter prior to the restoration, and compensate for this extra blurring of the Gaussian in the restoration procedure. We show how this pre- filtering improves the performance of the restoration algorithms. The experiments were performed on spheres convolved with a confocal point spread function and distorted with Poisson noise.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Analysis of defocused image data for 3D shape recovery using a regularization technique

Abstract: The reconstruction of three-dimensional (3D) information from defocused image data is formulated as an inverse-problem that is solved through a regularization technique. The technique is based on modeling the sensing of defocused images in a camera system using a three-dimensional (3D) point spread function (PSF). Many images are acquired at different levels of defocus. The difference (mean-square error) between this acquired image data and the estimated image data corresponding to an initial solution for 3D shape is minimized. The initial solution for 3D shape is obtained from a focus and defocus analysis approach. A regularization approach that uses a smoothness constraint is proposed to improve this initial solution iteratively. The performance of this approach is compared with two other approaches: (1) gradient descent based on planar surface patch approximation, and (2) a local error minimization based on a limited search. We exploit some constraints such as the positivity of image brightness unique to this problem in the optimization procedure. Our experiments show that the regularization approach performs better than the other two and that high accuracy is attainable with relatively moderate computation. Experimental results are demonstrated for geometric optics model of 3D PSF on simulated image data.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Experimental determination of CT point spread function anisotropy and shift-variance

Abstract: The authors propose an experimental method to quantitatively assess the shift-variance and anisotropy of commercial CT scanners. A custom phantom consisting of 89 wires arranged in concentric circles was built for this purpose. A separable Gaussian model, expressed in a polar coordinate system, is proposed along with the corresponding estimation method. The estimated model parameters provide the quantitative information sought. The authors conclude that the scanner presents a rotating blur, i.e., that the PSF is independent of angular position when expressed in a polar coordinate system, and that the PSF shape broadens more quickly in the tangential direction than along the radial axis. Both of these observations agree with theoretical studies.

Iterative blind deconvolution of extended objects

Abstract: This paper describes a technique for the blind deconvolution of extended objects such as the Hubble Space Telescope (HST), scanning electron and 3D fluorescence microscope images. The blind deconvolution mechanism is based on the Richardson-Lucy (1972, 1974) algorithm and alternates between deconvolution of the image and point spread function (PSF). This form of iterative blind deconvolution differs from that typically employed in that multiple PSF iterations are performed after each image iteration. The initial estimate for the PSF is the autocorrelation of the blurred image and the edges of the image are windowed to minimise wrap around artifacts. Acceleration techniques are employed to speed restoration and results from real HST, electron microscope and 3D fluorescence images are presented.

Astigmatism measurement apparatus and method based on a focal plane separation measurement

Abstract: A computer receives image data from a star-shaped optical target in the object plane and calculates angle-dependent boundary sharpness. The horizontal, x-direction, amplitude derivative and the vertical, y-direction, amplitude derivative are computed over a portion of each star pattern image from a Z panning sequence. A microscope slide stage, carrying the target, is moved vertically from a level just below where the target is in focus to a level just above where the target is in focus. For each small increment of vertical motion, Z panning, an image of the star pattern is captured for analysis. Computations are performed on the differentiated images to search for evidence of elongation of the point spread function and variation with stage Z position of the angle of long axis of such an out-of-round point spread function. The presence of a distorted point spread function that varies along the optical axis indicates astigmatism.