Showing papers on "Point spread function published in 1976"

Singular value decompositions and digital image processing

Abstract: The use of singular value decomposition (SVD) techniques in digital image processing is of considerable interest for those facilities with large computing power and stringent imaging requirements. The SVD methods are useful for image as well as quite general point spread function (impulse response) representations. The methods represent simple extensions of the theory of linear filtering. Image enhancement examples will be developed illustrating these principles. The most interesting cases of image restoration are those which involve space variant imaging systems. The SVD, combined with pseudoinverse techniques, provides insight into these types of restorations. Illustrations of large scale N2× N2point spread function matrix representations are discussed along with separable space variant N2× N2point spread function matrix examples. Finally, analysis and methods for obtaining a pseudoinverse of separable space variant point spread functions (SVPSF's) are presented with a variety of object and imaging system dagradations.

Computer Simulation Studies Of Compensation Of Turbulence Degraded Images

Abstract: Techniques have been developed which allow nearly all phases of the image forming process for turbulence degraded images to be computer simulated. The simulation includes generation of turbulence de-graded wavefronts, the degrading point spread function, and the sensor properties. Pre detection compensation of the wavefront and post detection compensation of the image can likewise be simulated. These simulation techniques have been used to determine fundamental limits of pre detection and post detection processing. Results of the application of these techniques to sun-illuminated objects will be presented in this paper.

A new restoring algorithm for the preferential enhancement of edge gradients

Abstract: We present here a new restoring algorithm that is based on use of a “median-window filter.” This is intrinsically a nonlinear operation that, when cyclically used with any linear restoring algorithm, can typically enhance edge gradients by a factor of 5:1 with nearly a complete absence of Gibbs oscillation.

Image retrieval from astronomical speckle patterns

Abstract: A technique has recently been proposed for processing astronomical photographs to remove the effects of turbulence. The method involves averaging over many short-exposure photographs of an object to reconstruct a diffraction-limited image of the object. A two-dimensional computer simulation of the reconstruction technique is presented. The results indicate that diffraction-limited imaging with a 100 in. telescope is possible even with 2 arc sec seeing conditions.

Measurement of surface roughness properties by using image speckle contrast

Abstract: As a practical monitoring tool for measurements of surface roughness properties, a new instrument is actually constructed on the basis of using the average contrast of image speckle patterns. With this instrument an experimental investigation on the contrast variation of image speckle patterns is conducted systematically for various surface roughnesses of the objects under different conditions of an optical imaging system. It is found that the roughness and correlation length of the objects can be precisely determined from the experimental curve of the contrast variation versus the point spread of the imaging system. The present method using image speckle patterns is superior to the previous one which uses the contrast variation of speckle patterns in the diffraction field.

Further computer simulation study of image speckle patterns with relation to object surface profile

Abstract: As a further extension of the previous paper and by considering actual objects, the intensity distribution and average contrast of image speckle patterns for two different kinds of objects having an asymmetrical random surface and a Gaussian random surface combined with a periodic sinusoidal variation are investigated on a computer as a function of the surface roughness properties of objects and the point spread of an optical imaging system. The computer simulation study for objects with the asymmetrical random surface indicates that a variation of the average speckle contrast as a function of the above parameters takes almost the same behavior as for objects with a purely Gaussian random surface. The study for objects with the combined surface of Gaussian random and sinusoidal variations shows that the average speckle contrast strongly depends on the combination ratio of these two variations characterized by their standard deviation and correlation length. It becomes again clear that the maximum value of the image speckle contrast is dependent upon the rms surface roughness of objects but not upon their surface profile.

Analytical illuminance and caustic surface calculations in geometrical optics.

Abstract: The analytical illuminance monitoring technique provides an exact expression within the geometrical optics limit for the illuminance over an image surface for light that has passed through a multiinterface optical system. The light source may be collimated rays, a point source, or an extended source. The geometrical energy distributions can be graphically displayed as a line or point spread function over selected image planes. The analytical illuminance technique gives a more accurate and efficient computer technique for evaluating the energy distribution over an image surface than the traditional scanning of the spot diagram mathematically with a narrow slit. The analytical illuminance monitoring technique also provides a closed form expression for the caustic surface of the optical system. It is shown by examining the caustic surface for anumber of lens systems from the literature that the caustic is a valuable merit function for evaluating the aberrations and the size of the focal region.

Super-resolution imaging system

Abstract: The resolution of an imaging system is greatly enhanced by radiating an object with a plane wave field from a coherent source variable in either frequency, angle or distance from the object, detecting the wave field transmitted through, or reflected from, the object at some point on the image of the object, with or without heterodyne detection, and with or without a lens system. The heterodyne detected output of the detector is processed to obtain the Fourier transform as a function of the variable for a direct measurement of the amplitude and surface height structure of the object within a resolution cell centered at the corresponding point on the object. In the case of no heterodyne detection, only intensity data is obtained for a Fourier spectrum.

SNR in photocounting images of rough objects in partially coherent light

Abstract: Image irradiance distributions from objects illuminated with partially coherent, quasi-monochromatic light, viewed against a spatially uniform background and received with a photosensitive detector are analyzed. A general expression for the SNR at the detector output is obtained as a function of the coherence of the illuminating light, the object surface roughness, the width of the telescope point spread function, and the aperture and integration time of the detector. The expression is evaluated for several limiting cases of coherence of illumination and of object surface roughness.

Reconstructed images of Alpha Orionis using stellar speckle interferometry.

Abstract: We discuss a recently developed technique to recover nearly diffraction-limited images of supergiant stars such as α Orionis (Betelgeuse) from speckle interferometry data. This method relies on the digital identification and coaddition of the brightest individual speckles within a large number of short-exposure speckle photographs. The resulting average speckle may be thought of as the convolution of a point source speckle profile with the actual object intensity pattern. By making use of this point we have derived angular diameters and limb darkening coefficients in addition to finding evidence of possible surface structure on the star. The limitations of this technique have been determined empirically and they are discussed in this paper.

Speckle interferometry on the 2.5 m Isaac Newton telescope

Abstract: A speckle interferometer for use at the Cassegrain focus of large telescopes has been constructed and used to observe binary star systems on the 2.5 m Isaac Newton telescope in England. The instrument has a four-stage, magnetically focused EMI image intensifier with a maximum light gain in excess of 106, good resolution, and low background noise. The final images are recorded on 16mm cine film. Both analog optical and digital analyses have been carried out. The digital analysis enables the transfer function of the speckle method to be found and divided into the image power spectrum. Examples are given of experimental results. Binary star separations have been measured for stars as close as 0.1 arc sec and as faint as magnitude 6.

X-ray 3-D coded aperture imaging: displaying the heart.

Abstract: The coded aperture imaging technique using an array of x-ray sources for display of three-dimensional is applied to a relevant medical object: the coronal vascular system of an isolated heart filled with contrast medium, coded according to a nonredundant point distribution. Arbitrary layers of the heart are decoded by correlation using a holographic system. The course of the vessels in the third dimension can clearly be followed, thus presenting a completely novel tool of medical diagnosis.

Modification of PSF by Polarisation Masks

Abstract: The nature of the image produced by an optical system depends upon the point spread function (PSF) of the imaging system. It is thus necessary to modify the PSF of the system in order to change its imaging quality. This may be achieved by a number of ways. The usual practice is to modify the phase and amplitude transmission of different zones of ihu pupil. In the present paper, we have shown that the effect of various phase and semi-transparent coatings may be simulated by making-the lens aperture by suitably oriented polarisation devices. Besides, the effective PSF of the lens may be continuously varied by changing the orientation of the analyser at the output.

Interference in astronomical speckle patterns

Abstract: Astronomical speckle patterns are examined in an atmospheric-optics context in order to determine what kind of image quality is to be expected from several different imaging techniques. The model used to describe the instantaneous complex field distribution across the pupil of a large telescope regards the pupil as a deep phase grating with a periodicity given by the size of the cell of uniform phase or the refractive index structure function. This model is used along with an empirical formula derived purely from the physical appearance of the speckle patterns to discuss the orders of interference in astronomical speckle patterns.

Image Tube Characteristics of Importance in X-ray Diffraction Studies

Abstract: Publisher Summary This chapter discusses the image tube characteristics of importance in X-ray diffraction studies. In protein crystallography, it is necessary to measure the relative intensity of diffraction spots occurring over the entire cathode area to an accuracy of a few percent with a dynamic range of the order of 1000:l. It is thus necessary to correct observed intensities for nonuniformities of image tube response over the sensitive region. For X-ray diffraction studies, however, the MTF has not been found to be as useful as a related measure of resolution, the point spread function (PSF). The PSF is easier to measure and interpret and it contains information that is often difficult to extract from the MTF. Two point spread functions can be combined to form a resultant point spread function with the aid of a simple computer program. For X-ray studies of crystalline proteins performed to date, it has been necessary to resolve diffraction spots approximately 0.5 mm in diameter, separated by at least 1 mm center to center, over an intensity range of 500:l.

Apparatus for copying graphic images including the processing of incoherent radiation with a spatial filter

Abstract: A linear optical processing system for copying graphic images which includes a spatial filter in the form of a Fourier transform hologram together with a lens for processing incoherent electromagnetic radiation. The system provides a given real, substantially-uniform low-pass two-dimensional optical transfer function for spatially filtering incoherent radiation within a range of spatial frequencies 0<ω<Ω. The spatial filter is made by forming a mask representing a point spread function which is the inverse transform of the desired optical transfer function, and employing the mask in a coherent system to produce as the filter a Fourier transform hologram of the mask. The spatial filter operates on light distributed from an object containing a graphic image illuminated by incoherent light and provides a substantially-uniform low-pass response for passing all spatial frequency components within an interval of spatial frequencies above zero spatial frequency extending to a specified cutoff spatial frequency ωc. The filter interacts with the lens to provide processed convolution and/or correlation images of such filtered light at a predetermined image plane to provide a graphic image of the object at the image plane in which higher spatial frequencies, within the interval, the reduction of which contribute to degradation or loss of fidelity in multigeneration copies, are substantially unaltered. In one embodiment, the spatial filter provides a plurality of discrete optical transfer functions for respectively filtering a plurality of discrete color components.

Characterization of the imaging properties of x-ray focal spots.

Abstract: The imaging characteristics of diagnostic x-ray systems are dependent, in part, upon the size, shape, and intensity distribution of the x-ray source. These properties, however, vary with such parameters as kVp (tube mA (tube current), and, because of the steep anode face, field position. We have investigated the voltage), characterization of the focal spot using the image of a random array of small radiopaque objects. The Fourier transform of this image is, to a good approximation, the modulation transfer function (MTF) of the focal spot. We have compared the accuracy and reliability of this technique with both pinhole and star imaging Focal spot variations with kVp, mA, and field position have been measured, and the use of this techniques. technique with microfocus x-ray sources has been demonstrated.

Fundamental Limits And Degrees Of Freedom Of Imaging Systems

Abstract: A space-variant or invariant point spread function of an imaging system is used to generate the grammian matrix from a continuous-discrete imaging model. If in) is a point in the continuous aperture of of the object plane and the i-th sample of the image array is given by gi = Is hi( )f(, )ddri (1) then the grammian matrix of the point spread function becomes [V] = s Toht(, (2a) where v. = ST h.(, )h..( (2b) The properties of [V] of interest are the singular values which define both the entropy and degrees of freedom of the imaging system. Using physical constraints on the point spread function, i. e. 0 < h. ( ) 1 (3a) N2 E) = 1 (3b) i= 1 1 we develop a set of experimental curves relating entropy and degrees of freedom. In addition a variable sampling procedure is hypothesized to increase the degrees of freedom for a fixed number of samples (Nx N= N2) to improve the data acquisition and processing efficiency and approach the fundamental limitations of the imaging system.© (1976) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Image Quality of Photoelectronic Imaging Systems and its Evaluation

Abstract: Publisher Summary This chapter discusses the criteria for evaluating image quality of photoelectronic imaging systems. An imaging system is used to form an image that can be interpreted by the eye in terms of the scene viewed, and which contains visual information that is not available to the eye without the device. Except for the purely optical systems, which form aerial images, imaging systems form their images on a surface, usually plane. The basic problem in evaluating image quality is to determine and quantitatively evaluate the ways in which the visual information content in the image varies from that in the scene. A complete description of the image quality of an imaging system requires evaluation at the points distributed over the volume in image space extending over the entire area of the image plane, and between the extreme positions of the image plane for all wavelengths for which the system can be used, and over the illumination range for which the system is suited. In this chapter, concepts related to the point spread function are discussed. Details of the line spread function, the edge gradient and acutance are also provided. It also elaborates the basics of optical transfer function, contrast transfer function, and limiting.

A Statistical Method For Post-Detection Compensation For Atmospheric Distortions Of Images Of Faint Scenes

Abstract: A new method for post-detection compensation of atmospheric distortions of images of faint scenes has been outlined and initially tested. A sequence of short exposure (0.01 to 0.1 sec) visible light images is processed in terms of the statistics of the Fourier transform amplitudes. A "master image" is derived that is iteratively compared with each image (in Fourier space) so as to align the set of images on the basis of features in the scene. Aperture synthesis can be used to decrease aperture redundancy since the alignment uses only Fourier amplitudes that are prominent in the joint set of master image and the raw image sequence. The master image has an effective point spread function (PSF) comparable to the best PSF in the sequence but the phases are strongly quieted by the statistics of large numbers if 30 or more images are spread over a time interval of 15 or more sec. Thus spatial frequencies in excess of 1 cycle per arcsec may yield reliable photometry after correction for contrast loss and telescope aberrations. The degree of enhancement may be optimized, based on a separation of signal and noise in the data so that noise may be estimated. At faint levels quantum noise is severe. Since that noise is correlated with signal, the noise spectrum is not white but falls with increasing spatial frequency.© (1976) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Fast automatic lens testing for extended-field image quality.

Abstract: Monitoring of high-volume lens manufacture for Polaroid Land photography requires an acceptance criterion that is both relevant to picture quality and suitable for automatic testing. In a hardware test based upon the modulation transfer function (MTF), the problem is to choose what not to measure, while obtaining enough information about a lens to assign it a useful figure of merit. The predominant defects of manufacture are field tilt, resulting from tilting or decentering of components, and field curvature, from small errors in spacing, thickness, radius, or index. Field tilt and curvature are not local defects of the image surface, insofar as they may be removed by a local refocus. We therefore need a test procedure that can (perhaps simultaneously) examine image quality over an extended field, but is principally called upon to evaluate focus differences. At multiple field locations a single-frequency modulation measurement adequately defines the MTF for the low spatial frequencies of interest and can be interpreted easily in terms of the rms blur of the point spread function. We will discuss the logic of 100% lens acceptance testing and two types of instrumentation we have used.