Aperture Imaging Research Articles

Probabilistic Diffraction Limited Imaging Through Turbulence

The possibility of considerably increasing the resolution of an extended image taken through a turbulent atmosphere by selecting random "lucky" diffraction limited exposures out of many normal-turbulence degraded-short exposures is experimentally explored. For an imaging aperture D, larger than the atmospheric spatial coherence length ro, the probability P of getting a diffraction limited short exposure image of a point source is found to be exp[-0.6(D/r0)2 ]. This result is in agreement with Fried's prediction. The angular isoplanatism is measured to be -X/ro. However, observed random wavefront tilts scramble the diffraction limited picture elements in an isoplanatic patch, thus complicating the probabilistic imaging method.

Longitudinal emission tomographic imaging using a three-pinhole aperture and an Anger camera.

Single-photon emission tomographic imaging subdivides into two classes of technique: transverse-section computed tomography and longitudinal-section tomography. Longitudinal tomography can be achieved by combining a gamma camera with a spatially or time coded aperture or with a segmented multiple-pinhole collimator (Budinger, 1980). Longitudinal imaging using the latter type of collimator generates spatially distinct camera images. The reconstruction of longitudinal sections from such non-overlapping images does not necessitate decoding of an aperture function. Instead, reconstruction proceeds by iteration and, in principle, provided criteria for depth resolution are met, complete decoupling of reconstructed planes (blur-free imaging) results. For identifying lesions the absence of blurring artefacts is a great advantage over coded aperture imaging in which it is difficult to completely decouple planes (Webb et al., 1978); additionally, multiple-pinhole tomography can quantitatively map out the uptake of a radionuclide in organs and tumours. To date several workers, notably including Vogel et al. (1978, 1979), have concentrated on the development and application of a seven-pinhole aperture for myocardial imaging. Limited by a very small field of view, the seven-pinhole aperture has not found wider application in nuclear medicine. In this note an imaging technique based on the use of an aperture comprising three pinholes arranged in a triangle is proposed which overcomes this limitation and can lead directly to an extension of quantitative, completely decoupled, longitudinal tomography into other important fields of nuclear medicine.

Cinematography — Image Area Produced by 65 and 70 mm Motion-Picture Camera Aperture and Maximum Projectable Image Area on 70 mm Motion-Picture Prints — Positions and Dimensions

Cinematography — Image Area Produced by 65 and 70 mm Motion-Picture Camera Aperture and Maximum Projectable Image Area on 70 mm Motion-Picture Prints — Positions and Dimensions

Unstable optical resonator with self imaging aperture

In this paper we describe conditions for a negative branch resonator with an infinite Fresnel number. This realizable resonator eliminates primary diffraction effects and its modes are very different from conventional unstable resonator modes. A numerical calculation for a strip resonator indicated large mode selectivity and a fundamental mode with almost uniform intensity.

Digital deconvolution of a coded image obtained with a nonredundant pinhole array

A coded image or pseudohologram, obtained optically with a nonredundant pinhole array imaging aperture, was digitized in a 512 x 512 format. Images were reconstructed digitally. The reconstruction scheme, a deconvolution in the Fourier domain, is presented. The applicability of this technique to a real-time imaging system is discussed.

Computer Simulations of the Image Synthesis for a Strip Telescope

A strip telescope, one kind of optical synthetic aperture, is studied by computer simulations. In order to synthesize the image of a distant object, which might be focused by a full aperture telescope, it is necessary to rotate the long, narrow aperture and record images at each azimuth. The image synthesis from the blurred ones thus obtained is conducted by computer. The characteristic properties of a strip telescope and the image synthesizing procedure are described and the influence of the strip width up on the synthesized image is also discussed.

Microdensitometer Optical Performance: Scalar Theory and Experiment

The scalar theory of microdensitometer performance developed in previous papers is revised and expanded to include coherent illumination. The central ideas of scalar microdensitometry are combined from several basic sources and summarized ; this paper serves as a convenient single source of microdensitometer theory. Eight distinct variations of instrument configuration and operation are identified, and the image characteristics and conditions for linear microdensitometry are developed for each. The concept of effective incoherence is summarized and discussed. Further consideration of the problems associated with aperture sizes, determination of the sampling aperture size and image vs sample scanning are presented. An experimental test program, carried out with the Mann-Data Microanalyzer, by experienced operators, under closely controlled conditions to test the theory, is reported. Among the conclusions is that the instrument manifests its best performance with overfilled optics and sample scanning, and that an increased reduction factor on the influx side would be useful with image scanning. Several fundamental problems surface during the course of the investigation. These are discussed and the need for further study in certain areas is emphasized.

On Image Formation with Coherent Light

Starting from linearity considerations and the Huygens-Fresnel principle, the use of ‘matched’ illumination in image formation with coherent light is described. As typical examples, the influence of a longitudinal mismatch of illumination on the images of small circular apertures is numerically studied. For comparison, the results on the defocused images of the same objects under ‘matched’ coherent illumination are also given.

A new imaging principle

It is shown that frequency sweeping a bistatic coherent transmitter-receiver system can give rise to a synthetic imaging aperture of effective length equal to the spacing between transmitter and receiver with an effective operating frequency that can approach the upper sweep limit. Practical considerations and areas of application are discussed.

Coded Apertures Derived from the Fresnel Zone Plate

Mertz and Young introduced the idea of using a Fresnel zone plate as a shadow-casting reticle, or coded aperture, in X-ray astronomy. More recently, considerable progress has been made toward using the zone-plate aperture for gamma-ray imaging in nuclear medicine. The most successful configuration has used an off-axis section of a zone plate in conjunction with a halftone screen. In this paper, we discuss a variety of closely related coded apertures, including an annulus, an inverted zone plate, a spiral zone plate, and the Girard grill. In most cases, the technique of grid-coded subtraction is used to suppress the zero-order (DC) background light usually associated with zone-plate imaging. The first application of this technique, reported by Stoner et al., used a sequence of two to four on-axis zone plates. In the present paper it is shown that the method can be extended to other apertures and is also very useful in synthesizing the spatial filters for optical decoding.

Three-Dimensional Impulse Response of Rectangular and Circular Imaging Apertures

The three-dimensional intensity impulse responses of rectangular and circular apertures are calculated. The calculations take into account the variation of magnification with the amount of defocusing. The results may be applied to images of thin clouds of particles with arbitrary depth.

Aperture Configuration and Imaging Performance

It is generally accepted for optical apertures that obstructions of small proportions are relatively harmless for good imaging performance and that larger aperture obstructions should be avoided. This work presents some quantitative data on this subject. Optical simulation was used to measure parameters of images created by different families of aperture configurations. Twenty-four apertures were selected, and the following data given: aperture shape, energy portion within an area equivalent to the Airy disk and to a circle including the third bright ring of the ideal diffraction image, modulation transfer function, and a photograph of the diffraction image. Design criteria are discussed for aperture configurations of large telescope systems.

Holographic image processing and aperture synthesis methods

Holographic image processing and aperture synthesis methods

Image Deblurring and Aperture Synthesis Using a Posteriori Processing by Fourier-transform Holography

Image deblurring and aperture synthesis using a posteriori processing by Fourier transform holographic spatial filtering

Imaging Performance and Surface Deviations*

The imaging performance of an error-free optical system is limited by deviations in the optical imaging surfaces. Large apertures are not diffraction limited. The irradiance distribution in image elements is derived theoretically for aperture diameters and surface deviations in a relative range of 1:25. The efficiency of large optical imaging apertures as a function of the optical precision is discussed.

Image formation in microscopy at high numerical aperture.

Experiments are described which illustrate the effect of aberrations of the objective, working at high numerical aperture, on the diffraction images of circular apertures in an opaque film. Measurements of the diffraction image of a straight opaque edge at high numerical apertures, in polarized and non-polarized light, show the breakdown of the scalar theory at angles of convergence greater than 30°.