1. Field of the Invention
This invention relates to the art of enhancing image resolution and creating a sense of depth perception through spatial filtering usually carried out within the image transform regions. Some of the better known prior art technologies are the schlieren, dark ground, phase contrast and modulation contrast methods.
2. Description of the Prior Art
Image resolution enhancement methods consist of emphasizing the higher spatial frequencies of the image transform. Methods for doing this exploit qualities unique to either diffraction or interference but on occasion they are combined. As used here, diffraction is the process of limiting a single wave in any way whereas interference is the interaction of two or more waves with each other.
As Abbe has shown, all information concerning the image of an object is contained in its diffraction pattern. Light reflected from any point of an image array a(u) is a complex quantity composed of real and imaginary components, amplitudes and phases respectively. Photography fails to preserve these relationships since film responds solely to light amplitudes. Only the absolute value of each element of array a(u) would be recorded. Object reconstruction from these diffracted images is possible only when their components are real, that is, when their phase components are either zero or 180.degree..
A phase object is an optical element having high light transmission and which causes a relative phase retardation at various points on its surface that is proportional to its thickness variation at those points. Required thickness variations are best obtained by controlled dielectric deposition on specified areas of a plate's surface. A shell cast plastic sheet and/or window glass pane have randomly distributed thickness variations that could, at times, favor their use as phase objects. Optical flat or even window glass are good examples. In a phase object, the vectors representing the complex amplitudes of all of an object's various points are of equal length but differ in phase angles. The vector sum of these vectors is the single vector that represents the zero order of the diffraction pattern.
Spatial image enhancement by the dark ground and schlieren methods suppress the zero order and its vector is therefore greatly reduced or eliminated while the image's new vectors become of unequal length and therefore represent an increase in contrast. There are a variety of ways of accomplishing this. Mechanical means such as discs so placed as to block out the central portions of an image is the technique of dark ground enhancement with its disc located on the object side of the lens. In the schlieren method, a knife edge at the image focal plane literally cuts its diffraction image, and thereby its transform, in half. In the phase and modulation contrast methods, the zero order vector is shifted by means of a phase object upon whose surface the location and thickness of a dielectric is varied.
The joint use of diffraction and interference together in the photographed image is of great theoretical and practical significance. French physicist Lippman used this to produce dyeless color images from black and white positives. He coated the rear surface of a photo plate with mercury so that the wave from any given point of the transform is reflected back through the film's emulsion and thereby interferes with its incoming wave. This results in standing waves throughout the emulsion whose wavelength at any point is that of the incoming wave at that point. When the developed positive is viewed at an angle under white light, the embedded standing waves function as spectral diffraction gratings, and extract from the white light, a color that corresponds exactly to that at the same point on the object. The result is a picture in exquisite natural color.
A hologram is the image captured on film of an interference created diffraction pattern. Light from a coherent source is divided into two paths. One contains the plane (reference) wave, while the other illuminates the object. The object's reflecting points cause their reflected (object) waves to be spherical. An intersecting plane (film) placed at a point where the spherical and plane waves are interfering, would have on its surface, a series of irregularly spaced concentric rings looking somewhat like a zone plate. The ring spacings correspond to the phase differences between the zero and first orders of the transform. When viewed under its reference light, the film's rings cause its diffraction and result in object reconstruction.