A method of optical filtering in order to provide for enhancement of x-ray images whose quality has been degraded because of the use of extended x-ray sources (penumbra effect) is considered. This method employs spatial filtering techniques in a coherent optical system. A description of the penumbra effect follows from the mathematical description of imaging in incoherent optical systems. With the use of this model the degradation can be described in terms of the effect of the extended source on the frequency content of the resulting image. This leads to a precise definition of the ideal filter in terms of the spatial intensity distribution of the source and a general description of the filtering operations which will be required in all cases, independent of the exact source distribution. It is shown that the form of the ideal filter is such that the same optical system used in the filtering process is ideally suited for generating filters with, approximately, the required transmission characteristics. Experi mental results are presented.
The performance characteristics, such as time delay, spurious response levels and bandwidth, of multiple-reflection-type delay lines are discussed. State of the art performance is indicated, and efforts to improve these characteristics are described. The time delay is determined by the path length of a given design and size limitations on the fused-quartz blanks. Multiple symmetry designs are derived from regular polygons by rotating facets to achieve long path lengths. Typical designs are presented, and the delays that can be achieved in readily available blanks of fused quartz are indicated. Spurious responses result from diffraction at the transducer and subsequent reflecting facets, and from reflections at the transducers. “Creation angle” spurious responses characterize multiple symmetry type designs, resulting in the use of simpler designs for many critical applications. Aperturization techniques may be employed to control the size of the reflecting facets, and hence to control certain diffraction effects. The delay-line bandwidth is a function of the transducer resonant frequency, bonding techniques and delay medium characteristics. The dependence of bandwidth on bonding parameters is discussed in terms of the transducer equivalent circuit. Medium losses increase with frequency, and limit the attainable bandwidth for long delays. The ultimate performance capabilities of multiple reflection type delay lines using materials and techniques currently available are considered. Limitations imposed on the time-bandwidth product and dynamic range are discussed. Practical upper limits are considered in terms of figures of merit for communications and radar applications.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.