As the opening authors at a two-day seminar concerned with imaging techniques, we have a responsibility to define some terms. Obviously, "imaging" is the first word that merits consideration. The dictionary (Ref. 1) associates many words with an image, including imitation, likeness, copy, and conception. A definition (Ref. 1) that seems to fit many of the topics to be discussed here is as follows: "An image is an optical appearance or counterpart of an object---analogous to an image formed by light rays." A definition such as this permits us to include test systems that employ light as well as other radiations (X- and y-rays, neutrons, acoustical, ultrasound, and thermal) in our image seminar. Indeed,a glance at the program quickly verifies that the majority of test systems to be discussed involve radiation other than light.
A real-time inspection system has been developed at HEDL for rapid and thorough x-ray inspection of FFTF fuel pins. The FFTF fuel pins consist of sintered UO2 - PuO2 pellets contained in seamless, wire-wrapped 316 SS tubes. Each pin is 0.23 inches OD, and 94 inches long, and is sealed by a cap welded at each end (Figure 1).
One of the authors (ESK) and his students (Refs. 1-4) have, for several years, investigated system dynamics information contained in the gamma ray field emerging from an operating nuclear reactor core. Early experiments were directed along the lines of obtaining the sys-tem transfer function (frequency response). Either a driven perturbation or the natural noise generated in a neutron chain reaction represented the input to the system. The emerging gamma rays represented the system output. In later experiments, two highly collimated detectors were used to respond to the gamma rays generated in two narrow, intersecting channels through the reactor core. As with the earlier experiments, either a driven perturbation or natural noise phenomena can be used as dynamic system input. Selectivity in gamma ray energy detection and correlation of the two detector outputs yield information regarding the fission rate in the volume of intersection of the two channels under surveilance provided that the input process correlation is of dimensions on the order of (or smaller than) the volume of intersection.
A number of direct imaging techniques were compared by analysis of radiographs, and density versus exposure curves were generated for the various techniques. Fiber optics face plates were used to improve the resolution of scintillator-produced neutron radiographs while calcium-tungstate scintillation screens were employed to enhance transfer radiographs. The use of gadolinium oxysulfide scintillation screens resulted in reduced exposure times compared to standard metal screens.
In the United States, as well as in Germany, England, and Japan, a limited effort was devoted during the last twenty years to the development of solid state radiographic converters (Ref. 1), which could replace the fluoroscopic screen or the radiographic film. In spite of the fact that the developed converters had some very attractive characteristics, they did not find routine applications in radiographic work (Ref. 2). One reason for this was that some characteristics, as the resolution, the speed and the sensitivity were not satisfactory for a wide application.Another reason was that a vacuum type X-ray image intensifier, which was developed earlier (Ref. 3), gained a widespread application, principally in medical radiology. However, as the work on solid state converters continued, their characteristics were improved so that they can advantageously be used in special applications (Ref. 4 and 5). In the following the working principles and the present state of the radiographic converter screen will be described.
A new silver halide material, 3M Company's Type HL film, for clinical x-ray imaging either with or without an intensifying screen, can be handled under relatively high room-illumination levels and therefore requires no dark-room. Evaluation of this new film in comparison to 3M Company's conventional Type R film shows Type HL to be equivalent or superior: The relative speeds are approximately the same, and the resolution and MTF of Type HL are slightly better than of Type R. Type HL is sensitive to the same wavelengths as the spectral emission of the intensifying screens but is insensitive to the wave-length emission region of present x-ray darkroom safe-lights or the new high-light-level filters supplied by 3M. Although the evaluation discussed here is based on clinical uses, the film promises to be advantageous in industrial radiology.
Though nature abounds with sonic detecting systems, men have been slow to recognize the potential use of acoustic energy as a means of visualization, that is, for "seeing" with sound. In the 18th Century a rather inventive Italian scientist named Spallanzani carried out a study of the remarkable ability possessed by bats for avoiding obstacles in the dark. (Ref. 1) He and his co-workers finally concluded that bats must have some unknown "sixth sense". One hundred years later the Frenchman Langevin proposed a sonic-echo system as a possible means of locating objects submerged in the sea, German U-boats in particular. Shortly after, Hartridge recognized and suggested a possible similarity between Langevin's system and the system used by bats (Ref. 2,3).
The capability of nondestructive testing techniques heavily depends on the way of collecting, analyzing and presenting data for defect evaluation. The more dependable evaluation calls for the more thorough information. It is often desirable to present data in graphical form, so that the location and the shape of defects within in the sample under test can be seen at a glance.
An ultrasonic imaging technique has been developed which produces an isometric projection of three dimensional objects on a two dimensional display device. The technique has been demonstrated by apparatus which combines X, Y and time or Z axis information from conventional C-scan type devices into the X' and Y' axes of the display device. This arrangement provides a combination of depth and spatial information in a single record which can be easily interpreted. The controls may be adjusted to view the image from any perspective. By recording the test information on a tape loop and playing it back at high speeds, the operator can select the optimum perspectives for interpretation of unknown objects and flaws. Orthographic projections can also be made. Applications and demonstrations (including successful imaging in liquid sodium) are described which clearly indicate the advantages of the ultrasonic imaging technique over conventional B- and C-scans. The technique also has some advantages over scanned ultrasonic holography in that the entire object is in focus and more accurate depth resolution can be obtained. It could also be used to good advantage to complement the capabilities of holographic techniques.
This paper discusses the application of various scanning configurations used in acoustic holography for imaging internal flaws or voids in thick metal sections; i.e., 10 to 25 cm. The following source-receiving scanning configurations were employed: 1. Simultaneous (inclined plane-wave) source and point receiver scan-ning; 2. Simultaneous (focused) source-receiver scanning employing a simulated acoustic off-axis reference; 3. Simultaneous point source-receiver scanning employing a simulated acoustic off-axis reference; and 4. Stationary (plane-wave) source and point receiver scanning employing a simulated acoustic off-axis reference. All of the above mentioned configurations impose a linear diffraction grating on the hologram and provide the necessary conditions for imaging flaws directly in the projected aperture and separation of the reconstruct-ed images. The flaw images appear two-dimensional as a result of the large difference between the construction and reconstruction wavelengths. Longitudinal magnification is much greater than the lateral magnification and the images appear stretched in depth. However, the hologram does contain the complete depth information of the flaws and this is easily observed by focusing on different planes in the metal sections parallel to the hologram plane (i.e., different depths within the metal section are viewed simply bymanipulation of the optical imaging lens in the reconstruction The images of the internal flaws are viewed by adjusting the lens until the image is in focus. The lateral and longitudinal dimensions of the flaws are then calculated using the conventional image location and magnification equations.
Recent advances in the field of acoustic holography have shown it to be a very promising technique for the non-destructive imaging of the interiors of opaque objects. High resolution acoustical holography would enable the mapping and characterization of minute hidden flaws in materials, and facilitate the study of their temporal propagation. Acoustical methods are particularly well suited to the visualization of flaws because the large impedance mismatch presented by such a defect causes a strong reflected signal and a pronounced display. It may well be possible to visualize features of a specimen which go undetected by conventional methods.
An improved method of quickly determining the flatness characteristics of disks to be used for magnetic recording in disk packs has been developed. The data is obtained optically so that there is no mechanical contact with the disk. The instrument has been made relatively insensitive to disk alignment by means of a retro-reflector. The entire surface can be quickly scanned by a combined circular and radial motion of the disk holder. Scattering of the optical beam by scratches on the disk surface does not affect the accuracy of the system. The system compares the resultant image formed from the light beams reflected from the test disk surface with values expected from a disk within specifications. The process has been automated by scanning the output of the system with various size apertures located in front of a photodetectoi The final output pulses may be recorded as well as displayed. Portions of the disk surface that are out of specification can be rapidly located. This method of evaluation is considered more meaningful than present methods. The relative ease with which data may be taken is considered even more significant. Correlation with data taken by existing methods will be discussed.
The microscope was invented several hundred years ago and stereo microscopes were developed about 75 years ago. These optical instruments usually only allow individual viewing. This makes inspection, testing, training, and other procedures difficult and time consuming since each person has to take turns viewing.
Within the past year, several papers have been presented concerning the advent of a new device for real-time non-contact thermal imaging of microscopic targets. This device, the BM-50 IR Microscanner has since been utilized in several new and interesting areas of non-destructive testing and it is the purpose of this paper to discuss these new applications and to project sore areas which show future promise.
Holographic interferometry has been the most immediate, significant application of laser holography. For the first time, optical interferometric measurements can be made on non-optical surfaces; such as castings, pipes, panels, composites, etc., as well as through diffusely illuminated transparent scenes. Like classical interferometry, holographic interferometric measurements are sensitive to path changes down to one tenth of a wavelength of light (0.07 micron or 3 microinches). The technique has made available a completely new method of non-destructive testing: automobile tire testing, testing of adhesive bonds, pressure vessels, microwave antenna testing, the determination of resonant frequencies of mechanical structures, transient deformation of surfaces, and the study of aerodynamic phenomena.
Holographic measurements of optical phase are used in primarily three forms, double-exposure holographic interferometry, real-time holographic interferometry, and time-average holographic interferometry. A fourth type which utilizes moire' principles and which has many variations is possible. This fourth type is essentially multiple-beam interferometry.
Many important quality tests on materials and components are made by visual assessment. Operators use microscopes to make visual assessments to describe, for example, the non-metallic inclusion content of steel or the grain size in a variety of materials. Human inspectors frequently have to watch miles of fast moving strips of paper, steel, cloth and other materials to spot and record visible defects. Also, Exr example, electron micrographs are examined to determine particle size distributions and radiographs are inspected visually to evaluate porosity.
There have been some technological improvements for identifying and interpreting side-looking radar imagery since the appearance of Hoffman's review article on radar technology in 1960 . These are marked by the refinement of traditional photo-interpretation techniques, and the use of false-color enhancement and isodensitracer [microdensitometer] attempting to correlate density patterns with terrain features . These methods proved to be more sophisticated for individual case studies, but not efficient for processing the massive inflow of SLR data from aerospace platforms as expected in the future. The purpose of this paper is to suggest an automatic pattern recognition system of SLR imagery based upon discriminatory analyses on the isodensitracer and TV scanned data.