Scanning

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Scanning

 

(time-base sweep), a method of representing the changes of a physical quantity variable over time by unambiguously converting it into another quantity that varies in space. Scanning is performed by a scanning element, which successively scans space according to specific laws so that certain spatial coordinates of the scanning element correspond to each instant of time and to a certain value of the physical quantity. The reverse transformations usually are also called scanning, obviously on the basis of the similarity of the techniques used in both cases.

Thus, in the picture tube (kinescope) of a television receiver, the electrical voltage at the control electrode, which is variable over time, is converted in a specific manner by means of scanning into a change in brightness over the surface of the screen. In a television camera tube, the reverse process is accomplished by scanning—the brightness of various sections of the image is converted into an electric current that varies accordingly. In both cases, the scanning element is the point on the surface of the screen at which the electron beam is focused. The movement itself of the scanning element is often called scanning.

The scanning element may be (1) a spot of light that moves over the image or screen on deflection of the light beam (optical scanning) or on displacement of the object of the image, (2) a small moving aperture in the screen that covers the image or the pen of an automatic recorder (mechanical scanning), or (3) a luminous point on the screen of a cathode-ray tube (electronic scanning). Since a single scanning system may use a combination of optical, mechanical, and other methods of producing and deflecting the scanning elements, scanning cannot always be classified exactly on the basis of these characteristics.

Scanning may also be classified according to the trajectory of the scanning element. In rectilinear scanning, the trajectory is a straight line, in circular scanning, a circle, and in spiral scanning, a spiral. If the trajectory of the scanning element forms a raster, then the corresponding scans are called raster scans, which are then classified according to the shape of the raster. If the scanning element moves along the image contour as if tracking it, then such a scan is called a tracking scan. The objects scanned may be both continuous and discrete physical quantities.

Scanning is widely used in oscillographs, automatic recording devices, radar displays, and long-distance data transmission equipment. Oscillographs chiefly use rectilinear scanning. The scanning is called multiple scanning if each scanning cycle is immediately and automatically followed by the next cycle. The scanning is called slave sweep if each cycle begins only when a special triggering signal is received.

In radar displays, two-dimensional scans, for example, panorama or radial-circular scans, three-dimensional scans, and other types are used to determine target coordinates on the display screen.

Television and facsimile chiefly use raster scans with a rectangular raster. Systems with tracking scanning are sometimes used in the transmission of contoured and line images in facsimile and in feeding graphic information to an electronic computer.

L. I. FREIDIN


Scanning

 

the controlled spatial displacement of a ray, such as a light ray, or beam, such as an electron beam, according to some definite pattern. Scanning is frequently used in various areas of science and technology. For instance, scanning by an electron beam is used to produce an image in cathode-ray tubes and in scanning electron microscopes. Scanning by a light ray is used in optical data-processing systems.

Scanning can be achieved by mechanical or nonmechanical methods. Mechanical methods are based on an angular displacement of the radiating system. In nonmechanical scanning the ray is displaced either by electrically controlling individual components of a stationary radiating device or by controlling the properties of the medium through which the ray is propagated. Alternating electric or magnetic fields that act upon a beam of charged particles are used to achieve scanning by such beams.

V. N. PARYGIN

Figure 1. Beam motion in helical scanning. The displacement of the beam is rotational with a constant angular velocity. The relative motion is oscillatory in a plane perpendicular to the plane of rotation and at a significantly lower velocity.


Scanning

 

in radar, the sequential inspection of a given spatial zone in some prescribed manner by displacing a radar beam or the solid angle of sight of a receiving antenna. The purpose of scanning is the detection and observation of objects located in the scanning zone.

Figure 2. Beam motion in zigzag scanning. Both the displacement and the relative motion of the beam are oscillatory, with varying velocity ratios (a and b).

Scanning can be achieved either with a fan or pencil beam. In a fan beam the beam angle is much smaller in one plane than in the other; a pencil beam is a narrow beam symmetrical with respect to the direction of maximum intensity of radiation. The most common scanning methods with a fan beam are azimuth scan, in which the beam rotates around a stationary axis, and certain methods using sector scanning, in which the beam oscillates periodically through a given sector. Multibeam scanning systems with fan beams are also used. In scanning methods that use a pencil beam the complex motion of the beam can be represented as two simple motions: a displacement motion around a stationary axis and a relative motion around a moving axis.

Figure 3. Beam motion in spiral scanning. The displacement of the beam is rotational, and the relative motion is oscillatory (here, with a lower velocity).

The principal types of scanning with pencil beams are helical scanning (Figure 1), zigzag scanning (Figure 2), spiral scanning (Figure 3), and sequential lobing (Figure 4). Conical scanning (Figure 5) is a special type of sequential lobing.

REFERENCES

See references under RADAR.

B. V. MALANOV

Figure 4. Beam motion in sequential lobing. The displacement of the beam is oscillatory, and the relative motion is rotational (here, at a significantly higher velocity).

Figure 5. Beam motion in conical scanning. The motion of the beam is rotational (circular). The direction of maximum intensity of radiation OA is displaced with respect to the axis of rotation OO’ by some constant angle α, Here, α is less than one-half the beam width in order to ensure a direction of constant signal along the axis of rotation.


Scanning

 

in radiology, the study of the distribution of radioactive preparations introduced into the body of a person or animal for purposes of diagnosis, treatment, or research. Scanning utilizes radioisotopes or their compounds, which, upon decaying, emit gamma quanta. A scanner is used to make the distribution of the radioactive preparations visible; it consists of a movable gamma-radiation detector (scintillation counters) and of systems for translating an electrical signal into a light signal. The resulting image is then recorded graphically or by means of black-and-white or color photography.

In clinical practice, scanning is used to obtain images of almost all internal organs and systems. It makes it possible to determine the position, shape, size, nature, and functional state of an organ’s internal structure and to detect the presence of sizable lesions. The method is safe, simple, and causes no discomfort.

V. Z. AGRANAT and F. M. LIASS

scanning

In radar, the motion of the radar antenna assembly when searching for targets. Scanning usually follows a systematic pattern involving one or more of the following:
i. In horizontal scanning (or search lighting), the antenna is continuously rotated in an azimuth around the horizon or in a sector (sector scanning). This is used to find the azimuth of the targets and generate plan-position-indicator-scope displays.
ii. Vertical scanning is accomplished by holding the azimuth constant but varying the elevation angle of the antenna. It is used in height-finding radars to generate the relative-height-indicator-scope display.
iii. For conical scanning, a somewhat off-center radiating element is rotated while its parabolic reflectors remain fixed in position, so that the radiated beam generates a conically shaped volume with the antenna at the apex. It is used to determine an accurate bearing and the elevation angle of targets and is employed in automatic-tracking radars.
iv. In helical scanning (or spiral scanning), the azimuth and the elevation angle of the antenna are constantly varied, so that at a given distance from the radar the radiated beam generates the surface of a hemisphere. It is used for radio direction finding, in certain types of search radars, and in tracking radars to search areas for targets. See sector scan, conical scanning, helical scan, and height-finder radar.
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