Radio Imaging

the production of a visible image of an object by means of radio waves. It is used to study the internal structure of objects that are opaque to waves in the optical region and to observe objects that are located in an optically opaque medium. Since the radio waves usually employed in radio imaging are in the millimeter and centimeter bands, rather fine details of an object’s structure can be distinguished in the optical image that is produced. The radio waves radiated—in passive radio imaging—or scattered—in active radio imaging—by bodies carry information regarding the bodies’ structure and condition. The waves may convey this information through, for example, their intensity and phase distributions, the nature of their polarization, or their time lag. The chief task of radio imaging is to collect the information and present it as a visible image. This is accomplished by special devices called radio introscopes; an example is the radio image device.

Figure 1. Schematic of a radio image device: (1) radio-frequency radiation, (2) casing of the device, (3) polyethylene terephthalate (lavsan) film, (4) aluminum layer, (5) ultraviolet rays, (6) sources of ultraviolet radiation, (7) phosphor layer

Various physical effects and phenomena are made use of in radio imaging. Thus one type of radio image device is based on the variation in intensity of luminescence of certain phosphors with temperature. The principal element of this device is the screen. The screen consists of a film of polyethylene terephthalate (lavsan) under tension; the film is spray-coated with a thin layer of aluminum, which is covered with a layer of the thermo-sensitive phosphor (Figure 1). The screen is irradiated on the phosphor-covered side with ultraviolet rays and emits a weak, uniform glow. When the screen is struck by radio waves having a complex spatial distribution of intensities, the waves are absorbed by the aluminum substrate, which is heated most strongly where the intensity of the radiation is greatest. When the phosphor is heated by the aluminum substrate, it emits less light, and a visible negative image appears on the screen. With such a radio image device objects can be “seen” with the same sensitivity by means of waves from the infrared to the super-high-frequency range. The sensitivity of the screen depends on the characteristics of the phosphor and the power of the radiation. The visual detection threshold of the device is about 1 milliwatt/cm². On the radio image device’s screen image details can be discerned that are of the order of tenths of a mm in length.

Sensing elements used in other radio introscope designs include liquid crystals, semiconductor single crystals, and special photographic films. For all such elements the action of the radio waves alters the optical characteristics—the reflection coefficient or the transmittance for visible light.

The radio images of objects are obtained most often by scanning the object with a narrow beam of radio waves and receiving the signals reflected from the object. The scanning may be accomplished, for example, by the mechanical rotation of the radiating and receiving antennas or by an electrical method. In the electrical technique, the phase of radio waves radiated from many sources is varied in such a manner that a narrow beam of radio waves is formed in space; this beam “looks over” the object or locality. Sometimes the radio waves reflected from an object are shaped by means of radio lenses as light waves are shaped in optics.

Radio imaging is used to detect and identify aircraft. It is also employed in the landing and takeoff of airplanes under unfavorable meteorological conditions, such as fog, rain, and snow. Its other uses include marine and river navigation and space research. It is used for the nondestructive inspection of materials and products in industry and for the diagnosis of various diseases in medicine. Radio imaging can be used to check the quality and alignment of sources of radio-frequency radiation. It is employed in the arctic and antarctic to determine the thickness and structure of ice sheets and is also used in mountainous regions. Present research on radio imaging is centered on the use of the principles of holography and on the achievement of color images.