(2) The scan technique allows small areas directly below the aircraft to be scanned
in a line. The converted line scan IR energy is then produced as successive lines on the
recording device. The area that can be viewed by the scanner at any given instant is called
the instantaneous field of view (IFV), as shown in Figure 1-5. The ground resolution capability
of an IR system depends on the size of its IFV. Such factors as inaccurate detector response,
lack of target-to-background contract, recorder error, and uncompensated aircraft motion all
tend to degrade IR target ground resolution.
b.
The detector transforms the incoming IR signals into proportional electrical signals.
These signals are then amplified and fed into a recording unit. The detector is the heart of an
IR reconnaissance system. Crystals sensitive to radiation of varying wavelength are used for
IR detection. Since all matter above absolute zero temperature radiates IR energy, the
detector crystal itself must be cooled to near absolute zero. This cool-down reduces receiver
noise caused by internal IR radiation. This noise reduction is necessary for the detection of IR
energy collected by the scanner system.
(1) IR wavelengths change with changes in temperature. The average temperature
range of specific types of targets must be considered in selecting detector materials. The
actual material used as a detector and the temperature to which it must be cooled depends on
the wavelength of the IR radiation under surveillance.
(2) When IR radiation is focused into the detector crystal, an electrical signal is
proportional to the fluctuation of the incoming IR energy along the scan line. These electrical
signals are amplified and transmitted to the IR set recording unit.
c.
The recording unit converts the amplified IR signals into a variety of usable forms
recorded on film, video tape, or a cathode-ray tube (CRT) viewing screen. A number of
methods are used to produce identifiable images from IR line scan information. A common
method is to use the amplified signal from the detector to develop a light source such as a
CRT (Figure 1-6).
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