system increases, and the reliability of the system
decreases. If the number of detectors decreases, the
amount of information that can be processed is
reduced. A compromise between a large number of
elements (increased cost) and a smaller number of
elements (reduced information) is the linear array that
scans in one direction only. Each detector scans one
line of the scene image. The complexity of the
electronics is reduced, and the amount of information
that is processed is increased. Thus, the size of the
scene to be viewed and the detail of the scene is
increased.
There are many types of mechanisms that can be
used to scan the scene. When you scan with two axes,
the two scanning motions must be synchronized. The
electronic signal that controls the sampling of the
detectors must also be synchronized with the scanning
motions.
Front End Optics
The front end optics collect the incoming radiant
energy and focus the image at the detectors. The
optics may be reflective or refractive, or a
combination of both. Many systems offer a zoom
capability, allowing a continuous change in
magnification of the image without changing the
focus. Spectral filters are used to restrict the
wavelength of light entering the system. This
prevents unwanted wavelengths of light from
reaching the detector and interfering with the imaging
process.
Refrigeration System
A refrigeration system is needed in imaging
systems because many types of infrared detectors
require low temperatures if they are to operate
properly. The two types of detector cooling that are
used are the open-cycle and the closed-cycle types.
In the open-cycle type of cooling, a reservoir of
liquified cryogenic gas is provided. The liquid is
forced to travel to the detector, where it is allowed to
revert to a gas. As it changes from a liquid to a gas, it
absorbs a great deal of heat from the surrounding area
and the detector.
In the closed-cycle type of cooling, the gas is
compressed, and the heat generated by the
compression is radiated away by the use of a heat
exchanger. The gas is then returned to the compressor
and the cycle repeats itself.
Image Processing Systems
The image processing system is used to convert
the data collected by the detectors into a video
display. Data from the detectors is multiplexed so
that it can be handled by one set of electronics. Then
it is processed so that the information coming from
the detectors is in the correct order of serial
transference to the video display. At this point, any
other information that is to be displayed is added.
In other image processing systems, the signals
from the detectors are amplified and sent to an LED
display, or they are optically amplified by
photomultiplier tubes and projected on a
phosphorescent screen.
FORWARD-LOOKING INFRARED
SYSTEM
Learning Objective: Recognize components
and operating principles of a typical FLIR
system.
A forward-looking infrared system is an infrared
detecting set (IRDS). The IRDS is a passive device
that operates on the IR principles of emissivity. The
terms FLIR and IRDS are synonymous so far as
system operation is concerned. Only the azimuth
coverage differs.
The FLIR system scans an
operator-selected portion of the terrain along the
aircraft’s flight path and displays a televised image of
the IR patterns of the terrain. The primary function of
the FLIR system is to give the operator an improved
capability to detect, identify, and classify targets of
interest that would otherwise be concealed from either
visual observation or radar detection. The visual
concealment could be due to darkness or camouflage.
The radar concealment could be due to extreme scope
clutter caused by inclement weather, rough seas, or
electronic jamming. Additionally, the FLIR system
emits no transmission for detection by an enemy.
Although there are various types of FLIR systems
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