IR to Composite Video Conversion
Figure 6-12 is a diagram showing the optics and
electronic components of a typical receiver-converter
required to perform IR detection and conversion into
a useable video signal.
Although the signal optical
path and conversion coincide, each path is discussed
separately for simplicity. You should refer to
figure 6-12 as you read the following paragraphs.
SIGNAL OPTICAL PATH.— Incoming IR
energy from a target enters the receiver through a
window and strikes one side of a double-sided scan
mirror. This mirror is controlled by a seamer module.
The IR signal either strikes the mirror directly or is
focused onto the mirror by optical lenses contained in
what is called an afocal optics unit. The operation of
the afocal unit is governed by a field of view (FOV)
switch on the operator’s infrared detecting set control
(IRDSC). In the wide FOV mode of operation, the
afocal optic lenses are not in the signal optical path.
In the narrow FOV mode of operation, the lenses are
in the path shown in figure 6-12. The lenses are
focused by a motor that is controlled by a focusing
module in the receiver-converter assembly. The
focusing module has two inputs to control the
operation of the focusing motor. One input is a target
range scale set by the operator on the IRDSC. The
other input is a feedback signal from a temperature-
sensing circuit in the afocal optics unit. Because the
index of refraction of an optical lens changes with
changes in temperature, the focusing module
monitors the temperature of the lens and maintains
proper focusing of the IR signal onto the scan mirror.
The scan mirror is controlled by a scanner
module. It is also positioned in line-of-sight (LOS)
position, along with the entire receiving head, by
Figure 6-12.-FLIR optical path and IR processing.
6-10
