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background detail is obtained with a separate conventional photography 
motor-driven 35 mm camera mounted alongside the IR camera to view a 
corresponding but 30 percent larger field than the IR camera. High-speed 
color film is used. 
The rest of the system consists of a custom-designed automatic 
exposure control unit (AECU) to control the synchronous exposure of the 
two motorized conventional cameras on demand and to supply digital display 
on the imagery of data on date, time, filter and aperture combinations, 
and exposure number. A third conventional 35 mm camera records aircraft 
instrument readings at the time of each picture. The IR system is run from 
an aircraft 12 or 24 V DC power supply through a stabilized 115 V AC sine 
wave static inverter. A magnetic tape recorder can be used to record an 
analog IR signal for later analysis, but for our operation of repeated passes 
over a point we found photographing the IR signal as displayed on the color 
TV monitor to be a sufficient record. 
A high speed twin engine light aircraft was required to transport 
the IR system to forest fires. We found the Piper Aztec (PA-23-250C) to be 
suitable because of its high cruising speed, which minimized data losses in 
getting to fires while they are still in the initial spread stage, and its 
good maneuverability in terms of ability to climb and turn quickly in the 
difficult terrain which often hampers fire surveillance. 
The IR and conventional cameras are mounted on a sliding platform 
which extends outside the aircraft fuselage through a modified cargo com 
partment door (Figures 1 and 2). The IR camera remains horizontal because 
of the requirement of liquid nitrogen cooling of the InSb sensor, but its 
view is vertical downwards through a 45 first-surface aluminum coated mirror. 
Figure 3 illustrates the mounting of the IR system components in 
the cabin of a six-passenger Piper Aztec. Pilot and navigator occupy the 
front seats, the mid seats and right rear seat being removed. The IR operator 
sits behind the color monitor, (left), viewing the screen in order to make 
gain control settings and activate conventional cameras for recording each 
pass over a fire. Below the color monitor is the DC to AC power inverter. 
On the right is the IR system control console showing the black and white 
cathode ray tube monitor which is sometimes photographed with a Polaroid 
camera to provide instant hard copy of fire imagery. The automatic exposure 
control unit is mounted on top of the control console. 
Analysis of the IR imagery for a particular fire involves a pre 
liminary step of drawing maps of fire perimeters from successive photographs, 
corrected for scale differences due to variable flying height and aircraft 
tilt and heading differences. Measuring of spread rate or advance of various 
portions of the fire front over discrete time intervals can then be readily 
done. In addition fire residence time or duration of burning in various fuels 
can be determined from time-apparent temperature functions drawn for particular