Since neither instant transmission nor rapid delivery of imagery to 
a fire control agency was required, we did not need the expensive capability 
of producing continuous hard copy or magnetic tape recording of IR video signals. 
For the same reason it was not necessary to deal with the problem of resolving 
low intensity ambient backgrounds on the same imagery with high intensity fire 
output. Gain controls could be set to match the IR output of the fire with 
separate conventional photography used to record ground detail. 
THE INFRARED SYSTEM 
Tests conducted in 1972 indicated that a commercially available IR 
imaging system, AGA Thermovision 680 System, a product of AGA Infrared Systems 
AB of Sweden, was one that could meet our needs. This system uses an indium 
antimonide (InSb) photovoltaic sensor which resgonds to infrared radiation in 
the spectral range of 2.0 to 5.6 microns. A 25 by 25 lens is used for our 
application and this lens has an instantaneous field of view (IFOV) of 2.5 
milliradians. 
This optical configuration is seldom limiting in resolution, but 
occasionally creates problems in terms of ability to photograph large fires 
in one frame because of aircraft flying height limitations. Fires as small 
as 0.2 ha may be mapped from 300 m above ground whereas at maximum flying heights 
of about 3700 m, the total field of view is about 270 ha. 
For our use as a fire mapping sygtem, the AGA Thermovision 680 
specification of thermal resolution of 0.2 C at an object temperature of 
30 C was not of as much concern as the ability to spatially resolve small 
targets with large temperature differences from background at large object- 
to-sensor distances. We found that the 25 IR lens could resolve 0.2 m 
burning charcoal targets from ambient backgrounds at 98 m flying heights 
and 1 m targets at 345 m. This indicated adequate resolution of the smallest 
fires would not be limiting, so long as timber canopy and angle of view do 
not combine to obscure the target. We found in operational use that only the 
smallest spot fires under dense timber canopies, such as lightning strikes 
less than 2 m in size under dense conifer stands, were obscured unless viewed 
very near the nadir. 
Conventional photography is employed for permanent recording on 
35 mm color film of the IR image which is produced on a ten-color cathode 
ray tube television monitor at a rate of 16 fields per second. 
The system can be preset for object temperatures ranging between 
-30 C and +850 C by selecting an appropriate combination of sensitivity 
(10 steps) and lens aperture (7 f/stops). Filters reducing IR transmission 
from very hot sources can increase the measurable temperature range up to 
2000 C. Because a given sensitivity-aperture setting combination cannot 
cover the full range of ambient background to forest fire temperatures, for 
our use the optimum setting for any particular fire is selected and ambient