Included in the scanner design is a thermal 
reference plate which is placed at one end of the 
linescan locus so that referencing takes place at the 
scan rate. This arrangement enables the selection of 
five sensitivity ranges covering the following 
temperature ranges: 2°, 4°, 8°, 16° and 32°. Cooling 
is carried out using high pressure compressed air 
supplied to a Joule-Thompson minicooler. Further 
design details can be found in Berry and Runciman, 
1981. 
4 DATA ACQUISITION 
To date, both the Barr and Stroud IR18 and Rank 
Pull in Controls thermal scanners have been evaluated 
for remote sensing applications. The initial 
assessment of the former system was performed in 
August 1984 by flying the camera, in a Cessna 172 
light aircraft, over several tests sites in the West 
of Scotland. 
The Cessna 172 is a four seat aeroplane with two 
access doors and one luggage door immediately behind 
the pilot's door. Several modifications were 
necessary to the aircraft to enable the IR18 to be 
used for the production of vertical imagery. The main 
features of the design which was used are described 
in Ekin (1984). Briefly the approach adopted by Ekin 
was to design a retractable camera carriage, attached 
to the luggage door, which could be extended in 
flight by the camera operator. A pair of rails, on 
which four wheels with V - shaped groves ran ensured 
the smooth movement of the rig into the extended 
position. Figure 4 illustrates the IR18 mounted in a 
slightly modified version of the rig described. 
Figure 4: Barr and Stroud IR18 mounted on retractable 
camera rig in a Cessna 172 light aircraft. 
The adoption of a light aircraft and an easily 
manipulated TVFS system of the type described offers 
several advantages to the user. Firstly, by using 
widely available light aircraft the operational costs 
of the survey can be significantly reduced. Secondly, 
once the site of interest has been located and the 
camera rig extended, the scanner operator can view, 
via a small monitor, the area being imaged by the 
instrument. Thirdly, after the survey has been 
completed immediate confirmation that the data has 
been recorded successfully can be achieved by playing 
back the video tape, either in flight, or more 
commonly when the aircraft returns to base. 
For other applications however, it may be 
unacceptable to use the scanner from a light 
aircraft, either because of the need for a more 
stable platform, or the need to use the camera in 
conjunction with several other sensors. More recent 
experience of using the IR18, for defence and 
commercial applications by ERSAC Ltd. has involved 
using the IR18 in a purpose built camera mount in a 
twin engined Piper Aztec aircraft. For maximum 
flexibility of operations and integration with a 
standard aerial camera, a special adapter plate was 
designed and deveolped by ERSAC and Barr and Stroud 
to enable the IR18 to be mounted and operated through 
a standard Wild RC8 aerial camera mount. This 
configuration is illustrated in Figure 4. The most 
significant advantage of this design is the 
capability for full yaw control; this enables higher 
precision to be achieved when flying parallel 
flightpaths for the acquisition of imagery over large 
are a s 
Figure 5: Barr and Stroud IR18 in an adapted Wild RC8 
aerial camera mount in a Piper Aztec aircraft 
5 DATA PROCESSING 
The processing of video based thermal data consists 
of three distinct phases: 
(a) conversion of the analogue video recording to a 
digital format, 
(b) pre-processing of the digital data to remove 
image inconsistencies, and/or to rectify and 
geometrically correct the image, and 
(c) processing and analysis of the data using 
conventional image processing techniques. 
The analogue to digital (A to D) conversion of 
video tape is carried out using a digital frame store 
(DFS) or 'frame grabber'. These instruments are now 
widely available at relatively low cost. The 
particular model used for these investigations was 
the Quantel 3001 DFS controlled by an IDP 3000 image 
processing system. 
The pre-processing, and processing stages, were 
carried out on a GEMS image processing system. One 
particularly useful pre-processing technique is that 
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