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temperature data.
The majority of high performance TVFS systems
operating in the 8 to 13 pm band have been developed
for avionic and military applications. Development of
the current range of UK military imagers began in
1976 and at that time the decision was made to adopt
a modular design philosophy. This is known as the
Thermal Imaging Common Module (TICM) programme. The
GEC V1010 TICM 11 is representative of this design.
Instruments of this type can be mounted in either a
fixed wing aircraft or a helicopter, in the latter
case the scanner is mounted in a pod to allow a
forward looking image to be obtained.
The Rank Pull in Controls (RPC) SS600 thermal imager
differs from the conventional TICM scanning design
which uses a flapping mirror to scan the.field in
elevation and a rotating polygon to provide the line
scan. In contrast, the RPC design uses two coaxially
mounted rotating polygons to form the two dimensional
scan pattern. This unique design (Figure 1) is
claimed to result in a lighter, more robust and more
compact imager suitable for use where severe
vibration may be experienced.
DETECTOR
10Omm
Figure 1. Rank Pullin Controls SS600-scanning system
(Lettington, 1985)
The Barr and Stroud IR18 is another TVFS system based
on a modular design approach. A narrower scanner
field of view is used compared with several other
equivalent systems. In this case also a novel
optical-mechanical scanning design is used which is
discussed in the following section of the paper.
3.3 The Barr and Stroud IR18 Mk II Thermal Video
Frame Scanner
The scanner used in these tests was developed
initially for military use, but it is now available
for commercial remote sensing applications. Table 2
outlines some of the technical specifications of the
sensor and Figure 2 illustrates the principle of
operation of the scanner.
To achieve a video compatible image, two main
criteria have to be met: firstly, the resultant image
must be rectilinear and, secondly, it should have a
625 line CCTV output (525 lines in the USA). In order
to achieve this, banded scanning is carried out using
a high speed rotor, turning at 39,000 rpm, to give
Figure 2. Barr and Stroud IR18 Mk 11 thermal video
frame scanner - principle of operation (Berry and
Runciman, 1981)
the line format, and a slower galvanometer driven
rotor to achieve the frame transfer at 50Hz. Petrie
(1983) has estimated, that the system has a data
collection rate equivalent to 19 X 10^ pixels/second.
The optics of the IR18 are used to collimate the
incoming radiation and, by using afocal telescopes,
enable a variety of magnification ratios to be used,
so producing various object field of views (Table
3). Such an arrangement ensures that the
incident radiation forms a bundle of parallel rays
which can then be focussed onto the frame scanning
rotor. A concave mirror then re-collimates the image
onto the rotating hexagonal rotor and subsequently
onto the detector.
Table 3: Barr and Stroud IR18 - object field of view
with various auxiliary telescopes
Magnification
Ratio
Object Field
of View
(H° XV 0 )
1
38.0 25.5
1.5
25.3 17.0
6
6.30 4.25
9
4.22 2.8
14
2.7 1.8
The detector used is an eight-element Mu Hard
’SPRITE’, although four elements are normally
preselected for use. The scanning sequence is such
that on each successive frame scan a different
element (of the four active elements) scans the same
part of the scene. A consequence of this is that if
the imager output is integrated over a number of
frames, detector non-uniformity errors can be
eliminated. Complex microprocessor controlled
circuitry controls the synchronisation of the rotor
speeds and signal outputs and the transfer of the
data onto video tape. A conventional U-Matic video
tape recorder is used to store the imagery.
