286 
3 THERMAL VIDEO FRAME SCANNERS (TVFS) 
A TVFS system consists of four basic components: 
(a) a scanning system which views the scene to be 
imaged and focusses the incoming infrared radiation, 
(b) a ferro-magnetic or semi-conductor detector 
which measures the thermal variations in the scene, 
(c) a CCTV compatible display system where the 
thermal variations are used to modulate the intensity 
of an electron beam, and 
(d) a video cassette recorder (VCR) to record the 
thermal imagery. 
Unlike thermal IR linescanning systems which are 
designed specifically for airborne use, the 
TVFS systems under consideration are designed for 
both ground and airborne operation. Consequently, the 
optical configuration, scanning mechanisms and 
detector arrays differ from those used for thermal 
linescanning and this leads to significantly 
different imaging systems being employed. 
3.1 Imaging techniques 
In general terms a thermal imager operates by 
mechanically scanning a focussed beam of incoming 
radiation onto an infrared detector. In its simplest 
form, such as in a thermal IR linescanner, the 
scanning is performed in one direction, perpendicular 
to the direction of flight, by an oscillating mirror 
or rotating prism. The incoming radiation is then 
focussed onto a single detector, normally of mercury 
cadmium telluride (HgCdTe). The 'frame' scanning in 
this case is performed by the forward motion of the 
aircraft. 
If the imager is to be used from, a static platform 
then some method of scanning in two dimensions is 
required. There are, however, several practical 
difficulties with such a single element detector 
design if a flicker free, high spatial resolution 
image is to be obtained. Consequently, if high 
resolution is required it is often more efficient to 
use a multi-element detector design. Three 
distinctive arrangements are possible: parallel, 
serial and matrix scanning. 
Parallel scanning involves using a single column of 
detectors arranged so that each detector element 
scans a single line in the image. This arrangement 
reduces the scanning speed. However, the performance 
of each of the detector elements needs to be similar 
if the formation of a 'streaky image' is to be 
avoided. Further significant reductions in scan speed 
can also be achieved by using band interlaced 
scanning techniques (Chiari and Morten, 1982). 
Serial scanning systems, in contrast, operate by 
using a single row of detector elements. The 
cumulative output is obtained by summing the 
individual signals from each detector. In order to 
achieve this it is necessary to include separate 
pre-amplifiers and delay line circuitry for each 
detector element. Although the electronics of this 
design are more complex, since the same detector 
element scans all lines in the image the uniformity 
of the image is more consistent than with the 
parallel system. 
A compromise arrangement which combines the scene 
uniformity advantages of the serial scan approach 
with the high scan speeds of the parallel arrangement 
is the mixed parallel/serial or 'matrix' design. 
Inevitably, as the number of detector elements 
increases, so the number of electrical connections 
also increases. Although this difficulty, and the 
consequent signal processing complications can be 
overcome, the design trend in recent years has been 
to use SPRITE detectors. 
As mentioned previously, with conventional serial 
scanning the output from each IR detector is 
pre-amplified, delayed and then added to the signal 
which is generated in the following element. SPRITE 
(Signal PRocessing In The Element) detectors overcome 
the need for these separate connections. In the 
SPRITE design the row of individual detectors is 
replaced by a single strip of HgCdTe with only two 
connections and one pre-amplifier. Eight element 
SPRITE detectors are used in most TVFS systems, such 
an arrangement is equivalent to a conventional array 
consisting of 64 discrete elements. 
Although SPRITE detectors eliminate many of the 
connections and much of the circuitry, they still 
require some form of mechanical scanning to be 
carried out. An area of considerable interest at 
present is the development of 'staring arrays' i.e. a 
matrix of infrared detectors. In this case the 
function of the optics is simply to focus the 
incoming radiation onto the matrix of detectors 
located in the focal plane of the camera. Whilst it 
is likely that such arrangements will eventually 
replace the SPRITE design, this is unlikely to occur 
until very dense matrices can be formed thus enabling 
high resolution imagery to be produced. 
3.2 System review 
A wide range of thermal imaging systems are currently 
available and Table 2 outlines the technical 
specifications associated with a selected sample. 
Table 2: Technical Specifications : Thermal Video 
Frame Scanning Systems. 
Spatial 
Resolution 
(mr) 
MRTD* 
(°C) 
Spectral 
Range 
(pm) 
AGA 
Thermovision 
5.8 
0.1 
3-5.6 
8-12 
GEC V1010 
TICM 11 
2.27 
0.1 
8-13 
Hawkeye 
HT4 
2.1 
0.15 
8-13 
Rank Pullin 
Controls 
SS600 
2.1 
0.15 
8-13 
Infremetrics 
IRTV-445G 
2.0 
0.4 
8-12 
FLIR Systems 
1000A 
1.87 
0.2 
8-12 
Barr and 
Stroud IR18 
1.73 
0.38 
8-13 
MRTD* = Minimum Resolvable Temperature Difference 
As mentioned in the literature review several 
previous authors have discussed the use of the AGA 
Thermovision range of instruments for airborne use. 
These instruments, of which model 782 is the most 
recent, are single detector, low spatial resolution 
systems. Two versions of this instrument are 
available for sensing in the 3 to 5 urn and 8 to 14pm 
regions of the spectrum. An image consisting of 100 
elements/line over a 280 lines/frame format refreshed 
at 25Hz and interlaced four to one is produced by the 
scanner. In addition to the low spatial resolution of 
the instrument the slow refresh rate creates image 
registration problems when sensing from a moving 
platform. A further disadvantage is the difficulty of 
producing a vertical image. In order to achieve this 
a 45° mirror has to be used so as to avoid the liquid 
nitrogen from the cooling system being expelled. 
The Inframetrics IRTV 445G can however be gimbel 
mounted for airborne use. It uses a 4 element HgCdTe 
detector and creates a 400 elements/line image over 
a 445 lines/freme format refreshed at at a frame rate 
of 30Hz. 
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