Thermal imager

A thermal imager comprises a scanner (13), detector (14) with mount (14A) cooled substantially below ambient temperature, and which via a reflector (16) on the locus (15) along which an image of the detector (14) is effectively scanned provides a characteristic radiation feature to the detector (14) in superimposition with the scanned radiation from the scanner (13). The scanner (13) operates with less than 50% efficiency so that in the detector output signal the scene-derived waveform has a duration T.sub.1 <50%T, where T is the period of the detector output signal. The characteristic radiation feature gives rise to a sync signal at the detector output and is located prior to and closely adjacent the scene-derived waveform at a time interval T.sub.2 prior to the end thereof. This sync signal is detected by a recognition circuit (20) and activates a monostable forming part of a clock circuit (22) for a duration T.sub.3 such that T.sub.2 <T.sub.3 <50%T. The clock circuit (22) controls the timing of signal processing circuitry (24) and a video output device (25) connected to the output of the detector (14).

This invention relates to thermal imagers. 
In thermal imagers the scene from which the thermal infrared radiation 
emanates is scanned continuously by an optical scanner across a detector 
the output signal waveform of which is fed through signal processing 
circuitry to a video output device. The scanner scans two-dimensionally 
but the detector output signal is substantially continuous being composed 
of successive waveform portions arising from the time interval that the 
detector is effectively scanned across the interior of the imager housing 
and the time interval that the detector is effectively scanned across the 
scene. The detector output signal therefore has a periodical structure the 
period T being conveniently measured from the end of the waveform portion 
due to a first scan across the scene to the end of the waveform portion 
due to the next scan across the scene. The waveform portions due to the 
scene-derived information are also of constant duration T.sub.1, the scan 
efficiency of the imager being the ratio T.sub.1 to T. 
In order to synchronise the operation of the signal processing circuitry 
with the detector it is customary to add a waveform of predetermined shape 
to the detector signal in a fixed position during each period T relative 
to the waveform portion due to the scene-derived information. This sync 
signal is then recognised by a recognition circuit and used to activate a 
monostable from which a clock signal for the signal processing circuitry 
is derived. In the known thermal imagers this sync signal is positioned 
immediately after the scene-derived information and this gives rise to a 
relatively complicated sync recognition circuit in order to ensure that 
any scene-derived information which has a waveform portion similar to that 
of the sync signal does not prematurely activate the monostable. 
Unfortunately, in practice, where such a waveform portion exists it is 
usually repeated in a substantial number of the successive scan lines 
(i.e. periods T) since it arises from the existence of a physical object 
of significant size in the scene being viewed. Thus the monostable and 
hence the clock signal becomes unsynchronised with the scene-derived 
information for a significant number of scan lines and valuable 
information is lost since the corresponding video picture becomes 
unintelligible. 
It is an object of the present invention to provide an improved form of 
thermal imager. 
According to the present invention there is provided a thermal imager 
wherein the detector output waveform has a period T, the scene-derived 
waveform has a duration T.sub.1, where T.sub.1 &lt;50%T, the sync signal is 
located prior to and closely adjacent the scene-derived waveform, the 
duration from the sync signal to the end of the scene-derived waveform is 
T.sub.2 and the monostable has a set time T.sub.3 such that T.sub.2 
&lt;T.sub.3 &lt;50%T. 
With the arrangement of the present invention the monostable is set for a 
duration greater than T.sub.1 but less than 50%T so that even if the 
monostable is prematurely activated it will have reverted to its off 
condition prior to the sync signal during the next period T of the 
detector output signal so that correct synchronisation will then occur, 
resulting in only a single line of unintelligible video. 
It will be noted that because the scan efficiency is less than 50% the 
waveform portion due to the detector being scanned across the interior of 
the imager housing has a duration greater than T.sub.1 and during this 
period the detector output requires to be devoid of a waveform which could 
be recognised erroneously by the recognition circuit. This however is 
easily achieved since the interior of the imager housing is of a 
predetermined nature. The sync signal may conveniently be provided by a 
mirror within the imager housing and reflecting infrared radiation from 
the detector mount (which is cooled substantially below ambient 
temperature) back onto the detector.

The drawing illustrates in FIG. 1 a thermal imager 10 comprising a housing 
11 having a window 12 through which thermal infrared radiation from the 
scene is incident on a scanner 13. The scanner 13 focusses this radiation 
on a detector 14 resulting in an image of the detector 14 effectively 
being scanned along a line locus 15 within the housing 11 on which lies a 
mirror 16 for providing a sync signal by the narcissus effect from the 
mount 14A of detector 14 since the latter is cooled substantially below 
ambient. Any alternative known form of arrangement for providing such a 
sync signal would suffice however. The detector output is fed along signal 
path 18 to a sync signal recognition circuit 20 which extracts the sync 
signal and uses the extracted sync signal to activate a monostable forming 
part of a clock arrangement 22. The detector output signal is also fed 
along path 23 to the signal processing circuitry 24 and hence to the video 
output device 25 both of which are clocked by clock arrangement 22 for 
synchronisation. 
The form of the detector output signal 30 is illustrated in FIG. 2, being 
formed of successive waveform portions 30A, 30B, 30C, 30D and 30E. Portion 
30A is representative of a first scan line containing scene-derived 
information; 30B represents the waveform due to the interior of the 
housing 11; 30C is the sync signal; 30D is a second scan line having a 
similar but not-necessarily identical waveform to portion 30A; and 30E is 
the same as 30B. It will be evident that the function of portions 30A and 
30B represents the beginning of a period T terminating at the junction of 
portions 30D and 30E when the next period commences. Portion 30D is of 
duration T.sub.1 such that T.sub.1 &lt;50%T and the sync signal is located 
closely adjacent and prior to the portion 30D. The duration T.sub.2 from 
the junction of portions 30B and 30C to the end of the period T is also 
less than 50%T. 
The sync signal recognition circuit 20, which conveniently may comprise a 
differentiator and threshold device recognises the presence of the sync 
signal portion 30C and sets the monostable of the clock arrangement 22 for 
a time interval T.sub.3 such that T.sub.2 &lt;T.sub.3 &lt;50%T as shown in FIG. 
2, the leading edge 28A of the monostable set portion being aligned in 
time with the sync signal and the trailing edge 28B of the monostable set 
portion being after the termination of T.sub.1 and T.sub.2 and T. 
It will be noted that waveform portions 30B and 30E are devoid of anything 
resembling the sync signal portion 30C whereas portions 30A and 30D, being 
scene-derived information contain peaks such as 31 which in the absence of 
the present invention could lead to erroneous operation of the sync-signal 
recognition circuit 20.