Thermal imager systems

A thermal imager system comprises a scanning system 1 for scanning a required scene 2 and an anamorphic optical system 3 for focusing the scanned scene onto a Sprite detector 4, the optical anamorphic system comprises a pair of optical prisms 17, 19 disposed between a collimating element 15 and a focusing element 21, one or both of the prisms 17, 19 being pivotally mounted whereby the anamorphic ratio and hence the focal length of said optical system 3 may be changed. (FIG. 1).

This invention relates to thermal imager systems and is especially 
applicable to such systems incorporating an anamorphic optical system. 
In GB Patent Application No. 2187301A and European Patent Application No. 
0256826A2 there are disclosed prior art anamorphic optical systems and 
thermal imager systems incorporating such systems. The anamorphic optical 
systems which are described comprise a pair of prisms which are inverted 
relative to one another in order to compensate for temperature and 
wavelength changes and are used in thermal imager systems which include a 
so-called "Sprite" detector. 
In thermal imager systems which use a Sprite detector, opto-mechanical 
scanning mechanisms are normally used the characteristics of which 
normally define the field of view (FOV) in the scanned direction. This 
scanned direction is normally in the azimuth plane. In this case the 
elevation FOV is determined by the detector channel separation and the 
focal length of the optical system. (In particular, the focal length in 
the elevation plane). 
In a conventional isomorphic optical system a short focal length must be 
employed to obtain a large FOV and this can lead to problems of 
resolution. The resolution of a Sprite detector in the direction of scan 
is closely related to the effective size of its readout and its diffusion 
characteristics and can be enhanced by using larger focal lengths in this 
plane. This is commonly accomplished by using an anamorphic optical system 
having an elevation focal length chosen for FOV and an azimuth focal 
length optimised for azimuth resolution. These focal lengths however must 
be chosen with f number in mind to ensure adequate radiometry. One 
disadvantage of changing the azimuth focal length is that the scan 
velocity and therefore the required bias voltage of the Sprite detector 
must also change. 
It is an object of the present invention to provide a thermal imager system 
incorporating an anamorphic optical system and a Sprite detector, which 
allows selection of the optimum azimuth focal length for the purpose of 
resolution and allows a variable elevation focal length to provide 
multiple field of view, without requiring the detector bias conditions to 
be changed. 
According to the present invention there is provided a thermal imager 
system comprising scanning means for scanning a required scene and an 
anamorphic optical system for focusing the scanned scene onto a Sprite 
detector, said optical system comprising first and second optical energy 
transmission elements arranged in series between a collimating element and 
a focusing element for said Sprite detector, at least one of said 
transmission elements being pivotally mounted whereby the focal length of 
said optical system may be changed. 
In a preferred arrangement it will be arranged that both of said first and 
second optical energy transmission elements are pivotally mounted for 
changing the focal length of said optical system, and the optical output 
from said second transmission element may be maintained parallel to the 
optical input to said first transmission element. 
In carrying out the invention it may be arranged that an optical reflector 
is disposed between said first and second transmission elements whereby 
the optical output from said second transmission element may be maintained 
co-axial with the optical input to said first transmission element. 
It may be arranged that said optical transmission elements take the form of 
optical prisms, conveniently of germanium material, and it may also be 
arranged that the first and second optical prisms are inverted relative to 
one another. 
Advantageously the said scanning means will comprise an opto/mechanical 
scanning arrangement which affords an optical input to said collimating 
element.

In FIG. 1 of the drawings there is depicted a known form of thermal imager 
system which incorporates a scanning system 1 for causing a required scene 
to be scanned in both the horizontal and vertical directions, and an 
anamorphic optical system 3 for focusing the scanned signal into a Sprite 
detector 4. 
The scanning system 1 of FIG. 1 comprises a rotating polygon 5, on a face 6 
of which an entrance pupil 7 is formed which, as the polygon 5 rotates 
causes the scene 2 to be scanned in the horizontal direction as shown at 
8. The scanned beam 9 is directed by the scanner 5 onto a bend mirror 10 
and thence to a relay mirror 11 and line mirror 12 which directs the 
scanned beam 9 onto a frame or vertical scanning mirror 13 which affords 
the optical input to the optical system 3. 
Vertical scanning of the scene 2 as shown at 14 in FIG. 1, is effected by 
causing the frame mirror 13 to be pivoted horizontally backwards and 
forwards. 
The scanned beam from the frame mirror 13 is passed through a stop 14 and 
is collimated by lens 15 to afford a collimated beam 16 to a first optical 
prism 17. The optical prism 17 causes the beam 16 to be deviated to afford 
a further beam 18, the size of which, relative to the size of the beam 16, 
depends on the incidence angle of the beam entering the prism 17 and also 
the angle and refractive index of the prism 17. The beam 18 is then 
further deviated by a second optical prism 19 to afford a further beam 20 
of possibly different beam size. The beam 20 is directed to a focusing 
lens 21 by means of which it is focused onto the surface 22 of the Sprite 
detector 4. 
It is arranged that the second prism 19 is inverted relative to the first 
prism 17 and the anamorphic effect of the system is governed by the 
relative sizes of the beams 16 and 20. The relative directions of the 
beams 16 and 20 are governed by the characteristics of the prisms 17 and 
19 and their relative orientation, and these may be chosen, for example, 
to afford beams 16 and 20 which are parallel to one another but are 
displaced laterally. 
It is well known that this arrangement of prisms can provide correction for 
the affects of wavelength and temperature changes. However, the anamorphic 
ratio of such optical systems is fixed so that the corresponding focal 
length is also fixed thereby defining a fixed field of view. As is well 
known, it is necessary for the Sprite detector 4 to be biased in 
accordance with this fixed focal length. 
If it is required to change the field of view of the imager, it is 
necessary to change the focal length of the optical system and this is 
achieved by changing the lenses of the optical system in accordance with 
the required focal length. However, existing systems based on this 
approach require also that the bias of the Sprite detector be changed to 
correspond to the new focal length. 
It has now been appreciated that the anamorphic ratio of the optical system 
3 of FIG. 1 can be changed by causing the prisms 17 and 19 to be rotated 
about their axes, which allows the focal length in the vertical direction 
only to be changed and allows a constant Sprite detector 3 bias to be 
used. 
In FIG. 2 of the drawings there is depicted the prisms 17 and 19 of the 
optical system of FIG. 1 and shows how a variable anamorphic ratio may be 
obtained by arranging that the prisms 17 and 19 are rotated. 
In FIG. 2 the optical prisms 17 and 19 are shown together with the input 
beam 16 and the output beam 20. Typically the prisms 17 and 19 may be of 
germanium material and may have an included angle of 9.degree.. When the 
incidence angle I of the prisms 17 and 19 is 0.degree., each prism 
provides an anamorphic change of 1.267, so that the combined anamorphic 
change between the beams 7 and 3 is 1.6 to 1. 
If the prisms 17 and 19 are each arranged to be centrally pivotally mounted 
on respective axes which are parallel to the front and rear major surfaces 
respectively of the prisms 17 and 19, and are moved to the positions 17' 
and 19' respectively, shown in broken lines, where the incidence angle is 
28.degree., the anamorphic ratio for each prism becomes 0.894 and the 
combined anamorphic ratio of the two prisms 17 and 19 becomes 0.8 so that 
the output beam 20, from the prism 19' is 0.8 times the size of beam 16. 
Thus, by changing the positions of the prisms 17 and 19, the anamorphic 
ratio has been changed from 1.6 to 0.8 and an effective change in focal 
length of 2. 
By careful choice of the pivot point of each of the prisms 17 and 19, the 
axes of entry and exit of the prisms can be held stationary as the 
anamorphic ratio and hence the focal length is changed. In general a wide 
range of focal length variations can be provided for any chosen 
configuration of axes, and it is possible that the variations could be 
continuous or in steps. 
In the optical system of FIG. 2, it will be appreciated that the beams 20 
and 20', although being parallel to the beam 16 are displaced laterally 
relative to it. 
In FIG. 3 there is depicted a modification of the arrangement of FIG. 2 
which enables this lateral displacement to be overcome. In FIG. 3 an 
optical reflector 22 is interposed between the prisms 17 and 19, by means 
of which the beam 18, from prism 17 is reflected as beam 18' to prism 19. 
By suitably adjusting the position of the reflector 22 relative to the 
prisms 17 and 19, any lateral displacement of the beam 20 or 20' can be 
eliminated.