WO1991019377A1

WO1991019377A1 - Improvements in thermal imagers

Info

Publication number: WO1991019377A1
Application number: PCT/GB1991/000891
Authority: WO
Inventors: Hillary Gil Sillitto; Alison Brown Lessells
Original assignee: Gec Ferranti Defence Systems Limited
Priority date: 1990-06-05
Filing date: 1991-06-04
Publication date: 1991-12-12
Also published as: AU7952891A; GB2244817B; GB2244817A; GB9012456D0

Abstract

A thermal imager is described comprising an invertible Galilean telescope (10), a Keplerian telescope (12) incorporating image de-rotating means (14), a scanning mirror (16) and image detecting means (60). The imager is of compact form and incorporates correction for chromatic and other aberration using positive lenses.

Description

Improvements in Thermal Imagers

This invention relates to thermal imagers. Known thermal imagers suffer from disadvantages in relation to their field of view, cold shielding of the infra-red detector, image rotation, size, and optical aberrations. All the known problems can be overcome by resorting to expensive solutions usually involving an increase in size and/or complexity.

It is an object of the present invention to provide a thermal imager, of low cost, wherein the aforesaid disadvantages are overcome or minimised.

According to the present invention, there is provided a thermal imager comprising a Galilean telescope, a Keplerian telescope, image derotating means and image detecting means located seriatim along the optical axis, and detector cold shielding means, wherein the Galilean telescope is mounted for inversion end for end on the optical axis to provide at least two fields of view, the image derotating means comprises a Pechan prism mounted for rotation about the optical axis, and the image detecting means comprises a radiation sensitive elongate strip, there being scanning means for scanning radiation transmitted by the Keplerian telescope across the detecting means.

The Galilean telescope, in a first position, may present a first magnification and in its second, inverted position may present a second magnification four times greater than the first magnification.

The Galilean telescope may have a third position, intermediate the first and second position, wherein it presents unity magnification of a field of view.

The Galilean telescope preferably comprises first and second Germanium lenses. The telescope may be achromatised for radiation in the 8-12 .m band. Alternatively, the telescope may be achromatised for radiation in the 3-5 .m band. If desired, using supplemental lens elements, the telescope may be achromatised for radiation in both the 3-5 .m and 8-12 .m bands.

It is preferred that the Keplerian telescope has an overall magnification close to unity and comprises three positive lenses all formed of Germanium.

Advantageously, an objective lens of the Keplerian telescope is aspherised on one surface to correct for spherical aberrations of the telescope. Alternatively, the objective lens may comprise a doublet of spherical elements.

A meniscus field lens may provide the second positive lens of the Keplerian telescope, advantageously controlling the pupil position, astigmatism and/or field curvature. A field stop may be provided before the meniscus field lens.

The Pechan prism is preferably made of zinc selenide and is mounted for rotation about the optical axis under, for example, gyroscope control, to counter rotation of the imager about the optical axis. Zinc selenide introduces negative chromatic aberration. Its use in the construction is preferably arranged to overcorrect for chromatic aberration introduced elsewhere in the system.

A third positive lens of the Keplerian telescope preferably acts as a collimating lens and may be made of Gallium Arsenide to provide a residual correction of chromatic aberration.

Folding of the optical path is conveniently achieved, to render the imager more compact, by the interposition of one or more path folding mirrors between the meniscus and collimating lenses.

The scanning means, preferably in the form of a surface silvered plane mirror, is advantageously mounted for rotation about an axis transverse to the optical axis and parallel to the direction of elongation of the radiation sensitive strip, in a position substantially coincident with the exit pupil of the Keplerian telescope.

The objective lens of the Keplerian telescope, and the Pechan prism are conveniently over-apertured, no unique aperture stop being provided in the system. The scanning mirror may operate as an aperture in the azimuth direction. If the consequent oversize is unacceptable, the field lens may be anamorphic, i.e. have cylindrical power, or an additional cylindrical lens may be provided.

The focal plane of the Keplerian telescope objective lens is conveniently arranged to lie between the Pechan prism and the field lens. Such an arrangement permits thermal clamps to form the field stop being located at each side of the sharply focused image of the field of view. The field of view is thereby maximised to the limit imposed by the acceptance angle/aperture product of the Pechan prism. Next along the optical path (folded by the scanning mirror) is the image detecting means. Preferably, the image detecting means comprises a detector lens assembly having a first and second positive Germanium lenses in a Petzval configuration. Each lens may include reflective cold shielding and that of the first lens being conveniently arranged to position its effective cold stop as close to the scanning mirror as is possible. The first and second lenses may each be aspheric to correct for aberrations. The aberrations should not change significantly as the.pupil of the detector means is scanned into the telescope by the scanning mirror.

The first lens and its cold shielding effectively provide an aperture, in the elevation direction, for the imager.

The detector may comprise a pixel wide elongated strip of a material sensitive to radiation in the infra-red. The strips may have a length of several hundred pixels, each pixel square being a separate radiation sensitive element. Alternatively, the strip may comprise a single elongate, pixel wide detector. Both forms are well known in the prior art. The detector is enclosed in a "DEWAR" enclosure. Such an enclosure, known in the art, contains a vacuum and has a cooling means for maintaining the detector strip at a low temperature, for example, 77°K. Cold shielding means blinker the detector strip at each side restricting the receiving angle to a focused image width of one pixel. However, the solid angle of impinging radiation to which the strip is exposed, in the direction of its elongation, is much greater, the shields at the ends of the strip being positioned so as to prevent vignetting. The reflective cold shield of the second positive lens of the detector assembly is preferably arranged to re-image the cold shield (of the detector DEWAR) upon itself. Such reflective cold shield ensures that the detector strip sees, apart from the desired image forming radiation, only other parts of the cryogenic DEWAR enclosure whereby the signal to noise ratio is greatly enhanced. The second lens is advantageously positioned close to the window of the DEWAR enclosure, is weakly curved to permit over-sizing, and its corresponding reflective cold shield is more gently curved.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which:-

Figure 1 is a diagrammatic plan view of a thermal imager constructed in accordance with the present invention; Figure 2 is a ray path diagram of the imager of Figure 1; and

Figure 3 is a diagram, to an enlarged scale, of an image detecting means of the thermal imager of Figure 1.

As shown in the drawings, a thermal imager comprises a Galilean telescope 10, a Keplerian telescope 12 incorporating a Pechan prism 14, a scanning mirror 16 and an image detecting assembly 18 arranged seriatim on an optical axis 20.

The Galilean telescope comprises a positive objective lens 22 and a negative eyepiece lens 24. The lenses 22, 24 are made of Germanium. The lenses 22, 24 are mounted in a housing 26 itself mounted on stub axles 28 for rotation about an axis 30, orthogonal to the optical axis 20, by a motor (not shown) through gears 32. The telescope 10 can thus be inverted, end for end, to provide two different fields of view, the fields of view may have magnification of

0.5 and 2.0 (as shown in Figure 1) giving an overall magnification ratio of 1:4. If the spacing between the lenses 22 and 24 is sufficient, the telescope may have a third position wherein its optical axis is orthogonal to the optical axis 20 of the imager. In this case, .the housing 26 is apertured (as indicated by the circle 34) to permit the Keplerian telescope 12 to look through the housing 26 at unity magnification.

The Keplerian telescope 12 comprises a singlet objective lens 36 of which one surface, for example, surface 38, is aspherised to correct for spherical aberration. Such a singlet lens 36 may, of course, be replaced by a doublet of spherical lenses, if desired. The singlet lens 36 (or doublet of spherical lenses) is made of Germanium.

The Pechan prism 14 is of known construction. It is mounted in bearings 38 for rotation about the optical axis 20 by a motor 40 through gearing 42 under the control of, for example, a gyroscope (not shown). The Pechan prism, upon rotation about the optical axis similarly rotates the radiation rays passing therethrough (see Figure 2). In this way, if, for example, the imager is mounted on an aircraft, the display can be made to remain stationary and upright relative to a display device irrespective of any roll of the aircraft.

The prism 16 comprises two unequal elements 44, 46 of zinc selenide to form the prism symmetrical laterally about the optical axis of the imager. The convoluted optical path within the prism may be, for example, 115 mm long but the actual axial length of the prism may be as little as 50 mm.

At the exit of the prism 14, field stops 48 are provided at each side of the optical path. The stops 48 may be formed as thermal clamps or references to provide uniform signal values when perceived by the detector.

A field lens 50 of meniscus form is provided downstream of the field stops 48. The meniscus field lens 50 controls the position of the exit pupil of the Keplerian telescope 12 and serves also to control astigmatism and field curvature of the imager. The lens 50 is advantageously formed of Germanium.

The last element of the Keplerian telescope 12 is a collimating lens 52 which is preferably formed of Gallium Arsenide. The use of zinc selenide for the Pechan prism 14 introduces a strong negative chromatic aberration correction which .more than corrects for the Germanium lenses of the imager. The lens 52 is designed to eliminate the residual over-correction of chromatic aberration introduced by the Pechan prism 14. The distance between the lenses 50 and 52 is sufficient to allow the introduction of one or more mirrors (not shown) to fold the optical path and reduce the overall length of the imager.

The exit pupil of the Keplerian telescope 12, is positioned (by the meniscus field lens 50) to be at or slightly beyond the location of the scanning mirror 16.

The scanning mirror 16 is a plane, surface silvered mirror mounted for pivotal movement about an axis 54 transverse to the optical axis 20. To simplify the illustration of the imager as shown in Figures 2 and 3 of the drawings, the fold of the optical path, occasioned by the mirror 16, is not shown.

The imager does not have a unique aperture along the optical axis 20. The scanning mirror provides an aperture in, for example, the azimuth direction (and the detector provides the aperture in the elevational direction). Such an arrangement requires the objective lens 36 and the Pechan prism 14 to be over-apertured. If this oversize is unacceptable the meniscus field lens may be anamorphic so as to provide some cylindrical power or, alternatively, an additional anamorphic or cylindrical lens (not shown) may be provided.

Referring now to Figure 3, the image detecting means of the imager comprises a lens assembly, in Petzval configuration, in the form of first and second detector lenses 56 and 58. The lenses 56 and 58 have at least one of their surfaces aspheric. Both are formed of Germanium and each has an associated reflective cold shield.

The lenses 56, 58 serve to image the field of view into a plane containing a radiation detector 60. The detector 60 is an elongate (in a direction parallel to the axis 54) strip of an infra-red sensitive material. The strip 60 is one pixel wide and may comprise, along its length, a plurality of individual radiation sensitive cells, each one pixel long, or a continuous radiation sensitive element. Both forms of detector 60 are well known in the prior art. The detector strip 60 is mounted in a DEWAR enclosure 62. Such an enclosure has it interior maintained at low temperature and the detector 60 is positively cooled to a temperature of, for example, 77°K. The interior surface of the enclosure 62 is arranged to shield the detector 60 from infra-red radiation from any source other than the desired image radiation. To this end, the front surface (adjacent the lens 58) has an elongate radiation transmitting window wherethrough desired image radiation may pass. As such window is elongate and therefore has a large acceptance angle in its plane of elongation, there is a possibility of radiation from other sources entering the enclosure 62. To prevent this the reflective cold shields associated with the lenses 56 and 58 are provided. The reflective cold shielding of the detector lens assembly is designed to re-image the physical cold shield in the enclosure 62 upon itself. The shields may take the form of separate elements (not shown) but can take the form of surface silvering of the front surface 64 of the lens 56 and the rear surface 60. of the lens 58. In each case, a window is provided in the silvering, in alignment with the window in the enclosure 62 and the detector strip 60.

Radiation at the exit pupil of the Keplerian telescope 12 is swept by the scanning mirror 16 across the front surface 64 of the lens 56. Radiation, corresponding to a narrow strip of the field of view of the imager, is instantaneously passed by the windows of the lenses 56, 58 and enclosure 62 and is imaged on the detector strip 10. An output (or outputs) of the detector strips 10 is clocked in synchronism with the scanning speed of the mirror and fed to a signal processor and display device (not shown). The invention is not confined to the precise details of the foregoing example and variations may be made thereto. For instance, the Pechan prism 14 may be made of Germanium instead of zinc selenide, the latter being preferred. The prism 14 also imposes a limitation on the diagonal of the field of view. If electronic correction of the output is provided, vignetting at the corners can be tolerated.

Achromatisation in two bands, 3-5 .m and 8-12 .m, can be achieved either by introducing extra elements simultaneously reducing the number of aspheric surfaces, or by replacement of the singlet objective lens of the Keplerian telescope 12 with a doublet and/or forming the lens 22 of the Galilean telescope 10 of silicon.

Athermalisation of the imager may be provided by permitting one lens to be moved axially. Passive athermalisation using known elements may be provided.

A domed window may be provided in front of the scanning mirror.

Other variations are possible within the scope of the present invention. It will be appreciated that the invention provides a thermal imager with at least two fields of view, image derotation, scanning, achromatisation, aberration correction and excellent resultant transmission, in a compact form. The single thermal clamp 48 and the image derotation operate for all fields of view and in all modes. By appropriate selection of materials for the optical elements, achromatisation is achieved without the use of negative lenses.

Optical data for a specific, non limitative example of the Keplerian and detector optical elements is given below:-

Claims

1. A thermal imager comprising a Galilean telescope, a Keplerian telescope, image de-rotating means, image detecting means, located seriatim along the optical axis, and detector cold shielding means, wherein the Galilean telescope is mounted for inversion end for end on the optical axis to provide at least two fields of view, the image de-rotating means comprises a Pechan prism mounted for rotation about the optical axis, and the image detecting means comprises a radiation sensitive elongate strip, there being scanning means for scanning radiation transmitted by the Keplerian telescope across the image detecting means.

2. An imager as claimed in claim 1 wherein the Galilean telescope, in its inverted position presents a magnification of up to 4 compared with its non-inverted position.

3. An imager as claimed in claim 1 or 2 wherein the Galilean telescope has a further position, transverse to the optical axis, wherein it present unity magnification of a field of view.

4. An imager as claimed in claim 1, 2 or 3 wherein the lenses of the Galilean telescope are formed of Germanium.

5. An imager as claimed in any of claims l to 4 wherein the telescopes are achromatised for transmission of radiation in the 8-12 urn band.

6. An imager as claimed in any of claims 1 to 5 wherein the telescopes include further lens elements and are achromatised for transmission of radiation in the 3-5 um and 8-12 um bands.

7. An imager as claimed in any preceding claim wherein the Keplerian telescope .has a magnification substantially equal to unity, comprises three positive lenses and all the lenses are formed of Germanium.

8. An imager as claimed in any preceding claim wherein an objective lens of the Keplerian telescope is aspherical on one surface to correct for spherical aberration.

9. An imager as claimed in claim 7 wherein the second positive lens of the Keplerian telescope comprises a Germanium meniscus field lens serving to correct for field curvature and for astigmatism.

10. An imager as claimed in claim 9 wherein the meniscus field lens is arranged to control the position of the exit pupil of the Keplerian telescope and has a field stop adjacent its first surface and in the focal plane of the first positive lens.

11. An imager as claimed in claim 10 wherein the field stop is arranged to provide thermal references laterally of the field of view.

12. An imager as claimed in any preceding claim wherein the Pechan prism is formed of zinc selenide to introduce negative chromatic aberrations.

13. An imager as claimed in any preceding claim wherein means are provided in the Keplerian telescope for folding the optical path.

14. An imager as claimed in claim 7 wherein the third positive lens of the Keplerian telescope is formed of Gallium Arsenide instead of Germanium, the third lens acting as a collimating lens.

15. An imager as claimed in claim 9 wherein the Pechan prism is located between the first and second positive lenses of the Keplerian telescope.

16. An imager as claimed in claims 9, 13 and 14 wherein the optical path folding means are located between the second and third positive lenses of the Keplerian telescope.

17. An imager as claimed in any preceding claim wherein the scanning means comprises a surface silvered plane mirror mounted for rotation about an axis transverse to the optical axis and parallel to the direction of elongation of the radiation sensitive elongated strip and positioned on the optical axis adjacent the exit pupil of the Keplerian telescope.

18. An imager as claimed in claim 17 wherein the scanning mirror acts as an aperture in one direction, the direction of its pivotal axis, of scan.

19. An imager as claimed in any preceding claim wherein the image detecting means comprises first and second positive Germanium lenses in a Petzval configuration, each lens having associated reflective cold shielding.

20. An imager as claimed in claim 19 wherein the first lens of the image detecting means, and its associated reflective cold shielding, is positioned closely adjacent the scanning means .

21. An imager as claimed in claim 18 or 19 wherein the second positive lens of the image detecting means, and its associated reflective cold shielding, is positioned closely adjacent a DEWAR enclosure of the radiation sensitive strip, the cold shielding of the second lens serving to re-image physical cold shielding of the DEWAR enclosure upon itself.

22. An imager substantially as hereinbefore described with reference to the table of optical data.

23. A thermal imager substantially as hereinbefore described with refeernce to and as illustrated in the accompanying drawings.