GB2433608A

GB2433608A - Ancillary optical system for imaging optics in the infrared spectral region

Info

Publication number: GB2433608A
Application number: GB0625217A
Authority: GB
Inventors: Joerg Baumgart
Original assignee: Diehl BGT Defence GmbH and Co KG
Current assignee: Diehl BGT Defence GmbH and Co KG
Priority date: 2005-12-22
Filing date: 2006-12-19
Publication date: 2007-06-27
Anticipated expiration: 2026-12-19
Also published as: DE102006004490A1; FR2895525B1; FR2895525A1; GB2433608B; GB0625217D0; DE102006004490B4

Abstract

An ancillary optical system 10 for imaging optics (46) for the infrared spectral region, comprises a lens system. In the direction from an object side to an image side, this comprises two groups of lenses 12, 14. The group of lenses 12 on the object side has a negative effect and the group of lenses 14 on the image side has a positive effect on an incident beam path. In addition, the group of lenses 14 on the image side has a lower refractive power than the group of lenses 12 on the object side. The focal point on the image side of the group of lenses 12 on the object side coincides with the focal point on the object side of the group of lenses 14 on the image side. Moreover, the design of the lens system is such that its exit pupil 42 is located on the image side of the group of lenses 14 on the image side. Folding elements, such as a Bauernfeind prism (50), may be disposed between the two groups. Furthermore, the invention relates to imaging modules (44) with imaging optics (46) for the infrared spectral region and with the above-described ancillary optical system (48).

Description

Ancillary Optical System The invention relates to an ancillary optical

system for a lens system for the infrared spectral region. The invention also relates to imaging modules having such an ancillary optical system.

So-called wide-angle converter lenses are known from photography and video technology for the spectral region visible to the human eye. These are mounted as lens attachments in front of an existing camera system to enlarge the field of view captured by said camera system. The wide-angle converter lens is then an attachment for the objective of the camera system, by which the effective focal length of the camera system is shortened so as to obtain a wider angle of image and thus more of the subject on the film.

EP 0 595 153 Bi, for example, discloses a wide-angle converter for a zoom objective, said converter consisting of two lenses, by means of which an afocal magnification of more than 0.8 can be achieved.

Mso of interest for the infrared spectral region are wide-angle converters taking the form of ancillary optical systems for retrofitting or supplementation of existing imaging optics so as, for example, to be able to capture large fields of view even in conditions of poor visibility or at night.

It is therefore the objective of the present invention to propose an ancillary optical system for imaging optics for the infrared spectral region, by means of which a large field of view can be captured and which can be simply attached to the following, largely unknown imaging optics without there being any need to interfere with said imaging optics.

This objective is achieved according to the invention in respect of an ancillary optical system for imaging optics for the infrared spectral region in that the ancillary optical system comprises a lens system which, in the direction from the object side to the image side, comprises two groups of lenses, in which case a. the group of lenses on the object side has a negative effect and the group of lenses on the image side has a positive effect on an incident beam path, b. the group of lenses on the image side has a lower refractive power than the group of lenses on the object side, c. the focal point on the image side of the group of lenses on the object side coincides with the focal point on the object side of the group of lenses on the image side and d. the design of the lens system is such that an exit pupil is located on the image side of the lens system.

In the context of the present invention, a negative effect of a group of lenses should be understood to mean that the group of lenses has a diverging effect on incident beam paths, whilst a positive effect of a group of lenses should be understood to mean that the group of lenses has a converging effect on incident beam paths.

In a first step, the invention is based on the fact that enlarged images of an object scene can be achieved with a Galileo telescope and that the magnification of a Galileo telescope depends on the ratio of the focal length of the objective to the focal length of the eyepiece, i.e. on the ratio of the focal length of a group of lenses with positive effect to the focal length of a group of lenses with negative effect. A Galileo telescope specifically ensures an enlargement of the angle of the field of view, i.e. the angle which the outermost beams emanating from a viewed object form with one another.

The invention also proceeds from the observation that, by modif'ing the focal lengths of the two groups of lenses of a Galileo telescope, variability in the field of view from 0 to 1800 is possible, as a result of which the Galileo telescope can easily be adapted to various requirement profiles in respect of enlargement.

In a third step, the invention proceeds from the knowledge that with an inverse Galileo telescope -that is to say with a lens system having the aforementioned features a, b and c -an imaging beam path entering said telescope at a steep angle exits it again at a small angle.

A lens system along the lines of an inverse Galileo telescope thus allows coverage of a large field of view. Depending on the selected focal lengths of a group of lenses on the object side and a group of lenses on the image side of such a lens system, a strong or moderate wide-angle optical system can be achieved via the lens system together with the following imaging optics.

In a following step, the invention proceeds from the knowledge that stops or exit pupils located inside a lens system can lead to extreme degradation of the image quality in respect of a following imaging optical system for the infrared spectral region, since each stop or image of a stop constitutes a radiation source in the infrared spectral region. If this happens to lie within a lens system and vignettes, that is to say it restricts a pencil of rays passing through the lens system, then it is also imaged via the lenses of the lens system and can lead to distortions and impairments of the field of view, as viewed with the following imaging optics for the infrared spectral region, since for example it irradiates said field of view and blanks it out. In addition, diffraction can occur at such stops, as a result of which an impairment of the point image occurs.

In a further step, the invention proceeds from the observation that a freely accessible exit pupil located on the image side of a lens system does, for example, make it possible to have this coincide with an actual, i.e. physical, stop. Consequently, defined radiometric conditions can be produced, as a result of which significantly improved stray-light behaviour can be achieved, which ensures qualitatively higher-value images when an ancillary optical system comprising such a lens system is attached in front of imaging optics for the infrared spectral region. Attachment of such an ancillary optical system to following imaging optics is thus significantly facilitated, since it is possible to have the exit pupil coincide with a stop of said imaging optics.

Ideally, so as to produce optimum radiometric conditions, care should be taken that the exit pupil of the ancillary optical system coincides with the entrance pupil of the following imaging optics for the infrared spectral region.

It is advantageous if the exit pupil has a diameter which is the same size or larger than the diameter of a first lens, in the direction from the object side to the image side, of the imaging optics. Also with largely unknown imaging optics, the diameter of the first lens of same can usually be detennined by viewing from above. Thus a connection of the ancillary optical system to following imaging optics can be achieved which ensures that the exit pupil does not vignette the beam path of the imaging optics. A vignetting by the exit pupil in the infrared spectral region would give rise to the exit pupil acting as a radiation source which distorts the image and irradiates the actual field of view to be imaged.

In a further advantageous embodiment of the invention, a number of folding elements are disposed between the group of lenses on the object side and the group of lenses on the image side. A number of folding elements should, in this case, be understood to mean that it may involve a single folding element or a plurality of folding elements. Since the distance between the group of lenses on the object side and the group of lenses on the image side can be varied based on the focal lengths of the two groups of lenses, folding elements can be accommodated in the space between them. The folding elements can, for example, invoLve prisms, deflecting mirrors, gratings or the like. Depending on the desired purpose, the folding elements can enable a change in direction of the imaging beam path and also, if necessazy or desired, the stopping down of a specific spectral region.

It is practical for the folding element to involve a prism. By the use of a prism, a complex, double deflection of an imaging beam path can be achieved over a small distance. Thus, in restricted spatial conditions, the installation space available in the longitudinal and transverse directions can be used effectively and a compact ancillary optical system can be produced.

It is particularly advantageous if the prism is a Bauemfeind prism. The Bauernfeind prism belongs to the group of reflecting prisms which correspond in performance to the combination of a number of flat mirrors. The Bauernfeind prism is a deflecting prism with constant deflection, i.e. the angle of deflection is in this case independent of a rotation of the prism. Deflection of the imaging beam path through a defined angle can be achieved depending on the geometric construction of the Bauernfeind prism. With a Bauernfeind prism it also involves a non-dispersive angle. Thus the angle of the beam deflection is not dependent on the wavelength of the incident radiation. A Bauernfemd prism is also distinctly smaller in size than a simple deflecting prism, as a result of which no valuable installation space is wasted and the ancillary optical system can be kept compact.

It is advantageous if the group of lenses on the object side and/or the group of lenses on the image side comprise, in the direction from the object side to the image side, a first lens with a negative effect and a second lens with a positive effect. Owing to the fact that the group of lenses on the object side and/or the group of lenses on the image side involve a doublet consisting of two lenses, with clever selection of the shape of the negative and positive lenses, mutual compensation of their image defects can be achieved without additional lenses being necessary for the purpose. This leads to a reduction in costs as well as a compact structure of the ancillary optical system due to a reduced space requirement. In addition, it can be shown that an optical system, which is disposed mirror symmetrically about a diaphragm, is free of the image defects coma, distortion and longitudinal chromatic aberration. Even with a slight break in this symmetry, these three image defects can be largely eliminated.

It is particularly practical if the structure of the group of lenses on the object side is virtually symmetrical relative to the structure of the group of lenses on the image side, that is to say if it virtually corresponds with the latter, i.e. the two groups of lenses each comprise a first lens with negative effect and a second lens with positive effect, in which case particularly the first and second lenses are in each case made of the same material.

Thus a lens system can be produced which is athermal over a wide temperature range.

Temperature-dependent changes in refractive power can be completely or at least partly compensated by the combination of a negative lens with a positive lens. If the temperature-dependent change in refractive power leads, for example, to an increase in the diverging effect of the negative lens, then at the same time it also leads to an increase in the converging effect of the second lens. Compared to a lens system which is not subjected to the change in temperature, an imaging beam path incident on the first lens is then diverged more strongly by this, but is in turn converged more strongly by the second lens so that ultimately -depending on the degree of temperature change -there is no significant difference from a lens system which is not subjected to a change in temperature.

It is a further advantage if at least one of the two outer faces of the first lens of the group of lenses on the object side and/or on the image side has an aspherical shape. By means of aspherical forming of outer faces, controlled aberrations can be introduced into the lens system, which can, for example, specifically eliminate the aberrations of other optical elements of the ancillary optical system. As a result, the optical performance of the ancillary optical system can be improved.

In a further advantageous embodiment of the invention, the two outer faces of the second lens of the group of lenses on the object side and/or on the image side have a spherical shape. Spherical outer faces can be produced with greater geometric accuracy and in particular with less roughness than, for example, aspherical outer faces. As a result, degradation of the image quality of the ancillary optical system, due to geometric deviations of the outer faces from the desired form and owing to rough surfaces, can be kept to a minimum. The level of stray light distorting an image can be further reduced. In addition, the manufacture of spherical outer faces is easier and thus more cost-effective than the manufacture of aspherical outer faces.

The person skilled in the art knows that, for obtaining good imaging qualities in respect of lenses, it can be necessary in groups of lenses to use lenses with different shaping of their outer faces, i.e. spherical or aspherical, or to use a lens having one outer face which is spherical and the other outer face aspherical. In this way, it is possible to combine the inherent advantages of one shape with the inherent advantages of the other. Furthermore, it is possible to compensate at least in part for the disadvantages associated with one shape by means of the other shape. For example, it is known to the skilled man that the aperture defect produced by a lens with outer faces having a spherical shape can be reduced by a lens having outer faces with an aspherical shape.

in a preferred embodiment of the invention, the outer faces lying opposite one another of the first, negative lens and of the second, positive lens of the group of lenses on the object side and/or of the group of lenses on the image side are provided with the same radii of curvature. As a result, it is possible to dispose a biconvex, pianoconvex or concavo- convex second lens so that it rests directly against a biconcave, piano-concave or convexo-concave first lens. As a result, the mounting and adjustment of the lenses relative to one another is made considerably easier. Degradation of the imaging quality of the ancillary optical system due to mounting/adjustment errors can thus be largely eliminated.

It is practical if, with the group of lenses on the object side and/or on the image side, the first lens is made of germanium and the second lens is made of zinc selenide. Germanium and zinc selenide are materials which are transparent in respect of the infrared spectral region and thus can be used for producing an ancillary optical system for the infrared spectral region. Furthermore, a colour correction of the colour defects produced by the second lens can be effected by means of the lens made of germanium, as a result of which the imaging properties of the ancillary optical system can be improved. Also, with lenses made of germanium and zinc selethde, the change in imaging properties caused by material dispersion is so small that an ancillary optical system based on such lenses can be used both in a spectral region of 3 to S m (mid IR) and in a spectral region of 8 to 12 jun (long-wave IR). If the imaging optics located downstream of such an ancillary optical system can be refocused, then operation is possible in both spectral regions with the same structure comprising imaging optics and ancillary optical system.

With loss of some of the advantages mentioned above, it is also conceivable to make the lenses of the lens system of said ancillary optical system from other materials which are transparent in the infrared spectral region, such as for example silicon, zinc sulphide, calcium fluoride, magnesium fluoride, sapphire etc. (barium fluoride, calcite, caesium bromide, caesium iodide, germanium, potassium bromide, potassium chloride, potassium iodide, KRS-5 and KRS-6, lithium fluoride, lithium niobate, lithium tantalate, magnesium oxide, sodium chloride, sodium fluoride, crystalline quartz, rubidium bromide, rubidium chloride, rubidium iodide, silver bromide, silver chloride, strontium fluoride, thallium bromide, titanium dioxide (rutile), YAG (yttriwn aluminium garnet), zinc selenide, zirconium dioxide or Amtir (chalcogenide glass) are also possible).

The invention also relates to an imaging module with imaging optics for the infrared spectral region, having an ancillary optical system as described above and with a swivelling mechanism for swivelling the ancillary optical system in front of the imaging optics and away from the imaging optics.

Such an imaging module can, for example, be used for regional reconnaissance, in particular in the military field for purposes of terrain reconnaissance and surveillance. If, for example, specific objects are being looked for in an initial phase of a terrain reconnaissance operation, it is useful to swivel the ancillary optical system in front of the imaging optics via the swivelling mechanism so as to be able to capture and analyse as large a field of view as possible. As soon as an object of interest is found which needs to be more closely appraised or identified, then the ancillary optical system can be swivelled away from the imaging optics via the swivelling mechanism and thus the object can be viewed more closely using only the imaging optics. Therefore, by means of the imaging module with swivelling mechanism, a so-called dual field-of-view lens system is achieved by means of which a large or small field of view can be captured depending upon the situation.

Moreover, the invention focuses on an imaging module with imaging optics for the infrared spectral region and with an ancillary optical system as described above with a folding element between its group of lenses on the object side and its group of lenses on the image side, the ancillary optical system being rotatable about the optical axis of the imaging optics.

An imaging module designed in such a way constitutes a compact scanner by which, due to beam deflection by the folding element and rotation of the ancillary optical system about an optical axis of the imaging optics, a large area of terrain, for example, can be scanned for objects of interest. However, as with a detector on which an image is formed, the imaging optics for the infrared spectral region can be mounted so as to be structurally fixed. Owing to the fact that only the ancillary optical system must be capable of rotating, the portion of the imaging module that has to be rotated to perform a scanner function is negligible in respect of its weight. This means a more simple design of rotary mechanism, since fewer demands are made on it in terms of the load to be carried. In addition, the rotary movements can be executed more precisely since there is not such a large mass to be accelerated and then slowed down again. As a result, such an imaging module ensures a good imaging quality.

The aforementioned imaging module, with the ancillary optical system having a folding element between its group of lenses on the object side and its group of lenses on the image side, can be fitted not only with a rotary mechanism for the ancillary optical system but also with a swivelling mechanism for swivelling the ancillary optical system in front of the imaging optics and away from the imaging optics. As a result, the advantages of the two aforementioned imaging modules are combined. Such an imaging module can, for example, be operated as a scanner when the ancillary optical system is swivelled forward and rotated and, when the ancillary optical system is swivelled away, can be used for the

detailed analysis of a small field of view.

Working examples of the invention are described in detail below with the aid of the drawings which are as follows: Fig. 1 shows an ancillary optical system having a group of lenses on the object side comprising two lenses and a group of lenses on the image side comprising two lenses, and Fig. 2 shows an imaging module with imaging optics for the infrared spectral region, with an ancillary optical system with a Bauernfeind prism disposed between the first and the second group of lenses, and with a swivellinj mechanism.

Functionally identical components are provided with the same reference numbers in the drawings.

Table 1 shows the design values of the ancillary optical system according to Fig. 2.

In Fig. 1, an ancillary optical system 10 is shown which comprises a group of lenses 12 on the object side and a group of lenses 14 on the image side. The group of lenses 12 on the object side consists of two lenses 16, 18, and the group of lenses on the image side consists of two lenses 20, 22. The lenses 16 and 20 are made of germanium and the lenses 18 and 22 are made of zinc selenide.

The lens 16 is a negative lens and the lens ISis a positive lens. As can be seen in Fig. 1, as a whole the group of lenses 12 on the object side has a negative effect on an imaging beam path 24. The lens 20 is a negative lens and the lens 22 is a positive lens. As a whole, the group of lenses 14 on the image side has a positive effect on the imaging beam path 24.

The lens 16 has a convex, aspherical outer face 26 on the object side and a concave, aspherical outer face 28 on the image side. The lens 18 has spherical outer thces 30, 32, the outer face 30 on the object side being convex and the outer face 32 on the image side being concave. The lens 20 is biconcave and has an aspherical outer face 34 on the object side and a spherical outer face 36 on the image side. The lens 22 is biconvex and has spherical outer faces 38, 40. Because of the identical radii of curvature of their outer faces 36 and 38, the lenses 20 and 22 rest against one another.

The ancillary optical system 10 according to Fig. I has a magnification of one third, as a result of which the field of view of following imaging optics is trebled. The ancillary optical system 10 is designed for a total field of view of 130 . An even larger total field of view can be achieved if an application can accept a small loss in imaging quality. By altering the focal lengths of the groups of lenses 12 and 14, it is possible for a skilled man to enlarge the total field of view or even to reduce it if required.

The design of the ancillary optical system 10 according to Fig. 1 is such that there is a freely accessible exit pupil 42 located on the image side of the group of lenses 14 on the image side. More detailed design data of the lenses of the ancillary optical system 10, specifically the radii of the outer faces of the lenses 16, 18, 20, 22, their aperture radii and the material from which they are made, can be seen in Table I. Fig. 2 shows an imaging module 44 with schematically represented imaging optics 46 for the infrared spectral region and with an ancillary optical system 48. The ancillary optical system 48 comprises the same functional elements as the ancillary optical system 10 represented in Fig. 1, but a Bauernfeind prism 50 is located in the space available between the group of lenses 12 on the object side and the group of lenses 14 on the image side.

After the lens 18, the Bauernfeind prism 50 deflects the imaging beam path 24 incident on the object side twice through a total of 45 degrees towards the following group of lenses 14 on the image side.

It can be seen that the exit pupil 42 of the ancillary optical system 48 coincides with a first lens 52 -represented in broken lines -of the imaging optics 46. Other elements of the imaging optics 46 are not shown in the interests of greater clarity of the drawing. The lens 52 then at the same time forms the entrance pupil of the imaging optics 46. The diameters of the exit pupil 42 and of the lens 52 are the same size.

As symbolized by the arrow 54, the ancillary optical system 48 is rotatable about an optical axis 56 of the imaging optics 46 by means of a rotary mechanism which is not shown. Also only schematically indicated is a swivelling mechanism 58, via which the ancillary optical system 48 can be swivelled both in front of the imaging optics 46-as shown in Fig. 2-as well as away from the imaging optics 46.

The precise design data of the ancillary optical system 48 of the imaging module 44 can be seen in detail in Table 1. The distance specified in the table of 0.1 mm between the lenses and 22 does not actually exist and merely represents a simulation artefact. The aspherical outer faces 26, 34 of the lenses 16 and 20 respectively are defined in accordance with the following formula for aspherical surfaces: z cvr2 +adr4+aer6+afr8+agrl0 I +[l -cv(cc+ I)r2 in which r designates the radius of an outer face, cv the curvature, cc the conic constant and ad, ae, af, ag the aspherical coefficients. Aspherical coefficients not specified in Table I are zero in the present working example.

Table 1 also gives the slope and decentring values for the Bauernfeind prism 50, dcx, dcy and dcz indicating the decentring of the respective plane of reflection in the corresponding direction (x, y and z), and tia, tib and tic indicating the tilting of the respective plane of reflection about the corresponding axes (a, b and c) -in degrees. Again values not mentioned are zero.

Table!: Design data of the ancillary optical system according to Fig. 2 Radius Thickness (mm) Aperture Material Comments (mm) or radius (mm) ___________ Distance (mm) ___________ _____________ ___________________________ Object plane __________ _____________ __________ Air 1 _____ _______ _____ Air 2 278.8261 4 16 Germanium Lens 16 3 30.371 2 14 Air Distance to the next lens 4 52.035 5 14 Zinc selenide Lens 18 347.16 4 15 _____________ Distance to Bauernfeind prism 6 __________ 16 14 Silicon Bauernfeind prism 50 7 ___________ -21 20 ____________ First reflection 8 _________ 13.777 20 ___________ Second reflection 9 ___________ 4 20 ____________ Distance to the next lens -765.6672 4 18 Germanium Lens 20 11 148.878 0.1 18 Air (Distance to the next lens) 12 148.878 6 18 Zinc selenide Lens 22 13 -42.917 3 18 ____________ Air Aperture 10.491871 Exitpupil42 stop ____________ ________________ Image plane __________ _____________ __________ 10 Aspherical data(conic and polynomial) cc ad ae af ag __________________________ ___________ 4.176E-07 -2.0129E-08 8.69413-11 ____________ Outer face 26 ____________ 1.13613-06 -6.41 5E-09 2.4012E-1 1 ____________ Outer face 34 Slope/Decentring _____________ __________ ____________ Bauernfeind prism 50 ___________ dcx dcy dcz ____________ First reflection ___________ tla 49 tlb tic ____________ ___________ dcx dcy dcz ___________ Second reflection _____________ tla -24.5 tlb tic _____________ tia 49 Tilting of the plane of reflection relative to the preceding surface about a-axis through 490 dcx Decentring of the plane of reflection in x-direction List of Reference Numerals Ancillary optical system 12 Group of lenses on the object side 14 Group of lenses on the image side 16 Lens 18 Lens Lens 22 Lens 24 Imaging beam path 26 Aspherical outer face 28 Aspherical outer face Spherical outer face 32 Spherical outer face 34 Aspherical outer face 36 Spherical outer face 38 Spherical Outer face Spherical outer face 42 Exit pupil 44 Imaging module 46 Imaging optics for the infrared spectral region 48 Ancillary optical system Bauernfeind prism 52 Lens 54 Arrow 56 Optical axis 58 Swivelling mechanism

Claims

Patent Claims An ancillary optical system (10, 48) for imaging optics

(46) for the infrared spectral region, comprising a lens system, which, in direction from an object side to an image side, has two groups of lenses (12, 14), in which a. the group of lenses (12) on the object side has a negative effect and the group of lenses (14) on the image side has a positive effect on an incident beam path, b. the group of lenses (14) on the image side has a lower refractive power than the group of lenses (12) on the object side, c. the focal point on the image side of the group of lenses (12) on the object side coincides with the focal point on the object side of the group of lenses (14) on the image side, and d. the design of the lens system is such that an exit pupil (42) is located on the image side of the lens system.

2. An ancillary optical system (10,48) according to Claim 1, characterized in that the exit pupil (42) has a diameter which is the same size as, or larger than, the diameter of a first lens (52), in the direction from the object side to the image side, of the imaging optics (46).

3. An ancillary optical system (48) according to one of the preceding claims, characterized in that a number of folding elements is disposed between the group of lenses (12) on the object side and the group of lenses (14) on the image side, one folding element preferably being a prism, and in particular a Bauernfeind prism (50).

4. An ancillary optical system (10, 48) according to one of the preceding claims, characterized in that the group of lenses (12) on the object side and/or the group of lenses (14) on the image side, in the direction from the object side to the image side, comprises a first lens (16, 20) with a negative effect and a second lens (18, 22) with a positive effect.

S. An ancillary optical system (10, 48) according to Claim 4, characterized in that at least one of the two outer faces (26, 28, 34) of the first lens (16, 20) of the group of lenses (18, 20) on the object side and/or on the image side has an aspherical shape.

6. An ancillary optical system (10, 48) according to Claim 4 or 5, characterized in that the two outer faces of the second lens (30, 32, 38, 40) of the group of lenses on the object side and/or on the image side have a spherical shape.

7. An ancillary optical system (10, 48) according to one of Claims 4 to 6, characterized in that outer faces (36, 38) of the first lens (20) and of the second lens (22) which face one another are provided with the same radii of curvature.

8. An ancillary optical system (10, 48) according to one of Claims 4 to 7, characterized in that the first lens (16, 20) is made of germanium and the second lens (18, 22) is made of zinc selenide.

9. An imaging module (44) with imaging optics (46) for the infrared spectral region, with an ancillary optical system (48) according to one of Claims 1 to 8 and with a swivelling mechanism (58) for swivelling the ancillary optical system (48) in front of the imaging optics (46) and away from the imaging optics (46).

10. An imaging module (44) with imaging optics (46) for the infrared spectral region and with an ancillary optical system (48) according to one of Claims I to 8 with a folding element between its group of lenses (12) on the object side and its group of lenses (14) on the image side, the ancillary optical system (48) being rotatable about the optical axis (56) of the imaging optics (46).

11. An ancillary optical system as substantially described herein with reference to figure 1 of the drawings.

12. An imaging module as substantially described herein with reference to figure 2 of the drawings.