GB2501818A

GB2501818A - Zoom objective and camera system

Info

Publication number: GB2501818A
Application number: GB1305522.3A
Authority: GB
Inventors: Martin Gerken; Bertram Achtner
Original assignee: Cassidian Optronics GmbH
Current assignee: Hensoldt Optronics GmbH
Priority date: 2012-03-26
Filing date: 2013-03-26
Publication date: 2013-11-06
Anticipated expiration: 2033-03-26
Also published as: GB2537070B; GB201305522D0; GB2501818B; DE102012005939A1; GB201612225D0; GB2537070A; DE102012005939B4

Abstract

A zoom objective 1 has a common imaging path or axis 2, an optical zoom group 4 displaceable along the imaging path 2, and a beam splitter element 10 to separate the common beam path 2 into first and second paths along first and second optical axes 11,12, one inclined relative to the other. First and second optical imaging groups 13,15 along the first and second optical axes 11,12 respectively, form object images on first and second detectors 14,16. The beam splitter may be for polarisation dependent separation.

Description

Zoom objective and camera system The invention relates to a zoom objective for forming an object image on separate detectors. The invention is concerned with increasing the information gain of the object by forming an object image on separate detectors-In addition the invention relates to a camera system with such a zoom objective for observing an object with separate detector& The information gain of an object to be observed can be increased if the object is observed and imaged on separate detectors. For example. the observation of an object with separate detectors in different spectral regions permits an improved analysis of the intrinsic absorption, reflection, transmission and/or radiation characteristics of the object, so that a specific property or the object per se can be more readily identified. Thus, for example, two objects appearing identical in the visible region of the spectrum can differ from one another in an infra-red (IR) region of the spectrum if they are at different temperatures.

Illuminated objects that are invisible in one region of the spectrum can appear spectacularly in another region of the spectrum on account of their increased reflection or absorption capacity there. Also, objects can be extracted from a background image if these for example have warmed up differently in sunlight compared to the surroundings on account of their specific absorption or reflection capacity.

Technical implementations of multispectral objectives are known. For example, multispectral objectives with a fixed focal length are known from EP 0 935 772 BI for the visible (VIS) and the infra-red (IR) range, from US 5,781,336 for a wavelength range from 0.55 pm to 5.35 pm, and from US 6,950,243 B2 for a wavelength range from 0.7 pm to 5.0 pm. By a suitable choice of material it is possible in particular to keep the chromatic aberrations low for the spectral range to be imaged. Furthermore, a multispectral reflecting objective with a fixed focal length is known from US 5,847,879. There a wide-angle objective for the visible and for the IR ranges is disclosed. In this connection a beam splitter element separates the visible region from the lR region. Both spectral regions are reflected onto a compact detector.

Multispectral objectives are as a rule comparatively complicated and expensive.

In addition they generally have t9 be designed in each case specifically for the particular application. Objectives with a fixed focal length are not suitable for the variable imaging of objects at different distances onto separate detectors of predetermined size. The use of separate objectives for object imaging on separate detectors leads disadvantageously to a large installation space and a high weight. For a camera system with two separate objectives a high electrical output is undesirably required. Also, it is difficult to adjust two objectives forming images on separate detectors. If two separate objectives are used this causes parallax defects in the image. The alignment of the objectives with respect to one another proves to be complicated, especially under the effects of temperature, vibration or shock. The image contents of the separate detectors can be compared or related to one another only with difficulty.

The object of the invention is to provide an objective that allows a variable imaging of an object on separate detectors. The objective adjustment should be able to be carried out as simply as possible, and the image contents of the separate detectors should be able to be related to one another in a simple way.

A further object of the invention is to provide a corresponding camera system including such an objective.

The first-mentioned object is achieved according to the invention by a zoom objective, which includes along a common imaging path at least one optical zoom group displaceable along the imaging path, and a beam splitter element that is adapted to separate the beam path into a first path along a first optical axis and into a second path along a second optical axis that is inclined with respect to the first optical axis. Furthermore there are disposed along the first optical axis a first optical imaging group that is adapted to form an object image on a first detector and along the second optical axis a second optical imaging group that is adapted to form an object image on a second detector.

The invention envisages arranging along a common imaging path a single zoom optics system that splits all image paths to form an image of the object on separate detectors. This common zoom optics system comprises at least one optical zoom group displaceable along the common image path. The displaceable zoom group serves to change the focal length, focussing and also air pressure and temperature compensation for all separate image paths.

The splitting of the beam path into separate paths takes place in the beam splitter element. The optical imaging groups present after separation in the separate paths allow the focal length to be adapted in each case to different detector sizes. Also, a specific correction of aberrations can be carried out in each case in the separate optical imaging groups.

Together with the separate further optical imaging groups the zoom optics system thus form along the various paths an object image on separate detectors. The use of common optical components for the imaging on separate detectors allows a smaller installation space compared with separate objectives.

The focal length and thus the object section imaged on the detectors are changed simultaneously and intrinsically for all spectral ranges. Apart from the detector size, no further complicated mathematical conversion is necessary for comparing the image contents of the separate detectors. Thanks to the use of the same zoom path for different image paths, the separate detectors supply identical contents not only as regards the field of view, but also as regards a distortion and other optical influences. Processing the separate image contents with each other, for example by an overlapping, by a fusion or by some other mathematical algorithm, is possible in particular in real time. Such an objective is suitable in particular for a camera system used for filming.

The optical imaging groups comprise in each case a number of lenses or a lens group. The displaceable zoom group is preferably adapted fpr a zoom factor of at least 10, preferably however of 20 or more. The zoom factor is in this connection defined as the ratio of the longest and the shortest adjustable focal length...

Advantageously the zoom optics system includes along the common imaging path also an optical front group and a common optical imaging group, between which is arranged the displaceable zoom group. Such a front group and/or such a common imaging group enables the length of the objective overall to be maintained constant for different focal lengths. Also, the zoom range can be increased in this way.

The zoom objective is furthermore preferably designed for a high aperture. The diaphragm factor f is, by suitably choosing the optical components, preferably less than 8, more preferably less than 2.8.

In addition the optical zoom group advantageously includes a displaceable variator group and a displaceable compensator group. Different focal lengths are adjusted by means of the variator group. The variator group corrects the image position for different focal lengths and accordingly different positions of the variator group.

In order to correct aberrations, lenses with aspherical surfaces can expediently be arranged in the front group and in the displaceable optical zoom group. The front group as well as the displaceable optical zoom group are adapted, in collaboration with the common imaging group, for a high zoom factor.

The beam splitter element is preferably formed as a partially transmitting reflector plate or as a splitter prism. The splitter prism is for example formed as a cubical beam splitter, in which the separation takes place by interface effects.

Advantageously the beam splitter element is also used to correct aberrations. In particular it is envisaged for this purpose to align an inlet surface and an outlet surface of the beam splitter element at an angle to one another so as to form a wedge-shaped structure. Due to such a wedge shape the astigmatism correction of the imaging can be improved as long as the beam splitter element is not situated in a parallel beam path. In the latter case a plane-parallel configuration of the beam splitter element is expedient.

In a further preferred embodiment the zoom objective comprises several second imaging groups, which are respectively arranged angularly displaced with respect to one another about the common imaging path, and which are adapted in each case for imaging at least one partial region of the second spectral range on a respective second detector.

By rotating a rotatably arranged beam splitter element the annularly arranged further second imaging groups and detectors can be illuminated with the reflected out' partial beam. The zoom objective forms an image in each case on the first detector and on one of the second detectors. Various detectors can be used as regards their wavelength sensitivity or as regards their response tima In a preferred embodiment the beam splitter element is adapted for a polarisation-dependent separation of the beam path into the first path for a first polarisation direction and into the second path for a second polarisation direction. The image contents on the separate detectors thus correspond to an observation of the object with different polarisation directions. In particular the beam splitter element splits an object beam into a beam with p-polarisation (linear polarisation parallel to the incident plane) and into a beam with s-polarisation (linear polarisation perpendicular to the incident plane). An object is then observed separately on the separate detectors with s-polarised and p-polarised light.

An observation of the object with s-polarisation and p-polarisation can be employed in particular to reduce vapour effects in the case of remote objects.

An object illuminated by daylight scatters unpolarised light onto the zoom objective. On the path between the object and the objective polarised light is however scattered from the object beam according to Rayleigh's law. More remote objects therefore appear on images to be lying behind a haze. If such an object is observed with a polarisation filter, then depending on the selling of the polarisation filter the iniage has a minimum brightness in one position and a maximum brightness in another position. By separately observing the object in s-polarisation and in p-polarisation the "haze" can be deducted, so that the object information and not the scatter information appears in the foreground.

The contrast as regards the observed object can be improved. The objective is enhanced for daytime visibility.

In addition, due to a polarisation-separated observation of the object on separate detectors the object identification by means of the zoom objective can be improved. For example, reflecting objects can be filtered out from the background if the object is observed only with polarised light. Whereas the background image becomes dark, a reflecting object appears bright when observed with polarised light, since reflected light becomes linearly polarised Thus, metallic objects in particular that have the same colour as the surroundings can be identified with the zoom objective. Under normal observation such objects cannot be differentiated as regards their colour

compared to the background.

In a further advantageous embodiment a polarisation optics system is arranged in front of the zoom group, and is adapted so as to decouple two object beams of different observation angle and polarisation along the common image path.

Due to such a polarisation optics system an object is observed with a parallax, so that the two object beams contain spatial information on the object. The two object beams differing in their observation angle are also polarised differently to one another. After passing through the common zoom optics system the two beams are split again at a polarisation beam splitter and an object image is formed on the two separate detectors. A spatial image of the recorded object can be determined by processing the image contents of the two separate detectors with each other.

Since the object imaging on the two separate detectors takes place simultaneously on account of the common zoom optics system and the observed field of view as well as further optical artefacts produced by the zoom objective can intrinsically change at the same time on both detectors, the image contents of the two detectors can be processed in real time to form a three- dimensional image. This allows the creation of a camera system in which three-dimensional recordings can be made in real time. Such a camera system is of great interest in particular for the film industry for producing 3-D films.

Expediently the polarisation optics system includes at least one first mirror element and one second partially transmitting mirror element, wherein the first mirror element and the second mirror element are arranged spaced from one another at right angles to the common image path. Via the first mirror element and by reflection at the second mirror element a p-polarised first object beam along the common image path is coupled into the zoom objective. The second object beam is coupled close to the axis by the second partially transmitting mirror element and accordingly has an s-polarisation.

In another preferred embodiment an optical phase analyser is arranged along the second optical axis, which is adapted for imagining a local phase deformation onto the second detector. The beam splitter element is in this connection provided in particular for splitting the object beam onto two separate detectors. The image of the object is observed via the first detector. A phase deformation of the wave front can be observed with the second detector. The beam splitter element can expediently be designed so that the reflected out intensity that reaches the second detector is chosen to be lower than the transmitting intensity that is incident on the first detector.

In addition the phase analyser is preferably formed with an aperture array or a micro lens array. via which the reflected out object beam can be imaged and focussed onto the second detector. When using an aperture array one also speaks of a so-called Hartmann-sensor. If a micro lens array is used, such a phase sensor is also termed a Shack-Hartmann sensor. A position-resolved focussing of partial beams is performed via the aperture array or the micro lens array-The resultant image points change their position with respect to one another on the detector depending on the phase relationship of the partial beams. The position of the individual image points and their change over time is observed and determined via the second detector. The finer the resolution of the aperture array or of the micro lens array, the higher the local resolution of the determined phase deformation of the wave front.

If a phase deformation of the wave front is observed by means of the second detector, this allows a calculated reduction of fuzziness or image deformations in the image content of the first detector caused by temperature or density fluctuations in the atmosphere. The observation and evaluation of a phase deformation by means of an optical phase analyser thus enables the image quality of the imaging of the zoom objective to be improved.

Alternatively or in addition, in order to offset the observed phase deformation an adaptive optical element is preferably arranged along the first optical axis. Such an optical element alters its optical properties, in particular the focal length or a lens shape per se, if suitably activated. Due to a corresponding control of the adaptive optical element, the quality of the imaging on the first detector can immediately be optically improved depending on the observed phase deformation, since the adaptive optical element is able to cancel the phase deformation if the time constant of the activation is small compared to the time change of the phase deformation.

The offsetting of the phase deformation and of the image content of the first detector and a corresponding control of the adaptive optical element preferably takes place in real time. A camera system can thereby be created that immediately provides an improved image quality over the known prior art.

Image deformations caused by atmospheric effects can be significantly reduced.

In an expedient modification the zoom objective is designed for imaging in a wavelength range between 400 nm and 2.5 pm. This enables an optical glass to be used for the imaging components.

Furthermore, at least one of the optical imaging groups is preferably displaceably arranged along the respective optical axis. This allows a focal intercept compensation. If it is desired that both image contents are sharp, it is advantageous to displace the respective imaging group so as to focus in the observed spectral range.

The second mentioned object is achieved according to the invention by a camera system with a zoom objective of the aforedescnbed type, a number of detectors and an image processing unit for evaluating the image contents of at least the first and second detectors.

In this connection the advantages mentioned in each case regarding further developments of the zoom objective can be applied as appropriate to the camera system. Such a camera system is of compact construction, and has a low weight and a low energy requirement. CMOS and/or CCD detectors are preferably used as detectors.

Furthermore the image processing unit is preferably adapted to process the image contents of at least the first detector and a second detector with each other. Advantageously, the image processing unit is adapted to process the separate image contents of at least two of the detectors with each other in real time. By a suitable choice of image processing mechanisms the image contents of the different detectors can be electronically superimposed in real time, combined, interfering artefacts can be deleted and the picture quality can thereby be improved. Due to the common imaging path it is ensured by the zoom optics system that the image contents recorded by the separate detectors can be correspondingly simultaneously imaged at any time as regards the image field or the image section, an optical distortion, or other optical influences.

Exemplary embodiments are described in more detail with the aid of the drawings, in which: Fig. 1: shows schematically a zoom objective for imaging an object image on two separate detectors, Fig. 2: shows schematically the zoom objective corresponding to Fig. 1 with a polarisation optics system, and Fig. 3: shows schematically a further zoom objective with an optical phase analyser.

Fig. 1 shows schematically the general structure of a zoom objective I for forming an object image on two separate detectors.

The zoom objective 1 includes along a common imaging. path 2 a common zoom optics system for all imaging paths. This zoom optics system comprises at least one zoom group 4 displaceable along the common imaging path 2, which group comprises a variator group 5 and a compensator group 6. In addition a front group 3 as well as a common imaging group 7 can be provided.

as can be seen from Fig. 3, and between which the displaceable zoom group 4 is arranged.

After the zoom optics system the separation of the beam path into separate paths is performed by a beam splitter element 10 designed as a splitter prism.

This separates the beam path into a first path along a first optical axis 11 and -11 -into a second path along a second optical axis 12. The splitter prism 10 is formed for example as a polarisation beam splitter, so that a partial beam with p-polarisation is reflected along the second optical axis 12 and a partial beam with s-polarisation is transmitted along the first optical axis 11.

The first partial beam is imaged by means of the first optical imaging group 13 on a first detector 14. The reflected second partial beam is imaged with a second optical imaging group 15 on a second detector 16.

The image contents of the two separate detectors 14, 16 are processed with each other, in particular in real time, with an image processing unit 20. The image processing unit 20. detectors 14, 16 and zoom objective 1 together form a camera system 21.

By means of the zoom objective 1 an object with variable focal length can be observed simultaneously on the two separate detectors 14, 16. Adapting the image contents of the two detectors 14, 16 to one another takes place intrinsically with a changed adjustment of the variable zoom group 4. In particular the same field of view is always observed on both detectors 14, 16. A laborious computational matching of the image contents to form a fusion or overlapping is not necessary. In any case, the size of the employed detectors 14, l6can differ.

The zoom objective 1 corresponding to Fig. 1 allows in particular a camera system 21 to be provided, in which the image processing unit 20 processes the image contents of the two separate detectors 14, 16 in real time and emits and/or stores image information or an image representation.

The observation of an object with polarisation-different observation on separate detectors 14, 16 allows, as described hereinbefore, daytime visibility enhancement, since in the case of remote objects scattering effects due to atmospheric particles can be taken into account or can be computationally suppressed. Also, a polarisation-separated observation allows the discrimination of specific objects, in which the polarisation properties of the light reflected or scattered at these objects can be utilised Fig. 2 shows schematically the zoom objective 1 and the camera system 21 corresponding to Fig. 1 with an additional facility A polarisation optics system 22 is arranged in front of the optic zoom group 4, which observes an object under two different angles of observation and couples both object beams along the common image path 2 into the zoom objective 1.

The polarisation optic system 22 includes in this connection a first mirror element 23 and a second, partially transparent mirror element 24 perpendicular thereto arranged spaced from the image path 2, the element 24 being formed so that the reflected polarisation is orthogonal to the transmitted polarisation.

For example, this can be implemented so that the reflected beam is s-polarised and the transmitted beam is p-polarised. A first object beam is collected via the first mirror element 23 and is deflected by reflection onto the second mirror element 24. After further reflection at the second mirror element 24 the first object beam 26 is coupled into the zoom objective 1. At the same time a second object beam 28 close to the axis, which passes through the partially transparent polarisation-dependent second beam splitter element 24, is also coupled into the zoom objective 1. Via both object beams 26, 28 an object is thus observed under a parallax. The two object beams 26, 28 contain spatial information on the object.

Through the double reflection of the first object beam 26 this receives a p-polarisation parallel to the plane of incidence. The second object beam 28 receives on passing through the second beam splitter element 24 an s-polarisation perpendicular to the plane of incidence.

The first object beam 26 is reflected at the beam splitter element 10 and is focussed by means of the second imaging group 15 onto the second detector -13- 16. The second object beam 28 is focussed via the first imaging group 13 onto the first detector 14.

The image contents of both detectors 14, 16 are processed by means of the image processing unit 20. The image processing unit 20 generates in this case in real time a three-dimensional image of the recorded object. The camera system 21 is suitable in particular for three-dimensional filming.

A further zoom objective 30 is illustrated in Fig 3 This differs from the zoom objective 1 corresponding to Figs. 1 and 2 inter alia in that the zoom optics system has been augmented by a front group 3 and by a common imaging group 7. The beam splitter element 10 reflects 20% of the object beam and allows 80% to pass through to the first detector 14.

The second imaging group 15 is formed as an afocal lens system 31. An aperture diaphragm 32 is arranged in the afocal system. An optic phase analyser 34 is arranged between the afocal lens system 31 and the second detector 16. This analyser includes a micro lens array, which corresponding to the phase relation of partial beams focuses in a locally resolved manner onto the second detector 16. The aperture diaphragm 32 in the afocal system allows an unambiguous phase measurement on the optic axis.

The image points generated by the individual micro lenses vary in their position on the detector 16 depending on the phase relation of the imaged partial beams with respect to one another. By observing the image content of the second detector 16 it is therefore possible to observe the phase deformation of the incident wave front in real time.

An object is observed via the first detector 14. Due to atmospheric fluctuations, for example temperature or density fluctuations, image deformations occur in the imaging on the first detector 14.

The image processing unit 20 takes into account the information on the phase deformation of the wave front obtained by means of the optical phase analyser 34 and back-calculates the image deformations. The determination of the phase deformation as well as the calculation for reducing the image deformation take place directly. The corrected, improved image is output by the image representation unit 35 in real time. The image processing unit 20. image representation 35, the detectors 14, 16 and the zoom objective 30 together form a camera system 36.

Alternatively the first imaging group 13 can be formed as an adaptive optical element or can include such an element. This adaptive optical element is controlled by the image processing unit 20 depending on the information on the phase deformation of the wave front obtained by means of the optical phase analyser 34, so that the image deformations caused by atmospheric fluctuations are corrected optically. In other words, the adaptive optical element is controlled as it were in a complementary manner, so that on passing through the adaptive optical element the phase deformations of the wave front are cancelled. The corrected image is output by the image representation unit.

List of reference numerals I Zoom objective 2 Common imaging path 3 Front group 4 Zoom group Variator group 6 Compensator group 7 Common imaging group Beam splitter element 11 First optical axis 12 Second optical axis 13 First imaging group 14 First detector Second imaging group 16 Second detector Image processing unit 21 Camera system 22 Polarisation optics system 23 First mirror element 24 Second mirror element 26 First object beam 28 Second object beam Zoom objective 31 Afocal lens system 32 Aperture diaphragm 34 Phase analyser Image representation 36 Camera system -16-

Claims

Claims Zoom objective (1, 30) comprising along a common imaging path (2) at least one optical zoom group (4) displaceable along the imaging path (2) and a beam splitter element (10) which is adapted to separate the beam path into a first path along a first optical axis (11) and into a second path along asecond optical axis (12) that is inclined relative to the first optical axis (11), as well asalong the first optical axis (11) a first optical imaging group (13) that is adapted to form an object image on a first detector (14), and along the second optical axis a second optical imaging group (15) that is adapted to form an object image on a second detector (16).
2. Zoom objective (1, 30) according to claim 1, wherein the beam splitter element (10) is adapted for a polarisation-dependent separation of the beam path into the first path for a first polarisation direction and into the second path for a second polarisation direction.
3. Zoom objective (1, 30) according to claim 2, wherein the beam splitter element (10) is adapted for a polarisation-dependent separation of the beam path into the first path and into the second path for two mutually orthogonal polarisation directions.
4. Zoom objective (1, 30) according to any one of the preceding claims, wherein a polarisation optics system (22) is arranged in front of the zoom group (4), which is adapted for a coupling of two object beams (26, 28) differing in the angle of observation and in the polarisation, along the common imaging path (2).
5. Zoom objective according to claim 4, wherein the polarisation optics system (22) includes at least a first mirror element (23) and a second, partially transparent mirror element (24), and wherein the first mirror -17-element (23) and the second mirror element (24) are arranged spaced from one another perpendicular to the common imaging path (2).
6. Zoom objective (1. 30) according to any one of the preceding claims, wherein an optical front group (3) and a common optical imaging group (7) are furthermore provided along the common imaging path (2), between which groups is arranged the displaceable zoom group (4).
7. Zoom objective (1, 30) according to any one of the preceding claims, wherein the optical zoom group (4) includes a displaceable variator group (5) and a displaceable compensator group (6).
8. Zoom objective (1, 30) according to any one of the preceding claims, wherein an optical phase analyser (34) is arranged along the second optical axis 12, which is adapted to form an image of a local phase deformation on the second detector (16) Zoom objective (1, 30) according to claim 8, wherein the second imaging group (15) includes an afocal lens system (31) with an aperture diaphragm (32), and wherein the phase analyser (34) is formed with an aperture array or a micro lens array, which forms an image on the second detector (16).10. Zoom objective (1, 30) according to any one of the preceding claims, wherein an adaptive optics element is arranged along the first optical axis (11).11. Zoom objective (1, 30) according to any one of the preceding claims, wherein the optical components are made of an optical glass.12. Zoom objective (1, 30) according to any one of the preceding claims, wherein at least one of the optidal imaging groups (13, 15) is displaceably arranged along the respective optical axis (11, 12).13. Camera system (21, 36) with a zoom objective (1, 30) according to any one of the preceding claims, comprising a number of detectors (14, 16) and an image processing unit (20) for evaluating the image contents of at least the first and the second detectors (14, 16).14. Camera system (21, 36) according to claim 13, wherein CMOS and/or COD detectors are used as detectors (14, 16).15. Camera system (21, 36) according to claim 13 or 14, wherein the image processing unit (20) is adapted to process the image contents of the first detector (14) and of the second detector (16) with each other, in particular in real time.16. Camera system (21, 36) according to any one of claims 13 to 15, wherein the image processing unit (20) is additionally adapted to control the adaptive optics element.