GB2501817A

GB2501817A - Multispectral zoom objective and camera system

Info

Publication number: GB2501817A
Application number: GB1305521.5A
Authority: GB
Inventors: Bertram Achtner; Martin Gerken
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: GB2501817B; DE102012005938B4; GB201305521D0; DE102012005938A1

Abstract

A multispectral 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 along with the first and second detectors 14,16. First and second optical imaging groups 13,15 may be provided along the first and second optical axes 11,12 respectively to form object images on the first and second detectors 14,16.

Description

I

Description

Multispectral zoom objective and camera system The invention relates to a multispectral zoom objective for imaging at least a first spectral region on a first detector and a second spectral region on a second detector. The invention addresses the issue of increasing the information about an object by imaging the object in various spectral regions. The invention further relates to a camera system with a multispectral zoom objective of this type for observing an object in various spectral regions.

The information about an object to be observed can be increased if this is observed or imaged in various spectral regions. The observation of an object in various spectral regions generally allows an improved analysis of the object's own absorption, reflection, transmission and/or radiation characteristics, such that a specific property or the object as such can be identified more easily. So, for example, two objects which appear identical in the visual spectral region may be differentiated from one another in an infrared (IR) spectral region if they have different temperatures. Illuminated objects which seem unremarkable in a spectral region may appear spectacular in another spectral region due to their increased capacity for reflection or absorption in this area. Objects can also be extracted from a background image if they have heated up for example in the sunlight due to their specific absorption or reflection capacity as compared with the environment.

There are known technical implementations of multispectral objectives. For example, multispectral objectives with a fixed focal length are known from EP 0 935 772 SI for the visible (VIS) and infrared (IR) region, from US 5,781,336 for a wavelength region of 0.55 pm to 5.35 pm and from US 6,950,243 52 for a wavelength region of 0.7 pm to 5.0 pm. By selecting a suitable material, it is possible in particular to keep the chromatic imaging errors low for the spectral region to be imaged. A multispectral reflective objective with a fixed focal length is further known from US 5,647,679. This describes a wide-angle objective for the visible and IR region. Here, the beam splitter element separates the visible region from the IR region. Both spectral regions are reflected onto a compact detector.

Multispectral objectives are generally comparatively complex and expensive In addition to this, they are designed specifically for each concrete area of application. Multispectral objectives with a fixed focal length are not suitable for the variable imaging of objects at various distances on detectors of given sizes.

The object of the invention is to provide a multispectral objective which enables a variable imaging of an object in various spectral regions by means of separate detector& A further object of the invention is to provide a corresponding camera system with an objective of this type.

The first object named is achieved in accordance with the invention by a multispectral zoom objective which comprises along a common imaging path at least one optical zoom group which can be displaced along the imaging path and a beam splitter element which is configured to separate the beam path for a first spectral region along a first optical axis and for a second spectral region along a second optical axis which is inclined with respect to the first optical axis and which further comprises a first optical imaging group along the first optical axis, which first optical imaging group is configured for imaging the first spectral region on a first detector, and a second optical imaging group along the second optical axis which is configured for imaging the second spectral region on a second detector.

In a first step, the invention is therefore based on the knowledge that the use of twozoom objectives for the variable imaging of an object in different spectral regions undesirably leads to a large design space and a heavy weight. If two separate camera systems are used, this requires high electrical capacity at the same time. Further, the use of two separate zoom objectives leads to parallax errors in the image. Aligning the zoom objectives to one another, in particular in terms of temperature, vibration or shaking, becomes complex. The image content of the separate detectors is difficult to compare or offset.

In a second step, the invention starts from the consideration to arrange along a common imaging path for both spectral regions one single zoom optic which enables the imaging of the object on the separate detectors with variable focal lengths. This common zoom optic comprises at least one optical zoom group which can be displaced along the imaging path and which is the same for all spectral regions to be imaged. The displaceable zoom group carries out the focal length alteration, focusing and air pressure and temperature compensation for all spectral regions.

The division into separate spectral regions is carried out in the beam splitter element. The optical image groups present in the separate optical paths after separation allow the focal length to be adapted to different detector sizes in the respective spectral regions. A fine chromatic correction for the different spectral regions can also be carried out in the separate optical imaging groups.

Together with the separate additional optical imaging groups, the zoom optic forms an object in various spectral regions on different detectors. The use of common optical components for a plurality of spectral regions enables a reduced installation volume as compared with separate objectives. The focal length and therefore the section of the object imaged on the detectors are altered for all spectral regions simultaneously. With the exception of the size of the detector, no additional, complex conversion is necessary to process the image content of different spectral regions. By using the same zoom path for the different spectral regions, the separate detectors yield the same content in terms of the field of vision and in terms of a distortion and other optical influences. Processing the separate image contents is possible for example by overlaying, by fusing or by using another mathematical algorithm, in particular in real time.

In a preferred improvement, the beam splitter element is rotatable about the common imaging path. This makes it possible to observe the reflected spectral region using additional imaging groups on an additional detector and to supply an additional evaluation. In particular, a selection of a section from the reflected spectral region can be carried out along a further optical path, which can further increase the level of information gained about the object. The rotatable arrangement of the beam splitter element makes possible in particular a further arrangement of second imaging groups and second detectors. By rotating the beam splitter element, a further image can be formed on any other of the second detectors. In this case, the beam splitter element reflects the second spectral region along the or a further second optical axis.

The optical imaging groups each comprise a number of lenses or a group of lenses. The displaceable zoom group is preferably configured for a zoom factor of at least ten, more preferably however of 20 or more. The zoom factor is defined from the ratio of the longest and the shortest focal lengths which can be set.

The zoom optic along the coñimon imaging path advantageously further comprises an optical front group and a common optical imaging group between which the displaceable zoom group is arranged. A front group of this type and/or a common imaging group of this type enables the length of the objective to be kept constant overall for various focal lengths. The zoom region can also be increased in this way.

The zoom objective is further preferably configured for a high opening. The number of apertures Ut is preferably kept less than 8, preferably less than 2.8 by selection of the optical components.

Further advantageously the optical zoom group comprises a displaceable variator group and a displaceable compensator group. Different focal lengths are set using the variator group. The compensator group corrects the image position at different focal lengths and accordingly different positions of the variator group.

In order to correct imaging errors, lenses with aspherical surfaces may be expediently arranged in the front group and in the displaceable optical zoom group. The front group and the displaceable optical zoom group are configured for a high zoom factor in cooperation with the common imaging group.

The beam splitter element is preferably provided as a partially transparent reflector plate or as a splitter prism. The reflector plate is for example coated to reflect the second spectral region, whereby it is tranSmissively configured for the first spectral region. The splitter prism is for example formed as a beam splitter cube, whereby the separation of the two spectral regions is achieved through interfacial effects. the beam splitter element is also advantageously used to correct imaging errors for the first spectral region. In particular, it should be provided that an entry and an exit surface of the beam splitter element are inclined to one another at such an angle that a V shape is formed. A wedge shape of this type improves the astigmatism correction of the image provided that the beam splitter element is not in a parallel beam path. If this is the case, a plane parallel shape of the beam splitter element is expedient.

In a preferred embodiment, a further beam splitter element is arranged along the first optical axis, hich beam splitter element is configured to separate a third spectral region from the first spectral region along a third optical axis which is inclined with respect to the first optical axis, and wherein a third optical imaging group is arranged along the third optical axis to image the third spectral region on a third detector. This design makes it possible to selectively fade out and image a further, third spectral region from the first spectral region for which the upstream (first) beam splitter element is transmissively formed. This image can for example be used for laser-based distance detection. Using the common zoom path ensures that the object irradiated by the laser beam is imaged on the third detector at the same time as the other detectors.

A fourth imaging group can expediently be arranged in front of the further beam splitter element in the above mentioned embodiment variant. This then forms the first spectral region in combination with the first optical imaging group and the third optical imaging group on the respective separate detectors.

In a further preferred embpdiment, several second imaging groups are contained by the multispectral zoom objective, which second imaging groups are arranged displaced at an angle to one another about the common imaging path, and which are each configured for the imaging of at least a partial region of the second spectral region on a respective second detector.

By turning a rotatable arranged beam splitter element, the further second imaging groups which are arranged in a ring and the detectors can be illuminated with the reflected spectral region. The multispectral zoom objective forms an image on the first detector and on one of the second detectors. If filter elements are arranged along the second optical path, or if the optical elements, such as in particular the lenses, of the second imaging group are selectively coated, then further spectral regions from the spectrum reflected by the rotatable arranged beam splitter element can be selectively imaged on the respective second detector. Different detectors can also be used on the basis of their wavelength sensitivity, such that in this way selected spectral regions from the reflected spectrum can also be observed. In this respect, an optical filter element is preferably arranged along at least one of the optical axes in the multispectral zoom objective. At least one wavelength selective coated image element is expediently contained within one of the optical imaging groups.

In a further advantageous embodiment, at least one of the optical imaging groups is configured to correct a chromatic aberration of the respectively imaged spectral region by means of the zoom optic which is arranged upstream in the common imaging path. This is recommended when the optical components of the zoom optic in front of the beam splitter element lead to imaging errors in one of the spectral regions observed, as these components are only optimised for an observed spectral region in terms of the imaging. If an optical imaging group is formed for the additional correction of the chromatic aberration in the upstream zoom optic then the colour errors thereof are taken into account. The imaging of the respective spectral region on the detector is then achieved with the desired quality by means of correspondingly optimised optical components of the imaging group.

The optical imaging group configured for the correction of the chromatic aberration of the common zoom optic preferably comprises a correction lens with a diffractive surface. A diffractive surface of this type, for example a grid structure, formed by varying the layer thickness, the refraction index or the transmissivity, for which in particular by means of the requirement for constructive interference at different focal lengths for beams of different wavelengths and can therefore be used to correct chromatic imaging errors. In particular, the diffractive surface of the correction lens is formed with a kinoform profile.

More preferably, the beam splitter element is configured to separate the beam path into a visible spectral region and an infrared spectral region. For example, a visible spectral region of 0.4 pm up to 0.7 pm can be transparently imaged on the first detector, while an IR region, for example the NIR region (0.7 pm to 1.4 pm) or the SWIR region (1.4 pm to 3.0 pm) is reflected by the beam splitter element. Other variations of the respective reflected and transparent spectral regions from the visible and IR region are also conceivabla In particular, the multispectral zoom objective is designed for imaging a wavelength region between 400 nm and 2.5 pm. This means that an optical glass can be used for the components to be imaged.

More preferably, at least one of the optical imaging groups is arranged displaceably along the respective optical axis. This enables a back focal length compensation when spectral regions which lie at a distance to one another are imaged on the first and the second detector. If it is wished for both image contents to be sharp, it is advantageous to displace the respective imaging group in order to focus on the spectral region being observed.

The second object is achieved according to the invention by a camera system with a multispectral zoom objective of the type described above, a number of detectors and an image processing unit to evaluate the image content of at least the first and the second detector.

Each of the advantages given in relation to the improvements to the multispectral zoom objective correspondingly can also be transferred to the camera system. A camera system of this type has a compact structure, is light weight and requires a small amount of energy.

CMOS andlor CCD detectors are preferably used as detectors for a VIS/NIR spectral region and lnGaAs detectors are used as detectors for an SWIR spectral region.

Furthermore, the image processing unit is preferably also configured such that the image cbntent of at least the first detector and a second detector can be computed. The image processing unit is advantageously configured such that the separate image contents of at least two of the detectors can be processed with each other in real time. The image contents from the different detectors can be overlaid or fused with one another electronically in real time by means of suitable image processing mechanisms. The zoom optic through the common imaging path means that the image contents recorded by the separate detectors are correspondingly simultaneously imaged at any time in terms of the image field and the image detail, an optical distortion or other optical influences.

Exemplary embodiments are explained in greater detail using figures. Figures: FIG 1: is a schematic view of a multispectral zoom objective to image different spectral regions on three detectors with a first alignment of a beam splitter element, FIG 2: is a schematic view of the multispectral zoom objective according to FIG 1 with a second alignment of the beam splitter element, FIG 3: is a schematic view of a further multispectral zoom objebtive for the imaging of different spectral regions on three detectors, which zoom objective comprises two beam splitter elements, FIG 4: is a specific design for the multispectral zoom objective in accordance with FIG 1, FIG 5: is a further specific design for the rnultispectral zoom objective in accordance with FIG I with a first alignment of the beam splitter element and FIG 6: is a specific design for the multispectral zoom objective in accordance with FIG 5 with a second alignment of the beam splitter element.

FIG 1 is a schematic view of the general structure of a multispectral zoom objective 1 for imaging different spectral regions on separate detectors.

The multispectral zoom objective I comprises a zoom optic which is common to all spectral regions along a common imaging path 2. This zoom optic has a front group 3, a zoom group 4 which can be displaced along the imaging path 2, which zoom group comprises a variator group 5 and a compensator group 6, a common optical imaging group 7 and an iris diaphragm 8 used as an aperture diaphragm.

After the zoom optic, the spectrum is separated into separate spectral regions by a beam splitter element 10 in the shape of a splitter prism. This separates the spectrum observed into a first spectral region along a first optical axis 11 from a second spectral region along a second optical axis 12. To this end, the splitter prism 10 is designed in a transmissive manner for the first spectral region and in a reflective manner for the second spectral region.

The first spectral region is focused on a first detector 14 using the first optical imaging group 13. The reflected second spectral region is focused using a second optical imaging group on a second detector 16.

When rotated 180° about the common imaging path 2, the multispectral zoom objective 1 further comprises an additional second optical imaging group 20 along an additional second optical axis 19, which second optical imaging group focuses on an additional second detector 21. A filter element 22 is arranged between the beam splitter element 10 and the additional second optical imaging group 20.

FIG 1 is the multispectral zoom objective 1 with an alignment of the beam splitter element 10 in such a way that the reflected spectral region is formed on the second detector 16. FIG 2 shows the same multispectral zoom objective 1, wherein the beam splitter element 10 is now in a position rotated 180° about the common imaging path 20. This means that the reflected spectral region is imaged via the additional second optical imaging group 20 on the additional second detector 21.

Figures 1 and 2 show that the zoom optic is the same for all imaged spectral regions on the detectors 11, 12, 19. The respective spectral regions are each completely imaged on the corresponding detectors 11, 12 or 21 by the zoom optic with the optical imaging groups 13, 15, 20. In each case, two detectors 11, 12cr 11, 19 are illuminated at the same time, depending on the position of the beam splitter element 10. The filter element 22 allows a third spectral region to be selected from the reflected second spectral region.

FIG 3 is a schematic view of a further multispectral zoom objective 30, which is also configured for imaging different spectral regions on three detectors.

The multispectral zoom objective 30 in turn comprises a common zoom optic along a common imaging path 2. Similarly to figures 1 and 2, this zoom optic comprises an optical front group 3, a displaceable optical zoom group 4 comprising a variator group 5 and a compensator group 6, a common optical imaging group 7 and an iris diaphragm 8.

The beam splitter element 10 reflects a second spectral region along the second optical axis 12. The second spectral region is focussed on the second detector 16 by means of the second optical imaging group 15. An additional beam splitter element 31 is arranged downstream of the beam splitter element along the first optical axis 11, which beam splitter element reflects a part from the first spectral region along a third optical axis 33. This third spectral region is focussed on a third detector 35 using a third optical image group 34. In addition to this, a third optical image group 36 is active between the beam splitter element 10 and the additional beam splitter element 31.

In the multispectral zoom objective 30 according to FIG 3, the reflected second spectral region is formed on the second detector 16 via the common zoom optic and the second imaging group 15. The third spectral region reflected from the transmitted first spectral region by means of the additional beam splitter element 31 is imaged on the third detector 35 by the zoom optic and the third optical imaging group 34. The remaining part of the first spectral region is imaged on the first detector 14 using the first optical imaging group 13.

An image processing unit 37 is provided for a camera system 39, which image processing unit is designed for processing the image content of the detectors 11, 12, 35 by fusion and/or overlaying in real time.

For example, the spectral region of 400 nm to 1800 nm should be imaged on the three detectors 11, 12, 35 using the multispectral zoom objective 30 in accordance with FIG 3 in order for an object to be observed. On the beam splitter element 10, the region from 400 nm to 700 nm is reflected as a second spectral region and imaged on the second detector 16. The remaining spectral region from 700 nm to 1800 nm penetrates the beam splitter element 10 and impinges on the additional beam splitter element 31. This reflects a region between 1500 nm and 1600 nm as a third spectral region at 70% on the third detector 35. The remaining light is imaged on the first detector 14 as a first spectral region.

Figures 4 to 6 each show specific embodiment variants of the multispectral zoom objective I shown schematically in figures 1 and 2. An image processing unit 40 is provided for each camera system 41, which image processing unit is configured for the common processing of the image contents of the respective illuminated detectors in real time. To this end, corresponding algorithms are used to fuse and/or overlay the image content in the image processing unit 40.

The progression of the field bundle for imaging an object on the different detectors is clearly shown in each case, The front group 3, the variator group 5, the compensator group 6 and the optical common imaging group 7 are each formed by a group of lenses with an obvious number of lenses. The same applies to the additional optical imaging groups 13, * l5and2O.

In accordance with the specific embodiment in FIG 4, the region from 400 nm to 700 nm is reflected on the beam splitter element 10 as a second spectral region from an observed spectral region of 400 nm to 1700 nm and imaged on the additional second detector 21. The spectral region from 700 nm to 1700 nm is imaged on the first detector 14 as a first spectral region.

According to FIG 5, the additional second detector 21 is illuminated using the beam splitter element 10. The second detector 16 which is arranged in a rotated manner 180° about the common imaging path 2 is not illuminated. The beam splitter element 10 can be rotated about the common imaging path 2. In FIG 6, the beam splitter element 10 is rotated 180° in relation to FIG 5. Accordingly, the second detector 16 is now illuminated.

In figures 5 and 6, a spectral region of 400 nm to 1700 nm is imaged on three detectors. The beam splitter element 10 reflects as a second spectral region 400 nm to 1400 nm. A first spectral region from 1400 nm to 1700 nm is imaged on the first detector 14. A partial region of 400 nm to 700 nm is imaged on the second detector 16 from the second spectral region. The additional spectral region from 700 nmto 1400 nm is imaged on the additional detector 21.

The distribution of the second spectral region on the detectors 16 and 21 is carried out by coating the lenses in the second optical imaging groups 15 and by means of long-pass and short-pass filters (not shown) and by selecting the corresponding detectors 16 and 21. The coating of the optic elements in the second optical imaging group 15 is selected such that transmission in the spectral window of 400 nm to 700 nm is maximised. The coating of the optic elements in the additional optical imaging group 20 is selected such that transmission in the spectral window of 700 nm to 1400 nm is maximised. By rotating the beam splitter element 10, optionally the second detector 16 or the additional second detector 21 can be illuminated.

Reference number list 1 Zoom objective 2 Common imaging path 3 Front group 4 Zoom group Variator group 6 Compensator group 7 Common imaging group 8 Aperture Beam splitter element 11 First optical axis 12 Second optical axis 13 First imaging group 14 First detector Second imaging group 16 Second detector 19 Additional second optical axis Additional second imaging group 21 Additional second detector 22 Filter Zoom objective 31 Additional beam splitter element 33 Third optical axis 34 Third imaging group Third detector 36 Fourth imaging group 37 Image processing unit 39 Camera system Image processing unit 41 Camera system

Claims

Claims Multispectral zoom objective (1, 30) comprising along a common imaging path (2) at least an optical zoom group (4) which can be displaced along the imaging path (2) and a beam splitter element (10) which is configured to separate the beam path for a first spectral region along a first optical axis (11) and for a second spectral region along a second optical axis (12) which is inclined with respect to the first optical axis, and a first optical imaging group (13) along the first optical axis (11), which first optical imaging group is configured for imaging the first spectral region on a first detector (14), and a second optical imaging group (15) along the second optical axis, which second optical imaging group is configured for imaging the second spectral region on a second detector (16).
2. Multispectral zoom objective (1, 30) according to Claim 1, wherein the beam splitter element (1) is rotatable around the common imaging path (2).
3. Multispectral zoom objective (1, 30) according to either one of claim 1 or claim 2, wherein an optical front group (3) and a common optical imaging group (7) are further provided along the common imaging path (2), between which common optical imaging group (7) and optical front group (3) the displaceable zoom group (4) is arranged.
4. Multispectral zoom objective (1, 30) according to any one of the preceding claims, wherein the optical zoom group (4) comprises a displaceable variator group (5) and a displaceable compensator group (6).
5. Multispectral zoom objective (1, 30) according to any one of the preceding claims, wherein a further beam splitter element (31) is arranged along the first optical axis (11), which beam splitter element is configured to separate a third spectral region from the first spectral region along a third optical axis (33) which is inclined with respect to the first optical axis (11), and wherein a third optical imaging group (34) is arranged along the third optical axis (33) to image the third spectral region on a third detector (35).
6. Multispectral zoom objective (1, 30) according to Claim 5, wherein a fourth optical image group (36) is arranged along the first optical axis (11) in front of the additional beam splitter element (31).
7. Multispectral zoom objective (1, 30) according to any one of the preceding claims, comprising a plurality of second imaging groups (15, 20) which are displaced at an angle to one another about the common imaging path (2). and which are each configured to image at least a partial region of the second spectral region on a respective second detector(16, 21).
8. Multispectral zoom objective (1, 30) according to any one of the preceding claims, wherein the beam splitter element (10) is configured such that it separates the beam path into a visible spectral region and an infrared spectral region.
9. Multispectral zoom objective (1, 30) according to any one of the preceding claims, wherein an optical filter element (22) is arranged along at least one of the optical axes (11, 12, 19).
10. Multispectral zoom objective (1, 30) according to any one of the preceding claims, wherein a wavelength selected coated imaging element is contained by at least one of the optical imaging groups (13, 15, 20, 34).
11. Multispectral zoom objective (1, 30) according to any one of the preceding claims, wherein at least one of the optical imaging groups (13, 15, 20, 34) is configured for correcting a chromatic aberration of the spectral region imaged in each case by means of the optical components arranged upstream in the common imaging path (2).
12. Multispectral zoom objective (1, 30) according to Claim 11, wherein at least one of the optical imaging groups (13, 15, 20, 34) comprises a correction lens with a diffractive surface to correct the chromatic aberration.
13. Multispectral zoom objective (1, 30) according to any one of the preceding claims, wherein the optical components are manufactured from an optical glass.
14. Multispectral zoom objective (1, 30) according to any one of the preceding claims, wherein at least one of the optical imaging groups (13, 15, 20, 34) is arranged movably along the respective optical axis (11, 12, 33).
15. Camera system (39, 41) having a multispectral zoom objective (1, 30) according to any one of the preceding claims, and having a number of detectors (14, 16, 21, 35) and having an image processing unit (37, 40) to evaluate the image content of at least the first and the second detector (14, 16, 21, 35).
16. Camera system (39, 41) according to Claim 15, wherein CMOS and/or CCD detectors are used as detectors (14, 16, 21, 35) for a VIS/NIR spectral region, and lnGaAs detectors are used as detectors (14, 16, 21, 35) for a SWIR spectral region.
17. Camera system (39, 41)accordingto either one of claim 15 or claim 16, wherein the image processing unit (37, 40) is configured to process the image content of the first detector (14) and a second detector (16, 21) with each other, in particular in real time.