GB2488245A

GB2488245A - Re-imager with symmetrical lens arrangement.

Info

Publication number: GB2488245A
Application number: GB1202877.5A
Authority: GB
Inventors: Holger Vogel; Harry Schlemmer; Alexa Schilling
Original assignee: Carl Zeiss Optronics GmbH
Current assignee: Hensoldt Optronics GmbH
Priority date: 2011-02-21
Filing date: 2012-02-20
Publication date: 2012-08-22
Anticipated expiration: 2032-02-20
Also published as: DE102012202541B4; GB201202877D0; DE102012202541A1; GB2488245B

Abstract

Re-imager with symmetrical lens arrangement.A thermal imaging camera with re-imager 7 has a two stage optic arrangement formed of a front objective 8 and relay or switching or inverting optics 9 as objective subsystems 10, which include a plurality of lenses 17a-17h, where the inner lenses of the subsystems 10 are arranged so that their refractive powers are symmetrical about an intermediate image plane 11.

Description

Description:

Optics for Thermal Imaging Cameras The invention relates to optics for thermal imaging cameras, in particular to a re-imager having a front objective and relay optics as objective subsystems, which include a plurality of optical elements, a beam path and an intermediate image plane provided between the objective subsystems in the beam path. The invention also relates to a thermal imaging camera having such optics.

Such optics for thermal imaging cameras or such a re-imager, i.e. a telescope with an intermediate image plane, are/is known from DE 196 00 336 Al.

The re-imager is optically designed in two stages and has an intermediate image plane. For optical imaging, as a rule two objective subsystems are used, with the object scene being imaged onto the intermediate image plane by means of a first objective subsystem, the front objective, and an optical imaging of the intermediate image into the actual image plane is carried out by means of a second objective subsystem, the relay optics. The realisation of the intermediate image plane allows here a simplified optical intervention in the intermediate image of the object scene. For example, an optical stop may be introduced into this plane, which is in particular also the case in thermal imaging systems, in order to reduce the incidence of interference radiation onto the detector, wherein for example a cooled stop in the intermediate image plane of the thermal imaging system is also referred to as a narcissus stop.

Observation devices for medium infrared, that is to say in the wavelength range of approx. 3 pm to 5 pm (hereinafter also referred to as MWIR / Mid Wavelength Infra Red), have a focal range error of ± 2 to 5 1. The typical field of vision error of such systems is also ± 2 to 5 % and the typical distortion is ± 1 to 3 1. The athermalisation of typical observation optics in MWIR is carried out actively, i.e. as a rule an optical element or a group of optical elements is moved into a position determined as a function of temperature by means of a temperature sensor, so that the image position remains unchanged. The same also applies to continuous zoom systems.

The position of the image centre may typically vary by ± 2 %.

In the case of LWIR (Long Wavelength Infra Red) imaging optics, i.e. for example in the case of re-imagers for the wavelength range of approx. 7.5 pm to 10.5 pm with a similar focal length, focal length accuracies of up to ± 0.1 mm can be achieved. In typical fixed focal lengths of re-imagers for an LWIR of 20 mm to 40 mm, a focal length accuracy of ± 0.1 mm corresponds to a focal length fluctuation of ± 0.2 % to 0.5 %.

Analogously to a focal length accuracy of ± 0.2 1 to 0.5 1, an accuracy in the same order of magnitude is achieved for the transferred field of vision. The passive athermalisation of LWIR re-imagers relates in the first instance to a stable image position and a constant focal length. The accuracy of the image centre position is ± 2 1. In a comparable field of vision, an image rotation of 1 1 to 2 1 is typical.

Apart from observation purposes, re-imagers can also be used for measurement purposes. By means of imaging the object scene using a re-imager, in particular lateral distances can be recorded. If for example length or speed information is needed in a particular three-dimensional measurement volume, for instance for recording flight data on birds, for example a thermal image measurement system on the basis of stereoscopy may be used. In such a stereoscopic measurement system, at least two re-imagers are used, which are spatially separated from each other by a respective so-called base and image substantially the same object scene. Moreover, it is also possible for a larger object scene to be realised by way of linking a plurality of individual fields of vision with each other with or without an overlap area. In these measurement applications, the imaging performance requirements to the re-imagers have to be clearly higher so as to ensure that the measurement task can be carried out at all. The stereoscopic measurement approach utilises in particular the differences between the two camera images for determining distance and speed parameters and is therefore particularly sensitive and therefore also interference-sensitive in respect of any possible influences that might have an effect on a comparison of these camera images. In principle, an initial calibration can be carried out, in the course of which some optical imaging errors may be corrected. However, the optical imaging properties have to be constant in particular also in terms of time and in respect of thermal influences to make sure the measurement results do not get distorted. If there is a need to replace one of the re-imagers because of a defect, the optical properties of the overall optical measurement system may also only be insignificantly changed by this replacement process.

The demands made here can be ensured only by an extremely high temporal and thermal stability of the line-of-sight (LOS), the field of vision, the focal length, the distortion as well as the focus position. Moreover, the optical imaging parameters that cannot be corrected by an initial calibration have to be close to the physically optimal limit values. Amongst other things, in particular the field of vision distortion and the chromatic imaging errors of the re-imagers should be extremely low.

DE 10 2008 058 798 Al relates to a stereo camera unit having at least two adjusted thermal imaging cameras which are arranged and positioned at a defined distance to each other, and such thermal imaging cameras have to be provided with a calibration unit for the continuous and automatic calibration thereof.

It is the object of the invention to meet the extremely high requirements made to the re-imagers of a stereoscopic measurement system in relation to thermal stability, chromatic imaging performance and distortion.

According to the invention, this object is achieved by means of optics for thermal imaging cameras which have the features listed in claim 1.

As a result of the fact that the distribution of the refractive powers in the objective subsystems in relation to the intermediate image plane is realised to be at least approximately symmetrical, an extremely low distortion can be achieved. A stereoscopic image evaluation by means of a stereo camera unit having two re-imagers according to the invention can thus be carried out in an advantageous manner in respect of orientation/position in the observation space with a sufficient level of accuracy.

In the symmetrical distribution of the refractive powers, the sequence of the magnitudes and/or the signs of the refractive powers of the individual optical elements, in particular lenses (positive lens or negative lens) in relation to the intermediate image plane of the objects may be carried out to be symmetrical, in particular the sequence of the signs of the radii of curvature of the individual optical elements (concave or convex) may be formed to be symmetrical.

The expression at least approximately symmetrical' as used herein means that on the one hand there is a certain tolerance range and on the other hand the objective subsystems differ from each other in the area of the image side and of the objective side, i.e. they are not strictly symmetrical. At least the core objective, i.e. the inner lenses, can be strictly symmetrical, with the level of symmetry can decrease outwards, because the distance from the object on the object side may for example be several hundred metres, whereas on the image side, the distance from the image of the remote object may only be several centimetres.

The sequence of the materials used for the individual optical elements of the objective subsystems in relation to the intermediate image plane may be realised to be symmetrical.

The optical properties of the optics according to the invention for thermal imaging cameras, in particular the focal length, the image position, the field of vision and/or the distortion, may for a predetermined temperature range be designed to be at least approximately invariable, in particular passively athermal, preferably with an amount of deviation that is equal to or smaller than 0.5 %, and/or in a predetermined wavelength range, in particular of approx. 3 pm to approx. 5 pm, to be at least approximately invariable, in particular achromatic. As a result of these measures, an achromatic and athermal design, in respect of image position, focal length, field of vision and/or distortion, is made possible in the predetermined wavelength and temperature ranges. The temperature range may be approx. ±5 K, in particular ±2 K about a predetermined constant coefficient of approx. 10°C to approx. 40°C, preferably 22°C.

At least one surface of at least one optical element of a respective objective subsystem, which element is preferably made from silicon, may be formed to be aspherical. The optical element of a respective objective subsystem, which is closest to the intermediate image plane, may be provided with the at least one aspherically formed surface.

According to the invention it may be provided for the objective subsystems to have respectively at least three optical elements, in particular lenses. It may further be provided for the objective subsystems to have respectively a maximum of four optical elements, in particular lenses.

The sequence of the materials used for the individual optical elements of the objective subsystems in relation to the intermediate image plane may be designed to be symmetrical.

At least one negative lens that is preferably made from zinc sulphide may be provided for each objective subsystem.

The entrance pupil of the optics according to the invention may be positioned at a sufficient distance in front of the front objective or the front lens in the direction of the incident radiation.

It is advantageous if at least one optical element of the objective subsystem is made from chalcogenide glass, preferably 1G4 or 1G6. One or a plurality of correction lenses for image errors may, due to their relatively neutral thermal and chromatic properties, be made from chalcogenide glass, preferably from 104 or 106, which is produced by the company Vitron GmbH, Jena.

At least one positive lens may be present for each objective subsystem, which is preferably made from silicon. Out of the range of materials available for an achromatic design, which in a predetermined temperature range is also athermal, silicon may be used for one of the lenses with a positive refractive power.

The optics for thermal imaging cameras according to the invention may: -have an amount of focal length accuracy that is equal to or smaller than 0.2 %, and/or -have an amount of variation of the position of the image centre compared to a mechanical reference that is equal to or smaller than 0.2 1.

The optics for thermal imaging cameras according to the invention may advantageously have an at least approximately temporally constant distortion, which is equal to or smaller than 0.12 % over the entire image area. As a result, the extremely high requirements made to the optics according to the invention within the context of a stereoscopic measurement system with regard to distortion may be met.

A narcissus stop may be inserted in the intermediate image plane.

Claim 14 discloses a thermal imaging camera having optics of a thermal imaging camera according to the invention.

The thermal imaging camera may have an amount of image rotation relative to a mechanical reference that is equal to or smaller than 0.5 %.

The dependent claims relate to advantageous embodiments and developments of the invention. An exemplary embodiment of the invention will be explained in more detail below with reference to the drawings, wherein: Fig. 1 shows a schematic view of a stereo camera unit; Fig. 2 shows a lens section of optics for thermal imaging cameras according to the invention; Fig. 3 shows a polychromatic MTF diagram for the optics according to the invention; and Fig. 4 shows a diagram for illustrating the distortion of a thermal imaging camera over the entire detector.

Fig. 1 shows a stereo camera unit 1 of a monitoring device (not shown in more detail) for runways and landing strips and/or air lanes of airports with a stereoscopic detection of approaching birds 2 or flocks of birds, wherein parameters such as flying altitude, flight direction, flying speed and type/size of the birds 2 or the flocks of birds may be determined. One or a plurality of such stereo camera units 1 are arranged in the area of such runways and landing strips and/or air lanes and have at least two thermal imaging cameras 3a, 3b which are arranged at a defined and adapted distance relative to each other and which run synchronously during operation. The recording times of the thermal imaging cameras 3a, 3b are at least approximately identical and the respective fields of vision 4a, 4b thereof have an overlap area 5. In the overlap area 5, a bird 2 is detected as an object. The two thermal imaging cameras 3a, 3b are adjusted and calibrated relative to each other. Thermal imaging ranges such as LWIR, MWIR, VIJWIR, FIR and SWIR, NIR can be considered for the thermal imaging cameras 3a, 3b.

The stereo camera unit 1 includes an image processing device 6 that is provided for processing the image data recorded using the two thermal imaging cameras 3a, 3b. Further, the cameras 3a, 3b each have optics 7 according to the invention, which are indicated in fig. 1 in a highly simplified manner, for thermal imaging cameras or a re-imager.

The stereo camera unit 1 principally operates autonomously.

However, the information and the recordings are available also externally of the stereo camera unit 1. This data is transferred mainly to air traffic control.

On the image processing unit 6 of the stereo camera unit 1, amongst other things, an observation method for runways and landing strips and/or air lanes at airports is executed, by means of which approaching birds 2 or flocks of birds are stereoscopically detected using the monitoring device or the stereo camera unit 1, and parameters such as flying altitude, flight direction, flying speed and type/size of the birds 2 or the flocks of birds or the density of the flocks may be determined. The parameters are determined by way of stereo evaluation. In this process, by means of the at least two angular fields onto the area 5 captured by the at least two thermal imaging cameras 3a, 3b of the stereo camera unit 1, absolute spatial points of the birds 2 or flocks of birds to be detected are determined. The flying speed of the birds 2 or of the flocks of birds is determined by way of observation over an appropriate period of time. It is also possible to detect birds 2 or flocks of birds that are present at greater distances, and for this purpose a correspondingly longer focal length is used for the two thermal imaging cameras 3a, 3b. In addition, flight objects such as model planes, stunt kites or the like can be detected by means of the stereo camera unit 1 (not shown) The parameters are used to carry out an evaluation and, if needed, to output a corresponding warning message.

Fig. 2 shows a lens section of optics 7 according to the invention for thermal imaging cameras having a fixed focal length (presently 40.09 mm) with a beam path 7a, which can be used for optical imaging onto a planar thermal imaging camera 3a, 3b. The fixed focal length can be adjusted to an accuracy of ±0.1 mm, which corresponds to a focal length accuracy of ±0.2 3. The field of vision has to be adjusted to a comparable accuracy. A fixed focal position and a fixed and accurate image position can be achieved by means of a corresponding adjustment accuracy, and each individual set of optics 7 is designed to be passively athermal and achromatic in terms of an invariable image position and focal length. The set of optics 7 is provided for a wavelength range of from 3.5 pm to pm. The thermal imaging camera 3a, 3b may have for example 640 x 512 pixels with a lateral pixel size of 15 pm. What can be clearly seen in fig. 2 are the two-stage design of the imaging system and the division into a front objective 8 and relay optics 9. Between these two objective subsystems 10, an intermediate image plane 11 is arranged. In front of the image plane 12, which at the same time constitutes the detector plane, a filter 13 and a detector window 14 having a cold shield are arranged upstream in the beam path 7a of the exemplary embodiment shown, in order to improve the imaging guality. Moreover, a filter plane 15 is also located at the location of the exit pupil of the optics 7, and the entrance pupil of the optics 7 is located at a sufficient distance in front of a front lens 17d. What is not shown in fig. 2 is a narcissus stop in the intermediate image plane 11. Each objective subsystem 10 of the optics 7 shown here has four individual lenses l7a -17d or 17e -17h as optical elements, and the distribution of the refractive powers of these individual lenses 17a -17d or 17e -17h is almost symmetrical in relation to the intermediate image plane 11. As a result of this comparatively strict symmetric design it becomes possible to minimise distortion. Due to the symmetric distribution of the refractive powers, the sequence of the magnitudes and/or of the signs of the refractive powers of the individual lenses 17a -17h (positive lens or negative lens) in relation to the intermediate image plane 11 of the optics 7 may be designed to be symmetrical, in particular the sequence of the signs of the radii of curvature of the lenses l7a -17h (concave or convex) may be formed to be symmetrical. In the present exemplary embodiment example, the following symmetric sequence is achieved in the objective subsystems 10: positive lenses 17a, l7e with a first convex surface facing the intermediate image plane 11 and with a second concave surface facing away from the intermediate image plane 11, negative lenses 17b, 17f with two concave surfaces, positive lenses 17c, 17g with a first concave surface facing the intermediate image plane 11 and with a second convex surface facing away from the intermediate image plane 11 as well as positive lenses 17d, 17h. The positive lenses l7d, l7h differ in the sequence of the signs of the radii of curvature, because they are in the area of the image side or in the area of the object side. At least the core objectives, i.e. the inner lenses 17a -17c and 17e -l7g, however, are strictly symmetrical.

In the objective subsystems 10, in particular also aspherical lens surfaces 18 may be used, which is here the case with the lenses that are closest to the intermediate image plane 11.

The lens materials are selected such that the extremely high requirements with regard to chromatic correction and thermal stability are met. The optics 7 are athermalised in particular by passive methods, and to this end, in particular also special materials are selected for the individual lenses l7a - 17h. For example, also the glass types with the abbreviated designations 1G4 and P36 (both chalcogenide glasses), ZnS (zinc sulphide) and ZnSe (zinc selenide) may be used. Negative lenses 17b and l7f are made from zinc sulphide. Since in the exemplary embodiment mentioned the optics 7 are integrated in a climatic chamber (not shown in more detail), also silicon may be used as an optical material for the lenses 17a and 17e.

In the exemplary embodiment shown, there is also a sequence of materials that is symmetrical relative to the intermediate image plane 11. The positive lenses 17a and 17e that are closest to the intermediate image plane 11 can each include silicon, whereas the negative lenses l7b and 17f, which respectively immediately follow on the side facing away from the intermediate image plane 11, can be made from zinc suiphide. It is also possible to use diffractive optics, with exclusively refractive lenses 17a to 17h being used for the optics 7 shown here. An active correction of the relevant imaging parameters by way of mechanical corrections is not provided in this example, and due to the limited temperature range as a result of the climatisation of the optics 7, special frame material may be dispensed with here.

Fig. 3 shows a calculated polychromatic Modulation Transfer Function (MTF) for the optics 7 for thermal imaging cameras 3a, 3b as shown in fig. 2. The illustration includes a plurality of calculated curves for different measurement points in the optical measurement field and also the physical diffraction limit of the image. For the various measurement points, there is respectively a very good course of the Modulation Transfer Function close to the physical limit (also diffraction limit) Fig. 4 shows a special illustration variant of the distortion over the entire detector of a thermal imaging camera 3a, 3b, wherein the occurring distortion of the optics 7 viewed here is indicated by arrows in a much exaggerated manner. The occurring average distortion is approx. 1 pm (exactly 0.0012608 mm), the minimum distortion is 0.16376e-26 mm and the maximum distortion is approx. 6 pm (exactly 0.0061428 mm) with a standard deviation of 0.0010903 mm. This meets exactly the high requirements imposed on the present measurement task.

The adaption to enlarged observation corridors/observation spaces may be carried out by two or a plurality of sets of optics 7 according to the invention, the fields of vision of which may be combined in such a way that an image combination of two or a plurality of detector images may be carried out with one or a plurality of overlap areas. If four sets of optics 7 are combined in a measurement system, two sets of optics 7 are respectively arranged on top of each other for the overlap, in order to cover the required field of vision range in height. The top and bottom sets of optics 7, respectively, from the one overlap arrangement are coupled via the stereo base to the upper and lower associated sets of optics 7, respectively, of the second overlap arrangement at the other end of the stereo base for calculations from the measurement data. For a correct evaluation of the measurement data from various sets of optics 7, an extremely high level of image accuracy is required.

The position of the image centre compared to the mechanical reference in relation to the vertical pixel number (presently 512) only varies by ±0.2 3. In this way, the overlap between the upper and lower sets of optics 7 may be kept small. In particular, the distortion must be very small for an image comparison of one, two or a plurality of camera images from the stereo arrangement with sub-pixel accuracy to be carried out. The distortion is less than 0.12 3 over the entire image and does not vary.

The position of the detector of the thermal imaging camera 3a, 3b can be adjusted in all degrees of freedom during installation. The focus position as well as the line of sight of the thermal imaging camera 3a, 3b may be determined during installation of the detector, in particular by way of a translational displacement or tilting of the same.

List of Reference numerals 1 Stereo camera unit 2 Birds 3a, 3b Thermal imaging cameras

4a, 4b Fields of vision

Overlap area 6 Image processing unit 7 Optics 7a Beam path 8 Front objective 9 Relay optics Objective subsystem 11 Intermediate image plane 12 Image plane 13 Filter 14 Detector window Filter plane 17d Front lens/positive lens 17a, 17c, 17e 17g, 17h Positive lenses 17b, 17f Negative lenses 18 Aspherical surface

Claims

Claims: 1. Optics (7) for thermal imaging cameras, in particular re-imagers having a front objectIve (8) and relay optics (9) as objective subsystems (10), which have a plurality of optical elements (l7a-17h), a beam path (7a) and an intermediate image plane (11) provided between the objective subsystems (10) in the beam path (7a), characterised in that the distribution of the refractive powers, in particular the seguence of the magnitudes and/or the signs of the refractive powers of the individual optical elements (17a-l7h) in the objective subsystems (10) in relation to the intermediate image plane (11), is designed to be at least approximately symmetrical.
2. Optics for thermal imaging cameras according to claim 1, the optical properties of which, in particular the focal length, the image position, the field of vision and/or the distortion, are in a predetermined temperature range at least approximately invariable, in particular passively athermal, preferably with an amount of deviation that is equal to or smaller than 0.5 3, and/or in a predetermined wavelength range, in particular of approx. 3,5 pm to approx. 5 pm, at least approximately invariable, in particular achromatic.
3. Optics for thermal imaging cameras according to claim 1 or 2, characterised in that at least one surface (18) of at least one optical element (17), which is preferably made from silicon, of a respective objective subsystem (10) is formed to be aspherical.
4. Optics for thermal imaging cameras according to claim 1, 2 or 3, characterised in that the optical element (17a, 17e) of a respective objective subsystem (10) that is closest to the intermediate image plane (11), is provided with the at least one aspherically formed surface (18)
5. Optics for thermal imaging cameras according to any one of claims 1 to 4, characterised in that the objective subsystems (10) have at least three optical elements, in particular lenses (17a-17h)
6. Optics for thermal imaging cameras according to any one of claims 1 to 5, characterised in that the objective subsystems (10) respectively have a maximum of four optical elements, in particular lenses (17a-17h)
7. Optics for thermal imaging cameras according to any one of claims 1 to 6, characterised in that the seguence of the materials used for the individual optical elements (17a-17h) of the objective subsystems (10) is designed to be symmetrical in relation to the intermediate image plane (11)
8. Optics for thermal imaging cameras according to any one of claims 1 to 7, characterised in that at least one negative lens (17b, 17f) is provided for each objective subsystem (10), which negative lens is preferably made from zinc sulphide.
9. Optics for thermal imaging cameras according to any one of claims 1 to 8, characterised in that the entrance pupil thereof is disposed at a sufficient distance in the direction of the incident radiation in front of the front lens (17d)
10. Optics for thermal imaging cameras according to any one of claims 1 to 9, characterised in that at least one optical element (17a-17h) of the objective subsystems (10) is made from chalcogenide glass, preferably 1G4 or 106.
11. Optics for thermal imaging cameras according to any one of claims 1 to 10, characterised in that at least one positive lens (17a, 17c, 17d, 17e, 17g, 17h) is provided for each objective subsystem (10), which positive lens is preferably made from silicon.
12. Optics for thermal imaging cameras according to any one of claims 1 to 11, characterised by: -an amount of focal length accuracy that is equal to or smaller than 0.2 1, and/or -an amount of variation of the position of the image centre in relation to a mechanical reference that is equal to or smaller than 0.2 1.
13. Optics for thermal imaging cameras according to any one of claims 1 to 12, characterised by at least one approximately temporally constant distortion that is equal to or smaller than 0.12 3 over the entire image area.
14. Thermal imaging camera (3a, 3b) having optics (7) for thermal imaging cameras (3a, 3b) according to any one of claims 1 to 13.
15. Thermal imaging camera according to claim 14, characterised by an amount of image rotation in relation to a mechanical reference that is equal to or smaller than 0.5 3.