Device for optically imaging a scene with protection function against intense interference radiation

Abstract

A device for optically imaging a scene has at least one optical detector and an optical arrangement arranged in front of the optical detector, by means of which the scene can be imaged in a focal plane of the optical arrangement. The detector surface of the optical detector is arranged outside the focal plane and the optical arrangement has an amplitude or phase mask which enables improved algorithmic reconstruction of a sharp image of the scene from a blurred image acquired by the detector. In an alternative, the device additionally has an optically nonlinear filter arranged in the region of the focal plane which, when subjected to incident optical radiation with an intensity above a threshold intensity, changes its optical properties such that it blocks or at least attenuates this radiation.This device creates an optical sensor system that has an improved protective effect against interference radiation, in particular incident laser radiation, while maintaining almost unchanged size.

Description

Technical Field of ApplicationThe present invention relates to an apparatus for optically imaging a scene, which apparatus comprises at least one optical detector and an optical arrangement arranged in front of the optical detector, wherein the detector surface of the optical detector is arranged outside the focal plane of the optical arrangement and the optical arrangement comprises an amplitude or phase mask which enables an improved algorithmic reconstruction of a sharp image of the scene from a blurred image captured by the detector.Optical sensor systems for imaging a scene can be temporarily dimmed or even irreversibly destroyed by high-energy laser radiation. This is due, among other things, to the fact that the laser radiation as well as the radiation to be imaged is focused by the optics of the optical sensor system onto the detector plane. This produces very high intensities in the detector plane, which saturates the detector elements and can even exceed the destruction threshold of the detector. The optical sensor system is thus no longer usable during and possibly also permanently after the glare.Prior ArtHitherto, various approaches have been known for avoiding or reducing these problems. For example, DE 20 2010 002 568 U1 describes a device for limiting a transmitted optical power, which has at least one focusing optical unit that focuses incoming light onto an intermediate focus. A functional element is then arranged in the intermediate focus, which has a greater absorption at higher intensities than at low intensities. In this way, an element located behind the device, such as a CCD camera, can be protected from damage by interference radiation with an excessively high optical power. However, by additionally generating an intermediate focus, the overall length of the optics required for an optical sensor system is significantly increased.Another known possibility for better protection of the detector of an optical sensor system against intense interference radiation is to shift the detector surface of the detector out of the focal plane of the optical arrangement, so that the detector captures an unsharp image of the scene. From the acquired unsharp image, an image as sharp as possible is then reconstructed with a suitable reconstruction algorithm. Such an algorithm employs a deconvolution method using the optical array's point spread function (PSF). To improve the reconstruction, it is known additionally to arrange an amplitude or phase mask in the optical arrangement, which mask modifies the PSF in a suitable manner. For example, J. H. Wernh et al., "PSF Engineering for Sensor Protection", in Frontiers in Optics 2017, show such a procedure with a suitable phase mask.RITT, Gunnar et al.: "Protection performance evaluation regularing imaging sensors hardened against laser dazzling"; Proceedings of SPIE, Vol. 9249, 2014, Art. No. 924908, 8 pp. - ISSN 0277-786X use a combination of a digital micromirror array (DMD) for beam deflection with wavelength multiplexing by dispersive elements to protect a sensor from laser radiation.DE 10 2019 135 485 B3 describes a device for protecting a sensor from laser radiation, in which two shutter devices made of DMDs are used, via which incident laser radiation is deflected when detected by a further sensor.The object of the present invention is to specify a device for optically imaging a scene which offers an even improved protection of an optical detector used in optical imaging against intense interference radiation, in particular against laser glare and laser destruction, without significantly changing the overall size of the device compared to a device without additional protective measures.SUMMARY OF THE INVENTIONThe object is achieved with the device according to claim 1 and claim 2. Advantageous embodiments of the device are the subject matter of the dependent patent claims or can be gathered from the following description and the exemplary embodiments.The proposed apparatus for optically imaging a scene has, in a known manner, at least one optical detector, for example a CCD detector, and an optical arrangement arranged in front of the optical detector, by means of which an scene can be (sharply) imaged into a focal plane of the optical arrangement. In the device according to the invention, the detector surface of the optical detector is displaced away from the optical arrangement relative to the focal plane of the optical arrangement, that is to say is arranged outside the focal plane or at an optical distance from this focal plane. The optical distance is understood here to mean the optical path length between the focal plane and the detector surface, since an optical deflection element can also be arranged between the two, for example. By this displacement or the optical distance, the detector detects a blurred image of the scene. In the proposed device, the optical arrangement further has an amplitude mask or phase mask which is designed and arranged in such a way that it enables an improved algorithmic reconstruction of an image of the scene that is as sharp as possible from the blurred image captured by the detector. The improved reconstruction is to be understood here in comparison with a reconstruction without such an amplitude or phase mask. In an alternative, the proposed device additionally has, in the region of the focal plane, preferably in the focal plane, an optically nonlinear filter which is transmissive for incident optical radiation of lower intensity, as occurs when a scene is illuminated by daylight, and, when incident optical radiation of higher intensity, changes its optical properties beyond a certain threshold intensity in such a way that it blocks or at least attenuates the incident radiation, preferably attenuates it to such an extent that the detector is not damaged or only minimally damaged by this radiation, so that it remains operable. The optically nonlinear filter is thus located between the optical arrangement and the detector and is designed such that it additionally protects the detector from incident optical radiation of higher intensity.In the proposed device, also referred to below as an optical sensor system, the protection of the optical detector takes place in a plurality of stages. On the one hand, the detector surface of the detector is displaced out of the focal plane of the optical arrangement. The displacement of the optical detector surface from the focal plane is preferably effected by an optical path length at which a reconstruction with sufficient quality is still possible, at a focal length of 75 mm, for example by an optical path length of 5 mm. This reduces the intensity of incident interfering radiation on the detector by several orders of magnitude. In order that a sharp image representation is also possible, incident radiation is suitably encoded by an amplitude mask or phase mask. With the aid of this coding, a sharp image can be reconstructed again from the unsharp image. This measure protects the detector above all from destruction when the optical sensor system is irradiated with a continuous wave laser. With the design of the amplitude or phase mask (i.e. the transmission profile or the height profile), the impulse response of the optical system, i.e. the PSF (point spread function), can be directly influenced. In this case, impulse responses are advantageous which have as few or no zeros as possible in the spatial frequency range and a low variance. This ensures that only little information is lost due to the blurring and an algorithm reconstruction using deconvolution methods can provide good results. An example of a suitable amplitude mask is known under the term in MURA (Modified Uniformly Redundant Array). A so-called spiral mask can be used as the phase mask, for example. Known deconvolution methods such as Wiener deconvolution are suitable for reconstruction.By moving the optical detector out of the focal plane, this focal plane or the immediate area around this focal plane can be used in an alternative to arrange an optically nonlinear filter which, at high or very high intensities which can block or destroy the detector, changes its optical properties such that it prevents incident optical radiation of these high or very high intensities from passing or attenuates or attenuates it so much during passing that this radiation can no longer block or destroy the detector. This non-linear attenuation of the optical radiation may be caused by various processes, such as non-linear absorption, non-linear scattering or phase change, or a combination of several of these processes, which are dependent on the irradiance. Examples of suitable materials for the optically nonlinear filter are nonlinearly absorbing materials, for example via an inversely saturable absorption or via a two-photon absorption, such as, for example, phthalocyanines, porphyrins, nonlinearly absorbing polymers or chalcogenides. A further possibility is offered by non-linearly scattering materials, such as, for example, conductive or semiconducting nanoparticles or carbon-based nanomaterials or materials with a non-linear refractive index change.The protective effect by this optically nonlinear filter is best when the filter is located exactly in the focal plane. It can of course also be displaced with respect to the focal plane. This filter represents a further protection mechanism which protects the detector against incident laser radiation, in particular against laser radiation of pulsed lasers, which can temporarily deliver radiation intensities higher than continuous wave lasers by several orders of magnitude.In a development of the proposed apparatus in this alternative, the optically nonlinear filter can have an adjusting device, by means of which it can preferably be displaced or rotated parallel to the focal plane. This can be realized, for example, by means of a linear displacement or by using a filter wheel. This is preferably carried out automatically via an activation and a suitable, for example electromotive, drive. As a result, after a potential attack by laser radiation on the optical sensor system, an undamaged filter section-or a filter section which has not changed its transmission properties-can be moved in front of the detector in order to continue the image acquisition. In a further advantageous embodiment, the filter is designed as a liquid suspension which is thereby self-regenerated. The suspension is filled into a suitable optically transparent container.In a second alternative of the proposed device, which can also be realized in combination with the first alternative, a spectrally selective optical element is arranged between the optical arrangement and the optical detector, which splits incident radiation into at least two spectral components, of which a first spectral component impinges on the optical detector and a second spectral component impinges on a further optical detector of the device. This further optical detector is preferably likewise arranged at the same optical distance from the focal plane as the first optical detector. The spectrally selective optical element can be, for example, a dichroic prism or a dichroic beam splitter which spectrally separates the incident radiation and directs the separated components onto the at least two optical detectors. The two optical detectors are arranged outside the focal plane and are thus, as already described above, already largely protected thereby from destruction by laser radiation. With this embodiment, only one of the two detectors is blended in each case when irradiated with a narrowband laser, if the other measures described are not yet sufficient. The other detector can then continue to provide an undisturbed image. Of course, more than two optical detectors and more than one spectrally selective optical element can also be used in this configuration, wherein the spectrally selective element(s) optical element(s) then must separate the incident radiation into correspondingly more than two spectral components and direct it onto the respective detectors.The proposed apparatus preferably also comprises an image processing device which is connected to the optical detector or detectors and reconstructs an image of the scene which is as sharp as possible from the respectively captured unsharp image using a suitable reconstruction algorithm.With the proposed device or the proposed optical sensor system, improved protection of the optical detector from intense interference radiation is possible due to the combination of a plurality of protection mechanisms. Since in the first alternative only the focal plane released by displacement of the optical detector is used for the optically nonlinear filter, the overall size of the optical sensor system is hardly changed by this additional measure. The displacement of the detector out of the focal plane also significantly reduces the laser back scattering when the laser radiation arrives. The ability to clarify such a system by optical detection systems is thus also reduced.The proposed device is suitable mainly in the military context for protecting its own sensor system against disruption and destruction by open persons. Here, focus is placed on obtaining the reconnaissance performance. However, the device can also be used in autonomous systems (also civil) such as, for example, autonomous vehicles, aircraft, drones, robots, monitoring cameras, etc., in order to protect the detectors from interfering radiation. Here, very sensitive detectors are often required, while at the same time there is a need for laser-assisted methods (for example LIDAR). Thus, the optical detectors can be protected against the own lasers or lasers of other autonomous systems or also against intentional interference.Brief Description of the DrawingsThe present invention is briefly explained again below on the basis of exemplary embodiments in conjunction with the drawings. The following are shown here: FIG. 1 shows a first example of an embodiment of the proposed device in schematic illustration; and FIG. 2 shows a second example of an embodiment of the proposed device in schematic representation.WAYS OF CARRYING OUT THE INVENTIONFigure 1 shows an exemplary optical sensor system constructed in accordance with the present invention. The optical sensor system has an optical detector S 1, in front of which an optical arrangement in the form of an objective O is arranged, which images a scene SZ to be imaged onto the focal plane F of the objective O, as is indicated by the dashed lines in FIG. 1. The optical detector S1 is arranged outside the focal plane. The objective O is in this example a two-lens system with an entry aperture E and an exit aperture A. Viewed from the direction of the scene SZ to be imaged, a mask M is located in front of the entry aperture E, which mask is designed as an amplitude or phase mask in order to reconstruct an unsharp image of the scene captured by the detector S 1 in such a way that an image of the scene SZ that is as sharp as possible is achieved. In principle, the mask M can also be arranged at another location, for example behind the entry aperture E. In the proposed optical sensor system, an optically nonlinear filter N is located in the focal plane F, which filter changes its optical transmission at high or very high incident light intensities such that it weakens the optical radiation very strongly during passage.FIG. 2 shows a further possible embodiment of the proposed optical sensor system, in which the same elements as from FIG. 1 are denoted by the same reference numerals. In this exemplary further embodiment, a spectral beam splitter B, in the present case a dichroic beam splitter, is additionally located between the focal plane F and the optical detector S 1. This filter B separates the incident optical radiation into two spectral components, a first spectral component of which is transmitted through the beam splitter B and impinges on the optical detector S 1. The second spectral component is reflected by the beam splitter B and impinges on an additionally present second optical detector S 2, which is arranged at the same optical distance from the focal plane as the first optical detector S 1 in the optical sensor system. Since laser radiation is generally narrowband radiation, incident interfering radiation will therefore strike either only the detector S 1 or only the detector S 2 by a laser, so that the respective other detector remains fully functional. This is a further protection mechanism, in particular with respect to narrow-band interference radiation, which further increases the protection of the optical sensor system against interference radiation.A possible further embodiment differs from the configuration shown in FIG. 2 in that the spectral beam splitter B is arranged not behind but in front of the focal plane F. As a result, each of the two detectors S 1 and S 2 can (optionally) be equipped with its own nonlinear filter. An advantage here is that only one non-linear filter is damaged in the case of incident laser radiation with high intensity. The detector, which monitors in the wavelength band complementary to the laser radiation, can record an undisturbed image of the scene.In the embodiments with splitting of the incident radiation by the dichroic beam splitter, the non-linear filter is optional. Protection against damage is already provided by the off-focus position of the detector.For the reconstruction with a suitable reconstruction algorithm, it is advantageous to carry out a plurality of calibration measurements for different object planes in advance and to store the corresponding results, so that a sharp image can then be reconstructed in different object planes on the basis of the PSFs stored for the different object planes.List of reference charactersS1 First detector S2 Second detector O Objective E Entry aperture of the objective A Exit aperture of the objective SZ scene M Mask F Focal plane N Optically nonlinear filter B Spectrally selective beam splitter

Claims

Apparatus for optically imaging a scene (SZ), having at least - an optical detector (S1) and - an optical arrangement arranged in front of the optical detector (S1), by means of which arrangement the scene (SZ) can be imaged into a focal plane (F) of the optical arrangement, wherein a detector surface of the optical detector (S1) is arranged outside the focal plane (F) and the optical arrangement has an amplitude mask or phase mask (M) which enables an improved algorithmic reconstruction of a sharp image of the scene (SZ) from a blurred image captured by means of the detector (S1), and wherein an optically nonlinear filter (N) is arranged in the region of the focal plane (F), the optical properties of incident optical radiation having an intensity above a threshold intensity change in such a way that it blocks or at least attenuates this radiation.Apparatus for optically imaging a scene (SZ), having at least - an optical detector (S1) and - an optical arrangement arranged in front of the optical detector (S1), by means of which arrangement the scene (SZ) can be imaged into a focal plane (F) of the optical arrangement, wherein a detector surface of the optical detector (S1) is arranged outside the focal plane (F) and the optical arrangement has an amplitude or phase mask (M) which enables an improved algorithmic reconstruction of a sharp image of the scene (SZ) from a blurred image captured by the detector (S1), and wherein a spectral-selective optical element (B) is formed and arranged in such a way between the optical arrangement and the optical detector (S1), splitting incident radiation into at least two spectral components, a first spectral component of which impinges on the optical detector (S1) and a second spectral component of which impinges on a further optical detector (S2) of the device, which is likewise arranged outside the focal plane (F).Device according to claim 1, characterised in that the optically non-linear filter (N) is formed as a liquid suspension in an optically transparent container.Apparatus according to Claim 1 or 3, characterized in that the optically nonlinear filter (N) has an adjusting device by means of which it can be displaced or rotated parallel or obliquely with respect to the focal plane (F).Device according to one of Claims 1, 3 or 4, characterized in that a spectrally selective optical element (B) is formed and arranged between the optical arrangement and the optical detector (S1) in such a way that it splits incident radiation into at least two spectral components, a first spectral component of which impinges on the optical detector (S1) and a second spectral component of which impinges on a further optical detector (S2) of the device, which is likewise arranged outside the focal plane (F).Device according to one of Claims 1 to 5, characterized in that the optical detector or detectors (S1, S2) are connected to an image processing device which can carry out a reconstruction of a sharp image of the scene (SZ) from an unsharp image of the scene (SZ) captured by the respective detector (S1, S2).Device according to one of Claims 1 to 6, characterized in that the optical arrangement has an objective (O) which is designed for imaging the scene (SZ) into the focal plane (F).

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