Device for optically imaging a scene with protective function against intensive interference radiation

Abstract

A device for optically imaging a scene (SZ) has at least one optical detector (S1) and an optical arrangement which is arranged in front of the optical detector (S1) and by means of which the scene (SZ) can be imaged in a focal plane (F) of the optical arrangement. The 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 improved algorithmic reconstruction of a sharp image of the scene (SZ) from a blurred image captured by the detector (S1). In the device, in an alternative an optically non-linear filter (N) is additionally arranged in the region of the focal plane (F) and, in the case of incident optical radiation with an intensity above a threshold intensity, changes its optical properties in such a way that it blocks or at least attenuates said radiation. By means of said device, an optical sensor system is realized which has an improved protective effect against interference radiation, in particular incident laser radiation, with virtually unchanged overall size.

Description

[0001] Device for optically imaging a scene with

[0002] Protection function against intense interference radiation

[0003] Technical field of application: The present invention relates to a device for optical imaging of a scene, comprising 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 has an amplitude or phase mask that enables an improved algorithmic reconstruction of a sharp image of the scene from a blurred image captured by the detector.

[0004] Optical sensor systems for imaging a scene can be temporarily blinded or even irreversibly destroyed by high-energy laser radiation. This is due, among other things, to the fact that the laser radiation, like the radiation being imaged, is focused onto the detector plane by the optics of the optical sensor system. This results in very high intensities in the detector plane, which saturate the detector elements and can even exceed the detector's destruction threshold. The optical sensor system is therefore unusable during and possibly permanently after the blinding. State of the art

[0005] Various approaches to avoiding or mitigating this problem are known. For example, DE 20 2010 002 568 Ul describes a device for limiting transmitted optical power, which includes at least one focusing optic that concentrates incoming light onto an intermediate focus. A functional element is then positioned at the intermediate focus, exhibiting greater absorption at higher intensities than at lower intensities. In this way, an element located behind the device, such as a CCD camera, can be protected from damage caused by interference radiation with excessive optical power. However, the additional creation of an intermediate focus significantly increases the overall length of the optics required for an optical sensor system.

[0006] Another known method for better protecting the detector of an optical sensor system from intense interference radiation is to shift the detector surface out of the focal plane of the optical arrangement, so that the detector captures a blurred image of the scene. A suitable reconstruction algorithm is then used to reconstruct the sharpest possible image from this blurred image. Such an algorithm employs a deconvolution process using the impulse response or point spread function (PSF) of the optical arrangement. To further improve the reconstruction, it is known to additionally incorporate an amplitude or phase mask into the optical arrangement that appropriately modifies the PSF. For example, J. H. Wirth et al., “PSF Engineering for Sensor Protection”, in Frontiers in Optics 2017, demonstrate such an approach using a suitable phase mask.

[0007] The object of the present invention is to provide a device for optical imaging of a scene which offers further 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 size of the device compared to a device without additional protective measures.

[0008] Description of the invention

[0009] The problem is solved by the device according to claim 1 and claim 2. Advantageous embodiments of the device are the subject of the dependent claims or can be found in the following description and the exemplary embodiments.

[0010] The proposed device for optically imaging a scene comprises, in a known manner, at least one optical detector, for example a CCD detector, and an optical arrangement positioned in front of the optical detector, by which a scene can be imaged (sharply) into a focal plane of the optical arrangement. In the proposed device, the detector surface of the optical detector is shifted away from the optical arrangement relative to its focal plane, i.e., it is located outside the focal plane or at an optical distance from this focal plane. The optical distance here refers to the optical path length between the focal plane and the detector surface, since, for example, an optical deflection element may also be arranged between them. Due to this shift or optical distance, the detector captures a blurred image of the scene.The optical arrangement of the proposed device further includes an amplitude or phase mask, which is designed and arranged in such a way as to enable an improved algorithmic reconstruction of the sharpest possible image of the scene from the blurred image captured by the detector. The improved reconstruction is to be understood as a comparison to a reconstruction without such an amplitude or phase mask.In an alternative version, the proposed device additionally features an optically nonlinear filter in the region of the focal plane, preferably in the focal plane itself. This filter is transparent to incident optical radiation of lower intensity, such as that occurring when a scene is illuminated by daylight. When exposed to incident optical radiation of higher intensity, the filter's optical properties change above a certain threshold intensity, such that it blocks or at least attenuates the incident radiation, preferably to such an extent that the detector is not damaged by this radiation or is only minimally damaged, thus allowing it to remain functional. The optically nonlinear filter is therefore located between the optical arrangement and the detector and is designed to additionally protect the detector from incident optical radiation of higher intensity.

[0011] In the proposed device, hereinafter also referred to as an optical sensor system, the optical detector is protected in several stages. Firstly, the detector surface is shifted from the focal plane of the optical arrangement. This shift is preferably by an optical path length at which reconstruction is still possible with sufficient quality; for example, with a focal length of 75 mm, this would be an optical path length of 5 mm. This reduces the intensity of incident interference radiation on the detector by several orders of magnitude. To ensure that a sharp image is still possible, incident radiation is appropriately encoded by an amplitude or phase mask. Using this encoding, a sharp image can be reconstructed from the blurred image.This measure primarily protects the detector against damage when the optical sensor system is irradiated with a continuous-wave laser. The impulse response of the optical system, i.e., the point spread function (PSF), can be directly influenced by the design of the amplitude or phase mask (i.e., the transmission profile or height profile). In this case, impulse responses with as few or no zeros and low variance as possible in the spatial frequency domain are advantageous. This ensures that little information is lost due to blurring and that algorithmic reconstruction using deconvolution methods can yield good results. An example of a suitable amplitude mask is known in MURA (Modified Uniformly Redundant Array). A spiral mask, for example, can be used as the phase mask.For reconstruction, well-known deconvolution methods such as Wiener deconvolution are suitable.

[0012] By shifting the optical detector away from the focal plane, this focal plane, or the immediate area around it, can be used to arrange an optically nonlinear filter. At high or very high intensities that could dazzle or destroy the detector, this filter alters its optical properties in such a way that it either blocks or attenuates incident optical radiation of these high or very high intensities to such an extent that it can no longer dazzle or destroy the detector. This nonlinear attenuation of the optical radiation can be caused by various processes, such as nonlinear absorption, nonlinear scattering, or phase change, or a combination of several of these processes, which depend on the irradiance.Examples of suitable materials for the optically nonlinear filter include nonlinearly absorbing materials, for example via reverse saturable absorption or two-photon absorption, such as phthalocyanines, porphyrins, nonlinearly absorbing polymers, or chalcogenides. Another possibility is nonlinearly scattering materials, such as conductive or semiconducting nanoparticles, carbon-based nanomaterials, or materials with a nonlinear refractive index change.

[0013] The protective effect of this optically nonlinear filter is best when the filter is located precisely in the focal plane. It can, of course, also be shifted relative to the focal plane. This filter represents an additional protective mechanism that shields the detector against incident laser radiation, especially radiation from pulsed lasers, which can temporarily deliver irradiances several orders of magnitude higher than those of continuous-wave lasers.

[0014] In a further development of the proposed device in this alternative, the optically nonlinear filter can have an adjustment mechanism by which it can preferably be moved or rotated parallel to the focal plane. This can be achieved, for example, by linear displacement or by using a filter wheel. This is preferably automated via a control system and a suitable drive, for example, an electric motor. This allows an undamaged filter section—or a filter section whose transmission properties have not changed—to be moved in front of the detector after a potential laser radiation attack on the optical sensor system, in order to continue image acquisition. In a further advantageous embodiment, the filter is designed as a liquid suspension that regenerates itself. The suspension is contained in a suitable optically transparent container.

[0015] In a second alternative of the proposed device, which can also be implemented in combination with the first alternative, a spectrally selective optical element is arranged between the optical arrangement and the optical detector. This element splits the incident radiation into at least two spectral components, a first of which strikes the optical detector and a second of which strikes a further optical detector of the device. This further optical detector is preferably also arranged at the same optical distance to the focal plane as the first optical detector. The spectrally selective optical element can, for example, be a dichroic prism or a dichroic beam splitter that 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 described above, largely protected against damage from laser radiation. With this configuration, when irradiated with a narrowband laser, only one of the two detectors is affected if the other described measures are insufficient. 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, in which case the spectrally selective optical element(s) must split the incident radiation into more than two spectral components and direct them to the respective detectors.

[0016] The proposed device preferably also includes an image processing device which is connected to the optical detector(s) and which reconstructs the sharpest possible image of the scene from the respective blurred image captured using a suitable reconstruction algorithm.

[0017] The proposed device, or optical sensor system, offers improved protection of the optical detector against intense interference radiation through the combination of several protective mechanisms. Since the first alternative utilizes only the focal plane freed up by shifting the optical detector for the optically nonlinear filter, this additional measure hardly alters the size of the optical sensor system. Shifting the detector out of the focal plane also significantly reduces laser backscattering upon arrival of the laser radiation. This, in turn, reduces the detectability of such a system by optical detection systems.

[0018] The proposed device is primarily suitable for military applications, protecting friendly sensors from interference and destruction by adversaries. The focus here is on maintaining reconnaissance capabilities. However, the device can also be used in autonomous systems (including civilian ones) such as autonomous vehicles, aircraft, drones, robots, surveillance cameras, etc., to protect detectors from interference. These applications often require highly sensitive detectors, while simultaneously necessitating laser-based methods (e.g., LiDAR). Thus, the optical detectors can be protected against friendly lasers, lasers from other autonomous systems, or even deliberate interference.

[0019] Brief description of the drawings

[0020] The present invention is briefly explained below with reference to exemplary embodiments in conjunction with the drawings. These show:

[0021] Fig. 1 shows a first example of a schematic embodiment of the proposed device; and

[0022] Fig. 2 shows a second example of a schematic embodiment of the proposed device.

[0023] Ways to implement the invention

[0024] Figure 1 shows an exemplary optical sensor system designed according to the present invention. The optical sensor system comprises an optical detector S1, in front of which an optical arrangement in the form of an object 0 is arranged. This object 0 maps a scene SZ to be imaged onto the focal plane F of the object 0, as indicated by the dashed lines in Figure 1. The optical detector S1 is arranged outside the focal plane. In this example, the object 0 is a two-lens system with an entrance aperture E and an exit aperture A. Viewed from the direction of the scene SZ to be imaged, a mask M, configured as an amplitude or phase mask, is located in front of the entrance aperture E to reconstruct a blurred image of the scene SZ detected by the detector S1 in such a way as to achieve the sharpest possible image of the scene SZ. In principle, the mask M can also be placed elsewhere, e.g.The proposed optical sensor system is located behind the entrance aperture E. In the focal plane F, an optically nonlinear filter N is situated which, at high or very high incident light intensities, changes its optical transmission such that it strongly attenuates the optical radiation as it passes through.

[0025] Figure 2 shows another possible embodiment of the proposed optical sensor system, in which the same elements as in Figure 1 are designated with the same reference symbols. In this exemplary further embodiment, a spectral beam splitter B, in this case designed as a dichroic beam splitter, is additionally located between the focal plane F and the optical detector S1. This filter B splits the incident optical radiation into two spectral components, the first of which is transmitted through the beam splitter B and strikes the optical detector S1. The second spectral component is reflected by the beam splitter B and strikes an additional second optical detector S2, which is arranged at the same optical distance to the focal plane as the first optical detector S1 in the optical sensor system.Since laser radiation is generally narrowband, any incident interference from a laser will therefore only reach either detector S1 or detector S2, so that the other detector remains fully functional. This is an additional protection mechanism, particularly against narrowband interference, which further increases the protection of the optical sensor system against interference.

[0026] Another possible embodiment differs from the configuration shown in Figure 2 in that the spectral beam splitter B is arranged not behind, but in front of the focal plane F. This allows each of the two detectors S1 and S2 (optionally) to be equipped with its own nonlinear filter. The advantage here is that, in the case of high-intensity incident laser radiation, only one nonlinear filter is damaged. The detector observing in the wavelength band complementary to the laser radiation can then acquire an undisturbed image of the scene.

[0027] In configurations where the incident radiation is split by a dichroic beam splitter, the nonlinear filter is optional. Protection against damage is already provided by the off-focus position of the detector.

[0028] For reconstruction using a suitable reconstruction algorithm, it is advantageous to perform several calibration measurements for different object planes beforehand and to save the corresponding results, so that a sharp image can then be reconstructed in different object planes based on the PSFs saved for the different object planes.

[0029] Reference symbol list

[0030] 51 First Detector

[0031] 52 Second detector 0 Lens

[0032] E Entrance aperture of the lens

[0033] A Exit aperture of the lens

[0034] SZ scene

[0035] M Mask F Focal plane

[0036] N optically nonlinear filter

[0037] B spectrally selective beam splitter

Claims

Patent claims 1. Device for optically imaging a scene (SZ) , with at least - an optical detector (Sl) and - an optical arrangement arranged in front of the optical detector (Sl) by which the scene (SZ) can be imaged into a focal plane (F) of the optical arrangement, wherein a detector surface of the optical detector (Sl) 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 with the detector (Sl), characterized in that an optically nonlinear filter (N) is arranged in the region of the focal plane (F), which, when optical radiation with an intensity above a threshold intensity is incident, changes its optical properties in such a way that it blocks or at least attenuates this radiation.

2. Device for optically imaging a scene (SZ) , with at least - an optical detector (Sl) and - an optical arrangement positioned in front of the optical detector (Sl), by which the scene (SZ) can be imaged into a focal plane (F) of the optical arrangement, wherein a detector surface of the optical detector (Sl) is arranged outside the focal plane (F) and the optical arrangement has an amplitude or phase mask (M) which enables improved algorithmic reconstruction of a sharp image of the scene (SZ) from a blurred image captured by the detector (Sl), characterized in that a spectrally selective optical element (B) is designed and arranged between the optical arrangement and the optical detector (Sl) such that it splits incident radiation into at least two spectral components, of which a first spectral component hits the optical detector (Sl) and a second spectral component hits a further optical detector (S2) of the device, which is also arranged outside the focal plane (F).

3. Device according to claim 1, characterized in that the optically nonlinear filter (N) is designed as a liquid suspension in an optically transparent container.

4. Device according to claim 1 or 3, characterized in that the optically nonlinear filter (N) has an adjustment device by which it can be moved or rotated parallel or obliquely to the focal plane (F).

5. Device according to one of claims 1, 3 or 4, characterized in that a spectrally selective optical element (B) is designed and arranged between the optical arrangement and the optical detector (Sl) such that it splits incident radiation into at least two spectral components, of which a first spectral component hits the optical detector (Sl) and a second spectral component hits a further optical detector (S2) of the device, which is also arranged outside the focal plane (F).

6. Device according to one of claims 1 to 5, characterized in that the optical detector(s) (Sl, S2) are connected to an image processing device which can perform a reconstruction of a sharp image of the scene (SZ) from a blurred image of the scene (SZ) captured by the respective detector (Sl, S2).

7. Device according to one of claims 1 to 6, characterized in that the optical arrangement has a lens (0) which is designed to image the scene (SZ) into the focal plane (F).

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