Visual improvement through deliberate incomplete circular polarization with incomplete suppression of backscatter.

Abstract

The invention relates to an optical detection system comprising at least one light-emitting device and at least one light-receiving device. The invention further relates to a searchlight of such an optical detection system, an optical add-on module for mounting on a searchlight, the use of an optical detection system, a searchlight, or an optical add-on module, and a method for optically detecting objects, particularly in maritime search and rescue operations and / or in poor visibility conditions, using an optical detection system. The invention enables improved visibility through deliberate incomplete circular polarization with incomplete suppression of backscatter.

Description

The present invention relates to an optical detection system having at least one light emitting device and at least one light receiving device. The invention also relates to a searchlight of such an optical detection system, an optical add-on module for attachment to a searchlight, the use of an optical detection system, a searchlight or an optical add-on module and a method for optically detecting objects, in particular in the case of sea emergency and / or in the case of poor weather vision, by means of an optical detection system.In sea rescue, search lights are used at night or in darkness / twilight in order to find persons driving in water, lifeboats or debris parts. After successful discovery, the most accurate possible explanation is required to prepare the rescue action, for example how many persons are located in a lifeboat and how the state is (panic, quiet behavior, persons already located in the water, etc.). The rescue works must then be monitored and monitored in order to be able to react as promptly as possible to incidents and accidents.In these scenarios, humidity is often high and fog or rain occurs. Even under moderate viewing conditions, significant back scattering from the water droplets in the air is frequently already dominant and leads to significant visual impairment of the illuminated scene. Frequently, the operating personnel stand directly on the search headlight or, if a camera is used, the latter is often mounted next to the search headlight and is therefore exposed to the back scattering in almost 180°, whereby the contrast of the observed scene is reduced and the finding or the like is reduced. Persons or objects are made more difficult.There are a number of technical tools for these scenarios, starting from passive methods such as thermal cameras and residual light amplifiers, which manage without active illumination (search lights), up to technically demanding methods with active illumination such as the range gating or LIDAR technologies. These methods are technically very complicated and expensive.Another promising method for improving visibility is the use of polarized radiation. Here, on the one hand, there are more complex polarization difference imaging technologies. On the other hand, the backscattered signal can also be very easily suppressed by using polarized active light sources in combination with polarization filters in front of the receivers if the prevailing backscatter is caused by small water drops in the air, since the backscatter in 180° is completely polarization-preserving in these cases. That is, at the receiver, the direction of polarization that was emitted in the case of linear polarization or the correspondingly complementary handiness in the case of circular polarization is blocked, since the direction of movement and thus the handiness change during the reflection on the water drop. The resulting image then consists only of the diffusive back scattering acting depolarizingly at the target object.The latter method is potentially suitable for improving vision in the scenario described. However, a further special feature of the scenarios at sea and in particular in the context of sea emergency is that retroreflectors are to be found on life vests, clothing and boats. These are very often likewise polarization-maintaining, so that they are likewise suppressed, i.e. invisible. Complete suppression is therefore undesirable, since these retroreflectors naturally have to be seen.Furthermore, particularly in the case of sea notes, it is desirable to search for with his own eyes or a binoculars and not to have to operate complex optical sensors. Since spendfinanced organizations are common in this area, the desire for low cost technologies is also logical.US 2019 / 0 064 414 A1 discloses a polarized illumination system. U.S. Pat. No. 10,666,916 B1 discloses a modeled viewing system with circular polarization. DE 102 30 955 A1 discloses methods for differentiated, target-structure-dependent illumination and viewing of surfaces and their sub-structures and a device for carrying out the method. US 2019 / 0 116 355 A1 discloses a system and a method for reducing sparkle effects.The object of the invention is to improve the optical detection of objects in poor visibility conditions.This object is achieved by an optical detection system according to claim 1. This includes an optical detection system having at least one light transmitting device and at least one light receiving device having the following features: a) the light transmitting device has a light source with a light emission spectrum in the range from a first wavelength to a second wavelength, b) the light transmitting device has at least one transmitter-side polarization device which is configured to generate incomplete circular polarization of the light emitted by the light source, c) the light receiving device has at least one receiver-side polarization filter which is configured to filter out received circularly polarized light and to incompletely suppress back-scattering of the light emitted by the light transmitting device.The light emitted by the light-emitting device is thus only incompletely circularly polarized by the transmitter-side polarization device. The invention makes it possible to clearly improve the visual recognizability of objects in various fields of application in which difficult viewing conditions prevail, for example in the case of sea emergency, but also in other fields of application, such as military use. The invention is generally suitable for use in poor weather vision conditions, i.e. when a substantial proportion of aerosols are in the air, for example in fog. In this case, in particular, no complete optical suppression of the reflection of specific elements of the object, such as retroreflectors, for example, takes place, so that these elements likewise still remain visible.The optical detection system according to the invention can be implemented in practice in particular with simple and cost-effective components. It can be easily inserted and easily carried by the user. The undesired back scattering is suppressed to the greatest possible extent.According to the invention, light with incomplete circular polarization is generated and emitted on the light transmission side. This can be realized, for example, by a combination of circularly polarized and linearly polarized light. In an advantageous embodiment, the light with incomplete circular polarization can have an elliptical polarization.Further advantages of the invention are that the method works just as well in the visible spectral range as in the invisible wavelength range. This also makes it possible to use special relatively simple eyeglasses instead of expensive camera systems, or to equip binoculars with corresponding attachments, so that an observer standing behind the headlamp, using cost-effective technology (attachment for headlamps and binoculars or eyeglasses for the eyes), already achieves a noticeable improvement in vision tailored to the application.The word lambda or the symbol λ is used to designate the wavelength of the light in the following.The light emitting device may emit light in a light emission spectrum that is in the visible wavelength range, i.e. in the range from 380 nm to 780 nm, and / or in a non-visible wavelength range, such as in the infrared range, e.g. in the short-wave infrared range (SWIR). In an advantageous embodiment of the invention, the light source is configured to generate visible light. Accordingly, light is emitted from the light source in which the minimum of the first wavelength is >380 nm and the maximum of the second wavelength is <780 nm. In an advantageous embodiment, the light source is configured to generate unpolarized light. In a further advantageous embodiment, the light source is designed as a broadband light source, in which the difference between the first wavelength and the second wavelength is relatively large, e.g. at least 100 nm or at least 200 nm or at least 400 nm. In particular, the light source can be configured to emit white light. The light color should be chosen to be neither green nor red, since these colors are used for position lights of ships and it can therefore lead to confusion.The incompletely circular polarization of the light emitted by the light-emitting device can be generated, for example, by superposition of the light of light sources of different designs. In an advantageous embodiment of the invention, the transmitter-side polarization device can have a transmitter-side polarization filter. In an advantageous embodiment, this is arranged in front of the light source in the light emission direction.The light of the at least one light source is converted into light with incomplete circular polarization by the polarization filter on the transmitter side.According to an advantageous embodiment of the invention, it is provided that the polarization device on the transmitter side has a lambda / 4 delay plate which is tuned to a wavelength in the range between the first wavelength and the second wavelength of the light emission spectrum of the light source, in particular to a value in the range of the center of the light emission spectrum. By means of such a lambda / 4 delay plate, the incomplete circular polarization on the transmitter side and / or the desired incomplete suppression of back scattering of the light emitted by the light transmission device on the receiver side can be realized in a comparatively simple manner with a small required installation space.According to an advantageous embodiment of the invention, it is provided that the transmitter-side polarization device and / or the receiver-side polarization filter has a linear polarizing filter. On the transmitting side, the linear polarizing filter can be arranged in front of the lambda / 4 delay plate on the transmitting side in the light beam direction. From the receiver side, the linear polarizing filter can be arranged behind the lambda / 4 delay plate on the receiver side in the light beam direction. In an advantageous embodiment, in the transmitter-side polarization device and / or the receiver-side polarization filter, the respective lambda / 4 delay plate can be rotated by 45° with respect to the polarization axis of the associated linear polarizing filter.Such lambda / 4 retarding plates may be purchased commercially and are provided for various applications in different configurations. In an advantageous embodiment, the transmitter-side polarization device and the receiver-side polarization filter can each have an identically designed lambda / 4 delay plate. Since the lambda / 4 retardation plate generates an exact circular polarization of the light emitted by the light source only at a wavelength defined on the production side, an elliptical polarization is generated in the wavelengths deviating from the specific wavelength of the lambda / 4 retardation plate due to the broadband nature of the light source, i.e. the light emission spectrum from the first wavelength to the second wavelength.According to the invention, it is provided that the receiver-side polarization filter has a lambda / 4 retardation plate, the retardation (retardation) of which at a defined wavelength in the range between the first wavelength and the second wavelength of the light emission spectrum is 0.25λ, wherein the lambda / 4 retardation plate generates a retardation at the first wavelength and / or the second wavelength, which retardation deviates in terms of amount at least by a minimum value from 0.25λ. As a result, the generation of incompletely circular polarization can be further optimized, in particular to the effect that at the receiver end, e.g. at least 1% or 2% leakage intensity, i.e. suppressed back scatterings of the light emitted by the light source, can pass through the receiver end polarization filter (e.g. scattered light or reflections). This can ensure that retroreflectors and reflections are sufficiently visible on the receiver side without transmitting the other objects to be detected, and at the same time ensures sufficient suppression of the atmospheric backscatter signal.Such a deviation from the 0.25λ value, which may be present on the receiving side and / or the transmitting side, can produce a defined detuning between the light transmitting device and the light receiving device, as regards the phase delay. As a result, the incompletely circular polarization of the light used according to the invention can be further optimized.Alternatively or additionally, it can be provided that the transmitter-side polarization device has a lambda / 4 retardation plate, the retardation (retardation) of which at a defined wavelength in the range between the first wavelength and the second wavelength of the light emission spectrum is 0.25λ, wherein the lambda / 4 retardation plate generates a retardation at the first wavelength and / or the second wavelength, which retardation deviates in terms of amount at least by a minimum value from 0.25λ.According to the invention, it is provided that the aforementioned minimum value is 0.02λ. Such a minimum value can be realized when designing the optics on the transmitting side and / or the receiving side.According to an advantageous embodiment of the invention, it is provided that the light source is designed as one or more high-power light-emitting diodes. In this way, the light source and thus also the light-emitting device can be provided in a simple, cost-effective and compact manner. In addition, the energy requirement is relatively low.According to an advantageous embodiment of the invention, it is provided that the light-emitting device has at least one focusing optical unit, by means of which the light emitted by the light source is focused to a reduced emission angle. As a result, the light-emitting device can be realized in a targeted manner as a search headlight. For example, the focusing optics can be configured to focus the light emission angle of the light source onto a light cone with an angle of at most 2° or at most 3°. In the light beam path in the light transmission device, the focusing optics can be arranged, for example, between the light source and the polarization device on the transmitter side.According to an advantageous embodiment of the invention, it is provided that the transmitter-side polarization device is configured to generate an elliptical polarization of the light emitted by the light source. In this way, the desired incomplete circular polarization can be generated particularly efficiently on the light transmission side.According to an advantageous embodiment of the invention, it is provided that the light receiving device is formed as part of a pair of eyeglasses, a binoculars or a camera to be worn by a user of the optical detection system, or as an optical unit to be placed on a binoculars or a camera. In this way, the optical detection system can be used very easily by the user. The production of the optical detection system is also possible in a simple and cost-effective manner.According to an advantageous embodiment of the invention, it is provided that the light-emitting device is arranged directly on the light-receiving device and / or is connected to the latter to form a structural unit. The light-emitting device and the light-receiving device are thus located in the same location (monostatic arrangement). This can ensure that the light transmitting device and the light receiving device always face in the same direction, whereby the scattering at the 180° angle can be suppressed. For example, the distance between the light transmitting device and the light receiving device may be <10 cm.According to an advantageous embodiment of the invention, it is provided that the optical detection system is designed as a marine emergency system and / or as a poor weather vision system. In this way, performance of sea rescue actions and other rescue actions can be substantially improved and made more secure.The object mentioned at the outset is also achieved by a search headlight of an optical detection system of the type explained above, in particular a search headlight for sea emergency and / or in the case of poor weather vision, wherein the search headlight is designed according to claim 10.The advantages explained above can also be realized in this way. The searchlight can be further developed with one or more of the features explained above for the light-emitting device of the optical detection system. The invention can also be implemented as an optical add-on module according to claim 11 for an existing searchlight. The optical additional module can then be placed on the search headlight. The optical add-on module can then have the transmitter-side polarization device which is configured to generate incomplete circular polarization of the light emitted by the light source.The object mentioned at the outset is also achieved by the use of an optical detection system, a searchlight or an optical add-on module of the type explained above in the case of sea emergency and / or in the case of poor weather vision for the optical detection of objects.The object mentioned at the outset is furthermore achieved by a method for optically detecting objects, in particular during sea emergency and / or during poor weather vision, by means of an optical detection system of the type explained above, having the following features: a) light of a light source having a light emission spectrum in the range from a first wavelength to a second having incomplete circular polarization is emitted by means of the light-emitting device, b) received circularly polarized light is filtered out by means of the light-receiving device, with back-scattering of the light emitted by the light-emitting device being incompletely suppressed.In the realization of the invention, for example, no laser of suitable wavelength should be used as the light source in combination with lambda / 4 delay plate of the same wavelength, because this would lead to pure circular polarization. Instead, a combination of a broad emission spectrum and a lambda / 4 retardation plate is proposed, so that the retardation (retardation) typically varies in the range of 0.2-0.3 over the light emission spectrum. With a delay of exactly 0.25, purely circular polarization results and theoretical complete suppression of back scattering and retroreflectors. The farther away from 0.25, the more inefficient the cancellation becomes.In practicing the invention, achromatic lambda / 4 delay plates designed to provide as uniform a delay as a function of wavelength should not be used. Preferably, a multiple-order (multiple-order) or zeroth-order (zero-order) retardation plate should be used, which exhibits a linear course of the retardation as a function of the wavelength. This allows intentional circular polarization to be produced only for one wavelength, typically close to the maximum of the emission spectrum, while the remaining wavelengths of the emission spectrum result in elliptical polarization.Multiple Order Delay Plate:For multiple order delay plates (waveplates), the total delay is the desired delay plus an integer. The excess integer portion has no effect on performance. Although multiple order retardation plates are made of only a single birefringent material, they can be relatively thick, facilitating handling and system integration. However, the high thickness makes multiple order waveplates more susceptible to retardation shifts due to wavelength shifts or changes in ambient temperature.Zeroth-order delay plate:For zero order delay plates, the total delay corresponds to the desired value without excess. Zero-order quartz retardation plates consist, for example, of two multiple-order quartz retardation plates whose axes cross each other, so that the effective retardation is the difference between them.The standard zeroth-order retardation plate, also referred to as a composite zeroth-order retardation plate, is composed of a plurality of retardation plates of the same birefringent material which are arranged so as to be perpendicular to the optical axis. The lamination of multiple retardation plates compensates for the retardation shifts occurring in the individual retardation plates and improves the stability of retardation in wavelength shifts and changes in ambient temperature. Standard zero order retardation plates do not improve the retardation shift caused by another angle of incidence.True zero order retardation plates, such as polymer waveplates, are made of a single birefringent material which has been processed into an ultra-thin plate which can be only a few microns thick to achieve a certain amount of zero order retardation. While the small thickness of the retardation plate may make handling or assembly of the wave plate difficult, true zero order retardation plates provide better retardation stability in wavelength shifts, changes in ambient temperature, and a different angle of incidence than other retardation plates.Birefringence retardation plates are made of birefringent materials, typically crystal quartz. Birefringent materials have slightly different indices of refraction for light polarized in different directions. Thereby, they separate the incident unpolarized light into its parallel and orthogonal components.In the system according to the invention, it is not necessary to use a camera with a corresponding polarization filter. Although this is also possible without problems, suitable spectacles can also be used in combination with the naked eye. The eyeglasses may preferably have the same lambda / 4 retardation plates and linear polarizers used in the light emitting device. Since the hand of circularly polarized light changes when reflected on water droplets, the eyeglasses suppress right-circularly polarized light (if left-circularly emitted) or left-circularly (if right-circularly emitted). Unlike 3D kino glasses, where one spectacle lens captures right-polarized light and the other spectacle lens captures left-polarized light in order to generate the 3D image, the same handiness of the light is thus captured here in the case of both spectacle lenses, i.e. the polarization filters are aligned identically on both spectacle lenses. A third variant is a detachable attachment which contains the polarization optics on the receiver side, for example for placing on a commercially available binoculars or a camera.The invention thus allows visual improvement by intentional incomplete circular polarization with incomplete suppression of back scattering.Incomplete circular polarization and intentional incomplete suppression of back scattering and retroreflectors can also be achieved in another way, for example by slightly adjusting the linear polarizing filters so that a few percent leakage intensities pass through. This would also enable the entire method with a laser. However, lasers are comparatively expensive and are not a standard for search lights. In contrast, LEDs which always emit with a broad light spectrum are more advantageous. This reduces costs and the eye safety is higher than in lasers. Due to broader emissions, some suppression inefficiency is intrinsic to LEDs at the same time, which is conducive to the invention.The invention is explained in more detail below with reference to an exemplary embodiment using the drawing FIG. 1. FIG. 1 shows an optical detection system for optically detecting objects in a schematic illustration.FIG. 1 shows a light-emitting device 1 which has a light source 2 from which broadband unpolarized light 3 is emitted. In the light beam path of the light 3 there is a polarization device 4, 5 on the transmitter side, which has a linear polarizing filter 4 in the light beam direction and behind it a lambda / 4 delay plate 5. Linearly polarized light is generated by the linear polarizing filter 4, the E field of which oscillates in the plane inclined by an angle α (with respect to the reference plane). The lambda / 4 retardation plate 5 is rotated with its almost axis relative to the polarization axis by 45° (i.e. by 45°+α relative to the reference plane). Light along the fast axis passes faster than in the perpendicular slow axis, causing phase retardation and producing circularly polarized light. Light 6 with incomplete circular polarization, in particular with elliptical polarization, is then emitted into the environment by the light-emitting device 1.The light-emitting device 1 can also have at least one focusing optical unit 15 by means of which the light 3 emitted by the light source 2 is focused to a reduced emission angle. The optics 15 can be arranged between the light source 2 and the polarization device 4, 5.The light 6 that impinges on a search object 8 can be unpolarized (diffused light) or polarized reflected back (retroreflector) at the search object 8, for example. This light 9 reflected back by the search object 8 can thus have diffuse components or further polarized components of the light 6.It is also assumed that an aerosol is located in the light beam path of the light 6 up to the search object 8, for example water droplets due to mist or the like. The aerosol 7 likewise carries out polarized back scattering 10 of the light 6.The optical detection system further includes a light receiving device 11 on which both the light 9 and the light 10 are received. The light receiving device 11 includes a receiver-side polarizing filter 12, 13. For example, the receiver-side polarization filter can first have a lambda / 4 retardation plate 12 in the light beam direction, through which, in turn, as in the light transmission device 1, light passes more quickly along the axis α+45° than perpendicular thereto; if the retardation for a wavelength is again 0.25 lambda, the total retardation is 0.5 lambda and light which is linearly polarized in α+90° is produced from the light previously after the linear polarizing filter 4 in the direction α. Behind the lambda / 4 retardation plate 12, a linear polarizing filter 13 can again be arranged with its polarization axis at the angle α. This consequently now blocks the linearly polarized light in α+90°. This results in a filtering out of circularly polarized light received at the light receiving device 11 and an incomplete suppression of back scattering of the light 6 emitted by the light transmitting device 1 for those wavelengths for which the lambda / 4 delay plates used have a delay different from 0.25 lambda. The image represented by the system then consists only of the diffuse (unpolarized) back scattering from the target object 8 and the non-suppressed portions of atmospheric back scatterings 10 and any likewise non-suppressed portions of specular reflections on the target object 8.If visible light of the light source 2 is used, the received light can be perceived and evaluated directly with the eye 14, for example, at the light receiving device 11. The light receiving device 11 can then be formed as part of an optical detection system to be worn by the user, e.g. spectacles, or part of a binoculars or a camera or an optical unit to be placed on a binoculars or a camera.The invention thus enables a visual improvement when using search lights in darkness in scattering environments (fog, rain, mist), wherein the polarization of the backscattered light is largely retained (or the handiness is reversed in the case of circular polarization). Such scenarios occur, among other things, in maritime environments because the back scattering of water droplets is polarization-preserving.The presented method improves with little technical complexity above all the perception of diffuse reflection caused by the target object to be detected in scattering (noble) environments, but also the contrast between environment and retroreflectors by shifting the signal heights (see explanation below). Cross-fades and halo effects by retroreflectors are likewise reduced in this case and the situation observed can be more clearly visualized, for example situation in a fully populated lifeboat with life vests and persons equipped with retroreflectors; monitoring rescue measures and recovery work. Owing to the simplicity, use is conceivable above all in the area of sea emergency, but also in autonomous and manned ship travel.In an advantageous embodiment, a combination of intentionally incompletely circularly polarized searchlight can be used in combination with either (a) a camera, (b) matching spectacles or (c) a cover for a binocular with polarization optics. The light is first elliptically polarized with a linear polarization filter of a subsequent zero- or multiple-order lambda / 4 retardation plate. In front of the receiver (camera or eye) are identical optics to those in front of the headlight, so that light of opposite hands (with respect to the emitted light) is blocked and does not contribute to the received image. What is new about the method is that the full width at half maximum (FWHM) of the emission spectrum of the LED headlight and the delay (retardation) of the lambda / 4 plate, which is a function of the wavelength, are intentionally slightly detuned, resulting in the incompletely circular polarization.Experiments have shown that a suitable combination typically has a retardation of 0.2 to 0.3 over the width of the light emission spectrum. More specifically: emission spectrum and lambda / 4 plate should be chosen such that a retardation of 0.25 (=λ center / 4) is present at the central wavelength (λ center). At this wavelength, fully circularly polarized light is generated and back scattering and reflections are completely suppressed. However, the edge wavelength should experience a retardation of at least 0.25 + / - 0.02 λ, since the design calculations below show that a few percent leakage intensity (i.e. scattered light or reflections) then passes through the filters, whereby retroreflectors and reflections are again visible. The light source should also be strong and adapted to the brightness sensitivity curve of the eye, i.e. emit where the eye is sensitive.The intentionally "impure" circular polarization for all wavelengths other than the design wavelength of the lambda / 4 plate results in incomplete suppression of the backscatter signal of the water drops, but also in incomplete suppression of potential retroreflectors. The detection of retroreflectors is important for use at sea and especially sea rescue. If pure circular polarization were used, the suppression of the back scattering (in the case of water drops) and of polarization-maintaining retroreflectors would be almost 100% and the received image would consist only of diffuse reflection.Advantageously, the linear polarizing filters of the light emitting device and the light receiving device should be parallel, the axes of the retardation plates should also be parallel to each other and rotated by 45° to the axis of the linear filters. This optimizes the intentional incomplete suppression of the backscatter signal. In the proposed method, the same component is delayed twice, which amounts to exactly half a wavelength at 0.25 delay, and thus, for example, a horizontal polarization is made up of vertical, which is blocked with the linear polarizing filter on the receiver side. However, at all other wavelengths or delays (e.g. 0.3), this leads only approximately to a shift by half a wavelength and therefore to leakage intensities.Retroreflectors normally have a stronger back scattering than fog or water drops and frequently saturate camera recordings, i.e. they do not exceed the fog signal in the representation in the same way as corresponds to physical reality. The proposed method reduces both signals, but does not completely cancel them out, so that in relation the retroreflectors usually still have a better contrast close to saturation. Saturation effects occur, among other things, in cameras because of the low "dynamic range". Alternatively, instead of a camera, the naked eye can be used with corresponding circularly polarized spectacles or a cover for binoculars. Due to the reduced back scattering, the eye can adapt better to darkness, the iris opens, and in relation retroreflectors are again better recognized. Also, the diffuse reflection is more prominent and gives a clearer image of the situation.Operation with spectacles / binoculars is particularly suitable for cost-effective use in the case that an operator of the searchlight stands directly on the headlight and operates (rotates and pivots) it. Alternatively, however, a commercially available visual camera can also be used provided it is equipped with the described polarization optics. Operation with a camera is suitable for compact systems in which the camera and searchlight are installed close together (e.g. in the same housing) and which are not manually operated but rather transmit their images to a position center or the command bridge. It is important that the operator / camera and the searchlight look in the same direction, since this method suppresses the back scattering in 180°. Scattering angles deviating therefrom tend to depolarize increasingly, depending on the wavelength and particle size, i.e. the improvement in visibility loses efficiency, the greater the angle between receiver and emitter.The advantage of circular / elliptical polarization is obtained, inter alia, when suitable spectacles and the naked eye are used as receiving optics, since the head can be tilted and the principle continues to function. This would not be the case with linear polarization.Mathematical Design Calculation:The mathematical design calculation is based on Stokes vectors and Muller matrices. The Stokes vector is defined asWhere I H and I v are the irritations in the horizontal and vertical directions, I 45 and I 135 are the irritations in 45° and 135°, and I R and I L, are the left and right circular components, respectively. The total intensity is S o( the first element of the vector).Optical elements such as polarizing films, reflections and lambda / 4 retardation plates are described by Muller matrices. The Stokes vector after the respective optical element is the input vector multiplied by the respective Muller matrix. An example calculation is carried out below using vertical polarization films and lambda / 4 plates as shown in FIG. 1. The Muller arrays of components are taken from a publication by Chipman et al. (2018), a standard work for polarizing optics.The Muller matrix for the vertical polarizing film is:The Muller matrix for a perfect mirror (simplification of the backscatter) is:The Muller matrix for a retardation plate of the retardation δ (in degrees or rad, i.e. 0.25λ=90°), the almost axis of which is at the angle θ, is:Starting point is unpolarized light of the I, i.e. the Stokes vector is:After passing through the first linear (here vertical) polarizer in FIG. 1, the Stokes vector is:After the filter, the absorbance is thus only half as great (the other half was absorbed), but for this purpose completely linearly polarized vertically.After passing through a lambda / 4 retarder with δ=90°, which is at an angle θ=5°, the following results:That is, all light is now circularly polarized to the left.After reflection, the following results:That is, the handiness of the light was reversed.On the return path, the light first passes a lambda / 4 plate which is parallel to the first. Since the coordinate system has now rotated, 135° instead of 45° must be used for this purpose in the above formula:All light is thus linearly polarized horizontally and is therefore completely absorbed at the vertically disposed polarization filter:This applies to the case where the delay is δ=90° (as 0.25λ). The above calculation was repeated numerically with a delay of 0.3 instead of 0.25. The result is the Stokes vector:That is to say, in this case, there is just 5% leakage intensity through the filters, with the result that retroreflectors and back scattering are reduced by a factor of 20.This calculation applies, however, only to a wavelength or a specific delay deviating from 0.25. In reality, one has a broad spectrum and has to perform the calculation for all wavelengths and weight with the intensity of the wavelength in the emission spectrum.

Claims

Optical detection system having at least one light-emitting device (1) and at least one light-receiving device (11), having the following features: a) the light-emitting device (1) has a light source (2) having a light emission spectrum in the range from a first wavelength to a second wavelength, b) the light-emitting device (1) has at least one transmitter-side polarization device (4, 5) which is configured to generate incomplete circular polarization of the light (3) emitted by the light source (2), c) the light-receiving device (11) has at least one receiver-side polarization filter (12, 13) which is configured to filter out received circularly polarized light and to incompletely suppress back-scattering of the light (6) emitted by the light-emitting device (1), characterized in that the receiver-side polarization filter (12, 13) has a lambda / 4 delay plate (12), their retardation (retardation) at a defined wavelength in the range between the first wavelength and the second wavelength of the light emission spectrum is 0.25λ, wherein the amount of retardation produced by the lambda / 4 retardation plate (12) at the first wavelength and / or the second wavelength deviates at least by a minimum value from 0.25λ, wherein the minimum value is 0.02λ.Optical detection system according to claim 1, characterised in that the polarisation device (4, 5) on the transmitter side has a lambda / 4 delay plate (5) which is tuned to a wavelength in the range between the first wavelength and the second wavelength of the light emission spectrum of the light source (2), in particular to a value in the range of the centre of the light emission spectrum.Optical detection system according to one of the preceding claims, characterized in that the light source (2) is designed as one or more high-power light-emitting diodes.Optical detection system according to one of the preceding claims, characterized in that the light-emitting device (1) has at least one focusing optical unit (15), by means of which the light (3) emitted by the light source (2) is focused to a reduced emission angle.Optical detection system according to one of the preceding claims, characterized in that the transmitter-side polarization device (4, 5) is configured to generate an elliptical polarization of the light (3) emitted by the light source (2).Optical detection system according to one of the preceding claims, characterized in that the light receiving device (11) is designed as part of spectacles, a binoculars or a camera to be worn by a user of the optical detection system or as an optical unit to be placed on a binoculars or a camera.Optical detection system according to one of the preceding claims, characterized in that the light-emitting device (1) is arranged directly on the light-receiving device (11) and / or is connected to the latter to form a structural unit.Optical detection system according to one of the preceding claims, characterized in that the light source (2) is configured to generate visible light.Optical detection system according to one of the preceding claims, characterized in that the optical detection system is designed as a sea emergency system and / or as a poor weather vision system.Searchlight of an optical detection system according to one of the preceding claims, in particular searchlight for sea emergency and / or in the case of poor weather vision, characterized in that the searchlight has a light transmission device (1) having the following features: a) the light transmission device (1) has a light source (2) having a light emission spectrum in the range from a first wavelength to a second wavelength, b) the light transmission device (1) has at least one transmitter-side polarization device (4, 5) which is configured to generate incomplete circular polarization of the light (3) emitted by the light source (2), wherein the transmitter-side polarization device (4, 5) has a lambda / 4 delay plate (5) whose delay (delay) at a defined wavelength in the range between the first wavelength and the second wavelength of the light emission spectrum is 0.25 λ, wherein a delay is generated at the first wavelength and / or the second wavelength by the lambda / 4 delay plate (5) which deviates in terms of amount at least by a minimum value from 0.25λ, wherein the minimum value is 0.02λ.An optical add-on module for attachment to a searchlight of an optical detection system according to one of claims 1 to 9, in particular searchlight for sea emergency and / or in case of poor weather vision, wherein the searchlight has a light transmission device (1) which has a light source (2) with a light emission spectrum in the range from a first wavelength to a second wavelength, characterized in that the optical add-on module has at least one transmitter-side polarization device (4, 5) which is configured to generate incomplete circular polarization of the light (3) emitted by the light source (2), wherein the transmitter-side polarization device (4, 5) has a lambda / 4 delay plate (5) whose delay (delay) at a defined wavelength in the range between the first wavelength and the second wavelength of the light emission spectrum is 0.25 λ, wherein a delay is generated at the first wavelength and / or the second wavelength by the lambda / 4 delay plate (5) which deviates in terms of amount at least by a minimum value from 0.25λ, wherein the minimum value is 0.02λ.Use of an optical detection system according to one of Claims 1 to 9, a searchlight according to Claim 10 or an optical add-on module according to Claim 11 in the event of sea emergency and / or in the event of poor weather vision for the optical detection of objects.Method for optically detecting objects, in particular during sea emergency and / or during poor weather vision, by means of an optical detection system according to one of Claims 1 to 9, having the following features: a) by means of the light-emitting device (1), light (6) of a light source (2) having a light emission spectrum in the range from a first wavelength to a second having incomplete circular polarization is emitted, b) by means of the light-receiving device (11), received circularly polarized light (6) is filtered out, with back-scattering of the light (6) emitted by the light-emitting device (1) being incompletely suppressed.

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