EP0250741B1 - Multi-spectral camouflage sheet - Google Patents

Description

The invention relates to a device for multispectral camouflage of objects against reconnaissance.

The camouflage of objects against reconnaissance by thermal imaging devices contains a special problem. In contrast to the visible, in the thermal infrared range the recognizability of an object is not only dependent on its surface properties (such as color, degree of reflection, roughness), but is also determined by the temperature of the surface and the temperatures of the surroundings, the background and the sky.

Low emissive coatings are used for camouflage. This measure reduces in proportion to the level of emissivity s of the surface, the heat radiation emitted by this object; In this way, the detectability can be reduced, particularly in the case of objects that are warmed up more.

In addition to the paints, other infrared camouflage agents with a similar effect are known: for example low-emitting textiles, laminated metal foils, infrared camouflage nets with metallic elements (layers, foils, threads), galvanic, low-emissive coatings and the like.

A common feature of these infrared-active camouflage agents is that the low-emitting effect is achieved by incorporating metallic layers or particles. Low infrared emissivities below about 70% only occur on homogeneous materials if they have a metallic character and a certain metallic conductivity.

Conventional metal-containing IR camouflage paints and IR camouflage agents have some typical disadvantages, which severely limit their possible uses and effectiveness: The metal component has the effect that the layers are generally opaque to electromagnetic radiation and show a strong reflection effect. In the optical field, the undesired reflection is usually suppressed with the help of color pigments, but this is not possible in the microwave and radio wave range, so that these IR camouflaging agents have no camouflaging effect compared to radar reconnaissance or the detectability is rather increased if the object itself is radar-neutral is.

For the same reason, conventional low-emissivity layers cannot be used to camouflage communication systems such as transmitting and receiving antennas, radar domes and other corresponding devices.

The present invention has for its object to provide a multispectral camouflage agent.

According to the invention, this object is achieved by the content of the characterizing part of claim 1, i.e. solved with the help of a coated plastic film, the coating consisting of non-metallic, infrared-transparent material. The layer thickness can be adjusted in relation to the refractive index in such a way that the heat radiation in the 2nd and / or 3rd atmospheric window is reduced due to interference effects. The arrangement has a high permeability to radiation in the microwave and radio wave range and, depending on the embodiment, also in other spectral ranges (visible light, near infrared), so that the multispectral camouflage effect is guaranteed or not impeded.

By lining up several film segments with different radiation (emissivities) and a suitable geometric shape, additional camouflage effects can be easily achieved by breaking down contours or generating any infrared signature.

The invention is described in more detail below with reference to figures.

• Figur 1 Schnitte durch den Aufbau der Folien in drei Ausführungsformen,
• Figur 2 den Spektralverlauf des Reflexionsgrades der drei Ausführungen in Figur 1,
• Figur 3 den Einsatz eines Mehrschichtensystems als Folienaufbau,
• Figur 4 die Darstellung einer Kuppel und die Möglichkeit der Simulation nichtvorhandener Strukturen auf der Kuppel,
• Figur 5 einen Schnitt durch den Aufbau einer Radarabsorbereinrichtung.

Show it:

• FIG. 1 sections through the construction of the foils in three embodiments,
• FIG. 2 shows the spectral profile of the reflectance of the three versions in FIG. 1,
• FIG. 3 the use of a multilayer system as a film structure,
• FIG. 4 shows a dome and the possibility of simulating non-existing structures on the dome,
• 5 shows a section through the structure of a radar absorber device.

Figure 1 shows the simple structure of the camouflage film in three different settings. An infrared-transparent, dielectric interference layer 4 is located on a preferably infrared-transparent carrier film 2 (for example made of polyethylene). The film covers the object 6 to be camouflaged against an observer. A protective layer 8 can optionally be used; in any case, it must be transparent in the IR frequency range of the application. The area 10 represents the air space between the film and the object. The layer thickness d of the interference layer determines the amount of heat radiation, the emissivity, the overall arrangement of the film and object for a given layer material. If a particularly low radiation, that is to say a cold surface, is to be simulated, the thickness should be set approximately to the value d = λ / (4 n) (FIG. 1a), where n means the refractive index of the layer and x wavelength of the radiation. As a rule, X is related to the center of the atmospheric window (approx. 4 µm and 10 11m). This results in low IR transmission and low heat radiation from the object to be camouflaged and the camouflage film. The opposite extreme case (high emission, high surface temperature) is set by the arrangement according to FIG. 1 and a medium state according to FIG. 1b. In FIG. 1 b, the thickness d is calculated as d = 3 • λ / (8 - n) and in FIG. 1 c, d receives the value d = X / (2 n). All other intermediate states can be easily implemented in this way.

For clarification, FIG. 2 shows the spectral course of the reflectance of the three arrangements outlined in FIG. 1. It can be seen how the reflection maxima are shifted in relation to the atmospheric windows and how the described effect arises.

Figures 1 and 2 describe the situation for interference on a layer. Reflex-reducing and reflex-enhancing effects can be increased even further by using systems with two or more interfering layers. However, the practice of infrared camouflage shows that in most cases it is not the extreme values, but rather average emissivities of e = 30-70% that can be generated with the single-layer interference.

A larger number of substances can be considered as possible layer material. The selection is based on the required transmission range in the infrared and optical spectrum, as well as on practical and technical aspects such as manufacturability, durability and costs. The group of semiconductors such as silicon, germanium, graphite, as well as metal sulfides, metal selenides and metal tellurides, which are also used as raw materials for compact IR windows, offers broadband camouflage and good stability. If transparency in the optical field is additionally desired, oxidic materials such as Si0 ₂ , Ai ₂ 0 ₃ , Sn0 ₂ , In ₂ O ₃ , Ti0 ₂ , Ce0 ₂ , MgO, fluorides such as MgF ₂ , PbF ₂ , BaF ₂ and others are Compounds with similar properties can be used.

Ist der Brechungsindex nicht frequenzabhängig (n₂ = n₃), so liegt das Maximum bei λ₂ = 5 jlm. Eine leichte Erhöhung des Abstandes der beiden Maxima (2,₂ - 1₃), der in der Regel erwünscht sein wird, erfordert, daß n₂ etwas grösser als n₂ ist.Another important criterion for the choice of material is the refractive index of the layer and its dispersion, i.e. its spectral course. High refractive indices are generally advantageous since the required layer thickness decreases with increasing n and the camouflage effect is still present even at increasingly oblique viewing angles. The dispersion must be taken into account if a simultaneous effect in both atmospheric windows is to be optimized. If, for example, a layer is set for maximum reflection (low emission) at A3 = 10 µm (d = λ ₃ / (4 • n3); n ₃ = average refractive index in the 3rd atmospheric window)), this layer also has a reflection maximum in the 2. atmospheric window, the exact location of which depends on the refractive index n ₂ (at 3 - 5 µm):

If the refractive index is not frequency-dependent (n ₂ = n ₃ ), the maximum is λ ₂ = 5 jlm. A slight increase in the distance between the two maxima (2, ₂ - 1 ₃ ), which will generally be desirable, requires that n _{2 is} somewhat larger than n ₂ .

FIG. 3 shows, instead of a compact inference layer, the use of a multilayer system in which two thinner films 4 with a high refractive index are separated by the layer 12 with a low refractive index at a distance d. The particular advantage of this embodiment is that a transparent plastic film can be used as layer 12, which e.g. is identical to the carrier film 2. In this case, the layers 4 can be made much thinner than the above λ / 4 layers, so that the arrangement is given greater flexibility, which has a very advantageous effect for many applications. However, the optical effect of this arrangement corresponds to the one-layer interference. The air gap 10 is located between the carrier film 2 and the object 6 to be camouflaged.

Further features and advantages of the invention are to be described on the basis of typical applications.

The camouflage film according to the invention can be used very advantageously for cladding radomes (radomes). The current construction of radomes has proven to be extremely unfavorable in terms of detectability in the IR range. Due to the low thermal conductivity and thermal capacity of the radome outer skin (plastic foam material or foils), the surface temperature is subject to strong weather-related fluctuations, which gives these objects an unusually well-defined thermal image signature. Countermeasures with conventional camouflage means without impairing the radar transmission are not known.

Figure 4 symbolizes this application. FIG. 4a shows a typical signature of a radome in the sun. The upper half of the sphere is warmed up and stands out against the much darker background. At night, the light-dark conditions are just the other way around due to the low sky temperature, but just as easily recognizable. With the help of the film according to the invention, which is attached to the outer surface of the radome, an effective contour decomposition is brought about by typical structures of the environment, such as e.g. rectangular areas in agricultural fields (Figure 4b, without background) or settlements, building structures (Figure 4c) or other landscape formations (horizon lines, ridges, forest areas, river courses) can be simulated.

A similar situation exists with the camouflage of other systems and components of the transmission technology (i.e. radio transmitters, telecommunications stations, satellite reception antennas, radio control systems or direction finding and reconnaissance systems). All of these systems, which are indispensable in the event of a defense and which have hitherto been regarded as easily recognizable and vulnerable, can be effectively camouflaged with the aid of the invention against thermal imaging, without any impairment of their function.

Other applications are in the IR camouflage of buildings, roads, bridges and similar facilities; Also strategically very important objects, which so far have not been able to be protected against thermal imaging observation or only at the expense of increased radar detectability. It is also advantageous here that the camouflage film according to the invention does not have to be present at all times — like a coat of paint — since, if necessary, it can be spread out and removed again very quickly. For some Objects, such as roads and airfield systems, are the only conceivable solution.

An application in which the permeability in the microwave range is also a crucial requirement is radar-absorbing materials and structures. These camouflage agents known today against radar reconnaissance are invariably good IR emitters and therefore easy to detect in the thermal image, but on the other hand they cannot be treated with low-emissive coatings based on metal, since the radar absorber effect is then lost.

FIG. 5 shows a cross section through this arrangement. The IR-active camouflage film with plastic carrier 2 and interference layer 4 is connected directly to the radar absorber material 14. The above-mentioned variants for contour decomposition and signature simulation can of course also be used advantageously here. An additional camouflage effect in the visible or near infrared is possible by using colored plastic films. If foils with good optical transparency are used, then the visual camouflage effect can be achieved by means of deposited and thus easily changed paint coats, or it is already given by the existing camouflage of the object.

In the conceivable application of the camouflage film according to the invention to vehicles, ships, airplanes, steel bridges, steel masts and the like, there is a special aspect. Due to their predominantly metal structure, these objects have a clear and characteristic radar signature. This problem can basically be solved by using radar absorbers and multispectral camouflage film, as described above. However, if radar absorbers are not desired or not possible for any reason (weight, cost, availability), then a combined IR-radar camouflage effect can be achieved with the camouflage film according to the invention by metallizing the film over the whole or part of the object. In this way, certain characteristic radar signatures of the object can be canceled or falsified.

Claims (8)

1. A device for the multi-spectral camouflaging of objects against reconnaissance, characterised in that a synthetic plastics sheet (2) with a non-metallic low heat emitting infra-red-transparent coating (4) is used, the low-emitting effect occurring in the temperature radiation range by an interference effect in the coating (4) or in a multilayer structure, the coated foil having high permeability in other spectral ranges neighbouring on the thermal infra-red range.

2. A device according to Claim 1, characterised in that the coating thickness(es) is/are so adjusted in proportion to the refraction index of the interfering coating(s) that the maximum reflection is of the lowest order, according to the application in the centre of the third or second atmospheric window.

3. A device according to Claims 1 and 2, characterised in that the reflection maximum is of the lowest order of the interference coating system in the third atmospheric window and in that a simultaneous effect is created in the second atmospheric window due to the reflection maximum of the next higher order.

4. A device according to Claims 1 to 3, characterised in that lower reflection values from the sheet, equivalent in significance to a higher emission of the overall arrangement, can be adjusted in that, the reflection maxima are more or less removed from the centre of the atmospheric windows.

5. A device according to Claim 1, characterised in that the object (6) which is to be camouflaged is covered or masked by a plurality of adjacently disposed segments of sheet of different degrees of emission so that a break-up of the infra-red contour and adaptation to the background can be achieved, these segments being constructed in any desired geometrical shape.

6. A device according to Claims 1 to 5, characterised in that it can be used for the heat image camouflaging of radomes and other aerial installations and transmission stations.

7. A device according to Claims 1 to 5, characterised in that it can be used for the heat image camouflaging of buildings, bridges, roads, airfields and other facilities.

8. A device according to Claims 1 to 5, characterised in that the sheet (2) is wholly or spotwise metallised on the object side and can be used for the multi-spectral camouflaging of predominantly metallic objects such as vehicles, ships, aircraft, bridges, masts and other corresponding objects.

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