EP1102028A2 - Infra-red camouflage system - Google Patents

Abstract

An infra-red camouflage system comprises a thermorefractive layer system or a thermorefractive material. The material has a negative temp coefft.

Description

The invention relates to a camouflage system for infrared (IR) camouflage for land targets. she is particularly suitable for camouflaging military objects, especially land vehicles, against thermal imaging devices and infrared search heads.

With thermal camouflage, the aim is to adapt the thermal radiation emitted by the object to be camouflaged to the level of the respective thermal background. Here, for example, attempts are made to influence the temperature of the observable surfaces by means of constructive measures (thermal insulation, insulation, ventilation). This enables improvements in the area of the active signature, ie for internal heat sources (engine, transmission, energy units). These measures do not provide a satisfactory solution with regard to solar heating (passive signature), since the heating behavior of military objects generally differs greatly from that of a natural background. Proposed solutions to compensate for these deviations by active post-heating or cooling, as described, for example, in DE 32 17 977 A1 , are not very practical, particularly because of the high energy consumption.

Other known solutions pursue the goal of achieving the signature reduction not by influencing the actual surface temperature, but by changing the emission behavior of the surface. It is known that the heat radiation of a body is determined not only by its temperature, but also by the emissivity ε of its surface. The use of low-emitting surface layers for infrared camouflage is known and is described, for example, in DE 30 43 381 A1 and EP 0 123 660 A1 . A problem with this type of camouflage with low-emitting camouflage means is that the IR reflectance ρ increases when the heat emissivity ε is reduced, in principle according to the formula ρ = 1 - ε, and thus there is an increased reflection of ambient radiation. This superimposes the self-emission, so that the heat radiation and thus the observable radiation temperature with the reduction of the emissivity increasingly depends on the temperatures of the reflected surrounding surfaces (ground temperature, sky temperature). In particular, reflections from areas close to the zenith have proven to be critical, since the radiation temperatures differ extremely depending on the cloud state and can strongly influence the signature. A known effect with low-emitting camouflage agents is the observation of "cold spots", ie surface areas with a radiation temperature that is too low compared to the background due to the reflection of cold sky parts.

To take this into account, EP 0 250 742 A1 describes a device with which the emissivity can be controlled. The heat radiation of an object can thus be set as desired within wide limits by controlling the heat reflection and emission components with very little energy expenditure. This greatly reduces the contrast between the thermal radiation and the background.

However, the high outlay for implementing corresponding systems is disadvantageous and the need for additional measuring and control devices.

• zum Boden geneigte Flächenbereiche
• horizontale oder zum Himmel geneigte Flächenbereiche
• senkrechte bzw. gering (bis ca. 25°) zum Himmel geneigte Flächenbereiche.

When using low-emitting infrared camouflage means, the geometric peculiarities of the object to be camouflaged must be taken into account. There are two main differences:

• Surface areas inclined to the floor
• horizontal areas or areas inclined towards the sky
• Surface areas that are perpendicular or slightly inclined (up to approx. 25 °) to the sky.

Basically, these surface areas require different embodiments the camouflage. For the case with mostly sloping surfaces the known low-emitting camouflage means with a fixed and if possible low emissivity can be used because it is independent of the observation point the floor temperatures in front of the object are reflected. The radiation temperature of the soil is generally the same as the rest of the thermal Background identical. By transferring this temperature to that Camouflaging object can therefore have a high contrast reduction with the corresponding Camouflage gain can be achieved. In this case, the known ones can be used LE - (= Low Emission) camouflage agents, such as low-emitting paints (LEP = Low Emission Paint) or low-emitting plastic films (LEF = Low Emission Foil).

The known ones are low for surfaces with a predominantly horizontal orientation emitting camouflage agents cannot be used easily. The problem is that these surfaces, if they are observable, are always predominantly near the zenith Reflect sky temperatures. Since these sky temperatures are very low, however, this can vary greatly depending on the state of cloudiness extreme dependence of the reflected heat radiation on the cloud cover. In many cases, therefore, horizontal surfaces are those with low emissivity Camouflage means are provided, "Cold Spots" if by reflection of the cold Heavens own emissions are overcompensated. Low emission behavior is only desired to the extent that due to increasing solar warming a reduction in the thermal radiation of the surface becomes necessary. Similar problems exist for surfaces that are oriented upwards (angle to the horizontal less than approx. 65 °). They can also reflect the sky radiation.

It is therefore an object of the invention to provide a camouflage system for essentially horizontal or to create upward-facing object surfaces with which without elaborate Measurement and control devices can be effectively camouflaged.

This object is achieved with the subject matter of patent claim 1. Beneficial Designs and a camouflage process using the invention Camouflage agents are the subject of further claims.

The solution according to the invention provides that on the surface of the camouflage device a material or a layer system is used, the heat emission ε (T) a strong temperature dependency with a negative gradient (dε / dT), hereinafter referred to as 'thermorefractive' material.

As is known, the total amount of heat Q emanating from a body is composed of the intrinsic radiation (product of ε and the fourth power of the surface temperature T _o ) and the reflected ambient radiation (product of 1-ε and the fourth power of the temperature of the reflected surrounding zone T _u , typically the sky here) Q (T) ∿ ε (T O ). T O 4th + (1 - ε (T O )). T U 4th The temperatures refer to the absolute temperature scale.

If the body is observed by a thermal imaging device, then this determines Law the brightness and the contrast function of the individual pixel and thus the IR signature of the object.

With normal surfaces with ε → 1, the natural radiation, which increases sharply with temperature, predominates. According to the invention, a negative temperature coefficient of the emissivity is introduced and thus the temperature response Q (T) is compensated for as far as possible. If only the natural radiation were present, the condition for ε (T) would be: ε (T) ∿ T O -4 , however, since the reflection term has to be taken into account, the function ε (T) can have a weaker power. More accurate estimates show that it is already a linear reciprocal function ε (T) ∿ 1 / T O produces a very useful camouflage effect in practice.

It is important that the emissivity of the entire system within a temperature interval, typically about 20 to 40 ° C, drops significantly, e.g. of values ε ≥ 0.7 to values ε≤ 0.5, e.g. from ε = 0.90 to ε = 0.5. The lower threshold temperature the transition area is advantageously the mean ambient temperature equated.

Various mechanisms are conceivable for the practical representation of a negative temperature coefficient. One possible embodiment is to use a material with a non-metal-to-metal phase transition (MNM transition). At ambient temperatures, the material is in the non-metallic or semiconducting state (IR-transparent) and - if the thermorefractive material is arranged in front of a highly emissive background - there is normal, highly emissive behavior. With increasing solar heating there is a transition to the metallic state (IR reflective), which is associated with a reduction in the emissivity. A material suitable for the invention that shows the described MNM transition is, for example, vanadium oxide (VO ₂ ).

Another embodiment for a thermorefractive material is in use a composite medium, a composite consisting of an IR-transparent Matrix, preferably made of polyolefin (e.g. polyethylene) and an embedded one second component. The second component consists of an alternative organic or polymeric material with the best possible IR transparency, but a different temperature curve for the refractive indices. Therefore can preferably liquid, waxy or partially crystalline hydrocarbons are used, but also other substances with low IR absorption in Wavelength range from 8 to 12 µm. The material pairing of matrix and storage is to be coordinated so that the refractive indices of both substances at ambient temperature are approximately the same, but increasing with increasing temperature differ from each other. Such a system shows the desired negative Temperature effect: At low temperatures, the material is homogeneously IR-transparent and it is - when the thermorefractive material is in front of a highly emissive Background is arranged - normal, high emissive behavior before, higher Temperature is increasingly scattering, leading to increased remission and thus leading to a reduction in emissivity. To make the scattering effect effective to design, the inclusions should be significantly larger than that for thermal imaging camouflage relevant infrared wavelength of about 10 microns. A a suitable size for the deposits is in particular the area larger than 20 μm.

As a result of the temperature-dependent self-control of the camouflage device according to the invention no additional control electronics such as Sensors, actuators, Control electronics and cabling required. Rather, they pose for one effective camouflage necessary emissivities and thus the radiation temperatures independently. The exact, spatially resolved determination of the surface temperature, the in a camouflage device mentioned at the beginning according to State of the art for setting the emissivity for each actively controllable camouflage IR element is required is not applicable.

• Es wird eine hohe IR-Tarnwirksamkeit für unterschiedlichste Objekte erreicht;
• Die erfindungsgemäße Tarnvorrichtung ist in Form kostengünstiger, robuster Elemente realisierbar;
• Eine zusätzliche visuelle Tarnung in beliebiger Farbgebung ist möglich.

Further advantages of the advantages according to the invention are:

• A high IR camouflage effectiveness is achieved for different objects;
• The camouflage device according to the invention can be implemented in the form of inexpensive, robust elements;
• Additional visual camouflage in any color is possible.

Fig. 1

ein Diagramm des temperaturabhängigen spezifischen Widerstands einer wolframdotierten VO₂-Schicht im Vergleich zu einer undotierten VO₂-Schicht;

Fig. 2

eine konkrete Ausführung der erfindungsgemäßen Tarnvorrichtung;

Fig. 3

eine weitere Ausführung der erfindungsgemäßen Tarnvorrichtung;

Fig. 4

die auf die Himmelsstrahlung bzw. auf die Bodenstrahlung entfallenden Anteile der an der Tarnvorrichtung nach Fig. 3 reflektierten Strahlung in Abhängigkeit von der Beobachtungsrichtung;

Fig. 5

die scheinbare Objekttemperatur in Abhängigkeit von der Beobachtungsrichtung bei Einsatz einer erfindungsgemäßen Tarnvorrichtung (Kurve b) im Vergleich zu einer bekannten Tarnvorrichtung (Kurve a).

The invention is explained in more detail with reference to drawings. Show it:

Fig. 1

a diagram of the temperature-dependent specific resistance of a tungsten-doped VO ₂ layer compared to an undoped VO ₂ layer;

Fig. 2

a concrete embodiment of the camouflage device according to the invention;

Fig. 3

a further embodiment of the camouflage device according to the invention;

Fig. 4

the proportions of the radiation reflected on the sky radiation or on the ground radiation reflected on the camouflage device according to FIG. 3 as a function of the direction of observation;

Fig. 5

the apparent object temperature as a function of the direction of observation when using a camouflage device according to the invention (curve b) compared to a known camouflage device (curve a).

An embodiment of the invention is explained in more detail below with reference to the vanadium oxide (VO ₂ ), which shows the MNM transition described. Below a certain transition temperature, the material is semiconducting and therefore IR-transparent. On a highly emissive substrate, such as anodized aluminum or a plastic film, the overall structure has high emissivity. When heated above a specific transition temperature, the phase change takes place and the VO ₂ layer shows metallic behavior with high IR reflectivity. The phase change takes place at a temperature in the range of approx. 68 ° C.
In order to be able to use this effect for IR camouflage, a targeted adjustment of the position of the transition temperature and the width ΔT of the transition area is necessary. This can be achieved by adapting the temperature-dependent electrical conductivity of the vanadium dioxide and the associated IR reflectivity. One possibility for this is the doping of VO ₂ with eg tungsten (GV Jorgenson, JC Lee, Solar Energy Mat. 14 (1986) 205-214). 1 shows the temperature-dependent change in the conductivity of a VO ₂ layer in comparison with a tungsten-doped VO ₂ layer. As you can see, the transition temperature is shifted to lower temperatures. A shift up to ambient temperatures is possible. It has been shown that the transition region can also be broadened by varying the production parameters of the layer. In this way, the emissivity of a layer can be adjusted over a wide range depending on the temperature.

A solution according to the invention for producing a self-adapting camouflage for horizontal or upward-facing surfaces, e.g. of a vehicle thus a camouflage element with a thermorefractive coating, the emissivity the layer is set so that taking into account the intended use of the vehicle and, if applicable, the season, at ambient temperatures highly emissive behavior and with increasing temperature the Emission decreases.

Fig. 2 shows an embodiment of the camouflage device according to the invention. it includes a support plate made of anodized aluminum, which has a high emissivity has (ε≈1). The carrier plate is at a distance from the object to be camouflaged assembled and ventilated or otherwise thermal to the object isolated. This decouples the camouflage device from its own temperature of the vehicle, i.e. their own temperature is largely independent from possible heat sources of the object to be camouflaged. The carrier plate is coated with the thermorefractive layer according to the invention. As an alternative to Direct coating of the sheet is also the manufacture of the thermorefractive Layer on a self-adhesive, temperature-resistant plastic film, e.g. out Polyimide, possible, which can then be glued to the carrier layer. To preserve The thermal camouflage effect can be visually disguised with an IR-transparent Top layer (e.g. a pigmented and matted polyethylene film) be provided. It forms the external conclusion of the system towards the Observer.

• Bei geringer Oberflächentemperatur (Nacht, starke Bewölkung mit geringer Sonneneinstrahlung) ist kein Tarnbedarf vorhanden und der Emissionsgrad der Anordnung ist hoch. Die scheinbare Oberflächentemperatur ist der umgebenden Lufttemperatur und damit der des Hintergrunds gut angepasst.
• Bei solarer Erwärmung nimmt die Vorrichtung mit steigender Temperatur geringere Emissionsgrade an und kompensiert damit die thermische Abstrahlung.
• Da bei Sonnenschein typischerweise geringe Bewölkung und damit niedrige Himmelstemperaturen vorherrschen, kann das temperaturabhängige Emissionsverhalten der thermorefraktiven Schicht relativ gut voreingestellt werden und somit die Temperaturkompensation sehr effektiv stattfinden.

The IR camouflage mechanism of this device consists of three effects:

• With a low surface temperature (night, heavy cloud cover with little sunshine) there is no need for camouflage and the emissivity of the arrangement is high. The apparent surface temperature is well adapted to the surrounding air temperature and therefore that of the background.
• With solar heating, the device assumes lower emissivities with increasing temperature and thus compensates for the thermal radiation.
• Since there is typically little cloudiness and therefore low sky temperatures when the sun shines, the temperature-dependent emission behavior of the thermorefractive layer can be preset relatively well and the temperature compensation can therefore take place very effectively.

However, the invention cannot only be used for camouflaging essentially horizontal or upward-facing surfaces. As will be explained in more detail below, the solution according to the invention can advantageously also be used for camouflaging essentially vertical surfaces (this includes surfaces slightly inclined towards the sky - up to approximately 25 ° to the vertical). It should be taken into account that the situation with predominantly vertical surface areas is a mixture of the relationships with horizontal or upward-facing surfaces on the one hand and with surfaces inclined to the ground on the other. Depending on the observation angle, the reflected heat radiation comes mainly from areas near the ground or from the sky. The problem here is that even small changes in the observation angle (or equivalent: slight change in the inclination of the surface, for example in the case of moving camouflaged objects) cause a strong change in the ratio of these parts.
By means of suitable surface structures, the vertical surface can be broken down into partial surfaces oriented towards the ground and towards the sky, advantageously the largest possible proportion of the radiation reflected on the camouflage device originating from the ground and the smallest possible proportion derived from the sky radiation. The reflection components should remain constant over the largest possible inclination angle range. This can be achieved by means of a surface structure which consists exclusively of two groups of partial areas, the partial areas of the first group being oriented downward and forming an angle α with the vertical between 5 ° and 45 ° and the partial areas of the second group being oriented upward and form an angle β between 40 ° and 85 ° with the vertical, where α + β <90 °. The partial areas within the same group can have different angles α and β.
The upward facing areas are coated with a thermorefractive material as described above, while the downward facing areas are coated with a material with low infrared emissivity. Typical values for this are ε≤ 0.5.

α: [5, 45]; bevorzugt [15,25]

β: [50,85]; bevorzugt [55,70].

A geometric structure that has these properties is shown in FIG. 3. It consists of a regular series of surveys with a triangular cross section, the hypotenuses (length L) of which are oriented essentially vertically. It is a groove structure with horizontally aligned asymmetrical grooves. The geometry of the structure is clearly defined by the angles α and β and by the structure size L. The angle ϕ is the point of view of an observer to the horizontal. Suitable value ranges for the angles α, β are:

α: [5, 45]; preferred [15.25]

β: [50.85]; preferred [55.70].

Taking into account the reflection ratios of the two observable Partial areas at different angles ϕ can be determined the proportions that at different angles α and β of the structure. Fig. 4 shows the percentages Proportion of the from the ground or sky at different observation angles ϕ reflected radiation of this structure for a particularly favorable geometry α = 15 ° and β = 65 °. As you can see, over a large angular range are the reflected ones Parts coming from the sky or from the ground are almost constant, the proportion of soil as desired is very high.

For maximum effectiveness, the larger, downward facing area is the Soil portions reflected, with a layer with the lowest possible emissivity, i.e. maximum IR reflectivity. The smaller, upward facing area reflects the sky and therefore becomes - as in the case shown above horizontal surfaces - equipped with thermorefractive properties, so that sets a lower infrared emissivity on hot surfaces, which leads to a desired reduction in the radiation level of the overall arrangement contributes.

5 shows the radiation temperatures of two surfaces with the same emissivity, which were measured at different observation angles ϕ, where the curve marked with a the measured values of an unstructured surface and the curve marked b the measured values of a structure according to the invention reproduces. It can be seen that the radiation temperatures of the unstructured sample strong from a certain angle due to reflection of a cold sky surface drops while the structured sample is in the same radiation environment practically shows no such angle dependency as desired.

The structure sizes of the surface structure are in particular between 12 μm and 1cm, preferably between 100µm and 1mm.

In a particularly advantageous embodiment, the structure sizes are selected so that that they are larger than the wavelength of infrared radiation and smaller than that Wavelength of radar radiation. A suitable size range for this is between 20 µm and 1 mm. This ensures that the radar reflection cross section is not negatively influenced by multiple reflexes.

To maintain the visual camouflage effect, an IR-transparent cover layer (eg a pigmented and matted polyethylene film) can be provided as the outer end of the camouflage device.
In addition, additional camouflage effects can be achieved according to the principle of spot camouflage, in which a contour outline is also introduced in the infrared. This can be produced very effectively by different thicknesses of the color-providing cover layer on top, so that a spot-like pattern of the infrared signature is superimposed at all temperature states of the system.

The (micro) structuring according to the invention can be produced depending on the structure size through various common processes such as embossing, milling, engraving or photolithographic processes take place. An appropriately structured tool can then e.g. for transferring the structure to a - preferably self-adhesive plastic film, e.g. by hot stamping in a calender. A high IR reflection is achieved by metallizing and then IR transparent colored top layer. Another option is one Painting the structure with low-emitting camouflage paint.

With very small structure sizes (L approx. 100µm) it is also possible to use colored ones Plastic films made of IR-transparent materials (e.g. polyolefins such as PE, PP) Hot stamping to provide the structure and the IR reflector through the back To apply metallization. In this case, the structuring also causes Matting necessary to reduce the visual gloss of the plastic film.

• Die nach unten gerichteten Flächenbereiche des zu tarnenden Objekts werden mit einem Material versehen, das einen niedrigen Infrarot-Emissionsgrad aufweist. Typische Werte hierfür sind ε≤ 0,5.
• Die nach oben oder horizontal ausgerichteten Flächenbereiche des zu tarnenden Objekts werden mit einem Material versehen, das bei Temperaturerhöhung einen Phasenübergang von einem metallischen Zustand in einen halbleitenden Zustand durchläuft und somit seinen Infrarot-Emissionsgrad ohne äußeren Regelmechanismus an die veränderten Umweltbedingungen anpasst.
• Auf senkrechten Flächenbereichen des zu tarnenden Objekts bewirkt (Mikro-) Strukturierung die Aufteilung der Fläche in nach oben gerichtete Anteile und nach unten gerichtet Anteile.

An overall system for camouflaging an object using the camouflage device according to the invention thus has the following structure:

• The downward-facing surface areas of the object to be camouflaged are provided with a material that has a low infrared emissivity. Typical values for this are ε≤ 0.5.
• The surface areas of the object to be camouflaged, which are oriented upwards or horizontally, are provided with a material which, when the temperature rises, undergoes a phase transition from a metallic state to a semiconducting state and thus adjusts its infrared emissivity to the changed environmental conditions without an external control mechanism.
• On vertical surface areas of the object to be camouflaged, (micro-) structuring causes the surface to be divided into parts pointing upwards and parts pointing downwards.

Claims (12)

Infrared camouflage system, characterized in that it comprises a thermorefractive layer system or a thermorefractive material whose heat emissivity has a negative temperature coefficient. Infrared camouflage system according to claim 1, characterized in that the negative temperature coefficient takes place through a phase transition from a non-metallic to a metallic state. Infrared camouflage system according to claim 2, characterized in that the phase transition material consists of vanadium dioxide which is doped with foreign atoms, for example tungsten. Infrared camouflage system according to claim 1, characterized in that the negative temperature coefficient results from a temperature-dependent scattering effect in a composite medium, the components of which have a low IR absorption and have approximately the same refractive indices at ambient temperature, but increasingly divergent refractive indices at higher temperatures. Infrared camouflage system according to claim 4, characterized in that the matrix component consists of a solid polymer and contains deposits of solid, waxy or liquid substances. Infrared camouflage system according to one of the preceding claims, characterized in that it has the following structure: a carrier with high infrared emissivity which is thermally insulated from the object to be camouflaged; such as a layer of the thermorefractive material, the heat emissivity of which has a negative temperature coefficient. Infrared camouflage system according to one of the preceding claims 1 to 5, characterized in that it has the following structure: a carrier with high infrared emissivity which is thermally insulated from the object to be camouflaged; a plastic layer glued to the carrier, for example made of polyimide; a layer of the thermorefractive material, the heat emissivity of which has a negative temperature coefficient. Infrared camouflage system according to one of claims 1 to 5, characterized in that it has a surface structure facing the observer, which consists exclusively of two groups of sub-areas, the sub-areas of the first group being oriented downwards and with the vertical an angle α between Form 5 ° and 45 ° and the partial surfaces of the second group are oriented upwards and form an angle β between 50 ° and 85 ° with the vertical, where α + β <90 °, the downward facets are formed from a material with low infrared emissivity; the upwardly oriented partial surfaces are formed by the thermorefractive material, the heat emission level of which has a negative temperature coefficient. Infrared camouflage system according to one of the preceding claims, characterized in that an infrared-transparent, pigmented and matted cover layer made of plastic, for example polyethylene, is present as the outer end of the infrared camouflage device. Infrared camouflage system according to claim 9, characterized in that the cover layer has patch-like areas of different thicknesses. Method for infrared camouflage of an object, characterized in that the substantially horizontal or upward facing surfaces of the Camouflaging object by means of the camouflage device according to one of claims 1 to 7 are provided. A method for infrared camouflage of an object according to claim 11, characterized in that the downward-facing surfaces of the object to be camouflaged are provided with a coating of a material with low infrared emissivity; the substantially vertically oriented surfaces of the object to be camouflaged are provided with the camouflage device according to one of claims 8 to 10.

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