WO1996004520A1

WO1996004520A1 - Apparatus for altering the infrared characteristics of a body

Info

Publication number: WO1996004520A1
Application number: PCT/GB1995/001726
Authority: WO
Inventors: Donald James Highgate
Original assignee: The Secretary Of State For Defence In Her Britannic Majesty's Government Of The United Kingdom Of Great Britain And Northern Ireland
Priority date: 1994-08-04
Filing date: 1995-07-21
Publication date: 1996-02-15
Also published as: GB9415921D0; AU2988195A

Abstract

Apparatus for altering the IR characteristics of a body (1) comprising a surface having at least two discrete areas (5a, 5b, 5c, 8, 9, 11a, 11b, 16) possessing different IR characteristics, masking means (3, 10, 15) for masking different portions of the surface, and controlling means for operating the masking means arranged such that operation of the controlling means alters the relative exposures of the two or more areas of the surface such as to impart desirable IR characteristics to an aspect of the body as detected from a remote point. In particular, an IR signalling apparatus comprising an inelastic surface (12) composed of alternating low IR emissivity elements (8) and high emissivity elements (9). The inner face of the surface (12) is bonded to a body (1) while the outer face is bonded to a polythene core (13) which is divided into individual nitrogen-filled chambers (14). Masking means (15) in the form of parallel inelastic strips (16) are bonded to the outer face of the core (13). These strips (16) have an inner side which is composed of low IR emissivity material (16a) and an outer side composed of medium to low IR emissivity material (16b) also coloured in the visible. The apparatus is mounted on a body (1) for signalling to a remote detector (6). The masking means (15) is located above the surface (12) so as to achieve a desired overall IR characteristic, and relative motion is introduced between the two at a known frequency. The detector (6) is filtered at this frequency.

Description

APPARATUS FOR ALTERING THE INFRA-RED CHARACTERISTICS OF A BODY

The present invention relates to methods of varying the IR characteristics of a body, more particuarly for altering the IR characteristics of a body so as to reduce its detectability or to increase its distinctiveness. The invention further relates to particular methods of use of such apparatus.

As is well known, bodies emit thermal radiation as a result of their own surface temperature. However this radiation is not emitted uniformly across all wavelengths, and the variation in its intensity at a given temperature with the wavelength being detected is described by Planck's curve. For most materials at temperatures between 25 and 1000'C, the wavelengths emitted lie within the IR region between about 1 and 20 μm.

Many modern reconnaissance and weapons systems employ radiation detectors which operate within the IR region (principally between 0.8 to 100 μm) in order to aid target acquisition and weapon guidance. Such detectors, for instance thermopiles or photographic films, may operate by passively receiving thermal radiation emitted by a body which is at a different temperature to its environment. Alternatively they may passively receive radiation reflected from the surface of the body from other, possibly distant, sources eg. the sky or sun.

It is therefore desirable to provide potential targets with thermal 'IR camouflage' which confuses such detectors by matching the IR image of the body to that of its immediate environment notwithstanding the fact that the body is hotter or colder than the ambient temperature.

The thermal radiation emanating from a body is dependent not only on its surface temperature, but also on a number of other factors including the nature of the material forming the surface of the body. For instance thermal radiation may be reflected, transmitted or adsorbed (and re-emitted) by the body's surface depending on the molecular characteristics of the material from which it is composed. These characteristics (reflectance, transmitance, absorbance and emissivity) will be an innate feature of the material in question a will have a profound affect on the intensity of the radiation emanating from the material at any given wavelength and temperature. The term 'IR characteristics' as used hereinafter is intended to denote one or more of these characteristic properties of a material body as exhibited for those wavelengths of the IR region of concern (principally between 1 and 20 μm) .

Thus known thermal camouflage materials generally have a low IR emissivity i.e. a low emissivity in the IR region of concern. Coating a potential target in these materials can confuse passive detectors because their low emissivity means that they emit relativ little energy irrespective of the fact that the actual surface temperature is significantly above the local ambient level.

Unfortunately as the IR characteristics of such camouflage are an innate feature of the material itself, they must be selected in ord to maximise efficiency in an 'average' situation. Thus the camoufl of a body coated in these materials can be undermined when it is travelling through varied terrain or when there is an unusually lar temperature differential between the body and its surroundings.

Additionally, low emissivity materials will also have a high reflectance because these properties are inversely related to each other. Thus the effectiveness of the camouflage provided by such materials is to some degree undermined by reflection of solar radiation (corresponding to a high temperature) and radiation from other parts of the sky (corresponding to a low temperature) which lead to increased detectability.

A further problem in employing such simple camouflage materials is to ensure that a weapon or reconnaissance system recognises whether a target which has been detected through IR or other means represents a 'friend or foe' .

Current methods of distinguishing friendly targets, at least in the IR region, have concentrated on the provision of an area of material on the potential target being formed into an agreed 'symbol of the day'. Owing to the relatively poor spatial resolution of existing IR detectors it is necessary that the area of material is large, and has markedly different IR characteristics to those of its background. By definition, both of these qualities will undermine the effectiveness of the target's camouflage and lead to increased detectability. In addition, the relative simplicity of such systems makes them relatively easy to mimic.

In addition to camouflaging potential targets, it is also desireable to provide decoy systems for distracting attention away from them. Such systems are required to complement the effect of IR camouflage, because even the best camouflage sytems may be degraded in use, for instance by the presence of a surface layer of dirt or mud. Decoy systems should have an IR image which is in contrast to their immediate environment.

Simple decoy systems are known which provide a strong thermal signature, i.e. an IR image of greater intensity than that of its surroundings, by using a small heat engine or motor to generate hrat. However such systems have the disadvantage that it is difficult to fine-tune the decoy to have an IR image consistent with that of the equipment which it is intended to mimic. Additionally, the use of a powered structure leads to a requirement for logistic support.

The present invention has now provided an apparatus for controllably altering the IR characteristics of a body, and methods of use of such apparatus, that addresses some, and in preferred forms all, of these problems.

According to one aspect of the present invention there is provided an apparatus for altering the IR characteristics of a body comprising a surface having at least two discrete areas having different IR characteristics, masking means for masking different portions of the surface, and controlling means for operating the masking means arranged such that operation of the controlling means alters the relative exposures of the two or more areas of the surface such as to impart desirable IR characteristics to an aspect of the body as detected from a remote point.

Thus the IR characteristics of a whole body, or one of its aspects, can be selectively and quickly altered so as to suit its current situation. The areas of the surface preferably have sufficiently different IR characteristics to provide an acceptable range of alteration of the overall IR characteristics of the body. In order to maximise effect, the apparatus should be used far enough away from the remote detector that its spatial resolution is unable to wholly distinguish the IR characteristics of the areas of the surface from those of each other or the masking means i.e. so the apparatus as viewed appears to have a fairly uniforr IR signature. Thus the distance at which the apparatus will operate efficiently will depend on the acuity of the detector, which may be as far away as an orbitin satellite. Existing detector technology would make elements having scale lengths of 5 - 10 mm practical. Similarly the distance between the surface and the masking means should not be large compared to thi value.

It is to be noted that the term surface embraces not just a continuou surface but also the outermost aspect of discontinuous or non-uniform formations. The phrase 'discrete areas' embraces any individually distinct areas, not just areas which are physically separated. Thus the IR characteristics of the surface may change abruptly from area t area, or alternatively the areas may merge into each other presenting a continuum of varying IR characteristics.

Preferably the discrete areas of the surface are arranged in a regular pattern, such as parallel stripes or checks. This may improve ease of manufacture, and will also improve the efficiency of the masking means in embodiments where the masking means incorporates a corresponding pattern.

The surface can be provided by regularly arranged rows of discrete, possibly elongate, elements arranged side by side. Each element is rotatable and has a regular cross section, for instance circular or triangular, while the outer face or faces of each element have a varying IR characterisitc. Since only part of these elements are exposed in given a aspect of the body (the rest being masked by the element itself) , rotation of the elements leads to different parts of the face or faces of the elements being masked from detection, and hence imparts different IR characteristics to that aspect of the body.

Preferably, however, the masking means is a separate entity to the surface and comprises non-IR-transparent material, eg. an adiathermonous material, interposed between the surface and the remote point at which the IR characteristics of the body are being assessed. Such materials are well known to those skilled in the art.

Most preferably the masking means incorporates regularly arranged apertures or IR transparent portions. These should correspond to the arrangement of the areas of the surface such that the imposition of relative movement between the masking means and the surface efficiently (i.e with the minimum of movement) alters the relative exposures of the areas. The relative movement can be transalation of the whole masking means with respect to the surface. Alternatively the size and/or position of the apertures could be altered, for instance by having a multi-element masking means in which discrete elements are moved with respect to each other eg. as with a 'venetion blind' .

For applications in which the speed of the alteration of the characterisitics is not paramount, the controlling means can take the form of manual adjustment of the surface and/or masking means by a human operator. In automated systems, the controlling means can suitably effect movement of a surface or masking means through a conventional servo motor, which could drive a pulley system anchored to the apparatus. Alternatively, if all or part of the surface or masking means consists of coated metal elements (which have a low emissivity) then these could be magnetised and therefore able respond to applied magnetic fields. Preferably the movement is effected by a device which does not compromise the IR or visual characteristics of the body, for instance which can operate from behind the apparatus (with respect to the detector).

The controlling means may alter the exposure of the surface by simply moving the masking means relatively slowly with respect to the surface. Alternatively the controlling means could initiate movement at a constant frequency while altering the amplitude of the relative movemants to give a more finely tuneable reponse.

In preferred embodiments the surface and the masking means are separated by IR transparent material eg. diathermanous material. The term 'IR transparent material' is intended to include any material that has a significant IR transmitance. This can merely be a void, but preferably this is a flexible plastic which can be conveniently 'coated' with a suitable surface on one side and masking means on the other eg. polyethene. This can then be attached to the body, with the masking means outermost, in sheets. The flexibility of the plastic will affect the energy required to impose relative movement between the surface and masking means. Preferably the plastic incorporates volumes of gas or liquid to increase its flexibility and -

width without increasing weight.

In a most preferred form the apparatus further comprises an environmental sensor, eg. a thermocouple or IR detector, and the controlling means is operated in accordance with data received from the environmental sensor

The present invention also makes available an IR decoy apparatus the purpose of which is to present a distictive IR signature against its surroundings. In this case the controlling means could be operated manually, or in reponse to a bimetallic strip or thermocouple. By altering the IR characteristics of the decoy, an apparent temperature difference between the decoy and the ambient temperature can be maintained using only unpowered or low power-consumption apparatus. The system could be used on a whole decoy body, or on small sections of a large installation in order to distract attention away from more sensitive points. The ability to fine-tune the IR characteristics of the decoy would allow the accurate engineering of an IR signature which was consistent with the equipment to be mimicked.

The invention also makes available an apparatus for controlling the temperature of a body comprising an apparatus for altering the IR characteristics of a body as disclosed above and further comprising an environmental sensor which is an internal temperature sensor. Such a system could be employed by a body for which it is desired, for instance, to maintain a constant internal temperature. During daylight hours a highly IR reflective characteristic could be adopted, while at night the apparatus could be used to maximise IR absorbtion from the environment.

The invention also makes available an adaptable IR camouflage apparatus comprising an apparatus for altering the IR characteristics of a body as described above and further comprising an environmental sensor which is an IR detector. This 'feedback' system will allow the IR characteristics of a body to be iteratively approximated to those of an environment in which it is present or about to move into.

Preferably at least one of areas of the surface comprises a low IR emissivity material, for instance of a type used in known IR camouflage systems. Most preferably the masking means also comprises a low or medium IR emissivity material so as not to undermine the IR camouflage of the exposed portions of the surface. The surface and masking means may also incorporate visual camouflage.

The invention further provides a method for achieving a desired relationship between the IR characteristics of a body and those of an environment comprising use of an IR detector to make a first measurement of the IR characteristics of at least one aspect of the body and a second measurement of the IR characteristics of all or part of the environment and operating the controlling means of an adaptable IR camouflage apparatus as described above such as to achieve a desired relationship between the first and second measurements.

Thus, for instance, a tank fitted with the adaptable IR camouflage apparatus could continuously alter its IR characteristics such as to approximate its thermal emissions or reflections to those of its background in accordance with data recieved from IR sensors directed at both. Such sensors could be mounted anywhere externally eg. on board the tank, on a different land or air vehicle, or on the ground (when the tank was stationary) . Different aspects of the tank (eg. view from above, view from the side) with their correspondingly different backgrounds could be under the control of different sensors. Thus the IR characteristics of a surface which, when detected from a remote point, is reflecting the sun, can be altered so as to reduce its reflectivity and hence the apparent contrast viewed by a detector. The actual control of the operating means in response to the data fro the sensor or sensors could be carried out by a suitably programmed microprocesso . The invention also makes available an IR signalling apparatus for communicating information to an IR detector at a remote point comprising an apparatus for altering the IR characteristics of a body as described above wherein controlling means is operatable in accordance with an identification system such as to impart recognisable IR characteristics to an aspect of the body as detected from the remote point. Preferably the surface of the apparatus comprises alternating areas of relatively high and low IR emissivity material, such as to maximise the overall breadth of IR characteristics impartable.

This apparatus could allow a body suitably equipped to be identified as a 'friend or foe' without significantly undermining the IR camouflage capability of such a body. The identification system may also allow the communication of other simple messages.

In its simplest form the identification system may simply involve a brief but large scale shift in the overall IR characteristics of the body or an aspect of the body eg. by increasing the IR emissivity of the body for a set number of seconds every minute.

In a preferred embodiment the identification system comprises operating the controlling means such as to introduce relative movement between the masking means and the surface at a recognition frequency recognisable by the IR detector. Thus the IR characteristics of a body could be altered at a predetermined frequency, or combination of frequencies, selected so that non-friendly detectors would see only a time average IR signature. Friendly detectors could be filtered at an appropriate frequency such that they could recognise the body as one of their own. Suitable frequencies would obviously be limited by the temporal resolution of the non- riendly detector system in question, but for a typical multi element system frequencies in the range of 1 - 100 Hz may not be too low. The invention also makes available material adapted for use in any of the apparatus described above comprising a surface layer having at least two discrete areas possessing different IR characteristics, a masking layer for masking different portions of the surface, and an I transparent flexible plastic layer between the surface and the maskin layers,

Thus the invention makes available apparatus, methods and materials for use in the alteration of the IR characteristics of a body. Applications include decoy systems and temperature control systems with reduced power consumption (compared to actively heated ones). Also camouflage systems which can adapt to their conditions, and identification systems which are difficult to mimic but do not compromise the camouflage system of the body to which they are fitted All of these applications may potentially be performed by the same apparatus.

The apparatus of the present invention will now be described, by way of illustration only, through reference to the following examples and figures. Other embodiments falling within the scope of the invention will occur to those skilled in the art in the light of these.

FIGURES

Fig 1 shows a cross section of the apparatus described in Example 1 Fig 2 shows a cross section of the apparatus described in Example 2 Fig 3 shows a cross section of the apparatus described in Example 3 Fig 4 shows a plan view of the apparatus described in Example 3

Example 1

An IR decoy apparatus was provided as shown in cross section in Fig 1(a) and 1(b). The apparatus is mounted on a body (1) having a high emmisivity covering (2). The apparatus comprises several identical rotatable elements (3) of equilateral triangular cross-section (side length 10 mm) each mounted on a separate axle (*!) . Each element has three faces (5a, 5b, 5c) which have different IR emmisivities (approximately 0.1, 0.5. 0.9 respectively).

In use the axles (O of the elements (3) are rotated in a synchronised manner such as to alter the relative exposures of the three faces (5a. 5b, 5c). The rotation of the elements (3). either manually or by means of a servo motor (not shown), permits a desirable overall IR characteristic to be achieved as detected by a remote detector (6).

Example 2

An adaptable IR camouflage apparatus was provided as shown schematically in cross section in Fig 2(a), (b) and (c). The apparatus comprises a surface (7) composed of alternating low IR emissivity elements (8) and high emissivity elements (9) each being 5 mm high. The inner face of the surface (7a) is bonded to a body (1) to be camouflaged. Masking means (10) comprising discrete 5 mm wide parallel inelastic strips (11) are arranged over the outer face of the surface (7b) , each strip being pivotally mounted over alternative junctions between the surface elements (8 and 9). These strips (11) have a first side which is composed of low IR emissivity material (11a) and a second side composed of medium to low IR emissivity material (lib) also coloured in the visible.

In use the strips (11) of the masking means (10) are pivoted in a synchronised manner by means of a pulley system. The pulley is operated, either manually or by means of a servo motor (not shown), i response to signals from a remote detector (6) which detects IR emanations from the body (1). The movement of the strips (11) alters the relative exposure of the surface elements (8 and 9) so as to give an overall variation in emissivity.

Example 3

An IR signalling apparatus was provided as shown in cross section in Fig 3 and in plan view in Fig 4. The apparatus comprises an inelast surface (12) composed of alternating low IR emissivity elements (8) and high emissivity elements (9) each being 5 πim high. The inner fa of the surface (12) is bonded to a body (1) while the outer face is bonded to a 3 mm thick (when relaxed) polythene core (13) which is divided into individual nitrogen filled chambers (14). Masking mean (15) in the form of 5 mm high parallel inelastic strips (16) are bonded to the outer face of the core (13). These strips (16) have a inner side which is composed of low IR emissivity material (16a) and an outer side composed of medium to low IR emissivity material (16b) also coloured in the visible.

In use the apparatus is mounted on a body (1) for signalling to a remote detector (6). The masking means (15) is located above the surface (12) so as to achieve a desired overall IR characterisitc an relative motion with is introduced between the two at a known frequency and an amplitude of 2.5 mm. The detector (6) is filtered this frequency.

Claims

1. Apparatus for altering the IR characteristics of a body comprising a surface having at least two discrete areas possessing different IR characteristics, masking means for masking different portions of the surface, and controlling means for operating the masking means arranged such that operation of the controlling means alters the relative exposures of the two or more areas of the surface such as to impart desirable IR characteristics to an aspect of the body as detected from a remote point.

2. An apparatus as claimed in claim 1 wherein the discrete areas of the surface are arranged in a regular pattern

3. An apparatus as claimed in claim 1 one 2 wherein the surface comprises a plurality of discrete rotatable elements providing at least two discrete areas possessing different IR characteristics.

4. An apparatus as claimed in any preceding claim wherein the masking means comprises non-IR-transparent material interposed betwee the surface and the remote point.

5- An apparatus as claimed in any preceding claim wherein the masking means incorporates regularly arranged apertures.

6. An apparatus as claimed in any preceding claim wherein the controlling means causes relative movement between the masking means and the surface.

7. An apparatus as claimed in any preceding claim wherein the surface and the masking means are separated by IR-transparent material.

8. An apparatus as claimed in claim 7 wherein the separating

material comprises a flexible plastic.

9. An apparatus as claimed in claim 8 wherein the plastic incorporates volumes of gas or liquid.

10. An apparatus as claimed in any preceding claim further comprising an environmental sensor wherein the controlling means is operated in accordance with data received from an environmental sensor.

11. An IR decoy apparatus comprising an apparatus as claimed in any preceding claim.

12. An apparatus for controlling the temperature of a body comprising an apparatus as claimed in claim 10 wherein the environmental sensor is an internal temperature sensor.

13- An adaptable IR camouflage apparatus comprising an apparatus as claimed in claim 10 wherein the environmental sensor is an IR detector

14. An apparatus as claimed in claim 1 at least one area of the surface comprises low IR emissivity material.

15. An apparatus as claimed in claim 13 or 14 wherein the masking means comprises low IR emissivity material.

16. An IR signalling apparatus for communicating information to an IR detector at a remote point comprising an apparatus as claimed in any of claims 1 to 9 wherein the controlling means are operatable in accordance with an identification system such as to impart recognisable IR characteristics to an aspect of the body as detected from the remote point.

17. An apparatus as claimed in claim 16 wherein the surface comprises alternating areas of relatively high and low IR emissivity material

18. An apparatus as claimed in claim 16 or 17 wherein the identification system comprises operating the controlling means such as to introduce relative movement between the masking means and the surface at a recognition frequency recognisable by the IR detector

19. An apparatus as claimed in claim 18 wherein the IR detector i filtered at the recognition frequency

20. Material adapted for use in an apparatus as claimed in any on of the preceding claims comprising a surface layer having at least tw discrete areas possessing different IR characteristics, a masking layer for masking different portions of the surface, and an IR transparent flexible polymer layer between the surface and the maskin layers.

21. A method for achieving a desired relationship between the IR characteristics of a body and those of an environment comprising providing the body with an apparatus comprising a surface having at least two discrete areas possessing different IR characteristics, masking means for masking different portions of the surface, and controlling means for operating the masking means arranged such that operation of the controlling means alters the relative exposures of the two or more areas of the surface and using an IR detector to make a first measurement of the IR characteristics of at least one aspect of the body and a second measurement of the IR characteristics of all or part of the environment and operating the controlling means such a to achieve a desired relationship between the first and second measurements.