GB2228566A - Infra-red sensors - Google Patents

Abstract

An infra-red sensor having a polarisation sensitive element (3 and 4) employs the polarisation characteristics of the infra-red radiation from an object (2) to classify the object. A polariser (4) is rotated and the intensity of the IR reaching a detector (3) at a series of angles of the polariser is analysed by data processor (6). The output of detector (3) is compared with that of a reference detector (7) to compensate for atmospheric and other changes. <IMAGE>

Description

Infra-Red Sensors This invention relates to infra-red sensors.

It is common practice for military and law enforcement personnel to use infra-red sensors to pick out human activity from its surroundings by looking for anomalous hot spots produced by heat sources such as vehicle engines.

A problem with the use of such sensors is that it is very difficult to distinguish these heat sources of interest from the clutter of hot spots provided naturally by the sun, fires and reflections and artificially by flares or deliberately set decoy fires.

It has been found that the polarisation characteristics of the infra-red radiation emitted or reflected from a surface vary with the properties of the surface.

These characteristics can generally be expressed in terms of the infra-red radiation having a number of different polarisations, the number, relative angle and relative intensity of these polarisations varying with the surface.

This invention is intended to produce an infra-red sensor employing this phenomenon to produce a sensor overcoming, at least in part, the problems of the prior art.

A first aspect of this invention provides an infra-red sensor which employs the polarisation characteristics of an infra-red radiation emitter to deduce at least one physical characteristic of the emitter.

The invention is further described with reference to the accompanying Figures in which; Figure 1 shows, in block diagram form, an infra-red sensor embodying the invention, Figure 2 shows, in block diagram form, an infra-red system as shown in Figure 1 used in conjunction with a scanning system, and Figure 3 shows a guided weapon employing the system of Figure 2.

Referring to Figure 1, an infra-red sensor 1 receives infra-red radiation from an object 2. This infra-red radiation may be radiated by the object 2 or may be radiated by some source (not shown) and reflected from the object 2.

The infra-red sensor 1 includes a first infra-red detector 3, a second infra-red detector 7 and a rotatable infra-red polarister 4. The infra-red polariser 4 can be rotated by a motor 5.

In operation infra-red radiation from the object 2 passes through the polariser 4 to the first infra-red detector 3.

The first infra-red detector 3 produces an electrical signal dependent on the intensity of the infra-red radiation incident on it, this signal is supplied to a data processing system 6.

Infra-red radiation from the object 2 is also incident on a second infra-red detector 7, which has the same field of view as the infra-red detector 3. The second infra-red detector 7 produces an electrical signal dependent on the intensity of the infra-red radiation incident on it, and supplies this signal to the data processing system 6.

As the infra-red polariser 4 rotates the proportion of the infra-red radiation from the object 2 that passes through it varies with the relative angles between the planes of polarisation of the infra-red radiation from the object 2. Information on the angular position of the infra-red polariser 4 is passed to the data processing system 6.

The data processing system 6 compares the relative intensities of the infra-red radiation incident on the first infra-red detector 3 and the infra-red radiation incident on the second infra-red detector 7, for a series of different rotational positions of the infra-red polariser 4. From this comparision the data processor 6 calculates the polarisation characteristics of the infra-red radiation from the object 2, and passes this information to a comparator 8. By comparing the relative intensities of the infra-red radiation travelling directly to the sensor and travelling via the polariser 4, variations in intensity due to external factors such as atmospheric conditions or changes in the intensity of infra-red radiation reflected or radiated by the object 2 are eliminated.

The comparator 8 compares the polarisation characteristics of the infra-red radiation from the object 2 with a number of sets of known polarisation characteristics and their classifications stored in a memory 9.

When a known set of polarisation characteristics equivalent to those produced by the object 2 are found this classification of the object 2 is sent by the comparator 8 to a display 10.

Referring now to Figure 2, an infra-red scanning system 11 has a field of view 12. A number of objects including an object 13 in the field of view 12 emit infra-red radiation 14 which is passed onto the infra-red sensor 1. The infra-red sensor 1 operates as described above and provides information classifying the objects in the field of view 12, including the object 13, to a computer 15 which issues instructions on lines 16 depending on the classes of the objects in the field of view 12.

Referring to Figure 3 a guided bomb 17 bearing an infra-red target classification system 18, as shown in Figure 2, is falling towards the ground 19. The classification system 18 has a field of view 20. Within this field of view are an armoured vehicle 21 and a plurality of infra-red radiating flares 22 which are intended to protect the armoured vehicle 21 by decoying infra-red targetted weapons.

Using polarisation characteristics the classification system 18 is able to classify the flares 22 as naked flames and the engine compartment of the vehicle 21 as a hot piece of steel. The bomb 17 is then guided to impact the heat source formed by the vehicle 21 rather than those formed by the flares 22.

An alternative, arrangement in the sensor shown in Figure 1 would be to arrange for the fields of view of detectors 3 and 7 to have the same boresight direction.

This can be achieved by use of a beam-splitter located between the rotating polariser 4 and the target 2, acting to split the radiation of the target 2 into two parts, one going to each of the detectors 3 and 7. By this means, intensity variations due to external factors will be identical at the two detectors, this benefit being achieved at the expense of a loss in intensity, since half the intensity will then be received at each detector.

Also, the infra-red scanning and detector system (11 and 1) may alternatively consist of a non-scanned matrix array of infra-red detector elements together with a fixed objective lens arrangement.

This would avoid problems of synchronisation between the rotating polariser 4 and the scanning mechanism. It would also be mechanically less cumbersome.

Claims (6)

1. An infra-red sensor employing the polarisation characteristics of an infra-red radiation emitter to deduce at least one physical characteristic of the emitter.

2. An infra-red sensor as claimed in claim 1 in which the polarisation characteristics used include the number of different polarisation angles the radiation has.

3. An infra-red sensor as claimed in claim 1 or claim 2 in which the polarisation characteristics used include the relative angle of the different polarisation angles of the radiation has.

4. An infra-red sensor as claimed in any preceding claim in which the polarisation characteristics used include the intensities of radiation at the different polarisation angles the radiation has.

5. An infra-red sensor substantially as shown in and as described with reference to Figure 1 of the accompanying drawings.

6. An infra-red guidance system substantially as shown in and as described with reference to Figures 2 and 3 of the accompanying drawings.