CA1308179C - Device for the selective detection of objects - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A device for the selective detection of objects, such as aircraft, missiles and helicopters, by means of rays from the objects, preferably IR-rays, comprises at least one sensor unit.
The device comprises at least two optical entrances at a distance relative to each other across a sight line from the device to the object. The sensor unit comprises at least one focusing means for focusing the rays to a focal plane. The sensor unit is equipped with at least one radiation-sensitive detector element arranged to emit signals corresponding to the radiation. The device is arranged to variably scan an angular field in azimuth and/or elevation. The device further comprises an evaluation unit arranged to receive said signals. The evaluation unit is arranged to select objects which seem to be of similar size with regard to the angle measured from the position of the device, large objects being at a greater distance, such as aircraft, and small objects being at a small distance, such as birds, by sup-pressing signals whose amplitude as a function of the scanned azimuth or elevation angle shows a relatively great extension compared with signals which show a relatively small extension.

Description

1 3081 7q The present invention relates to a devlce for the selective detection of ob~ects, such as aircraft, missiles and helicopters by means of rays from the ob~ects, preferably IR-rays, comprising at least one sensor unit, the device being arranged to receive the rays through at least two optical entrances arranged at a relative distance across a sight line from the device to the ob~ect, the sensor unit comprising at least one focusing means arranged to focus said rays to at least one corresponding focal plane, the sensor unit havlng at least one radiation-sensitive detector element positioned in a focal plane, which detector element is arranged to emit signals corre-sponding to incoming radiation, the device being arranged to variably scan an angular field in azlmuth and/or elevation, the devlce further comprislng an evaluatlon unit, arranged to receive said signals.

The inventlon is based on a conventional technique, whlch wlll here be descrlbed brlefly.

On the market there ls a devlce, a so-called scanner, for detectlng flylng ob~ects by means of IR-rays emltted from the ob~ects. Such a scanner comprlse~ a sensor unlt, an evaluatlon unlt and a dlsplay and control unlt. The sensor unlt recelves IR-rays from the ob~ects to be detected wlthln the momentarlly scanned solld angle. The sensor unlt transmlts the correspondlng slgnals to the evaluatlon unit, whlch processes the ~lgnals and upon the slgnlflcant detectlon of ob~ects, l.e. ln a mllltary context, targets, transmlts the corresponding slgnals lncludlng dlrectlon coordlnates to the dlsplay and control unlt. From lt the collected output data go, whlch ln a mllltary context partly takes the form of a vlsible plcture on a screen, and partly the form of signals, to be forwarded onllne and e.g. be used for guldlng a flre-control system for antl-alrcraft defence.

The evaluatlon unlt selects those slgnals, among the slgnals recelved from the sensor unlt, whlch are slgnlflcant, -- 1 -- ~

1 3n81 79 i.e. which indicate targets withln the scannlng ran~e of the sen-sor unit, and to indicate when such targets appear, and generally indicate their coordinates. The evaluatlon unit functions thus according to pre-selected and programmed criteria of what will be considered significant ob~ects, i.e. targets.

One way of regarding the functions of the evaluation unit is to separate them into filter function and declsion func-tlon. The filter starts from the signals which are received from the sensor unit ~measured intenslty as a functlon of dlrectlon), and ls deslgned to enhance the typlcal slgnals of targets. Dur-lng flltering the directlon lnformatlon ls kept. One can say that the fllter output slgnal for a certaln dlrectlon ls a mea-sure of the probablllty that there is a target ln the actual dlrection.
A practlcal example 19 a filter, whlch for every direc-tlon forms the dlfference between measured intensity in the actual dlrection and the average lntensity ln a two-dimensional 20 interval of the surrounding directions. Typical for a fllter in thls appllcatlon 19 partlcularly that the output slgnal for a certain dlrectlon ls a welghted sum of the lnput signals of the fllter ln an angular range ln and about that dlrectlon.

In a scanner whose purpose is to select slgnlflcant ob~ects, i.e. targets, in addition to the fllter, a declslon functlon 18 also required. That functlon ls needed to declds, whether a signiflcant ob~ect exlsts or not. The most common decision function ls to compare the output signal of the filter with a threshold level. If that ls exceeded, a slgnlflcant ob~ect, i.e. a target, 19 lndlcated. The mechanlsm whlch chooses the threshold level may also be lncluded ln the declslon func-tion. The threshold level i9 often determlned through some klnd of statlstlcal evaluatlon of the output signals of the filter wlthin a large range, posslbly the whole scannlng range. The alm ls to find a level whlch ls exceeded at an acceptably low fre-~ "

quency in the absence of signlficant ob~ects, i.e. targets(false-alarm frequency) and which yet is not too high for appear-ing targets to be indicated with certalnty.

Scanners of the type just described function preferably within the IR-spectral ranges 3 to 5 and 7 to 13 micrometer, respectively, which represent "windows" with regard to the trans-mission spectra of the atmosphere for IR-radiation. That means, that the focusing means of the sensor unit, which in itself can consist of a lens or mirror, usually consists of a silicon lens for the 3 to 5 micrometer range and a germanium lens for the 7 to 13 micrometer range, i.e. it is chosen with respect to the actual spectral range. In view of hitherto existing corresponding radi-ation-sensltive detector elements, such a lens must be made com-paratively large, in order that the sensor unit will produce sig-nals such that the evaluation unit can detect significant targets with any appreciable precision.

Such a scanner cannot distinguish between birds (insignificant ob~ects) at a relatively close distance and alr-craft (slgnlficant ob~ects) at a greater dlstance, which means that such blrds could cause a false alarm which is a great disad-vantage ln hitherto known scanners of the klnd described in the lntroductlon.

The present lnventlon provldes a devlce of the klnd mentloned by way of lntroductlon, so as to make it posslble to dlstlngulsh between relatlvely close ob~ects from which radlation i9 emitted, and such ob~ects whlch are located at a greater dis-tance, but which ob~ects, when observed, subtend a slmilar solidangle.

According to the inventlon the evaluatlon unit ls arranged to select ob~ects whlch seem to be of slmllar slze with respect to the angle measured from the posltion of the devlce, large ob~ects being a great distance, such as aircraft and small --"` 1 308 1 79 ob~ects being at a short distance, such as birds by suppressing si~nals whose amplitude as a function of the scanned azimuth or elevation angle shows a relatively great extension compared with signals which show a relatively small extension.

Thus, according to the present invention there is pro-vided a device for the selective detection of ob~ects, such as aircraft, missiles and helicopters, by means of rays from the ob~ects, comprising at least one sensor unit, including at least one focusing means arranged to focus said rays to at least one corresponding focal plane, and at least one radiation~sensitive detector element which is positioned in a focal plane, said detector element being arranged to emit signals corresponding to incomlng radiation, said device being arranged to variably scan an angular field in azimuth and/or elevation, and an evaluation unit, arranged to receive said signals and to select o~ects which appear to be of similar size with respect to the angle mea-sured from the position of the device, large ob~ects belng at a great distance, small ob~ects being at a small distance by sup-pressing slgnals whose amplitude as a function of the scannedazimuth or elevatlon angle shows a relatlvely great extenslon compared with slgnals that show a relatlvely small extension.

In one embodlment of the devlce accordlng to the lnven-tlon the devlce comprlses at least two sensor unlt9, having theiroptlcal entrances for the rays from the ob~ects at a relatlve dl9tance acro9s sald slght llne. In this case each sensor unit functlons ln ltself as a complete unlt. Naturally, a correspond-ing ~ignal processing ln the evaluatlon unit ls requlred, l.e.
the slgnals from the sensor unlt are added before effectlng fur-ther slgnal processlng in the evaluation unlt. It ls sultable to deslgn a sensor unlt such that each optical entrance comprises a deflection means, preferably a mirror, arranged to deflect the incoming rays of the entrance to the correspondlng focuslng means. If one mirror per optical entrance is used as a deflec-tion means, the mirrors used must naturally be so arranged that the radiatlon from each mlrror, usually positioned at an angle of 45 to the incoming rays, can, in fact, reach the focusing means.
That can be done through an arrangement being such that the aper-ture of the optical entrances are so dlsplaced not only in one dimension relative to said sight line, which is a condition for the function of the device, but also ln a second dimension per-pendicular to the former dimension. It is also possible to use mirrors which are partly transparent to the actual radiation. If it is desired to have the optical entrances arranged in two lo groups, one on each side of the focusing means, an extra mirror per group may be arranged to guide the radlation, which ls trans-mitted from the group, to the focusing means. That ls usually a lens, as in the case of IR-radiation made of e.g. ~ermanium, but concave mirrors are also possible, e.g. according to the Cassegraln-system. Other deflection means can be used, such as prisms arranged to totally reflect the lncoming rays. The opti-cal axes of the optical entrances shall be parallel.

The device according to the invention can, under cer-tain circumstances, by applied wlth one slngle detector element,but it mlght be suitable to use a so-called array conslstlng of a number of detector elements arranged ln a row. Such detector elements can al90 be arranged ln a plane, l.e. two-dimenslonally.

The devlce can be deslgned 80 as to be able to scan a small or large solld angle ln azlmuth and elevatlon. The detec-tor elements of the sensor unlt, elther one slngle or several elements arranged elther as a one-dimenslonal or two-dlmenslonal array, call for dlfferent arrangements for wldenlng the vlewed angle ln azlmuth or elevatlon.

In a sultable embodlment of the devlce accordlng to the lnvention the devlce 19 as a whole rotatable about an essentlally vertlcal axls, whereby a plurallty of optlcal entrances are arranged at a dlstance from sald axls, across the slght llne toward an lmaglnary ob~ect. The devlce 19 further movable ln elevation, e.g. step-by-step, so that it e.g. rotates once about in every chosen elevation position. The movement in elevatlon can alternatively be continuous.

The scanning range can be arbitrarily large or small, provided that the scanning can be effected across the ob~ect in at least one direction which lies tolerably well within the plane where the ob~ect and at least two optical entrances are situated.

Naturally, an angle position transducer for azimuth and elevation is required, which emits a position signal to the eval-uation unit for every instantaneous measuring direction.

Instead of rotating the device lt can be made to move e.g. forward and backward.

It ls further possible to let the devlce as a whole be lmmoblle and lnstead let the sensor unlt or, where appropriate, the sensor unlts comprlse an optlcal scannlng means, arranged to varlably scan an optlcal angle, ln addltlon to the angle whlch ls scannable with the correspondlng detector element, in azlmuth and/or elevatlon. One way of achlevlng that ls to arrange sald deflectlon means, e.g. the mlrrors, to be movable. In an extreme case a ~ensor unit can be arranged wlth one slngle detector ele-ment, havlng a scannlng means, such as a mlrror functlonlng as adeflectlon means, whlch mlrror ls movable about two axes perpen-dlcular to each other. Such an arrangement, however, should have a llmlted practlcal appllcablllty, even wlth a correspondlng evaluatlon unlt. In such a devlce, a better performance can be achleved, lf the detector elements are arranged as an area array, l.e. two-dlmenslonally.

In a preferred embodlment of the lnventlon lt comprlses optlcal entrances, arranged ln groups. These groups can sultably comprlse two optlcal entrances each. The dlstance between the optlcal entrances ln a group wlll thus be shorter than the dls-tance between two optical entrances belonging to differentgroups. Through such an arrangement a further suppressing of signals which originate from comparatively small, comparatively close ob~ects is achieved, which will be shown.

The invention will now be described more ln detail with reference to the accompanying drawings, which relate to an exam-ple of one embodiment of the device, l.e. a scanner designed for military use with the aid of IR-technique and in which:-Figure l is a block diagram of the scanner;

Figure 2 is a perspective view of the sensor unit inthe scanner;

Figure 3 shows the optical arrangement of the sensor unit;

Figure 4 illustrates the focusing means and detector elements;
Flgure 5 shows a linear array of detector elements;

Flgure 6 ls a schematic of the scanning method of the devlce;
Figure 7 is a block dlagram of the sensor unlt;

Figure 8 ls a block dlagram of the evaluation unit;

Figure 9 is a diagrammatic plan view of a sensor head;

Figure lO ls a plot of slgnal amplltude vla the four optlcal entrances ln Flgure 9 as a functlon of the azlmuth angle;

Flgure ll ls a plot of a detector output slgnal with ~'', ' . ~
!, ' ~

four and one optical entrance, respectively;

Flgures 12, 13, 14a and 14b are plots of a detector and filter output signal with four and one optical entrances, respectively, with the distances 500 m, 130 m and 10 km, respec-tively; and Figures 15 and 16 a and b show diagrammatically the function of a filter.

In Figure 1, 1 designates a sensor unit, 2 an evalua-tion unit and 3 a display and control unit. IR-rays received from ob~ects within the scanning range are designated by 4, the corresponding slgnals from the sensor unit to the evaluation unit by 5 and the evaluated signals for significant ob~ects by 6, transmitted to the display and control unit 3. The collected output data of this unit is designated by 7, which can take the form of a visible picture on a screen and also signals for e.g.
guiding a fire-control system for anti-alrcraft defence. The connectlon deslgnated by 8 transmlts startlng and stopplng com-mands, etc.

The sensor unlt in Figure 2 comprises a sensor head with apert-- 7a -- - ` 1 308 1 79 ures 10 to Four optical entrances. The sensor head is suspended from an elevation servo 11 For a step-by-step adjustment into different elevation angles by turning about a horizorltal axis.
The elevation servo 11 is rigidly connected with a vertical axis 12 supported in a stand 13. ln tl1e stand there is a motor arranged to drive the axis 12 with a constant number oF revolu-tions per second. In addition tllere is an angle transducer and the slip rings required for signal transmittance throuyl- a cahle 14 to tt1e other units.

The optical arrangements in the sensor head 9 can be seen in in figure 3, where 15 designates a focusing means in the forrn of a lens. In its focal plane a number oF detector elements are arranged into an array 16 with its axis 17 perpendicular to the horizontal principal axis 18 of the lens. Un the other side of the lens 15 four plain rectangular mirrors 19 are ar-ranged, whose symmetry axes 2~, which are parallel to the short sides of the mirrors, intersect the principal axis 18 of the lens and are parallel to the axis 17 Or the detector array 16. The mirrors are parallel relative to eacl1 otller and the mirror plane Form~ an angle of 45 to the plane defir1ecl by the axes 17, 18. The mirrors 19 are ~o clisplaced relatively to each other alony the fiymllletry axis 20, and are oF SUCIl size that partly the efFective total aperture oF the sensor unit i8 principally the same as the area oF the lens, and partly, all the mirrors contribute principally equally to said apertures.
As tl-e optical axis 21 Or tl1e sensor head, is here definecJ
the axis which meets the axes 1~, 20 at right angles by tlle mirror wl1icl1 is the closest to the lens. It is understoc)d that an object being on the optical axis 21 at a long di~tance will be reproduced as a dot at the center Or the cleteclor array 1fi.

In figure 4 the focusing means, i.e. the len9 15 ancl the detector array 16, is drawn with reallstic size relations. The detecl:or array has the length l and the lens the focal lengtll r. In the Figure has been drawn tho rays traced Froln thLee diflerellt small objects being at a long distance and so that they are reproduced in the extreme positions and tlle center respective-ly of the cletector array.

It is understood t~)at the field Or view cle oF the sensor ~Init 1 5 in the shown plane is 'e f In this example o~ = 0.16 radian or 160 milliradians.

In figure 5 the appearance of the detector array is sllown more in detail. In ttliS example tl~ere are 64 similar separate detector 10 elemellts of the size a x b. In tiliS example a = 0.2 mln and b = 0.5 mm. The lengtll oF tlle detector array is tllus 64 x 0.5 mm = 32 mm. As a consequence the focal length of the lell8 i9 200 mm. Each detector element covers a solid angle of 2.5 mrad x 1 mrad.

15 From wltat has been said witll respect to figures 2 to 5 it i9 evident that tne sensor unit can measure simultaneously l:lle incident-ray intensity in 64 direcl;ions 2.5 mrad from each otller withirl a sector uf 160 mrarJ in thr3 vertical plalle whereL y the resolution is 2.5 mrao in elevation. By rOtatillg tlle sensor 20 head about the vertical axi~ 12 in figure 2 measuring can be effecte(J for azimlJth angles througl10ut tlle wllole revolutiorl with an angular resolution of 1.0 mra(J. Between differellt revolutiolls the elevation of the sensor head is altered with tlle aid of the elevation servo 11. A cornlete scanning can e.g. com~rise 25 three revolutions i.e. a scanlling of a range oF elevation of 54û mrad or about 31. 5uch a scanning cycle i3 sllowll in figur 6.

A block diagram of the sensor unit i5 ShOWIl in figure 7. Ilere the sensor llead is designated by 9 wberca~ an azimuth motor 30 is designated by 22 an eLevation servo ly 11 an azilllutl1 1119 le transducer by 23, the mirrors by 19, the lens ty 15, the detector array by 16, amplifier by 24, a multiplexer by 25 and an A/D-converter by 26. Ihe sensor UI1it emits output signals 27, 2B.

The signal 27, then, is a digital signaI in series rorm whicIl in a certain sequence and with a certain scale factor designates the incident-ray intensity measured via the respective detector elements~ e signal 28 shows the azimutl1 direction Or the optical axis Or the sensor head. The elevation servo 11 i~
controlled by a signal 29. During a complete scanIlillg cycle of three revolutions the signals 27, 28 and 29 describe the measured radiation intel;sity as a function of direction in the whole scanned solid angle range, whicII, as discussed above, in this case covers 540 mrad in elevation and the whole revolu-tion in azimuth.

rhe measuring values produced by the sensor unit to the evaluation unit during a scanniIlg cycle, can mathematically be said to describe a matrix, here designated by A, wl1ose elemeIlI:s a desigr1ate the measured incident radiation intensity in the direction .. ..
azimuth = i x 0.5 milliradians elevation - j x 2.5 milliradians relative to a chosen reFerence direction. Below, said matrix will be referred to in connection with an exernplification of the filLer functioning.

Z5 The signal from each separate detector element i5 read with a spacing in azimuth whjcI) is simiLar to halF Or the angle width of the detector element, i.e. 0.5 mrad. Totally, a large nuIlIber of measured values will thus be emitted froIll detector elements in the form of digital signals to the evaluation UI1it, wlIere they are stored in a memory, and can be visualized on e.tJ. a cathode-ray tube in such a way that the picture sIIows a plane picture of the scene wl1ich is covered by thc scanIled raIl(Je. In ` 1 308 1 79 the picture the lumirlous iotellsity in a certair~ point i5 a measure of the measured incicJel1t I~-radiation interlsity in the measuring direction corresponding to the direction of the poil1t.

The evaluation unit 2 is drawn in rigure ~ in the form of a block diagrarn. In it a nnemory 30 is included, which in this connection is called image memory, a filter 31, a threshol~
calculator 32, a comparing means 33 antJ a target memory 34.
Via the sensor unit the digital signals from the tJetector ele-mer~ts are available. ln the imaye memory 30, dirrereot coml~ a-tions of digital sigr-als from tl1e detector elemerlts are slurc(l temporarily, according to a certain secluence, thus representing tJifferer1t parts oF the scanrled angular range, in such a way, that during a scannir1g cycle, the signals from all the parts oF the scanning range can be processed by the filter 31, which calculates, in a manner, in itself known, tl1e diFference between the signal intensity in a chosen direction and the signal intens-ity in the area surrourlding it, area by area of the scanlled range.

To doterrnine wbetl1er a significant object ha~ heen mrasure~l or not, a decision function is now used, cornprising the thresl1old calculator 32 and tl1e comparirlg mearls 33, wllicl~ coml)arillg means alr30 lla3 a direct connoctiorl witl~ tl~e filtor 31.

rhe function will be showrl by a couple of application examl)les.

In figure 9 are shown vertically from abovo the mirrors 1, the lens 15, tlle detector elemer1t 16, an object 36, the optical axes ùF the optical entrar1ces 37-40 and rays from the objt?ct3 to the optical entrances 41-44. In the figule a scar-lner i5 showl1 in the scarlr1ir1g position B = o, i.e. azimuLII po,itior-l 0.
Tht? 8CalU~irl9 i5 effected by Ineall~ of rotation in clockwi;e direction, i.e. according to tlle arrow 45. Ihe dic)lllctor uf 3U tlle len3 i8 Dopt, and the diStarlCe betWeerl tlle Illi.t'l'Ol'S d12, d~, d34. Tlle angular distance of the object florrl the rospoclivt?
~ 2~ ~3 antJ ~4 and tht? distanct-? to l:llo objt?ct --- ` 1 308 1 7q is ~. The extension Or the oi~ject is D bj.

As an example oF measured test results, some diagrams are given below whicl- show the function of the device.

The device has in this example the Following dimensiorls:

D = 0 20 m opt d12 = 0.25 m d23 = 0 50 m d34 = 0.25 m B1 = 2.00 mrad 10 If r~ is varied from 90 rn to 10 km a nulllber of diagrams will be obtained, where the vertical axis in all the cases is related to the signal amplitude.

As iB evident From the choice of d12, d23 and d34, t~1e mirrors in tl1is example are arranged in two pairs witl1 regard to to the distance. rhe distance withir1 each pair is 0.25 m. The dir~tarlce between the pairs i8 0.50 m, mear,uretJ as tl1e dlstance betwet3n the mitJdle two Or the four. rhe aim i~ to prove the efrect Or the mirrors beintJ positit)netl in this particolar way.

When choosilly tlle parameter values, as well as tlle exallll)les oF embodiments oF the scanller oti-erwise, the aim was not to de-scribe an optinnal solutiorl but merely to give olle solution to illur~trate the invention.

The Following examples, figure 10-14, relate to objects which, seen from the device, i.e. the optical entrallces of thc sensor unit, snow a similar solid angle extension and similar ratJiation interlsity, i.e. bird~ at a compalatively shorter distal1ce and aircraft at a lorlger distance.

In Figure 10 is shown, to tegin witl1, For a 9U-m tJistal1te, tlle Four optical signals 1-4 WhiCIl reacl) tl1e detector via lhc four 1 3n81 79 .

mirrors. The n~lmbering of the signaIs in rigure 10 corresponds to the numbering of the mirrors in figure ~. It is to be ob-served that the output signaI of the detector is equal to the sum oF those Four optical signals. The output signal o~ tl~e de-tector is shown in figure 11 a. In Figure 11 b the deLector sig-nal is shown il~ tl1e case oF one single optical entrance. rigure 11 relates to the distance 90 m, as does figure 1~.

Ihe Functioning of the Filter 31 will be described with reFerence to figure 15. The Figure shows a section oF the matrix ~ where each elernent in the matrix is represented by a s~uare checl<.
In the check pattern two areas have been indicated, designated core and frame, respectively. The core comprises three matrix elemer1ts, the element aij being in the middle. The frame com-prises the matrix elements whicll adjoin the core and surround it. The filter function is to calculate the difference between the highest elemel1t value in the core and the highest elemer1t value in tl1e frame. Tl1e calculation is done for every possible position (i, j~ in the matrix A. The result is a new matrix H witl1 the elemellts bij, which i5 the output signal of the filter.

rhe function of tlle filter 31 is ilIustrated in riyurc 16 a and 1G b. Ilore, the nulllorical values of the elomellt~ in tlle matrix A have been written. In fi~Jure 1G a the higllost vaLur in tho core i8 3 and the hiyl1est value in the Fraale is 3, and B0 the output signal of the rilter is zero. In figure 16 b the highest value in the core is 3 and in the frame 1 and the out-put signal i8 then 2. That 9how5 that the rilter tends to en-hance, i.e. give a big output signal for objects whicll in the matrix A nave an extensioll whicll is smaller thall or similar to the size of the filter core, wherea~ larger object~ are sul)-pre~sed, i.e. give a small output slgnal frorll the filter.

It could be pointed out, that For the preferred ful1cLioll it would be enougb to use a filter whicll weighs togothcr only -" ~308~7~

tlle signals from tlle p~ane wllicll contains the object arld tlle optical entrances. In tlle sllown example tlle Filter would be illustratecl by row No. i.

In Fiyure 15 the size Or the filter core has been give~rl, il~
azimuth 1.5 mrad arld in elevation 2.5 mrad, whicl1 values follow from earlier given sampling intervals 0.5 mrad arld 2.5 mrad in the respective directions. It can be pointed out here, that an aircraFt at a distance Or 10 km, seen frolll lhe scanller, sub-tends an angle of generally less than 1.U mrad, and therefore tlle aircraFt is well withirl the Filter core and will thererore be enhanced by the filter.

It is evident from Figure 11 that For the dislance 90 m, the amplitude Or the detector signal will be considerably smaller with four entrances than with one single entrance. The radiation energy, received by the detector, is the same in both cases, but in the case of Four entrances the energy is received withirl a larger angular range, i.e. at a ccrtain scanrlirlg rate, clurillg a prolonged time and thererore at a lower level of power.
The output signal oF the detector or, more exactly, the output 2rJ voltage in volts, is in every molrlerlt proportional to the incidel-lt power in watts. The received energy is represerlted in figure 11 by the area below the respective curves.

It can also be said, that figure 11 illustrates a situation where tl1e use of four parallel and laterally separated opticaL
entrances results in the suppression of the sigr-lal Frolrl an object at such a distance from the scanner tl1at the rays from the object reach the entrances of the scanl1er divergclltly.
By divergcnt rayr~ it is theroby urlderMtood rays wiLh a rolal:ive difference of anyle wl1icll is the same or larger thallllot l:oo small a fraction of the anyular resolution, which in this case is 1 rnrad.

In figure 12 are showr1, for the distallce 5BB m~ partly tho dctector sigr1al 1, partly the output sit~llal 2. ri9 12 a rerers -- 13o8l7~

to tl~e case Or rour en~rallces an(l rigure 12 b rerers to ooe singel eotrallce. ~ig 2 i~lustrates the runctioll oF tl~e com~ a-tion oF the rour optical entrances on the one hand and oF
tlle filter on the other l~and. From the rigure it is evi~ent that the filter reduces tlle sigllal level corlsiderably more in tlle case of Four entrallces than in tlle case oF one single entrance. The reason is tllat tbe four laterally separate~ ent-rances give a widening oF the signal pulse and tllat the filter is so arranged as to give a lower outl)ut signal whcn ll~e Flller inp-lt signal is wider.

In Fiyure 13 are slwwn, For tlle distallce 1~0 m, tlle siynals cor-responding to those in figure 12. Ilere, ~wo signal pulses are obtained, in the case of Four optical entrances, one for eacl palr oF mirrors. Figure 13 illustrates tlle functioll oF arrangillg the mirIols~ with respect to distance, in two pairs, in ~lle way described above. By arrangillg the mirrors in this way at this distance two wide pulses are obtained wllicll the rilter tends to suppress.

In Pigure 14 are shown, For tlle distance 10 km, tlle correspon-dir~g signals, as in rigure 12. Froln figuro 14 it is eviderlt that the output signal froln tlle Filter is almost a~ big with four entrances as with a sinyle one. Ihe reasoll is tllat the di~tance iu 80 groat rolative to the lengtll withill wllicll tlle Inirrors are positiolled, that the rays From the objects reacll Z5 the scanner parallel, principally. Targets can tllus be detecl;ed at long distances as effectively with four laterally separated entrances a~ with one single entrallce.

The optical entrances of the sensor unlt or units may be positioned vertically in a row.

3~

Even if digital tecllllique is advantageous, it is also conceivable that the signals need not be sampled, nor need tlley have a digital form. The Filter can be designed in diFferent ways. It can e.g. be an analogue filter and functioll by time instead oF
by angle.

The transfer function of tlle filter can be other than that described in the example oF enlbodimellt, sn long as it is ar-ranged to relatively enharlce signals From tl~e objects or parts oF objects which, when observed from the position oF
the sensor and in the spectral range oF the sensor, show an angular extension which is less than a cl-osen value, but sup-press or, to a smaller degree, enhance signals from objects which show a yreater extension tl-an that chosen.

The transFer function of tlle filter need not be fixed, but can be variable For adjustment, e.g. accorcling to the actual con-ditions in relation to tlle objects.

Tlle spectral range can be otller than the IR-range, e.c~. tile UV-range, tlle visible range or the mm-wave range.

Claims (17)

1. A device for the selective detection of objects, such as aircraft, missiles and helicopters, by means of rays from the objects, comprising at least one sensor unit, including at least one focusing means arranged to focus said rays to at least one corresponding focal plane, and at least one radiation-sensi-tive detector element which is positioned in a focal plane, said detector element being arranged to emit signals corresponding to incoming radiation, said device being arranged to variably scan an angular field in azimuth and/or elevation, and an evaluation unit, arranged to receive said signals and to select objects which appear to be of similar size with respect to the angle mea-sured from the position of the device, large objects being at a great distance, small objects being at a small distance by sup-pressing signals whose amplitude as a function of the scanned azimuth or elevation angle shows a relatively great extension compared with signals that show a relatively small extension.

2. A device according to claim 1, comprising at least two optical entrances arranged at a relative distance across a sight line from the device to the object.

3. A device according to claim 2, including at least two sensor units having their optical entrances for the rays from the objects at a relative distance across said sight line.

4. A device according to claim 1, 2 or 3, in which each optical entrance comprises one deflection means.

5. A device according to claim 1, 2 or 3, in which each optical entrance comprises a mirror arranged to deflect the incoming rays of the entrance to the corresponding focusing means.

6. A device according to claim 1, 2 or 3, in which a sensor unit comprises a plurality of detector elements arranged in one dimension.

7. A device according to claim 1, 2 or 3, in which a sensor unit comprises a plurality of detector elements arranged in two dimensions.

8. A device according to claim 1, 2 or 3, is adapted for scanning in azimuth in addition to the optical entrances' own field of view by being movably arranged.

9. A device according to claim 1, 2 or 3, is adapted for scanning in azimuth in addition to the optical entrances' own field of view by being rotatable.

10. A device according to claim 1, 2 or 3, is adapted for scanning in elevation by being movably arranged.

11. A device according to claim 1, 2 or 3, adapted for scanning in elevation by being stepwise movably arranged.

12. A device according to claim 1, in which a sensor unit comprises at least one optical scanning means, arranged to scan variably an angular range in addition to the field of view which is scannable by the corresponding detector elements in azimuth and/or elevation.

13. A device according to claim 12, in which said scan-ning means consists of a movably arranged deflection means.

14. A device according to claim 1, in which at least one sensor unit comprises one single focusing means.

15. A device according to claim 14, which comprises one single focusing means.

16. A device according to claim 1, which comprises optical entrances arranged in groups.

17. A device according to claim 16, in which the groups each comprise two optical entrances.