GB2257802A

GB2257802A - A two-dimensional optical-mechanical scanning system having a plurality of analysis fields

Info

Publication number: GB2257802A
Application number: GB8126038A
Authority: GB
Inventors: Fernand Rene Loy
Original assignee: Telecommunications Radioelectriques et Telephoniques SA TRT
Current assignee: Telecommunications Radioelectriques et Telephoniques SA TRT
Priority date: 1980-10-27
Filing date: 1981-08-26
Publication date: 1993-01-20
Anticipated expiration: 2001-08-26
Also published as: FR2681749A1; DE3142186C1; GB2257802B

Abstract

This system uses for two-dimensional scanning an objective 41, a plane reflector 42 rocking about an axis 43 for the frame scanning, a field mirror 44, rotary drums 46 and 47 with reflecting surfaces for the line scanning, and a radiation-sensitive detector 14, the image of which scans the field image in the focal plane of the objective. The system is designed to scan at least two fields, a normal field and a reduced field contained one inside the other, the reduced field corresponding to increased performance in respect of object identification, changeover from one field to the other being simply by replacement of the drum 45 by the drum 47 and variations in the reflecting mirror rocking amplitude, without any change of the focal length of the objective and the aperture angle of the beam on the image of the detector in the focal plane of the objective, said image being smaller in the case of the reduced field than in the case of the normal field. <IMAGE>

Description

A TWO-DIiENSIONAL OPTICAL-MECHANICAL SCANNING SYSTEM K,nVING A PLURALITY OF ANALYSIS FIELDS This invention relates to a two-dimensional opticalmechanical scanning system, more particularly for the infrared wavelengths band, of the kind which for two-dimensional scanning uses an objective, a plane reflector rockable about an axis parallel to its reflecting surface in order to scan in a first direction, a rotary drum provided with a plurality of reflecting surfaces for scanning in a direction perpendicular to the first direction, and a radiation-sensitive detector, said system being designed to scan or sweep a plurality of fields of different dimensions contained one in the other or pothers.

The existence of several fields has the advantage that the system can be used for the total examination of a scene in order to discover therein an object which then undergoes specific examination. Total examination uses a wide-angle field scan while the specific examination of the object uses a narrow-field scan.

The performance of this type of system is analysed in terms of detection performance and reconnaissance and identification performance respectively. The term detection which is carried out in the wide-angle field - denotes indication of the presence of an object in the field, the term "reconnaissance" of this object which is carried out in the narrow field indicates that it is possible to determine that this object belongs to a specific category, while "identification" - which also takes place in the narrow field means that this object has a distinctive sign in that category. For example, once detected the object will be recognised as belonging to the category of tanks and identified as being a certain type whether friend or enemy.

It is in particular the reconnaissance and identification performance of these systems that the invention proposes to improve.

The various performances of these systems are expressed in terms of detection, reconnaissance and identification at distances respectivelyA'hich the detection, reconnaissance and identification take place. These performances depend upon the angular resolution of the system, which is itself a function of (a) its signal-to-noise ratio, hereinafter referred to as its "sensitivity" and (b) the elementary analysis field angle which is the field angle subtended by the detector in the focal plane of the objective.

Of course, in the case of narrow-field scanning, good reconnaissance and identification performance requires a narrower elementary analysis field than in the case of wideangle field detection and it is generally preferable, in the case of the narrow field, that the sensitivity of the system should be at least as great as in the case of wide-angle detection.

The signal-to-noise ratio of the system and the elementary analysis angle being linked quantities, there are therefore a number of difficulties in embodying these systems which operate in a plurality of fields.

French Patent Application No. 78 06 503 filed 13 June 1979 by Messrs. N.V. PHILIPS GLOEILAMPENE < BRIEKEN describes a system of this kind. With a double analysis system it is possible to analyse two rectangular fields of different dimensions in the focal plane of a single objective, the two analyses being carried out with different angular resolutions.

The dimensions of each field are fixed (a) by the value of the rocking angle of a plane reflector for scanning in a first direction, the frame direction, and (b) by the scanning length of the representation of the detector in the focal plane of the objective in the line direction perpendicular to the first direction, which depends on the periphery of the drum and the number of facets. The system uses the same entrance pupil diameter in both fields and also the representation of the detector and hence also the elementary analysis field angle on the analysed line, is larger in the case of the wide-angle than in the narrow-angle field, so that the angular aperture of the analysis system on the detector and hence the signalto-noise ratio of the system is greater in the case of the wide-angle than in the case of the narrow-angle field.A syster of this kind can therefore have good detection performance in the case of the wide-angle field but in the light of the foregoing remarks there is the risk that the reconnaissance and identification performances in the case of the narrow-angle field will be very limited.

French Patent Application No. 78 31 352 filed on 6th November 1978 by Applicants also relates to a two-field analysis system of the kind according to the invention. There is a single analyser system with an oscillating mirror and a rotary drum while the objective is a bifocal, the long focal length being used for narrow-angle examination and the short focal length for wide-angle examination. The entrance pupil is smaller in the case of the wide-angle field than in the case of the narrow-angle field, while the reverse applies to the elementary analysis fields.The representation of the detector on the analysed line is the same in the case of the wide-angle field as in the narrow-angle field and the same applies to the angular aperture of the analysis system on the detector, so that the signal-to-noise ratio remains the same from one field to the other, so that the system effectively carries out its reconnaissance and identification functions in the narrow field. Typically, with this two-field system comprising a single analysis system associated with a twin focal-length objective, the detection distance is d in the case of the wide-angle field of angular value a , while in the case of the narrow field of angular value of 0.3 a the reconnaissance and identification distances are respectively 0.3d and 0.2d, and hence much less than the detection distance.

The invention proposes substantially to double these reconnaissance and identification distances by improving the angular resolution of the analysis system without changing the bifocal objective of the camera. To this end, the invention creates a third analysis field restricted to the centre of the narrow-angle field. Typically, this third field, hereinafter referred to as the "reduced" field as compared with the narrow-angle or "normal" field, is of the order of O.la.

analysis of the reduced field makes use of an elementary anslysis field which is smaller than in the case of the normal field. The diameter of the entrance pupil remains the same in both the reduced and normal fields as in the said French Patent Application 1To. 78 06 503, so that reduction of the elementary field also introduces a fall in the signal-to-noise ratio. according to the invention, this fall is partially compensated by highly redundant scanning of the field so as effectively to improve the reconnaissance and identification performances of the system. The display of this reduced field on a television standard TV monitor may be accompanied by an electronic zoom effect which further assists identification of the target. The analyser is a double analyser as in the case of the system according to the said Patent Application No.

78 06 503 but is embodied by very different and much simpler means, the replacement of the means by one another being effected simply by translatory movements without the aperture angle o the beam on the detector image in the focal plane of the objective being modified from one field to another.

The invention will be more readily understood from the following description which is given by way of example with reference to one embodiment of the system according to the invention with a plurality o variants, the description being accompanied by theoretical considerations and drawinas wherein: Figures lea, 13 and 1C illustrate the problem solved by the invention.

Figure 2 shows the ~unction of the reflecting facet rotary drum in different scanning cases.

Figure 3 illustrates curves snowing the radius of curvature and analysed line et against istance C of the detector image from the axis of rotation of the drum.

Figure 4 is a section of one embodiment of the system according to the invention through its plane of symmetry.

Figure 5 is a projection of the system according to Figure 4 parallel to the axis of rotation of the rotary drum restricted to the part corresponding to its normal-field operation.

Figure 6 is a similar view to Figure 5 restricted to the part corresponding to reduced-field operation.

Figure 7 is a section of the same system provided with a LED display system, the section being through the plane of sy try.

Figure 8 is a graph showing the amplitude of normalfield frame scanning and narrow-field frame scanning against time accordion to a first variant.

Figure 9 shows the same scanning according to a third variant.

Figures lA, 1B, and 1C together illustrate the problem that the invention solves. Each of these Figures is a diagram showing in the line direction a multi-field analyser according to the invention, operating with one of its fields.

Figures 1A and 1B show the double-field system produced by means of a bifocal objective and a single analyser according to French Patent Application No. 78 31 352. In Figure lA, the objective 11 of axis 12 of the system operates with a short focal length F1 and a wide-angle field. The line analysed is the line 13 of length 11 situated in the focal plane of the objective. This line is analysed by means of the linear image 15, of length d', parallel to line 13 of detector 14 having the length d, formed by the line analyser shown diagrammatically by the convergent lens 16 and operating with the magnification z 1 In actual fact, as will be seen hereinafter, this analyser includes a rotating drum, the periphery of which is provided with reflecting facets.The field of the system is al while the elementary analysis field is 6 , both being shown in Figure 1A. The aperture angle of the optical system on the detector is U'1 for an objective angle U1. The magnification of the analyser is Y U the latter being such that the system has a sensitivity giving good detection performance.

In Figure 1B, the objective 11 of the system operates with a long focal length F2 and a narrow-angle field (hereinafter referred to as- the normal field). The analysed line situated in the focal plane of the objective still has the same length 11. It is analysed as in the wide-angle field by means of the image 15 of the same length d' of the detector 14 of length d, the analyser remaining the same in both the wide and narrow fields.The total field of the system and the elementary analysis field are respectively α2 and 6a -2 which are respectively less than a1 and #α1 The entrance pupil of the objective is such that the objective aperture angle is as in the case of the wide-angle field U1 while the aperture on the detector is U'1, the analyser being unchanged.

Consequently, the sensitivity of the system is the s=e as in the case of the wide-angle field but on the other hand the angular resolution is better since #α 2 < #α1.

Figure 1C shows the purpose of the invention. The objective 11 still operates at a long focal length F2 with an aperture angle again U1, but only the central part a3 of the field a2 is analysed, the corresponding analysed line length being 12 < 11. The invention proposes to analyse this field, which will be termed the reduced field, with a better angular resolution corresponding to an elementary analysis field 6a a2. For this purpose, it is proposed to reduce the 3 2 length of the image 15 of the detector 14 of length d given by the analyser, so that it becomes d'3d', the analyser being modified to give it a magnification γ3 < γ1. The numerical aperture of the optical system on the detector becomes U'3 U'1 so that the sensitivity of the system is reduced.The inventioencounters the same difficulties as in the aforesaid Application No. 78 06 503. The invention modifies the analyser design so as to compensate for the narrow-angle field a3 fall-off in sensitivity introduced by reduction of the analysis field and so as effectively to benefit from the required angular resolution increase. The invention is based on the theoretical considerations set forth hereinafter. The sensitivities of an analysis system of the kind according to the invention are conventionally classified by the following to quantities: NETDnminimum detectable difference equivalent to the noise (measurable on the video signal) NETpnminimum visually perceptible temperature difference equivalent to the noise.

The expressions of these quantities are of the following form:

where the symbols have the following meanings: Cv, C: dimensions of the detector (V vertical, H horizontal, or alternatively frame and line); Fi: image frequency; N : number of points analysed per line, p N1: number of lines in the image; a : line overlap rate; P V' H: vertical and horizontal sweep efficiency, a Vw oH: elementary analysis field; i : diameter of entrance pupil; T : optical system transmission factor, M*: detector detectivity factor; number of elements in detector; eye integration time.

In order that the NETD and NETp may be compared more easily in the systems operating with the fields corresponding to the figures shown in 1B and 1C respectively (these fields will frequently be referred to hereinafter as the normal and reduced fields), the expressions of these quantities are hereinafter written by combining in a single factor k all the quantities which do not vary between the two system configurations.

A f representing the system pass-band.

These expressions show tht reduction of the elementary analysis field of angular dimensions aVt a H for finer analysis of the central part of the field a3, without any other modification, particularly of the analyser, results in an increasedNvTD and NETp, which is equivalent to a reduction in sensitivity.

The invention proposes various methods of at least partially compensating for this loss of sensitivity on reduction of the elementary analysis field, by modifying the analyser and sweep characteristics.

A first method is to reduce the pass-band d f so that ETD and NETp remain constant on changeover -rom field 5 to a The confrontation of the two expressions(1) and (2) 3.

shows that this band reduction must be carried out by reducing the total number of image points N1 x Np and/or by increasing the sweep efficiency #v #H or by reducing the line overlap rate # . The disadvantage of reducing the pass-band A f is that the signal electronic processing networks require modification. More particularly, if the detector is in the form of a mosaic of elements used for series or series-parallel line scanning, the delay lines required to restore the phases of the signals from the various series elements require replacement on operation in the central field a3. Despite this disadvantage, this is one of the options claimed by the invention.

second method is to modify the sweep characteristics, the pass-band being maintained. The numerator of the expression (1) then has a constant value. NETD necessarily increases when the elementary analysis field a a H is reduced.

In order to maintain good reconnaissance and identification sensitivity, the invention provides for > Tp to increase less rapidly than NETS. To this end, all that is required is for numerator of expression (2) to be minimal when the numerator of expression (1) has a constant value, A f itself being constant.

To this end, the invention reduces the number of image points N1 x Np, by increasing the sweep ef~iclencyp V /Zt increasing the image frequency Fi and increasing the line overlap rate According to the invention, these theoretical considerations are applied, the line analyser being of the prismatic rotary drum type with reflecting surfaces, as described in the aforesaid Patent Application No. 78 31 352.

According to this prior art, the rotary drum is disposed in the convergent beam of a detector image transfer system at a point A situated on or outside the drum axis. Each line of the field in the focal plane of the objective is analysed during the drum rotation by the image of said point A on one of the surfaces of the drum.

Figure 2 illustrates various sweep cases Q in the plane perpendicular to the drum axis through the point A. The distance d of point-A from the drum axis is a parameter. In the drawing is shown as a polygon 21, its axis is represented by point 0, the analysed line by refennce 13, the latter being analysed by the image A' of A in the drum surface 22, the drum rotating in the direction of arrow 23.

The development of the radius of curvature r of the analysed line 13 and of the length 1 of the analysed arc for a drum rotation a = 2 150 appears from GA to GF The distances d and D are shown in two columns on the right of the drawings, distance D representing the distance between 0 and the apex on the straight line oA of the scanned line in the different sweep cases, these distances being expressed as a function of the circle inscribed in the polygon 21.The analysed line is a Pascal spiral, with the following cartesian co-ordinates in the system of co-ordinates XOY indicated at B , as a function of the drum rotation angle : 2 X ( a) = d - 2 (d cos a + R cos a Y ( a) = 2 (d sin cos a + R sin a ) By means of the curves M and N Figure 3 gives the values respectively for r and ss/2 as a function of d, unity being the radius R of the inscribed circle in the polygon 21 of the drum, calculated by the following expressions::

in which the symbols such as X' and X" denote the first and second derivatives respectively of X with respect to a In the expressions of X(a ) and Y(a) , d is counted positively when the direction of 0 towards A corresponds to the positive direction of the axis OX, while r is counted positive or negative when the analysed axis turns its convexity or concavity respectively towards 0.

The top arm of the curve giving the value of r indicates (a) that r passes through a minum for d = 0, signifying that when d # 0 there are always two values of d -R of opposite sign between and +where we have the same 7 radius r of the analysis line and (b) different lengths / of the analysis line correspond to these two values of d. This property thus makes it possible to analyse a larger or smaller length of the focal surface of radius r of an objective simply by moving a single rotary drum perpendicularly to its axis of rotation so as to operate for each of these two values of d.

If the values +R and 3 are selected for example for d, the 3 ratio of the corresponding analysis line lengths is 3.

This gives a means of varying the number N of points in the p line in a ratio of 3. A ratio of more than 3 could obviously be obtained with other values of d. This same property can be used by varying d continuously between the two values of the pair selected. A kind of zoom effect can thus be obtained by continuously varying the analysis line length, for the radius r varies little in the region of the minimum of the top arm of the curve. It is thus possible to obtain a multi-field analysis system using a plurality of different analysis drums of radius R to obtain the same radius r of the analysis line. All that is required for this purpose is to select d by reference to the curve shown in Figure 3. This is therefore an additional parameter for varying the ratio of the lengths of the analysis lines. It should also be noted that it is not essential to use the entire length of the analysed line, but only its central part. When different drums are used it is also possible to select the number of reflecting surfaces on each drum to give the best sweep efficiency as a function of the analysis beam aperture and the analysed line length.

The drums may be coaxial and rotate at the same speed, or else they may be independent.

One embodiment employing these results is given by way of example in Figures 4 - 6. The system is constructed in principle as in the aforesaid French Patent Application No.

78 31 352 comprising a single analyser, but in this case the analyser is a double analyser, its two parts being used in normal and reduced fields a2 and a3 respectively of Figures 1B and 1C.

In Figure 4, the optical diagram of the system is shown in section through its plane of symmetry. An objective 41, e.g. for infra-red, possibly with two focal lengths corresponding to the fields a 1 and a2 of Figures 1R and 1C is in this case assumed to be operating with its long focal length and the normal field a2. The planeframe mirror 42 is movable about axis 4s perpendicular to the plane of the drawing. The field mirror a4 is movable about the axis 45 perpendicular to the plane of the drawing. There are two rotary drums with the same axis of rotation 48 and movable along this axis.One of these drums, for example, has six surfaces and is used in the normal field and is shown in two positions 46 and 46', the first being the one corresponding to its operative role. The other drum, for example with 24 surfaces, is used for the narrow field and is shown in two positions 47 and 47', the latter position being that of the operative role. In keeping with the provision of the two drums1 the system comprises two image transfer systems for the detector 14. One is used in the normal field and comprises the convergent optical element 55, the deflecting mirror 53, the convergent optical element 51, the image of detector 14 being formed at 9, The other, which is used in the reduced field, similarly comprises the convergent element 55, the deflecting mirror 54, the convergent element 52, with the image of the detector 14 forming at 50.

Figure 5 is a projection of Figure 4 parallel to the axis 48 of the drums, restricted to an illustration of the drum 46, the field mirror 44, the back surface of which is reflecting, the field mirror axis 45 of rotation and the analysis line 13 in the normal field, the ends of which are 57 and 58. Figure 6 is also a projection of Figure 4, with the system operating in a reduced field. The ends of the analysed line 13 are denoted by references 59 and 60.

The properties of the system and its operation are analysed below with respect to a number of its variants given by way of example.

The change from the normal field to the reduced field is produced by the following three movements: a) The two drums are moved from positions 46, 47 to positions 46', 47' along their common axis 48; b) The system comprising the convergent elements 51 and 52 and mirrors 53 and 54 is moved perpendicularly to the plane of Figure 4 so as to bring either the optical axis 39 of 51 or the optical axis 38 of 52 into the plane of Figure 4; c) The amplitude of rotation of the frame and field mirrors 42 and 44 respectively about the axes 43 and 45 is modified.

The position of the image 9 of the detector and the radius of the drum used for the normal field (in position 46), and the position of the image 50 and the radius of the drum used for the reduced field (in position 47') are so selected tat the analysis lines 57 - 58 and 59 - 60 exactly coincide, change of the analysis field introducing no defocussing.

The ratio of the focal lengths of 51 and 52, and the radii of the drums 46 and 47 are so selected that the image 50 of the detector is smaller than the image 49 so that the speed of movement c the image of the passage into the plane of the detector 14 is the same in both the normal and reduced fields.

The number of surfaces of the drums 46 and 47 is so selected that the analysis line 59 - 60 is shorter and contains fewer analysis points than the analysis line 57 - 58 and the analysis efficiency is as high as possible while retaining the same convergent beam aperture 37 on to the image 49 or 50 of the detector. It is also so selected as to analyse a number of images per second compatible with the image display television standard by suitable electronic processing (described hereinafter).

The field mirror 44 is preferably a catadioptric mirror, the rear surface 44' of which is reflecting. As in the aforesaid Patent Application No. 78 31 352, this field mirror combines (a) the exit pupil of the objective 41 with the entrance pupil 61 of the analyser and (b) the focal surface of objective 41 with the analysed line 57 - 58.

Pupip 61 is virtual. In the case of the normal field it is substantially situated on the surface of drum 46. It thus defines a fixed beam issuing from the fixed point 49 towards the lens 51 and then towards the detector 14. The beam 37 falling on the detector 14 is thus fixed. The field of view of the detector is minimal and the detectivity of the detector is optimalised. In the case of the reduced field, the entrance pupil of the analyser is again 61. It is still virtual and is indicated in Figure 6. The end rays of the beam 62 and 63 leaving the end 59 of the analysis line 13, after reflection from the field mirror 44, impinge on the pupil 61 since the field mirror is unchanged. After reflection on surface 24 of drum 47, these rays appear to come from the point 50, which is the image of the detector 14.

It will be seen that when the drum rotates, the beam 65, 66 is movable inside the beam 65, 67. By suitable selection of various design parameters, the field on the detector image can be made to be the same in both configurations. In the case of the normal field, the convergent beam on the detector is fixed while in the case of the reduced field this beam is movable but still included in the previous beam.

The amplitude of the frame analysis mirror 42 is reduced in the ratio of the length of the analysis lines 57 and 59 - 60 so as to retain the same image format on a change from normal field to reduced field. The field mirror 44 undergoes the same amplitude reduction as the mirror 42.

These two mirrors are mechanically linked for example. The rotary movement of the field mirror is intended conventionally to retain the combination of the objective exit pupil with the entrance pupil 61 of the analyser in the frame direction, for the movement of the frame mirror 42 makes the image of the exit pupil of the objective 1 seen by reflection from the mirror 42 movable.

The number of analysis lines swept per second is higher in the case of the reduced field than in the normal field Each surface of the drums 46 or 47 sweeping one analysis line, the number of lines scanned is proportional to the number of drum surfaces when the drums rotate at the same speed.

Obviously it is practical to leave the drum speed unchanged when the field is changed. It is also practical to have a common axis of rotation for both drums. Because of the choice of the number of surfaces of the drums 46 and 47, the number of points per line is less in the case of the reduced field than in the normal field. Consequently, in the case of the reduced field the image must contain fewer analysis lines if the ratio of the number of lines to the number of points per line is to be retainec -o the same image format.This means that in the case of the reduced field, if the drums 46 and 47 have the same speeds, a larger number of images per second must be analysed, which will no longer be analysed at the television standard. 10 retain the television standard for the display, the invention proposes to store the different analysed images in a single image memory In which all the images analysed are sw-imated point by point during the time of one television frame, and then re-read this memory at the television standard. The invention also proposes using a muiti-standard televisic-. mQr.içor, thE scanning characteristics of which are linked to those of the analysis in each field.

In this case, summation of the information corresponding to each point is effected by the afterglow of the TV tube and the observers eye. It is advantageous to display the reduced image with the same dimensions as the normal image, for the transfer function modulation of the TV monitor comes within the range of a heat camera. The effect of the transfer function modulation is reduced because of the magnification of the image on the monitor. Display of the reduced image on the TV monitor with the same dimensions as the normal image necessitates displaying each point and each line several times. The invention proposes a number of variants for storage in the image memory.

A first variant is described with reference to Figure 8. The amplitude &commat; e of of the frame mirror sweep is plotted along the y axis of a system of rectangular axes, against the time which is plotted on the x axis. In the case of the normal field, this amplitude varies to a train of linear sawteeth as shown at 81. In the case of the reduced field, the frame analysis frequency is increased and the amplitude reduced, the latter varying in accordance with the linear sawtooth train as shown at 82, the duration of which is less than 81. The increase in the frequency and the amplitude reduction are so selected that the ratio of the number of lines to the number of points per line is the same in both the reduced and the normal fields.On display of the reduced field the same line is displayed several times in order to have the same number of lines on the TV monitor.

According to a second variant, the frame sweep frequency is again increased and its amplitude reduced as previously, the duration of the saw-tooth 81 again being a whole multiple of that of 82, but the frequency increase and amplitude reduction are so selected as to retain the same number of lines in the case of both the normal and reduced fields. Display of the reduced field is simpler because there is no need to display the same line several times as in the first variant, but the image memory must be of a greater capacity.

According to a third variant, the frame frequency is unchanged, but the frame sweep amplitude is reduced as previously. The arrangement is such that n successive lines analyse the same line of the image so as to enable them to be summated by storage in an image memory. A first method of embodying this variant is explained with referenceto Figure 9 in which the saw-tooth 81, as in Figure 8, represents the frame sweep amplitude e in the normal field against the time.

In the case of the reduced field, the amplitude of this frame scan is shown by the succession of steps 91, n lines being scanned on each step. A second method of embodying this third variant comprises retaining a linear frame scan but constructing a drum 47 whose n successive surfaces are inclined differently by a small angle with respect to its axis of rotation so that the n corresponding lines analyse a single line in the object space. The drum 47 must then have a number of surfaces which is a multiple of n; on the other hand, the recording in the memory must be synchronised with the position of the drum so that the lines corresponding to the same object space line can be summated.

The following table summarises the main features of the embodiment of the scanning system shown by way of example in Figures 4, 5 and 6 and compares the performances in the case of the normal and reduced fields, the notations being those used for the expressions of NETD and NETp Nature of characteristic Normal field Reduced field

Elementary analysis field a V and a a V and a H V H 2 2 Number of points in the line Np Np 7 Horizontal field a Np Horizontal field H P a N total H t 4 Number of lines in the image -7 a Total vertical field a p N " p 4 Horizontal scan efficiency ss H 2 P H Vertical scan efficiency g V V Number of lines scanned per second N 4N Image frequency Fi 8 Fi Pass-band Af df NETD i 4 NETp 1 ff Reconnaissance distance dr ~ 1, 4 dr Identification distance d. 1. 4 di 1 Figure 7 illustrates a variant in which display is produced by LED's.This diagram differs from the diagram in at Figure 4 by an objective 411 operating/the wavelength of the display, the focal length of which is equal to or differs from that of the objective 41 operating at the analysis wavelength.

A dichroic mirror 78 reflects, for example, the analysis wavelength and transmits the display wavelength (or vice-versa).

The same applies to mirror 79. The LED's 14' have the same configuration as the detector 14 and are disposed symmetrically to it with respect to 79, each detector being connected to the corresponding LED by an electronic amplification network (not shown). The analysis and display rays coincide over the entire optical path of the analyser contained between the dichroic mirrors 78 and 79. Thus all the faults arising out of the construction, scan synchronisation and interference vibrations are automatically compensated mnd the restored image is exactly identical to the analysed Image. Obviously this optical system can operate only if the system is made achromatic for the analysis and the display wavelengths. It is advantageous to use transfer systems comprising mirrors rather than lenses, as shown in Figure 8 for clarity of the diagram. The display may be made either direct to the naked eye or through a magnifying or other eyepiece disposed behind the objective 41', or by means of a TV camera situated behind the objective 41'.

In both cases, the analysis system can operate at any standard unrelated to the TV standard. The changeover to the reduced field does not give rise to any display problem.

Summation of the images is effected either by the eye or by the vidicon of a TV camera.

Claims

Claims:

1. A two-dimensional optical-mechanical system for scanning at least two fields of view of different dimensions contained one inside the other or others, one such field being termed the normal and the other the reduced field, and for displaying the said fields, of the kind comprising - in the sequence of propagation of the light from the field of view an objective system, a plane reflector situated in a convergent beam between the objective and its focal plane, said reflector being rockable about an axis parallel to its reflecting'surface for the purposes of scanning or sweeping the field in the said focal plane of the objective in a first direction, i.e. the frame direction, a field mirror, a system for sweeping or scanning the field in the focal plane of the objective in a second direction, i.e. the line dIrection, perpendicular to the frame direction, of the kind comprising a rotary drum with reflecting facets, a detector image transfer system, a fixed detector comprising one or more elements, an infrared image display system, the field mirror being movable about an axis parallel to that of the frame scanning mirror and deflecting the beam towards the line scan system and combining (a) the exit pupil of the objective with the entrance pupil of the line scan system and (b) the focal surface of the objective with the analysed lines, the rotary drum of the line scan system being situated in a convergent beam in the detector image transfer system, the detector image in each reflecting surface of the rotary drum analysing one line of the field, characterised in that: a) there is a single objective operating with the same aperture in both the normal and reduced fields b) the frame analysis system uses the same reflecting mirror for the normal and the reduced fields, with a rocking amplitude which is greater for the normal field and smaller for the reduced field, in conjunction with a larger and smaller field mirror rocking amplitude respectively c) the line scanning system uses a drum which differs in respect of form and/or position with respect to the detector image by the image transfer system for each of the fields, the drum rotation axis remaining parallel to itself, means allowing one drum to be replaced by the other, the analysed lines having the same radius of curvature in both the normal and reduced fields, but different lengths d) the detector image transfer system comprises optical paths each associated with one of the fields, it being possible for the said paths to have common and non-common elements, means allowing the non-common elements to be replaced one group by another e) the detector is provided with an identical field of view in both the normal and reduced fields, the convergent beam on the detector in the case of the field being fixed whereas the convergent beam on the detector is movable in the case of the reduced field and contained in the previous beam.

2. A system according to claim i, characterised in that:a) the optical paths of the detector image transfer system coincide b) the drum of the line analysis system is unchangeable from one field to the other and for each of the fields occupies positions derived from one another by controlled movement parallel to the perpendicular of the detector image on the drum rotation axis, the distances d of the said image from the axis relative to these positions being those given by the top arm of the curve M in Figure 3 of the accompanying drawings and corresponding to the same radius of curvature value of the analysed line on the focal surface of the objective.

3. A system according to claim 2, characterised in that the analysed line radius r has a value close to that of the minimum presented by the top arm of the curve M in Figure 3 of the accompanying drawings and the amplitude of the reduced field of view is continuously variable, the movement of the axis of rotation being continuous and in a constant direction, said movement corresponding to a distance d which decreases from the positive value relative to the normal field.

4. A system according to claim 1, characterised in that the line scanning drums differ in respect of form, the reduced field drum comprising a larger number of reflecting facets, the two drums having a common axis of rotation, the two optical paths of the image transfer system do not coincide, the distance d of the detector image through said image transfer system to the axis of rotation being positive and negative for the normal and reduced fields respectively.

5. A system according to claim 4, characterised in that the drums rotate at the same speed, their respective radii and the nagnifications of the two optical image transfer system paths being so proportioned that the image of the detector on the analysed line is smaller in the case of the reduced field than in the normal field and the speed of movement of the image in the detector plane is the same in both the normal and reduced fields.

6. A system according to claim 5, characterised in that the respective number of surfaces of each of the drums is such that the analysed line in the case of the reduced field contains fewer analysis points than in the case of the normal field and the analysis efficiency is the highest possible.

7. A system according to claim 5 or claim 6 characterised in that the frame and field mirror rocking amplitudes in the case of the normal and reduced fields are in the ratio of the lengths of the analysis lines for the same fields so as to retain the same image format from one field to the other.

8. A system according to any one of claims 1 to 7, characterised in that the field image display means comprise a multi-standard TV monitor, the scanning characteristics of which are those of the analysis in the case of the normal and reduced fields respectively.

9. A system according to any one of claims 5 to 7, characterised in that the field image display means comprise a single standard TV monitor, whose scanning characteristics are those of the analysis in the case of the normal field, the normal field images being directly displayed on said monitor while in the case of the reduced field, before the images are displayed on the TV monitor - the number of such images per second being greater than in the case of the normal field - the are stored preferably in a single memory in which the analysed images are summated point by point during the period of one frame scan of the monitor, the said memory being re-read in accordance with the monitor scanning characteristics.

10. A system according to claim 9, characterised in that in the case of the reduced field the frame analysis frequency and the scanning amplitude are such that the ratio of the number of lines to the number of points per line is the same as in the case of the normal field, and after storage in the memory, on display on the monitor, the same line is displayed several times to give the same number of lines on the monitor as in the case of the normal field.

11. A system according to claim 9, characterised in that in the case of the reduced field the frame analysis frequency and the scanning amplitude are such as to retain the same number of lines in the case of the reduced field as in the normal field and then after storage in the memory each line is displayed several times on display.

12. A system according to claim 9, characterised in that the frame analysis frequency is the same in the case of the reduced field as in the normal field, the amplitude cf the frame scan being reduced and varying step-bv-step so that the line analyser analyses the same image line n times.

13. A system according to claim , characterised in that the frame analysis frequency is the same in the case of the reduced field as in the normal field, the frame scanning amplitude in the case cf the reduced field varies linearly against the time as in the case of the normal field but in the case of the reduced field n successive surfaces of the drum are inclined differently by the same small angle with respect to the axis of rotation so that the corresponding n lines analyse the same line in the object space, the number of drum surfaces being a multiple of n, means allowing synchronisation of the recording in the memory with the position of the drum so as to enable the lines corresponding to the same line as the object space to be summated.

14. A system according to any one of claims i to 7, characterised in that for the display the system is provided, at the inlet, between the analysis objective and the frame mirror, with a first diochroic mirror, the normal of which forms an angle with the optical axis of the system and which reflects the analysis beam towards the frame mirror and, at the outlet, on the detector side, a second diochroic mirror reflecting the analysis beam towards the detector, said second diochroic mirror forming an angle with the optical axis of the system, a light emitting diode or a mosaic of light emitting diodes situated symmetrically with respect to the detector with respect to the second diochroic mirror and controlled by the signal leaving the detector, a display objective situated at the inlet of the system symmetrically of the analysis objective with respect to the first diochroic mirror, said first and second diochroic mirrors reflecting the analysis wavelength and being transparent to the wavelength of the light emitted by the diode.

15. A system according to any one of claims 1 to 7, characterised in that the display for the system is provided, at the inlet, between the analysis objective and the frame mirror, with a first dichroic mirror, the normal to which forms an angle with the optical axis of the system and which transmits the analysis beam to the frame mirror, and at the outlet, on the detector side, a second dichroic mirror forming an angle with the optical axis of the system, a lightemitting diode or a mosaic of light-emitting diodes situated symmetrically with respect to the second dichroic mirror and controlled by the signal leaving the detector, a display objective situated at the inlet of the system symmetrical of the analysis objective with respect to the first dichoic mirror, said first and second dichoic mirrors reflecting the wavelength of the light emitted by the diode and being transparent to the analysis wavelength.

15. A system according to any one of claims i to 7, characterised in that the display for the system is provided, at the inlet, between the analysis objective and the frame mirror, with a first dichroic mirror, the normal to which forms an angle with the optical axis of the system and which transmits the analysis beam to the frame mirror, and at the outlet, on the detector side, a second dichroic mirror forming an angle with the optical axis of the system, a lightemitting diode or a mosaic of light-emitting diodes situated symmetrically with respect to the second dichroic mirror and controlled by the signal leaving the detector, a display objective situated at the inlet of the system symmetrical of the analysis objective with respect to the first dichoic mirror, said first and second dichoic mirrors reflecting the wavelength of the light emitted by the diode and being transparent to the analysis wavelength.

Amendments to the claims have been filed as follows 1. A two-dimensional optical-mechanical system for scanning at least two fields of view of different dimensions contained one inside the other or others, one such field being termed the normal and the other the reduced field, and for displaying the said fields, of the kind comprising - in the sequence of propagation of the light from the field of view - an objective system, a plane reflector situated in a convergent beam between the objective and its focal plane, said reflector being rockable about an axis parallel to its reflecting surface for the purpose of scanning or sweeping the field in the said focal plane of the objective in a first direction, i.e. the frame direction, a field mirror, a system for sweeping or scanning the field in the focal plane of the objective in a second direction, i.e. the line directions perpendicular to the frame direction, of the kind comprising a rotary drum with reflecting facets, a detector image transfer system to direct light from the rotary drum to a fixed detector comprising one or more elements, an image display system for displaying an image in accordance with the beam detected at the detector, the field mirror being movable about an axis parallel to that of the frame scanning mirror and deflecting the beam towards the line scan system and combining (a) the exit pupil of the objective with the entrance pupil of the line scan system and (b) the focal surface of the objective with the analysed lines, the rotary drum of the line scan system being situated in a convergent beam in the detector image transfer system, the detector image in each reflecting surface of the rotary drum analysing one line of the field, characterised in that: a) there is a single objective operating with the same aperture in both the normal and reduced fields b) the frame analysis system uses the same reflecting mirror for the normal and the reduced fields, with a rocking amplitude which is greater for the normal field and smaller for the reduced field, in conjunction with a larger and smaller field mirror rocking amplitude respectively c) the line scanning system uses, for each of the two fields namely the normal field and the reduced field, a drum, the drum used for each of the two fields being either the same drum located in a different position for each field, or different drums used selectively for the corresponding field, the position of the operative drum being in each case defined with respect to the detector image by the image transfer system for each of the fields, the drum rotation axis remaining parallel to itself, means allowing the drum to be shifted towards an operative position, the analysed lines having the same radius of curvature in both the normal and reduced fields but different lengths.

d) the detector image transfer system comprises optical paths each associated with one of the fields, it being possible for the said paths to have common and non-common elements, means allowing the non-common elements to be replaced one group by another e) the detector is provided with an identical field of view in both the normal and reduced fields, the convergent beam on the detector in the case of the normal field being fixed whereas the convergent beam on the detector is movable the case of the reduced field and contained in the confines of the beam on the detector in the case of the normal field.

2. A system according to claim 1, characterised in that:a) the optical paths of the detector image transfer system coincide b) the drum of the line analysis system is unchangeable from one field to the other and for each of the fields occupies positions derived from one another by controlled movement parallel to the perpendicular of the detector image on the drum rotation axis, the distances d of the said image from the axis relative to these positions being those given by the top arm of the curve M in Figure 3 of the accompanying drawings, and corresponding to the same radius of curvature value of the analysed line on the focal surface of the objective.

4. A system according to claim i, characterised in that the line scanning drums differ in respect of form, the reduced field drum comprising a larger number of reflecting facets, the two drums having a common axis of rotation, the two optical paths of the image transfer system do not coincide, the distance d of the detector image through said image transfer system to the axis of rotation being positive and negative for the normal and reduced fields respectively.

9. A system according to any one of claims 5 to 7, characterised in that the field image display means comprise a single standard TV monitor, whose scanning characteristics are those of the analysis in the case of the normal field, the normal field images being directly displayed on said monitor while in the case of the reduced field, before the images are displayed on the TV monitor - the number of such images per second being greater than in the case of the normal field - they are stored preferably in a single memory in which the analysed images are summated point by point during the period of one frame scan of the monitor, the said memory being re-read in accordance with the monitor scanning characteristics.

12. A system according to claim 9, characterised in that the frame analysis frequency is the same in the case of the reduced field as in the normal field, the amplitude of the frame scan being reduced and varying step-by-step so that the line analyser analyses the same imagine n times.

13. A system according to claim 9, characterised in that the frame analysis frequency is the same in the case of the reduced field as in the normal field, the frame scanning amplitude in the case of the reduced field varies linearly against the time as in the case of the normal field but in the case of the reduced field n successive surfaces of the drum are inclined differently by the same small angle with respect to the axis of rotation so that the corresponding n lines analyse the same line in the object space, the number of drum surfaces being a multiple of n, means allowing synchronisation of the recording in the memory with the position of the drum so as to enable the lines corresponding to the same line as the object space to be summated.

14. A system according to any one of claims 1 to 7, characterised in that for the display the system is provided, at the inlet, between the analysis objective and the frame mirror, with a first diochroic mirror, the normal of which forms an angle with the optical axis of the system and which reflects the analysis beam towards the frame mirror and, at the outlet, on the detector side, a second diochroic mirror reflecting the analysis beam towards the detector, said second diochroic mirror forming an angle with the optical axis of the system, a light emitting diode or a mosaic of light emitting diodes situated symmetrically with respect to the detector with respect to the second diochroic mirror and controlled by the signal leaving the detector, a display objective situated at the inlet of the system symmetrically of the analysis objective with respect to the first diochroic mirror, said first and second diochroic mirrors reflecting the analysis wavelength and being transparent to the wavelength of the light emitted by the diode.