GB1605268A

GB1605268A - Optical scanning device

Info

Publication number: GB1605268A
Application number: GB5145677A
Authority: GB
Inventors: Fernand Loy; Jean Sebastien Dubuisson
Original assignee: Telecommunications Radioelectriques et Telephoniques SA TRT
Current assignee: Telecommunications Radioelectriques et Telephoniques SA TRT
Priority date: 1977-01-26
Filing date: 1977-12-09
Publication date: 1987-02-04
Also published as: IT1203128B; NL190769C; NL190769B; FR2593657B2; DE2802950A1; DE2802950C2; NL7714501A; FR2593657A2

Abstract

In this optico-mechanical scanning device the frame scan is obtained with the aid of a flat mirror which rolls over a conic, one of the foci of which is at the centre of the exit pupil of the objective and the other focus of which is substantially at the vertex of a field mirror which bends the analysis beam back towards the line scan means. The field mirror is impressed with a motion synchronous with the motion of the frame mirror, such that there is constantly optical conjugation of the centre of the said pupil with a fixed point of the axis of rotation of the line scan means. Application: infrared viewing of objects. <IMAGE>

Description

(54) OPTICAL SCANNING DEVICE (71) We, TELECOMMUNICATIONS RADJOELECTRIQUES ETTELEPHONIOUES T.R.T., of 88, rue Brillat Savarin, 75013 Paris, France, a French Body Corporate, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention is a Patent of Addition to Patent Application No. 3370/76 (Patent No.

1605265).

The invention relates to an improvement of the device for optically scanning a field of vision and displaying the field, disclosed in the aforementioned patent application. The invention relates to the raster part of the scanning system in association with the motion of the field mirror.

The aforementioned patent application relates to a device for optically scanning a field of vision divided into different regions and for displaying the field, scanning being made in two perpendicular directions, i.e. "line" scanning in a direction x and "raster" or "image" scanning in a direction y, the device scanning along beams coming from different regions of the field and causing the beams to converge on to a detector sensitive to the radiation in the beams, the device comprising the following components in order, in the direction of the path of the central incident beam from the field of vision: an objective, means for raster scanning in the y direction and a system for deflecting the beams bounded by the detector and the aperture of the objective towards means for line scanning of the image field of the objective in the x direction, the display part of the device having optical means similar to the field of vision scanning means and comprising instead of the detector, an electro-luminescent diode actuated by a signal coming from the detector, the device being characterised in that: the optical axis of the objective is in a plane P containing the y direction and perpendicular to the x direction, the focal surface of said objective being curved and such that its centre of curvature is at the the centre of the exit pupil of the objective, the raster scanning means comprise a plane mirror rotating in reciprocation around an axis parallel to the x direction and disposed in a convergent beam behind the objective near the field image in the objective, the line scanning means comprise, firstly, a drum rotating uniformly around a stationary axis YY', contained in the plane P and bearing a number of flat reflecting surfaces regularly distributed around the drum periphery and, secondly, an image-conveying means symmetrical with respect to the plane P and forming an image of the detector at a fixed point A' along the drum rotation axis YY', the drum being placed in a convergent beam in the path of the image-conveying means on the image side of the detector, the point symmetrical with the point A' with respect to each surface of the drum, when the surface is perpendicular to the plane P, being in the neighbourhood of the point D which is symmetrical with the focus of the objective with respect to the raster mirror in a position parallel to the YY' axis, and the optical beam-deflecting system comprises a concave or "field" mirror having the plane P as the plane of symmetry, the apex of the mirror being near D on the ZZ' axis extending through D and perpendicular to the YY' axis, the mirror being so disposed that it conjugates the centre 0 of the exit pupil of the objective with a fixed point 0' on the YY' axis, point 0' being symmetrical with respect to the ZZ' axis, with the point where the optical axis of the objective intersects the YY' axis; in order, if required, to ensure that the idle scanning time between two consecutive lines is zero, the field mirror also has a width in the x direction which is slightly less than the length of the analyzed line, which in turn is equal to the distance between the images of the detector in two consecutive surfaces of the rotating drum, the mirror being moved if required for small distances in phase with the movement of the raster scanning means, the small motion comprising reciprocation in translation along the ZZ' axis in the neighbourhood of D and reciprocating rotation around an axis parallel to the x direction, which is symmetrical with respect to the ZZ' axis, the amplitude of the motion in translation being such as to correct the defocusing introduced by the raster scanning means and the amplitude of the rotation being such as to ensure that the field mirror holds 0', the conjugate of the centre 0 of the exit pupil of the objective, in a constant position during the reciprocating rotation of the raster scanning means.

The device prevents the analysis beam from coming out of focus near the point D during raster scanning, and ensures that the centre rays of the beams cross the line scanning elements at a constant incidence, irrespective of the direciton of the analyzed field, by carefully combining and synchronizing the reciprocating rotation of the raster mirror and the rotation and motion in translation of the field mirror.

In such systems, it is always desirable to limit the oscillating mass, i.e. the dimensions, of the raster mirror, which is thus placed as near as possible to the focus of the objective. This is the aim of German Patent Spec. No.2011 883, which discloses a device for scanning a field in a single direction, using a plane mirror pivoting the beam transmitted by an objective, the beam being permanently focused at a single fixed point. This result is obtained by moving the plane mirror so that it is always at a tangent to an elliptical curve, one focus of which is in the principal plane of the objective and the other focus at the aforementioned fixed point; according to the aforementioned patent, the focal length f of the objective is equal to the sum of the semi-axes a and b of the ellipse, i.e.

a=b.

Consequently The ellipse, from the purely theoretical point of view, is a circle and, according to the last-mentioned patent, the focal point is in the principal plane of the objective. Such a system, even if only approximated, leaves only a short distance between the objective and the focal point and it is thus difficult to ensure that the focal point coincides with the apex of the field mirror of the optical scanning system according to the main patent application, to which the present patent of addition is attached. Furthermore the centre rays of the analysis beam, after converging at the fixed point, are in directions which vary with the direction of the analyzed field. Consequently the device according to German Patent Specification 2011 883 cannot by itself be used for raster scanning according to the main patent. Another disadvantage of the device according to the German Patent is that the mirror moves along an ellipse which is practically circular near its minor axis, at a distance substantially half the focal length of the objective. Consequently, the width of the intercepted beam and the surface of the mirror are relatively large and the mirror is heavy and bulky.

In the present invention, the optical scanning system according to the main patent has a new version of the raster scanning means which does not have the disadvantages of the cited prior art. The means substantially comprises a plane mirror which is moved in dependence on the motion of the field mirror so as to focus the beam at a fixed point substantially at the apex of the concave field mirror and simultaneously reflect the centre ray of the beam in a constant direction, irrespective of the direction of the analyzed field.

The raster mirror is moved and pivoted along a curve so that it is always in a position very near the focus of the objective and so that the beam is focused at a fixed point irrespective of the direction of the analyzed field, as in the German Patent No 2011 883. However, the field mirror, on the other hand, is only rotated in synchronism with the raster mirror, so that the centre ray of the beam analyzed by the raster mirror is reflected by the field mirror along a fixed radius.

According to the present invention, in the case when the spherical focal surface of the objective is concave with respect to incident light the raster mirror rotates in reciprocation while remaining at a tangent to a portion of an ellipse in the plane P and having a major axis equal in length to the radius of curvature of the focal surface of the objective, the centre of the exit pupil of the objective being at one focus of the ellipse and the point D being at the other focus, and the field mirror rotates in reciprocation around an axis extending through the point D and parallel to the x direction, the rotation being synchronised with the motion of the raster mirror and such that the centre ray of each incident beam is reflected along a fixed direction DO'. The field mirror oscillates around an axis extending through the focal point in synchronism with the raster mirror, so as to reflect the centre ray of the beam in a fixed direction, so that the centre of the exit pupil is optically conjugated with a fixed point on the axis of rotation of the line scanning system.

In a simplified variant of the invention, of use when the raster scanning amplitude is limited, the raster mirror is driven along a circle at a tangent to the ellipse at the point of contact of the mirror corresponding to the central direction of the analyzed field, the circle being centred at the centre of curvature of the ellipse at the aforementioned point of contact. As a result, slight deformation is introduced on each side of the centre of the field. The deformation is reduced by moving the axis of rotation of the field mirror a very short distance relative to the focus of the ellipse.

The raster mirror can move along a conical curve different from an ellipse, depending on the curvature of the focal surface of the objective. For example, the curve is a parabola when the focal surface is plane and the objective exit pupil is at infinity, or a hyperbola when the focal surface is convex with respect to incident light and the exit pupil is behind the focal surface.

The invention will be more clearly understood from the following description of some embodiments of the device, accompanied by drawings in which: Fig. 1 is a cross-section through an embodiment of the optical scanning device according to the main patent; Fig. 2 is a cross-section of the raster scanning means in association with the field mirror according to the present invention; Fig. 3 shows an embodiment of the raster scanning means, and Fig. 4 is a curve relating to the previous embodiment, showing how defocusing is corrected in dependence on the field angle.

Fig. 1 is a general view in section along the plane of symmetry P of the device according to the main patent. Reference 11 denotes a fixed objective having an optical axis 12 and a focus F. Objective 11 is preferably made up of lenses.

As stated in the main patent, the objective may alternatively be made up of mirrors. The optical axis 12 intersects the YY' axis at point E. The YY' axis is the axis of rotation of a system comprising a drive device 13 and a drum 13' bearing a large number of lateral reflecting surfaces. As stated in the main patent, the drum can have a number of shapes. In the present case it is assumed to be prismatic, its reflecting surfaces being uniformly distributed around the YY' axis. At 17, one of its surfaces is represented in a position perpendicular to plane P. The plane mirror or "raster" mirror 14 can move around an axis perpendicular to the drawing and extending through E. Actually according to the invention, the axis does not necessarily extend through E. The feature represented in Fig. 1 is preferred only in the case where, as described in the main patent, the device is provided with additional means for directly displaying the field by means of electroluminescent diodes. Mirror 14 reflects every beam which comes from a region of the field substantially at mirror 16. In Fig. 1, a particular beam 15 is shown, having a central ray identical with the optical axis 12, the image forming at point D on the ZZ' axis perpendicular to the YY' axis, corresponding to the centre of the field.

The "raster" mirror 14 scans the field in they direction perpendicular to axis 12 and in the plane of the drawing.

The device comprises a concave mirror forming an image at A' of a detector 19 placed at A. The mirror is made up e.g. of two paraboloid portions 18 and 18' of revolution around the YY' axis, the reflecting paraboloid surfaces facing one another. The paraboloids have apices S, S' and foci A, A', all situated along the YY' axis. A' is symmetrical with D with respect to the drum surface 17.

When the drum 13' rotates, the system comprising the drum, mirrors 18, 18' and detector 19 is adapted for line-by-line analysis, in the direction x perpendicular to the plane of the drawing of the image of the field given by objective 11 and the raster mirror 14 in the neighbourhood of point D. Hereinafter, this system will be called the "line analysis" or "x analysis" system. Mirror 16, which is symmetrical with respect to the (ZZ', YY') plane and has its apex near D, optically conjugates the centre of the exit pupil 0 of the objective 11 with the fixed point 0' on the YY' axis which is symmetrical with E with respect to the ZZ' axis so that any beam reaching the exit pupil of objective 11 is made to converge on detector 19. Mirror 16 is called the "field mirror", since it limits the analyzed field.

In Fig. 2, the device is shown partially in section through its plane of symmetry P. The drawing shows the objective 11, its optical axis 12, and its focus F. Curve 21 shows the cross-section of the focal surface of the objective through plane P.

In order more clearly to show the difference from the prior art, it is assumed that the exit pupil is not identical with the principal plane of the device. The pupil is represented by reference 22, whereas the principal plane is assumed to be along 11. The centre of the pupil is the point 0. In the example, the focal surfance 21, relative to O, is concave and spherical. Its centre of curvature isO, Fig. 2 also shows YY', the axis of rotation of the line scanning means, and the point D which is identical with the image of the detector given by the line scanning means when scanning reaches the middle of a line. 0' on the YY' axis is a fixed point optically conjugated with point O by the raster mirror and the field mirror, the position and motion of which will be set out hereinafter. Reference 23 denotes an ellipse having foci 0, D and a major axis equal to OF, the radius of curvature at Fof the focal surface 21. The raster mirror, whose reflecting surface is opposite D, moves and always remains at a tangent to the ellipse, the point of contact between the mirror and the ellipse being the point of intersection between the ellipse and the centre ray of the analyzed beam. Fig. 2 shows two of the aforementioned beams - i.e.

(a) the central beam 15 converging at F on curve 21, in which case the raster mirror is in position T, the point of contact with the ellipse being E, the intersection of axis 12 with the ellipse 23 - and (b) an extreme beam 24 converging at Fl on curve 21, in which case the raster mirror is in position TI, the point of contact with the ellipse being El, the intersection of ray 25 and the ellipse. Points F2, E2 on 21, 23 respectively correspond to points FI, E1 in the case of the extreme analysis beam symmetrical with 24 with respect to the optical- axis 12. When the raster mirror moves alternately from E1 to E2 and from E2 to E1 along ellipse 23, it focuses the analyzed beam at the point D irrespective of its position on the ellipse. In this manner, the field is scanned in the raster direction without any defocusing at point D, which is identical with the apex of the concave field mirror 16, which is moved in rotation around an axis perpendicular to plane P and extending through point D, the motion being synchronous with that of the raster mirror and such that the centre ray of the analyzed beam, e.g. 12 or 25, is permanently reflected in the direction DO'. Preferably, in order to reduce the size and weight of the raster mirror, the arc El E2 of the ellipse used is near point D and remote from the minor axis of the ellipse.

In practice, the mirror moves e.g. along two tracks of identical elliptical cross-section and disposed in planes parallel to plane P and on either side thereof, so that the planes extend at right angles to P along tracks 23, mechanical pressure means ensuring that the mirror remains in contact with the tracks without sliding.

In a special embodiment of the invention, the analysed field is not very big. In that case, arc F, F2 is not very long and the centres of curvature CI, C2, C of the ellipse at points El, E2, E respectively are very close together. The elliptical arc El E2 can then be replaced by a circle of centre C and radius EC. The mirror can then be driven along the circle, a simple method of doing so being to secure the mirror to an arm of constant length, articulated around C. The motion results in slight defocusing. In Fig. 4, the focusing I relative to D is indicated by curve 41, in dependence on the angle a between the analyzed direction and the optical axis of the objective. In Fig. 4, land a are zero at the point F and are considered positive in the direction corresponding to point F2 and negative in the direction corresponding to point Fl. Maximum defocusing occurs at points Fl and F2 (points A and B on curve 41).

According to the invention, the amount of defocusing is reduced by slight, but adequate shifting in plane P of the centre of rotation of the field mirror with respect to point D.

Fig. 3 shows the shift, which is made so as simultaneously to eliminate defocusing when the direction of the analyzed field is F, F1 and F2 respectively. Fig. 3 shows the focal curve 21, the focus F and the image F1 of the field in an extreme direction of analysis, at an angle aM with the optical axis 12 containing the point E in Fig. 2. Fig. 3 also shows the centre 0 of the exit pupil and its conjugate 0' on the YY' axis.

The raster mirror is shown in position TI. The mirror is tangential to circle 31, of radius EC.

The image of F1 in the raster mirror in the position T1 is F1,, i.e. the defocusing introduced is DFI'. In order to eliminate this defocusing, the axis of the field mirror is moved parallel to plane P at the point I, i.e. the intersection between the bisector 34 of angle EDO" and the mid-perpendicular 32 of segment Do'1, the axis remaining perpendicular to plane P.

During raster scanning, the field mirror oscillates around point I on either side of DI, i.e. from position 32 to position 33 corresponding to points F1, F2 respectively of the field image in the objective. In this manner, the maximum deviations during focusing are substantially divided by two, the remaining deviations being those measured between curve 41 and line AB in Fig. 4.

Of course, the invention also extends to the case where the focal surface of the objective is spherical and convex with respect to incident light and the exit pupil is behind the focal surface, and also the case where the exit pupil is at infinity and the focal surface is plane.

According to the invention, the mirror moves along a portion of a hyperbola or parabola. In the case of the hyperbola, its foci are, as before, the centre 0 of the objective exit pupil and point D. In the case of a parabola, its focus is at point D and its axis is parallel to the optical axis of the objective.

Of course, in the latter case, use can be made of circles at a tangent to the parabola and hyperbola at the point of contact between the corresponding raster mirror and the central direction of the field, the circles having the same radius of curvature as the parabola and hyperbola. The invention also includes the measures taken to reduce the resulting defocusing, which are similar to those taken for a concave objective focal surface.

The invention, of course, also includes any application or industrial use of the raster scanning system in association with the previously-described motion of the field mirror.

WHAT WE CLAIM IS: 1. A device for optically scanning a field of vision divided into different regions and for displaying the field, scanning being made in two perpendicular directions, i.e. "line ' scanning in a direction x and "raster" or "image" scanning in a direction y, the device scanning along beams coming from different regions of the field and causing the beams to converge on to a detector sensitive to the radiation in the beams, the device comprising the following components in order, in the direction of the path of the central incident beam from the field of vision: an objective, means for raster scanning in the y direction, and a system for deflecting the beams bounded by the detector and the aperture of the objective towards means for line scanning of the image field of the objective in the x direction, the display part of the device having optical means similar to the field of vision scanning means and comprising instead of the detector, and electro-luminescent diode activated by a signal coming from the detector, the device being characterised in that:

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the point D irrespective of its position on the ellipse. In this manner, the field is scanned in the raster direction without any defocusing at point D, which is identical with the apex of the concave field mirror 16, which is moved in rotation around an axis perpendicular to plane P and extending through point D, the motion being synchronous with that of the raster mirror and such that the centre ray of the analyzed beam, e.g. 12 or 25, is permanently reflected in the direction DO'. Preferably, in order to reduce the size and weight of the raster mirror, the arc El E2 of the ellipse used is near point D and remote from the minor axis of the ellipse. In practice, the mirror moves e.g. along two tracks of identical elliptical cross-section and disposed in planes parallel to plane P and on either side thereof, so that the planes extend at right angles to P along tracks 23, mechanical pressure means ensuring that the mirror remains in contact with the tracks without sliding. In a special embodiment of the invention, the analysed field is not very big. In that case, arc F, F2 is not very long and the centres of curvature CI, C2, C of the ellipse at points El, E2, E respectively are very close together. The elliptical arc El E2 can then be replaced by a circle of centre C and radius EC. The mirror can then be driven along the circle, a simple method of doing so being to secure the mirror to an arm of constant length, articulated around C. The motion results in slight defocusing. In Fig. 4, the focusing I relative to D is indicated by curve 41, in dependence on the angle a between the analyzed direction and the optical axis of the objective. In Fig. 4, land a are zero at the point F and are considered positive in the direction corresponding to point F2 and negative in the direction corresponding to point Fl. Maximum defocusing occurs at points Fl and F2 (points A and B on curve 41). According to the invention, the amount of defocusing is reduced by slight, but adequate shifting in plane P of the centre of rotation of the field mirror with respect to point D. Fig. 3 shows the shift, which is made so as simultaneously to eliminate defocusing when the direction of the analyzed field is F, F1 and F2 respectively. Fig. 3 shows the focal curve 21, the focus F and the image F1 of the field in an extreme direction of analysis, at an angle aM with the optical axis 12 containing the point E in Fig. 2. Fig. 3 also shows the centre 0 of the exit pupil and its conjugate 0' on the YY' axis. The raster mirror is shown in position TI. The mirror is tangential to circle 31, of radius EC. The image of F1 in the raster mirror in the position T1 is F1,, i.e. the defocusing introduced is DFI'. In order to eliminate this defocusing, the axis of the field mirror is moved parallel to plane P at the point I, i.e. the intersection between the bisector 34 of angle EDO" and the mid-perpendicular 32 of segment Do'1, the axis remaining perpendicular to plane P. During raster scanning, the field mirror oscillates around point I on either side of DI, i.e. from position 32 to position 33 corresponding to points F1, F2 respectively of the field image in the objective. In this manner, the maximum deviations during focusing are substantially divided by two, the remaining deviations being those measured between curve 41 and line AB in Fig. 4. Of course, the invention also extends to the case where the focal surface of the objective is spherical and convex with respect to incident light and the exit pupil is behind the focal surface, and also the case where the exit pupil is at infinity and the focal surface is plane. According to the invention, the mirror moves along a portion of a hyperbola or parabola. In the case of the hyperbola, its foci are, as before, the centre 0 of the objective exit pupil and point D. In the case of a parabola, its focus is at point D and its axis is parallel to the optical axis of the objective. Of course, in the latter case, use can be made of circles at a tangent to the parabola and hyperbola at the point of contact between the corresponding raster mirror and the central direction of the field, the circles having the same radius of curvature as the parabola and hyperbola. The invention also includes the measures taken to reduce the resulting defocusing, which are similar to those taken for a concave objective focal surface. The invention, of course, also includes any application or industrial use of the raster scanning system in association with the previously-described motion of the field mirror. WHAT WE CLAIM IS:

1. A device for optically scanning a field of vision divided into different regions and for displaying the field, scanning being made in two perpendicular directions, i.e. "line ' scanning in a direction x and "raster" or "image" scanning in a direction y, the device scanning along beams coming from different regions of the field and causing the beams to converge on to a detector sensitive to the radiation in the beams, the device comprising the following components in order, in the direction of the path of the central incident beam from the field of vision: an objective, means for raster scanning in the y direction, and a system for deflecting the beams bounded by the detector and the aperture of the objective towards means for line scanning of the image field of the objective in the x direction, the display part of the device having optical means similar to the field of vision scanning means and comprising instead of the detector, and electro-luminescent diode activated by a signal coming from the detector, the device being characterised in that:

the optical axis of the objective is in a plane P containing they direction and perpendicular to the x direction, the focal surface of said objective being curved and such that its centre of curvature is at the centre of the exit pupil of the objective, the raster scanning means comprise a plane mirror rotating in reciprocation around an axis parallel to the x direction and disposed in a convergent beam behind the objective near the field image in the objective, the line scanning means comprise, firstly, a drum rotating uniformly around a stationary axis YY' contained in the plane P and bearing a number of flat reflecting surfaces regularly distributed around the drum periphery and, secondly, an image-conveying means symmetrical with respect to the plane P and forming an image of the detector at a fixed point A' along the drum rotation axis YY', the drum being placed in a convergent beam in the path of the image-conveying means on the image side of the detector, the point symmetrical with the point A' with respect to each surface of the drum, when the surface is perpendicular to the plane P, being in the neighbourhood of the point D which is symmetrical with the focus of the objective with respect to the raster mirror in a position parallel to the YY' axis and the optical beam-deflecting system comprises a concave or "field" mirror having the plane P as the plane of symmetry, the apex of the mirror being near D on the ZZ' axis extending through D and perpendicular to the YY' axis, the mirror being so disposed that it conjugates the centre 0 of the exit pupil of the objective with a fixed point 0' on the YY' axis, point 0 being symmetrical with respect to the ZZ' axis, with the point where the optical axis of the objective intersects the YY' axis, in order, if required, to ensure that the idle scanning time between two consecutive lines is zero, the field mirror also has a width in the x direction which is slightly less than the length of the analyzed line, which in turn is equal to the distance between the images of the detector in two consecutive surfaces of the rotating drum, the mirror being moved if required for small distances in phase with the movement of the raster scanning means, characterised in that the focal surface of the objective is concave with respect to incident light, the raster mirror rotates in reciprocation while remaining at a tangent to a portion of an ellipse in the plane P and having a major axis equal in length to the radius of curvature of the focal surface of the objective, the centre of the exit pupil of the objective being at one focus of the ellipse and the point D being at the other focus, and the field mirror rotates in reciprocation around an axis extending through the point D and parallel to the x direction, the rotation being synchronized with the motion of the raster mirror and such that the centre ray of each incident beam is reflected along afixed direction DO'.

2. A device according to Claim 1, characterised in that the ellipse portion used is mainly the half-ellipse limited by the minor axis and nearer the point D.

3. A device according to Claim 1 or 2, characterised in that the raster mirror moves along a circle at a tangent to the ellipse at the point of contact corresponding to the centre of the analyzed field, and the centre and radius of curvature of the circle being respectively equal to the centre and radius of curvature of the ellipse at the aforementioned point of contact, the resulting slight defocusing being reduced by moving the axis of rotation of the field mirror a short distance in translation parallel to plane P.

4. A device according to Claim 3, and substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings characterised in that the axis of rotation of the field mirror is shifted parallel to the plane Pat the point of intersection between the bisector of angle EDO" and the midperpendicular of the line segment Do'1, E being the point of contact between the raster mirror and its circle of movement corresponding to the centre of the analyzed field, and F' being the image of the field given by the objective and the raster mirror along one of the extreme directions of the analyzed field.

5. A device similar to the device according to Claim I or 2, characterised in that the exit pupil of the objective is at infinity and its focal surface is plane, and the raster mirror moves along a portion of a parabola having an axis parallel to the optical axis of the objective and a focus identical with the point D.

6. A device similar to the device according to Claim 1 or 2, characterised in that the focal curface of the objective is convex, the exit pupil of the objective is behind the focal surface, and the raster mirror moves along a portion of a hyperbola whose foci are respectively identical with the point D and the centre of the exit pupil of the device.

7. A device according to Claim 5 or 6, characterised in that the raster mirror moves along a circle at a tangent to the parabola or hyperbola at the point of contact corresponding to the centre of the analyzed field and the centre and radius of curvature of the circle being respectively the centre and radius of curvature of the parabola or hyperbola at the aforementioned point of contact, the resulting slight defocusing being reduced by moving the axis of rotation of the field mirror a short distance in translation parallel to plane P, as described in Claim 4.

8. A device according to any of Claims 1 to 7, characterised in that it comprises tracks on either side of plane P and parallel thereto, the cross-section of the tracks being identical with the conic curve enveloped by the raster mirror, the mirror moving on each track.

9. A device according to any of Claims 3, 4 and 7, characterised in that the raster mirror is rigidly secured to an arm of constant length, articulated around an axis extending through the centre of the circle.

10. A device for optically scanning a field of vision, the device being substantially as herein described with reference to, and as shown in, the accompanying drawings.