WO1993009463A1

WO1993009463A1 - Device for microscanning and infrared camera provided with such device

Info

Publication number: WO1993009463A1
Application number: PCT/FR1992/001039
Authority: WO
Inventors: Pascal Joffre; Bertrand Remy
Original assignee: Thomson-Csf
Priority date: 1991-11-08
Filing date: 1992-11-06
Publication date: 1993-05-13
Also published as: EP0611454A1; FR2683639A1; FR2683639B1

Abstract

The invention relates to a microscanning device for deviating periodically a convergent beam. The device according to the invention is comprised of an assembly (33) consisting of an even number of thin blades (21-28) which are opposite by pairs, and forming a regular polygon. The assembly has a symmetry axis ($g(D)), arranged perpendicularly to the direction (Rm) of the convergent beam (FC), and is actuated with a uniform rotation motion about the axis ($g(D)). The invention applies to the microscanning in one or two directions, according to the cross-sectional shape of the thin blades, for bidimensional sensors of infrared cameras.

Description

MICROBALAYΛGE DEVICE AND INFRARED CAMERA EQUIPPED WITH SUCH A DEVICE.

The invention relates to the field of infrared detection, more particularly to detection by infrared cameras using, as detectors, two-dimensional mosaics of elementary photosensitive cells.

The object of the invention is a micro-scanning device intended in particular for such infrared observation cameras.

Detectors with a two-dimensional mosaic made up of a large number of photosensitive cells (for example 256 x 256) have the advantage, compared to

10 detection, to be able to directly form a video signal, without requiring scanning.

Indeed, while a detection strip is made up of a very limited number of elementary cells, at least on one dimension, requiring the adapted scanning of a

15 image of the scene observed, the two-dimensional detectors, more recently developed, constitute real electronic retinas, and the image of the scene projected on the detector having in this case dimensions equivalent to those of the detector.

-0 However, the photosensitive cells of such mosaics are separated by non-sensitive zones which overall represent a large percentage of the total surface of the detector. For example, on a 128 x 128 mosaic of sensitive cells in the 3-5 μm band, the sensitive areas

25 occupy only 25% of the total area. The necessary presence of a blind "frame", constituted by the non-sensitive zones between the disjoint photosensitive cells, results from the constraints linked to the technology of the circuits integrated into the two-dimensional detector.

30 Indeed, the video signal, supplied by two-dimensional detectors, is only obtained after multiplexing and reading charges delivered by the cells, proportional to the light flux received in a given infrared spectral band (for example in the 3-5 μ or 8-12 μm band corresponding to atmospheric transmission windows). The multiplexing and the reading of the charges are carried out conventionally through charge transfer circuits CCD (initials of Charge Coupled Device, in English terminology) or CID (Charge Injection Circuit) made up of charge integration and transfer zones whose dimensions cannot be reduced arbitrarily.

In order to obtain spatial detection continuity despite the "frame" effect produced by the presence of this blind frame, a microbaying of the beam from the scene is necessary. According to the prior art, such a micro-scan is obtained by the vibration of a blade with parallel faces, disposed between a focusing objective and the detector. Solutions of this type are conventional and have been proposed, for example in patents EP 289 182 or JP 61 236 282 (Patent Abstracts Vol. TI, n ° 81 (E -488) [2528]). However, these devices are not stable over time due to the problems inherent in vibration (blurring effect, adjustment, accelerated wear, etc.).

The invention aims to obtain spatial continuity of the detection surface of a two-dimensional detector, making it possible to increase the number of pixels in the final image formed from a video signal delivered by the detector, while overcome the disadvantages of the vibratory systems of the prior art. To achieve this objective, the present invention proposes to carry out a mono or bidirectional microscanning using an assembly of thin blades, animated by a uniform rotational movement around an axis perpendicular to the convergent light beam coming from the scene observed.

More specifically, the invention relates to a microscan device for periodically deflecting the direction of a convergent light beam, characterized in that it comprises an even number of optical blades 2 to 2 parallel associated dp so as to constitute an assembly of polygonal section around an axis of symmetry, perpendicular to the mean direction of the converging light beam. The invention also relates to an infrared camera comprising such a micro-scanning device.

Other characteristics and advantages will appear on reading the description which follows, with reference to the appended figures which represent, respectively: - Figures la and lb, a conventional device for obtaining a large amplitude scan, arranged respectively in vertical section at first and second angular positions;

- Figure 2, an embodiment of the device according to the invention applied to a monodirectional microscan, this device being seen in section along a plane perpendicular to its axis of symmetry; FIG. 3, an example of opto-electronic organization of an infrared camera with two-dimensional detector according to the invention;

- Figure 4, a diagram illustrating the effect produced by a micro-scan in two directions;

- Figure 5, an embodiment of the device according to the invention applied to a bidirectional microscanning, the device being seen in section along a plane of symmetry.

In order to define the bases of geometric optics on which the present invention is based, it is interesting to describe, first of all, a solution conventionally adopted for obtaining a large amplitude scan on a detection strip.

This solution implements a polygonal refractive prism, such as that referenced 1 in Figures la and lb which represent it in horizontal section, respectively, according to a first and a second angular position. Such a prism is described in the work of G. GAUSSORGUES entitled "The infrared thermography ", published by LAVOISIER (TEC and DOC).

In FIG. 1a, a parallel beam FE, coming from the scene to be observed, passes through a converging lens 2, of center 0, which forms a converging conjugate beam FC thereof, projected at a focal point occupying a position Po. An average radius Rm of the beams FE then FC, defining their mean direction, is coincident with the optical axis AOP A 'shown in phantom. The converging beam FC penetrates into the polygonal prism 1 and emerges from this prism according to a beam of the same convergence which finally focuses on a detection strip 3, placed precisely in the focusing plane of the lens 2-prism assembly 1. The prism 1 is constituted by a refractive material cut according to a regular polygon comprising an even number of plane facets, for example 8 facets referenced 11 to 18. Thus each pair of opposite facets, such as 11 and 15, constitutes a blade with plane and parallel faces which does not modify the convergence of an incident beam. The effect used to perform a large amplitude scan, shown in FIG. 1b, is the offset of the optical path of a light ray caused by the crossing of a blade with parallel faces inclined with respect to this ray.

In Figure 1b, the prism 1 has rotated at an angle relative to its occupied position in Figure la.

The angle> _. is defined as the angular difference between the optical axis AA ', and the normal to the opposite facets crossed by the converging beam. The external beam FE which finally converges on the bar 3 is then inclined by an angle G and the beam FC, conjugated with FE, virtually converges at a point which then occupies a position P .. different from the position P. The mean radius Rm is no longer confused with the optical axis AA ^r .

The point of convergence is deviated laterally from its position Po occupied in FIG. The calculation of the deviation lateral P. P, denoted below x, is led from the path followed by the mean radius Rm of the light beam, as a function of the thickness. of crossed prism, of the refraction index n of the prism and of the angle. This deviation has the expression:

Cos ^• -. i_ x = e sin (1 -) (D

\ v / n ² - S _i • n ² Λ

For low values of λ, expressed in radian s, this expression takes the following simplified form:

Λ = e. -X. (1- 1 / n) (II)

Thus, when the angle X. varies, each elementary cell of the bar "sees" juxtaposed areas of space forming a strip of space called the "line" of the image of the observed scene. A complete image of the observed scene is thus scanned by the bar. The passage of the beam FC on an edge of the prism 1 separating two facets causes a discontinuity of the scanning which returns at the beginning of the image by passing from a pair of blades to the next, such as M-15 and 12 - 16 in FIG. .

The invention exploits this same possibility of obtaining a lateral deflection of a convergent beam by crossing an optical element with parallel faces and inclined with respect to the mean axis of this beam. But in order to cause only an angular scan of very low amplitude, also called microbay, opposite to the previous type of scan, the invention implements an assembly composed of thin blades. The thickness of these blades is very small compared to that of the prism 1 crossed, in order to greatly reduce the factor e of the formulas (I) and (II) and therefore to reduce the amplitude of the optical deflection obtained by the same amount. The plates are made of a material transparent to the converging beam to be deflected, at least in a spectral band covering at least the range of sensitivity of the detector used in the main application envisaged below.

The assembly generally has a polygonal configuration such as that shown in horizontal section, by way of example in FIG. 2.

In this figure the polygonal assembly 20 is formed of 8 identical thin blades 21 to 28 of thickness _, with rectangular, planar and parallel external faces. The traces of two successive blades form in the plane of the figure a dihedral with an apex angle equal to 135 °, so as to form an octagon overall, the sides of which are formed by thin blades 21 to 28 two by two opposite, and whose axis of symmetry A is perpendicular to the plane of the figure.

In FIG. 2, the references identical to those of FIGS. 1a and 1b, such as the beams FE and FC and the converging lens 2, designate the same objects. When this assembly 20 is driven by a uniform rotational movement around the axis _X, it causes a lateral deviation Δ x d focal point of the converging beam FC. The expressions of Δ x, introduced previously, are still applicable in the case of the deviation obtained using the thin-blade assembly 20. More precisely, the thickness e used in the expressions I or II is then equal to the sum of the thicknesses of the two opposite blades traversed, such as the blades 21 and 25 of FIG. 2. Indeed, the calculations of geometric optics applied to the crossing of a blade with parallel faces show that the effects produced on a beam given light do not depend on the position of the blade on the mean axis of this beam, whether it is convergent or not. A set of two opposite blades traversed, such as the blades 21 and 25, therefore behaves exactly like a single equivalent blade of thickness 2, double that of each of these blades.

The thickness of this equivalent blade representing a fraction of the thickness of the large amplitude scanning prism 1, described with reference to FIGS. 1 a and 1 b, by example a few percent, the amplitude of the lateral deviation, proportional to the thickness crossed according to formulas I or II, will be reduced in the same proportions. The arbitrarily as small as desired thickness of the blades of the assembly 20 according to the invention allows a sweep of amplitude almost as reduced as desired.

FIG. 3 represents an example of an opto-electronic organization of an infrared camera with a two-dimensional detector equipped with a micro-scanning device according to the invention. The same references designate the same objects as in the previous figures.

Conventionally, a parallel beam F coming from the observed scene is made convergent according to a beam FC and projected onto an infrared detector 31, of the two-dimensional mosaic type, by passing through a focusing lens 2. A polygonal microscan assembly 33 according to the invention, composed of six thin blades in the example shown, is interposed in the path of the convergent beam FC, that is to say between the focusing lens 2 and the detector 31, the axis of symmetry Δ of the assembly 33 being perpendicular to the direction of the converging beam FC. The assembly 33 is driven in rotation by the action of a motor M, the speed of rotation being linked to the multiplexing times (integration, transfer and reading) of the charges released in the detector 31 in proportion to the light flux received and multiplexed conventionally in a CCD circuit integrated into the detector 31. The reading rate is managed by a sequencer 32 which also adjusts the speed of rotation of the motor M through an angular encoder 34. The signal formed by the multiplexed charges, at the output of the detector 31, is processed in a circuit 35 which delivers, in connection with this sequencer 32 a video signal SV according to a TV standard adapted to a display monitor (not shown).

The rotation of the polygonal assembly 33 gives the FE beam, as explained above, a movement of horizontal microscanning in one direction Dx on the detector 31. In order to carry out a micro-sweep in two directions, for example in the directions Dx and Dy indicated in FIG. 3, an alternative embodiment of the polygonal assembly according to the invention is described below with reference to FIGS. 4 and 5.

FIG. 4 illustrates a schematic representation of the beginnings of the first and of the second row of photosensitive cells, C .., ^ _TO '^ 13' ^ 21 ' ^C 22' ^C 23 of a two-dimensional mosaic detector comprising nxp cells

Cij, distributed over a matrix of n rows and p columns, i varying from 1 to n, and j from 1 to p. The cells represented have for example a square shape of 25 μm on a side and are separated, according to the horizontal direction Dx, from 100 μm and, according to the vertical direction Dy, from 70 μm. With a monodirectional microscanning in the direction Dx, such as that described above, everything happens as if additional elementary photosensitive cells are "created", that is to say appear successively in time, between the cells of the detector and therefore on the same lines as these cells. In FIG. 4, the additional cells C. .. ... VS .. . ,- , CC. ..,. , and C ₂₁ -ι, ^? i '^ 21 V ^orιt ^ ^te represented by their center, respectively on microlines Ml and M2 connecting the centers of cells Cil and C12 on the one hand and C21 and C22 on the other hand.

However, the effect of the pairs of opposite and identical thin blades being identical, whatever the couple, the microlines, such as Ml and M2, are scanned identically 2 N times per rotation of an assembly with 2 N identical blades forming N pairs of blades opposite two by two.

In order to carry out a bidirectional microbay according to Dx and Dy, it is advisable to offset successively, in the direction Dy, the scanned microlines so as to "create", successively in time, additional micrognes between these microlines such as those referenced Mil and M12 between Ml and M2 on the figure 4. This creation makes the creation of as many elementary photosensitive cells, represented only by their center so as not to overload the figure. To achieve this offset of the microlines, it is necessary that each pair of thin blades, active for the entire period in which it intercepts the beam to be deflected, produces not only a variable offset in the direction Dx, so as to describe the microlines, but also a constant pitch shift of the microlines in the direction Dy, passing from an active pair of blades to the next pair. Such an offset is obtained, in the context of the invention, by giving the thin blades of the microbay assembly a slight prismatic angle, different from one pair of blades to another.

A bidirectional microscan assembly according to the invention is, globally, approximately like the monodirectional assembly 3 shown in FIG. 3. In order to bring out the respective characteristic shapes of the blades of the mono and bidirectional assemblies, these assemblies have been shown in FIG. 5, in section along a plane passing through the axis of rotation ___ of FIG. 3 and the median lines of a couple of opposing blades. In FIG. 5, the traces of the blades of the unidirectional microscan assembly 33 appear in dotted lines and that of a bidirectional microscan assembly 50 appear in solid lines. The thin blades of the assembly 50, such as 51 and 54 have a prismatic profile with an angle at the apex A, the interior faces of these blades, such as FI. and F4. , being vertical and coinciding with those of the blades of the assembly 33, the external faces, such as FI and F4 being inclined. The prismatic profile of the blades can also be obtained using inclined inner faces and vertical outer faces. In Figure 5, the converging beam FC from the focusing lens 2 has also been shown in section. It is well known to the laws of light propagation that the passage from a light ray to through a slightly prismatic plate with an angle at the apex A, such as 51 or 54, causes an angular deviation approximately equal to (n-1) A. This angular deviation results, for the focal point P5 of the convergent beam, by a vertical offset A y of approximate value:

A y = (n-l) A.l

l being the distance between the prismatic plate and the focal point, and n the refractive index of the optical material constituting the plate.

In Figure '5, the distances from the blades 51 and 54 to the focusing plane containing Ε-. are denoted, respectively, i and 1. The thickness of the blades, assumed to be small, being negligible compared to the distances L and l. , the vertical offsets, introduced by each of these blades, are added and the overall vertical offset has the expression:

If the prismatic angles are different from one pair of blades opposite to another, the vertical offset produced by each of these couples, successively intercepting the converging beam during the rotation of the assembly j is different and the corresponding scanned microline s 'is shifted by as much. The different values of the prismatic angles can form a progression such that the successive shifts of the corresponding microlines form a progression of constant pitch.

Thus, an assembly comprising N pairs of opposite blades, ie 2N blades, causes the scanning of N microlines when each of the N couples consists of prismatic blades of the same angle at the top, this angle being different from one couple to the other. Indeed, the N microlines are scanned twice at each rotation of the assembly since the same pair of opposite prismatic blades causes the same offset twice per rotation, the blades coming to the beam converge in a certain order and an inverted order for two positions of the assembly angularly separated by 180 °.

For example, if the angles at the top of the 3 pairs of blades of assembly 50 are, respectively, 0 ° (blades of constant thickness), A ⁰ and 2A °, two additional microlines are created since a couple opposite blades, such as 51 and 54, causes the same microline offset twice per rotation, respectively (nl) A (1 _{1 +} 1 ₄ ) and (nl) A (_ ₄ + l.).

The angles at the top of the prismatic plates being very small, a few degrees at most, the distortion on the scanning which results from the prismatic effect, caused by the presence of a variable deflection of a light ray as a function of the thickness crossed by this radius, remains negligible.

The invention is not limited to the embodiments described and shown. For example :

- As regards the implantation of the assembly with thin blades according to the invention, it can be envisaged to integrate this assembly with the focusing lens; it can also be envisaged to introduce inside the assembly with thin blades fixed optical elements, such as lenses;

- The assembly of the thin blades can be achieved by any known means, for example by gluing or by fusion between two machined flanges;

- the offset Δ y shown in FIG. 5 can also be achieved by assembling thin blades with flat and parallel faces such as the blades of the assembly 33 but with different values of inclination of the blades relative to the axis \, l the angle of inclination of a pair of opposite blades being identical and the different values of the angles forming a progression such that the successive shifts of the Corresponding microlines form a constant step progression.

Claims

1. Micro-scanning device for periodically deflecting the direction of a convprgen t light beam, characterized in that it comprises an even number of parallel optical blades 2 to 2 (21 to 28; 5 1 to 56) associated so as to constitute an assembly (33, 50) having a polygonal section around an axis of symmetry (Δ) perpendicular to the mean direction of the converging beam (FC).

2. Microbay device according to claim 1, characterized in that the assembly (33, 50) is driven by a uniform rotational movement around the axis (Λ) produced by a motor (M).

3. Device according to claims 1 or 2. characterized in that the assembly (33, 50)) is made of a material transmitting light in a spectral band covering the infrared.

4. Device according to one of the preceding claims applied to a monodirectional microscan, characterized in that the optical blades (21 to 28) are thin blades with distant rectangular external surfaces of a constant thickness (£ _).

5. Device according to claim 4, characterized in that the optical plates (21 to 28) have an angle of inclination relative to the axis Δ.

6. Device according to claim. 5, characterized in that the pairs of opposite thin blades (21, 25) have a constant inclination angle relative to the axis _. this angle being different from one couple to another.

7. Device according to one of claims 1 to 3, applied to a bidirectional microscanning, characterized in that the optical blades are thin blades with external surfaces distant rectangles of variable thickness corresponding to a very small prismatic profile with apex angle (A).

8. Device according to claim 5, characterized in that the pairs of opposite thin blades (51, 54) have an identical prismatic profile, having the same apex angle, the apex angles of the pairs of thin blades being different from one couple to each other.

9. Infrared camera comprising a converging device (2) intended to project the image of a scene to be observed on a two-dimensional infrared detector (31) consisting of a matrix of p columns and n rows of elementary photosensitive cells associated with a circuit multiplexing and reading integrated into the detector (31), and a processing circuit (35) which delivers a video signal (SV) from the signal supplied by the detector (31), characterized in that the infrared camera also includes a micro-scanning device (33, 50) according to one of the preceding claims, and in that the reading rate of the charges from the detector (31) and the speed of rotation of the micro-scanning device (33, 50) are synchronized with the using a sequencer (32).