Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B13/00—Optical objectives specially designed for the purposes specified below
  • G02B13/14—Optical objectives specially designed for the purposes specified below for use with infrared or ultraviolet radiation
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B13/00—Optical objectives specially designed for the purposes specified below
  • G02B13/06—Panoramic objectives; So-called "sky lenses" including panoramic objectives having reflecting surfaces

Definitions

• the invention relates to the field of shooting or observation requiring significant angular coverage. Such observations are required, for example, when it comes to monitoring or panoramic surveillance.
• the spectral range concerned is that of visible or infrared radiation depending on the operating conditions (night, day, in space, on the ground).
• the choice of the type of detectors used is then determined in particular as a function of the coincidence between its spectral range of sensitivity and the domain targeted radiations.
• the object of the invention relates, more particularly, to an omni-directional watch device covering the entire surrounding space using adjoining observation fields.
• the invention applies both to airborne and terrestrial surveillance, without limiting the spectral range:
• the cover is then limited to the upper half space
• the spectral band covered is a function of the type of dedicated mission: for example, the spectral band concerned is 3 - 5 ⁇ m by setting up a detector with platinum-silicon (Pt-Si) when one s 'at- task more particularly to the detection of hot spots, such as missiles or propellants.
• Pt-Si platinum-silicon
• the first method consists in associating a fixed objective with very large field, conventionally called "fish eye” in the field of photography, with a two-dimensional detector of the matrix type.
• a two-dimensional detector of the matrix type The use in the infrared band of such a lens (with a field of 112 ° and an aperture at F / 1.2) associated with a Pt-Si detector matrix of 512 ⁇ 512 elementary sensors is described in SPIE Journal volume 1488, pages 368 to 375.
• the main advantage of this method lies in the fact that it makes it possible to dispense with the use of an optomechanical scanning device: in fact, a matrix detector directly captures the entire image of the scene observed, without the need for scanning by projection of this image onto the detector using an appropriate focusing optical group.
• the exploration of the landscape can thus be carried out at a very high rate, limited only by the integration times of the charges released at the level of the sensor in proportion to the illumination which it receives, according to the well-known CCD technique.
• the major drawback of this type of solution is its low angular resolution. Angular resolution is conventionally defined as the minimum angle for which two objects, located at a distance corresponding to a given range, are seen distinctly by the same sensor. The larger the field to be covered, the lower the resolution.
• the resolution is in fact directly limited by the area of the elementary sensors of the matrix detector and therefore, for a matrix of given dimension, by the number of elementary sensors in this matrix.
• this resolution can only be considered as constant along the field, that is to say for the sensors located in the center of the matrix and whose fields of conjugated objects undergo practically no distortion.
• a second method implements an optomechanical scanning system associated with a detector making it possible to cover a wider field.
• the detector then takes the form of a longitudinal strip comprising a reduced number of rows of elementary sensors.
• the optomechanical system performs the scanning of the strip on an intermediate image formed by an optical head system which, depending on the dimensions of the strip, is carried out in one direction (raster scanning) or two directions (line and raster scanning).
• the scanning is carried out using an oscillating mirror or a rotating prism with a certain period defining the scanning rate. For example, a system with mirror scanning head has been produced, providing 120 ° x 80 ° coverage in the infrared radiation field at a rate of 2 hertz.
• a prism scanning system is also described in the Journal SPIE volume 782, pages 300, 38 and following. This is an exploration of 3.3 steradians at a frequency of 3.6 hertz.
• the prior systems do not offer total coverage of the entire surrounding space.
• the invention proposes using several wide-field objectives of the "fish eye” type, each objective being associated with a two-dimensional detector to form a detection module, the set of detection modules optimally covering an object space of dimension as large as desired; in addition, the distortion of each objective is adapted so as to make the object fields of two adjoining detector mosaics without overlapping, the number of detection modules being determined as a function of the space to be covered.
• the invention relates to an omnidirectional monitoring device with coverage optimum of the surrounding space by joining fields, comprising optical means for forming images on at least one two-dimensional detector of the CCD type capable of delivering a video signal for displaying the projected image, characterized in that the optical means comprise several wide angle lenses of the "fish eye" type, each wide angle lens being associated with a two-dimensional detector to form a detection module, in that the number of detection modules is adapted to the overall dimension of the 'surrounding space to be covered, and in that each wide angle lens has a distortion law chosen to join the limited object fields covered by two neighboring detectors.
• the omnidirectional vei-lle device covers a complete object sphere, that is to say a solid angle of 4H steradians, using six wide angle objectives associated with six detectors to form six detection modules arranged on the faces of a cube.
• the device according to the invention offers a satisfactory and uniform angular resolution whatever the position of the object observed in the surrounding space. Indeed, the multiplication of the number of objectives restricting the field covered by each, for example of the order of 2 steradians in the proposed embodiment, the angular resolution is not tainted ( ⁇ 0.5 °) and remains uniform due to the relatively conservative distortion of the wide-angle lenses used.
• the invention allows high operating rates (100 Hz to 1 KHz) for minimum mechanical complexity, without the need for a dome of delicate design, nor for a scanning system.
• the detectors have insufficient filling rates of elementary sensors, there is added to each detector a micro-scanning element, of the rotating blade type, to remove the areas of the object space made inaccessible due to the existence of blind zones si ⁇ killed between the elementary sensors of the detector.
• FIGS. 5a to 5d four positions of an elementary sensor of the same two-dimensional detector during microscanning.
• FIG. 1 An example of optical architecture of a wide angle lens of the "fish eye” type, operating in the spectral band 3 to 5 ⁇ m, is provided in FIG. 1.
• This objective comprises an optical group of divergent input 1 associated, along the same optical axis XX ′, with a second globally convergent optical group 2.
• Such an objective open to F / 1 covers a field of 110 °; the light rays coming from this field such as those symbolized by arrowed lines on the fi gure 1, pass through the optical groups 1 and 2 so as to be projected onto a detector D.
• the plane of the exit pu ⁇ pile P of the optical system coincides with the cold screen of a cooling cryostat C in which is arranged the detector D, so that the latter only "sees" the useful flux coming from the field of observation.
• a conventional matrix detector of 256 x 256 elementary sensors forming a network of spatial pitch equal to 50 ⁇ m can be used in such a module; under these conditions, the objective then has a focal distance of the order of 8.2 mm and an exit pupil diameter of the same value, for an aperture F / 1.
• each of the six detection modules, EF1 to EF6, each module is composed of a wide angle lens, respectively 01 to 06, and of a detector Dl to D6; has an axis of symmetry which crosses perpendicularly and in its center, a face, respectively FI to F6, of a cube K.
• the axes of symmetry of two opposite focusing sets are merged and form three axes XI X6, X2 X4 and X3 X5 which intersect at the center 0 of the cube.
• the cooling cryostats of the detectors can be either independent, or grouped together to form a single common cooling system.
• Processing circuits video detectors form six sequences of images viewed simultaneously on six different monitors, or alternatively can, by switching, form one of the sequences of images on a single monitor.
• the realization of such cooling systems or such switching circuits is within the reach of the skilled person.
• the distortion of each wide angle lens is adapted so that the object fields of two neighboring detectors are join ⁇ tive without overlapping.
• FIG. 3 which represents the angular coverage of a fish eye makes it possible to define the conditions so that the fields covered by two neighboring objectives are contiguous.
• the preceding cube K has been represented, centered at point 0, and the vertices MNPQ of the front face F3 crossed by the axis X30X5 of the front and rear detection modules.
• the detection modules EF1 to EF6 of FIG. 2 are not shown in FIG. 3 for reasons of clarity.
• Each of the wide angle objectives must cover a minimum field so that the sum of all these fields covers all of the surrounding space. This minimum field is at the same time maximum so that the fields exploded by the six detection modules remain contiguous without overlapping on 411 steradians.
• the frontal detection module covers the solid angle centered on O and based on the vertices M, N, P, Q of the cube K.
• the field angle covered by each of the wide angle objectives of an omnidirectional standby device according to the invention is modulated by adapting the distortion caused by each of the wide angle objectives.
• ⁇ is the half-angle of field aperture k ( ⁇ ) is a function adaptation coefficient of ⁇ f is the paraxial focal length k ( ⁇ ) .f is the equivalent focal length p is the distance to the axis of a - elementary detector sensor.
• a simple calculation of k for extreme values of p that is to say for the elementary sensors located at the edge of the detector at a vertex and in the middle of the sides of a matrix detector supposed to be square and supposed to be placed near from the center 0, corresponding for example to the angular field apertures respectively 0X3, OP and 0X3, 01, shows that the coefficient k is a slightly increasing function of ⁇ from 0 ° to about 55 °: the adapted distortion therefore increases the focal equivalent at the edge of the field, which corresponds to a cushion-type distortion.
• a uniform angular resolution of 0.37 ° over 4fi steradians can be obtained.
• the elementary sensors of the two-dimensional detectors are not contiguous. For technological reasons, they are separated by non-photosensitive blind zones.
• micro-scan making it possible to capture the light fluxes normally intended for these blind areas but deflected, thanks to this micro-scan, on the elementary photosensitive sensors.
• microba ⁇ layings are obtained using blades with parallel faces, the implementation of which is described for example in patents FR 2 647 995 or EP 289 182.
• a blade with parallel faces in the flux converge of each detection module, for example between the exit pupil of each wide field objective and the associated detector, according to a particular implementation illustrated in FIG. 4.
• a blade with a parallel face L has a thickness e and is inclined at a very small angle ⁇ , excessively enlarged in the figure for the sake of clarity.
• the blade L is rotated around the optical axis XX 'and is placed in a convergent beam of each wide angle objective of the standby system.
• a motor for example an asynchronous motor, performs the rotation of the blade using a suitable pinion.
• This inclined blade causes an offset of the optical beams.
• the central radius, RC represented by an arrowed line in FIG. 4
• ⁇ ' is refracted by crossing the blade with parallel faces L at an angle ⁇ 'and comes out of the blade with a shift ⁇ equal to e sin ( ⁇ - ⁇ ') / cos ⁇ '.
• the angle ⁇ is calculated so that the rotation of the blade prints on the elementary field, corresponding to each elementary sensor, a nutation allowing the coverage of the blind field zones. This results in a corresponding "virtual" nutation of each elementary sensor allowing the coverage of the blind zones located on the detector.
• FIGS. 5a to 5d four "virtual" extreme positions have been shown, of an elementary sensor, RSTU, successively taken during the rotation of the blade with parallel faces.
• the hatched areas which represent the blind areas surrounding the elementary sensor, are successively layered by portion during the rotation of the blade.
• the angle of inclination ⁇ of the blade is calculated so that, for example> point S performs the nutation represented felt in FIG. 5a by the circular curve drawn in dotted lines, during the rotation of the blade.
• the integration of the charges is carried out between the different extreme positions a, b, c, d of FIG. 5.
• the integration is then done four times per turn of the blade.
• the refresh rate of the complete image being effected at each turn of the blade the exploration rate is divided by four in the proposed embodiment. For example, for integration times between each pass of the order of 2.5 milliseconds, or 400 KHz, the exploration rates obtained are 100 KHz, which remains much higher than the performance of scanning systems. (of the order of a few hertz.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Health & Medical Sciences (AREA)
• Toxicology (AREA)
• Photometry And Measurement Of Optical Pulse Characteristics (AREA)
• Studio Devices (AREA)

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• 1992
  • 1992-06-12 FR FR9207112A patent/FR2692369A1/fr active Granted
• 1993
  • 1993-06-11 EP EP93913116A patent/EP0645020A1/de not_active Ceased
  • 1993-06-11 WO PCT/FR1993/000560 patent/WO1993025926A1/fr not_active Application Discontinuation

Non-Patent Citations (1)

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