Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
  • G02B27/01—Head-up displays
  • G02B27/017—Head mounted
  • G02B27/0172—Head mounted characterised by optical features

Definitions

• the present invention relates to an optical device for correcting aberrations affecting an image.
• a device according to the invention makes it possible to correct the distortion due to a spherical concave mirror inclined relative to the direction in which this mirror is observed.
• the invention applies in particular, but not exclusively, to a helmet finder for pilot of an aircraft or helicopter weapons or for operator of a training simulator.
• a helmet viewfinder is an image presentation device integrated into a helmet.
• the viewfinder allows the wearer of the helmet, such as an airplane pilot in flight, to observe visual information simultaneously with the view of the landscape, or of the cockpit, which he most often perceives through a visor of protection.
• the presentation of adapted information allows assistance with piloting and navigation.
• the presentation of a reticle provides assistance in aiming a weapon.
• the information may also consist of a landscape image acquired by sensors different from the eye of the helmet wearer such as infrared image sensors or light intensifiers to supplement or replace the direct view.
• an image generator comprises an imager whose screen, for example a cathode ray tube screen or a liquid crystal screen, makes it possible to display an image.
• the image is most often transported using a relay optic to a combiner which provides a presentation of the transported image superimposed on the landscape view.
• the latter is also focused at infinity by a co-animation optic.
• the coliimation of the image can be achieved by an optic placed between the imager and the combiner; such an embodiment of the prior art has the main drawback of requiring too bulky coliimation optics relative to the restricted field of view provided.
• a combiner having optical power has been proposed; such a combiner achieves for its user both the collimation of the image and the superimposition of the collimated image with the view of the landscape.
• the prior art is rich in numerous and varied devices comprising an optical power combiner. We are particularly interested in image presentation systems comprising a spherical concave mirror to collimate the image.
• a concave spherical mirror achieves an average quality coliimation of an image placed at a particular point in the space located on the axis of the mirror and at a distance from it equal to half of its radius of curvature.
• the eye located on the axis of the mirror receives rays from the imager after their reflection on the spherical mirror, these rays are parallel and lead to the perception by the eye of a coliimatée image.
• the mirror is semi-reflecting, it allows the same eye to observe the landscape by transparency.
• the imager should be on the axis of the semi-transparent spherical mirror and it would obscure the field of view of the user.
• the spherical mirror is tilted relative to the normal to the face and the user's eye is no longer on the axis of the mirror.
• This arrangement has the drawback of leading to a coliimated image affected by optical aberrations, in particular eccentricity, which it is necessary to correct at least partially.
• eccentric distortion of the second kind characterized by a convergence of verticals and an apparent curvature of horizontals.
• the prior art teaches us to correct the distortion of the image provided by an optical assembly to introduce reverse distortion at the level of the imager by electronic correction; this is easily achieved when the imager includes a cathode ray tube, but this solution is not suitable for an imager, such as for example a light intensifier, which does not have the necessary image adjustments.
• the particular surface of the aspherical mirror proposed makes it possible to modify the light rays in order to correct the effects of the spherical concave mirror on the horizontal and the vertical of the image observed and thus ensure a correction of the distortion.
• This correction is achieved by the introduction by the aspherical mirror of a second kind of eccentricity distortion to compensate for the distortion of the same type due to the spherical concave coliimation mirror used off axis.
• the effect of the aspherical mirror leads to making the vertical parallel and the horizontal rectilinear in the coliimated image.
• the image is straightened and orthoscopic, but the overall shape of the mirror locally causes an amplification of aberrations and in particular of astigmatism.
• the correction of the distortion that this invention allows is limited by a degradation of the resolution of the image.
• the problem consists in producing an image presentation device comprising an off-axis spherical collimating mirror, presenting a collimated image satisfactory for the user, that is to say devoid of annoying aberrations and having a large superior field of view. or equal to 40 degrees. It is a question of obtaining a coliimaté image which presents at the same time a good resolution and a good correction of the distortion.
• the spherical collimating mirror being observed at an oblique angle relative to its axis, it introduces a second kind of eccentric distortion characterized by an absence of symmetry of revolution.
• This distortion is particularly dangerous for a user driving a vehicle, since the perception of perspective is degraded.
• the difficulty consists in finding a means for correcting the distortion which does not degrade the quality of the image and such that the entire optical device has a reduced mass and size.
• the invention provides an optical device for a system for presenting collimated images to a user comprising an imager and a spherical concave mirror off-axis, characterized in that it comprises optical means for correcting the distortion of the image. presented to the user which is due to the spherical concave mirror off axis, said means comprising a diffractive field mirror.
• the diffractive mirror includes a reflection hologram. According to the invention, the correction of the distortion is made when the diffractive mirror is placed in the vicinity of an intermediate image of the optical device: it is a diffractive field mirror. Its diffractive effect near the intermediate image makes it possible to move the points of the image non-uniformly.
• the correction made by the diffractive mirror does not degrade the resolution of the image.
• the extent of the neighborhood is limited by the resolution, which is imposed by the rest of the device.
• the diffractive mirror is preferably located at the limit of the neighborhood fixed by the resolution. While being inside the vicinity of the intermediate image, the diffractive mirror is placed at a maximum distance from the intermediate image beyond which it degrades the resolution of the image presented to the user.
• the diffractive mirror can be, for example, located in the vicinity of the first intermediate image, that is to say that which is closest to the eye of the user of the device. However, it is preferably placed in the vicinity of the second intermediate image; this preferred arrangement allows the production of a lighter and less bulky device and in which the hologram is better protected.
• the hologram in the invention is for example digital digital with discrete variations, planar digital continuous profile, it can also be recorded in a photosensitive material.
• the substrate in which the hologram is shaped may be planar, but is preferably not planar; the curvatures of the surface taking care of part of the correction, the hologram ensuring a residual correction.
• the transparent substrate of the volume hologram in a photosensitive layer preferably has a variable optical index or a variable thickness.
• the device also includes one or more optical power or relay groups placed on the ray path between the imager and the spherical mirror, upstream and / or downstream of the diffractive field mirror.
• optical groups also ensure the precorrection of the astigmatism necessarily introduced into the coliimated image because the spherical mirror is observed at an angle inclined relative to the radius which defines the optical axis of this mirror.
• This astigmatism can be corrected for example by a spherical converging lens and a cylindrical lens, in an optical relay group located between the imager and the diffractive mirror. It can also be corrected by a diffractive lens placed in a power group between the diffractive mirror and the spherical mirror.
• the invention makes it possible to preserve an image of good resolution while ensuring a thorough correction of the distortion due to the spherical and inclined collimating mirror.
• the invention has the advantage of correcting the distortion of the image presented to the eye of the user for a large instrumental pupil, for example at least 15 millimeters in diameter, and for a wide field typically greater than 40 degrees.
• the instrumental pupil is the area of space in which the user of an instrument must place the pupil of his eye to use it. This correction is particularly interesting when a distortion cannot or cannot easily be imposed on the imager. Indeed in such a case an electronic correction of the prior art is not suitable.
• the first pupillary image of the device is inclined relative to the optical axis, the diffractive mirror according to the invention gives a second pupil image straightened on the optical axis.
• the invention can be integrated into a helmet sight having a wide instrumental pupil and a wide field.
• FIG. 1 schematically and partially represents an optical device with a spherical combiner mirror off the optical axis
• FIG. 2 represents the distortion that the invention corrects
• FIG. 3 represents a preferred embodiment of a device according to the invention
• FIG. 4 shows another embodiment of a device according to the invention.
• the optical diagrams are shown developed in a plane called the plane of symmetry of the optics. This plane contains the normal to the pupil of the entrance of the eye of the user and the center of the sphere supporting the spherical mirror.
• mirrors not shown, which do not introduce aberration, make it possible to direct the beams in three dimensions, to satisfy various space constraints: for example so that the device is adapted to the contour of the head of the user.
• a user of an optical device comprising a spherical mirror 1 is represented by the plane of the pupils 2 and the straight line 5 normal to this plane 2.
• the pupil 11 of the eye is generally located optically at 3 millimeters in removal of cornea 12 from the eye 3.
• the straight line 5 may correspond to the view of the user straight in front of him, or else to a view upwards, downwards, towards one side. or the opposite side.
• the spherical mirror 1 is placed in front of the user, its concavity is turned towards the user.
• the intersection of the observation axis 5 with the mirror 1 is designated by the reference 6.
• the spherical mirror 1 is supported by a sphere S whose center 4 does not belong to this straight line 5.
• the plane P of FIG. 1 is a plane of space which contains the center 4 of the support sphere of the spherical mirror 1 and the line 5 passing through the center of the pupil 11 of the eye 3. It is the plane of incidence of the line 5 on the spherical mirror 1, it is called the plane of symmetry of the optics. Most often this plane is coincident with the plane passing through the center of the pupil 11 and parallel to the theoretical plane of symmetry of the face of the user.
• the line 5 and the radius 7 of the sphere S passing through the point of intersection 6 are separated by an angle ⁇ .
• a non-zero value of this angle ⁇ characterizes an off-axis use of the spherical mirror 1.
• the spherical mirror 1 itself is said to be "off-axis".
• an image whose center 9 is placed at a distance equal to half of the radius of curvature of the sphere S on this optical ray is perceived by the user's eye 3 as coliimated in the first order because the light rays coming from the image thus placed are reflected by the spherical mirror 1 in the direction of l eye 3 in the form of a beam of substantially parallel rays.
• the center image 9 may have field curvature.
• the coliimation by reflection on the spherical mirror is not perfect, it is affected, in addition to the aberrations intrinsic to this mirror, an optical aberration of eccentricity due to the off-axis use of the spherical mirror 1.
• the spherical mirror 1 may be semi-transparent. In this case light rays 10 coming from the environment external to the spherical mirror 1, that is to say coming to strike the convex face of this mirror, are transmitted to the eye 3 by the spherical mirror 1. This spherical mirror 1 then produces a combiner which superimposes a coliimated image with the direct view of the environment. This arrangement is generally adopted in a helmet sight.
• the central field is defined as the beam of light rays coming from the center 9 of the image to be collimated.
• the path of this light ray is the optical axis of the device used.
• the optical axis is generally a broken line.
• Line 5 supports part of the optical axis. Most often, the image is presented straight in front of the user, the line 5 is then substantially normal to the face of the user, but the image can for example be presented at the top of the field of vision of rest at infinity of the user and the line 5 is then oriented in the corresponding direction.
• FIG. 2 represents the image perceived by the eye of the user of an optical device according to FIG. 1 in which an image centered on point 9 and comprising a square with a regular square grid is collimated.
• the perceived deformation is a second kind of eccentric distortion: the vertical lines which should be parallel lines are converging and the horizontal lines which should be parallel lines are curved. This particular distortion is due to the inclination of the spherical collimating mirror relative to the axis of observation; it presents an absence of symmetry of revolution.
• a driver uses an optical device according to FIG. 1 to steer his vehicle, he is greatly disturbed by the deformation between the image presented and the real landscape. Heights are overestimated and speeds are underestimated.
• FIG. 3 paths of light rays inside a preferred embodiment of a device according to the invention are shown.
• the imager comprises a screen such as for example the screen of a cathode ray tube or a liquid crystal screen.
• the screen can also be produced for example by a fiber optic beam section or a slide or the screen of a light intensifier tube.
• An image whose surface is arbitrary is displayed on the screen 20 of the imager represented by its tangent plane.
• the image provided by the imager can be flat, spherical or even have another shape. The paths of the light rays from the screen 20 of the imager to the eye 3 of the user are traced for this embodiment of the invention.
• the device comprises a spherical mirror 1 placed in front of the user's eye 3 and a field diffractive mirror 21 placed between the screen 20 and the spherical mirror 1.
• a diffractive mirror is a diffractive optic which works in reflection.
• the device also includes a power unit 22 between the diffractive field mirror 21 and the spherical mirror 1, as well as relay optics 29 between the screen 20 and the diffractive field mirror 21.
• the light rays coming from the screen 20 of the imager are received, after passing through relay optics 29, by the diffractive mirror 21; they are reflected and deflected by the latter then pass through the power unit 22 before striking the spherical mirror 1 off-axis which ensures a coliimation of the image finally perceived by the eye 3 of the user.
• the light rays coming from the center of the screen 20 of the imager form the central field of the imager.
• the optical axis of the device corresponds to the path of the radius of the central field which passes through the center of the pupil of the user's eye 3.
• the path of the light rays is now observed in the other direction, that is to say starting from the user's eye 3 and going up the various optical elements towards the screen 20 of the display.
• the rays from the eye are reflected on the spherical mirror 1 off-axis then form a first intermediate image 25.
• the image perceived by the eye is combined with the first intermediate image 25 by the spherical mirror 1.
• the optical axis which, in the example of FIG. 3, is horizontal on a first part 31 between the center of the pupil of the eye 3 and the spherical mirror 1 is also reflected on the spherical mirror 1.
• This part 31 of the optical axis and its reflection on the spherical mirror 1 define a plane called the plane of incidence of the optical axis on the spherical mirror 1 off-axis.
• the plane of incidence coincides with the plane of symmetry of the optics which is represented by the plane of FIG. 3.
• the plane of symmetry of the optics is a plane containing the path described by the optical axis between the imager and the pupil of the user.
• an embodiment of the invention is not limited to an optic in this plane; in the context of the invention, it is always possible to add additional plane mirrors allowing, for example, to exit optical elements outside the plane of the figure. Indeed, flat mirrors, also called folding mirrors, do not modify the optical function, they do not provide and do not correct aberration but they allow the optical rays to bypass obstacles such as the user's head.
• the rays reflected by the spherical mirror 1 strike, in this embodiment, a plane mirror 23 which allows the folding of the optical rays while respecting the plane of incidence of the optical axis on the spherical mirror 1.
• the invention can be produced without this plane mirror 23.
• the optical axis is oriented along a straight line 32 of the plane of incidence.
• first pupillary image 24 which is the image of the pupil of the eye 3 given by the spherical mirror 1 off axis.
• the normal to the plane tangent to this first pupil image 24 is not parallel to the corresponding section 32 of the optical axis.
• the first pupillary image 24 is inclined on the optical axis. This inclination is an effect of the distortion to be corrected.
• the power group 22 is placed for example so that the first pupil image 24 is on the path of the light rays between the spherical mirror 1 and the power group 22.
• the power group is preferably centered on the second part 32 of the optical axis. It includes at least one converging lens. And in the embodiment illustrated in FIG.
• the power unit comprises a diverging lens placed between a first and a second converging lens; these successive lenses, each having a reduced optical power, limit the aberrations introduced by the power unit 22 itself.
• the group 22 reduces the opening of the incident beam on the diffractive mirror 21. This opening is very small in comparison with the opening of the incident beams on the spherical mirror 1.
• the power group focuses the first intermediate image 25 on a second intermediate image 27. It affects the image and it allows the optical device according to the invention to present good image quality.
• This power group is an optical element close to the first pupillary image 24; it has little effect on the latter.
• the diffractive mirror 21 is placed in the vicinity of the second part 32 of the optical axis, the first pupil image 24 is on one side of the power unit 22 and the diffractive mirror 21 is on the other side.
• the diffractive mirror 21 reflects rays coming from the pupil of the eye towards the screen 20 of the imager.
• the plane of FIG. 3 is also the plane of incidence of the optical axis on the diffractive mirror 21.
• the diffractive mirror 21 is close to the second intermediate image 27 that the device forms from the image displayed on the screen 20.
• the mirror 21 imposes at each point on its surface a particular deviation from each light beam it receives.
• a point of the image 27 is formed by rays which are both reflected by the mirror 21 and deflected by the diffractive power of this mirror.
• the local phase difference applied by the mirror 21 to the light wavefront is recorded in a hologram and the pitch of the fringes interference is proportional to the derivative of the phase function.
• the deviation imposed on a light ray is all the more important as the fringes are tight.
• the mirror If the mirror is far from the image, which in this example is the second intermediate image 27, it imposes an overall deformation of the image which does not correct the annoying distortion.
• the deviations move the points of the image independently of each other.
• the proximity of the intermediate image makes it possible to separate the points of the field, the displacements of the points are not uniform and they allow a correction of the distortion distortion of the image.
• the distortion can only be corrected by large deviations; the phase function ensuring the correction then presents significant fluctuations, it is difficult to control and to carry out. In the extreme, when the mirror 21 is exactly on the image, the deviation imposed by the mirror at each point of the image is zero.
• the hologram placed in the vicinity of the intermediate image 27 deflects a light ray from the image without modifying the local focusing: it shifts the position of a point in the image without modifying the quality of the image.
• the diffractive mirror 21 according to the invention affects the distortion of the image without affecting the resolution.
• the diffractive mirror 21 makes it possible to correct the distortion of the image introduced by the spherical coliimation mirror 1 used off-axis.
• the mirror is preferably at a distance which corresponds for the center of the field to the resolution limit of the image, the vicinity of the image is limited by this distance depending on the resolution. At the edge of the field, a lower resolution is tolerated.
• phase function of the hologram is calculated by projection on a reference base, preferably a polynomial or Zernike type base. Such projection provides a slowly variable phase function. The calculated coefficients are then recorded on a substrate.
• the hologram is for example a reflective digital hologram: the phase function is digitized and recorded in a substrate in the form of a variation in thickness of the substrate.
• the variation can be discrete as for example in a digital digital hologram with discrete variations which can be obtained by attack of the substrate with through binary masks.
• the variation can be performed analogically, as for example in a flat digital hologram with a continuous profile, which is in particular produced with masks with variable transmission.
• the hologram is recorded on a substrate, for example glass; it may be a blade with flat and parallel faces, but the surface is preferably not planar, which has the advantage of relieving variations in incidence on the substrate and / or of performing part of the optical function of the mirror 21
• the reflective hologram can also be a volume hologram recorded in the photosensitive surface of a transparent support using a recording bench using two waves arriving on either side of a surface made of a photosensitive material such as for example a bi-chromated gelatin.
• the transparent support of the sensitive surface can be a glass slide with flat and parallel faces. But it can also have a variable thickness, or have a variable optical index depending on the position on the surface of the substrate.
• the substrate is a spherical plate and the hologram performs the residual correction that the spherical plate alone cannot provide.
• the hologram according to the invention is supported for example by an aspherical surface or a Mangin mirror.
• the position of the diffractive mirror 21 in the vicinity of the second intermediate image 27 makes it possible to place it fairly far from the eye of the user.
• the hologram is placed inside the helmet in a location protected from attack such as humidity or contact with the pilot's fingers.
• the third part 33 of the optical axis corresponds to the reflection of the second part 32 of this same optical axis on the diffractive mirror 21, one observes there, between the diffractive mirror 21 and the screen of the imager 20, a second pupillary image 30 which has a tangent plane substantially normal to the local optical axis 33.
• the diffractive mirror 21 transforms a pupillary image 24 inclined on the optical axis into an image pupillary 30 perpendicular to the optical axis.
• the diffractive mirror 21 allows the device according to the invention to present a good quality of pupil without affecting the quality of the image.
• the useful part of the diffractive mirror 21 has a tangent plane whose normal 28, belonging to the plane of incidence, is not parallel to the second part 32 of the optical axis.
• the diffractive mirror 21 is inclined relative to the optical axis, it is said to be off-axis.
• the opening around the axis 28 is sufficient to optimize the draft left available to place, for example, return mirrors between the diffractive mirror 21 and the nearest lens in the power group 22.
• the angle of incidence the optical axis on the mirror 21 also makes it possible to limit the useful surface and thus to maintain good image quality over the entire surface.
• the angle of incidence is preferably close to 45 degrees.
• the useful surface of the mirror 21 is for example estimated by a diameter of approximately 45 millimeters.
• the optical device according to the invention illustrated in FIG. 3 comprises a relay optics 29, placed between the diffractive mirror 21 and the screen 20 of the imager, to distance the screen 20 of the imager from the diffractive mirror 21.
• This distance is generally made necessary to meet space constraints. It allows for example for a helmet finder to place the entire imager, which can be a cathode ray tube, at a satisfactory position in the available volume of the helmet.
• the beams of light rays between the relay optics 29 and the diffractive mirror 21 have a very small aperture. These beams are downstream of the diffractive mirror 21 by considering the paths of inverted beams, that is to say from the eye to the imager. The opening is very small compared to that of the beams on the spherical mirror 1.
• the relay optics 29 is substantially aligned with the third part 33 of the optical axis. This essentially centered relay optic is simple to achieve.
• the relay optics 29 also has optical power functions for pre-correcting near the imager the astigmatism which will be introduced by the off-axis observation of the spherical mirror 1. In an alternative embodiment, this correction of the astigmatism is not performed at the relay optics 29 but in the power unit 22, which then comprises for example a diffractive lens and a converging lens.
• the relay optics 29 also comprises a mixing cube 26, or a semi-reflecting blade, which allows the mixing of the channel of the screen 20 with a channel of another not shown display. in FIG. 3.
• the cube 26 makes it possible, for example, to superimpose visual information from a cathode ray tube and that coming from an assembly (not shown) comprising a shooting objective and an image intensifier.
• the magnification between the two pupillary images 30 and 24 is preferably of a value close to one.
• the practically unitary pupillary conjugation has the advantage of reducing the size of the optical device, it allows minimization of the size of the optics along the optical path. This reduction in size is advantageous for the weight of the device and for its cost.
• the optical device comprises, between an inclined spherical collimating mirror 1 and the screen 20 of the imager, a diffractive mirror 41 placed near the first intermediate image 25.
• the substrate of the hologram is for example a flat glass slide with parallel faces.
• the optical assembly 42 provides a conjugation of the intermediate image 25 on the screen 20, it comprises several lenses including two aspherical lenses 43, 44.
• the assembly 42 may also include a mixer cube 45 associated with another light source.
• this embodiment of the invention requires a heavier and more expensive optical assembly than the preferred embodiment illustrated in FIG. 3.
• the spherical mirror 1 off-axis can be semi-transparent, in this case, the light rays emitted by the landscape or the environment in the field of view of the user are transmitted by this mirror and are received by the pupil of the eye simultaneously with the rays reflected by this same mirror and previously described.
• the semi-transparent mirror is a combiner. It is therefore a spherical combiner used off-axis.
• the combiner is preferably part of a visor for protecting the eyes and even the face of the user.
• a visor according to the invention has at least one spherical reflecting part off axis. In use position the visor is folded down so that the part corresponding to the spherical mirror 1 is placed in front of the user's eye.
• the entire device for presenting collimated images can be integrated into a helmet, for example for an airplane or helicopter pilot, and makes it possible to produce a helmet viewfinder.
• the viewfinder can be monocular if it presents the coliimated image to only one eye.
• the viewfinder can be binocular if it includes the presentation of an image for each eye. It has the advantage of allowing a pleasant vision when the overlap of the fields of view of the two images is total.
• a binocular viewfinder can also present a partial overlap of the two fields of view which allows for the same size of the optics to obtain a wider field of view without damaging the perception of the information presented.
• the distortion of an image having a grid leads to the distortion of the grid.
• the images presented to the user and of which the distortion inherent in the concave spherical off-axis visor is corrected are particularly advantageous for a helmet viewfinder because they respect the real dimensions of the objects represented. This is essential when the viewfinder presents an image superimposed on the direct view and is even more so when the image presented replaces the direct view for the user, for example in the case of night vision assisted by an intensifier. image, infrared vision or training simulator.
• the correction of this distortion has the advantage of allowing the user a good appreciation of the distances on the image he observes and of allowing him, for example, to pilot at night without positioning error.

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• 1998
  • 1998-10-06 FR FR9812499A patent/FR2784201B1/fr not_active Expired - Fee Related
• 1999
  • 1999-10-05 IL IL14243999A patent/IL142439A0/xx active IP Right Grant
  • 1999-10-05 EP EP99970171A patent/EP1127288A1/de not_active Withdrawn
  • 1999-10-05 WO PCT/FR1999/002378 patent/WO2000020913A1/fr not_active Application Discontinuation
  • 1999-10-05 US US09/806,936 patent/US6788442B1/en not_active Expired - Fee Related
• 2001
  • 2001-04-04 IL IL142439A patent/IL142439A/en not_active IP Right Cessation

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