NIGHT VISION SPYGLASS WITH ONE EYELECTRIFUGAL FOR TRANSPORTING A DISTORTION-CORRECTED IMAGE AND ASSOCIATED CALCULATION METHOD

Description

[0001] The field of the invention is that of night vision goggles incorporating a light intensifier. More specifically, the object of the invention relates to image-inverting devices.

[0002] Typically, night vision goggles consist of a lens, a light-intensifying device that transforms the image from the lens into a intensified image, and an eyepiece. The light-intensifying device includes a photocathode that converts the photons from the lens image into electrons, a microchannel plate that amplifies these electrons electronically, and a phosphor screen that converts the amplified electrons back into photons, as the electrons are accelerated between the microchannel plate and the phosphor screen. There are different types of photocathodes, such as GaAs or multi-alkali photocathodes. There are also different types of phosphors. P22 or P43 phosphors emit green light, while P45 phosphors emit white light.

[0003] The magnified image is viewed through an eyepiece. Typically, the field of view has a diameter of approximately 40 degrees. Its standard focal length is 27 mm, the same as that of the objective lens. The eye relief is 25 mm.

[0004] If the telescope contained only these elements, the image would be seen inverted by the observer. Therefore, the telescope must include an optical device that inverts the image from the objective lens. There are different types of such devices.

[0005] The first device consists of placing a pad of inverting optical fibers against the phosphorescent screen. A diagram of such a telescope is shown on the figure 1 This telescope comprises, in this order, the objective lens 1, the light intensifier 2, the fiber optic array 3 containing the inverting fibers 31, and the eyepiece 4. The intensifier 2 includes the photocathode 21, the microchannel array 22, and the phosphor screen 23. The observer is symbolized by an eye Y. The object O and its various images through the device are represented by oriented arrows. The path of the light rays is shown by thin lines. Currently, it is possible to obtain high-resolution fiber optic arrays. For example, an array can have a resolution of 64 lp / mm, "lp" meaning line pair. For a typical screen diameter of 18 mm, this results in a resolution of 2300 dots per line. These fiber optic wafers dedicated to intensification are specified by the MIL-I-49428 standard entitled "Military Specification: Image Intensifier assembly, 18 mm, microchannel wafer" (06-Nov-1989).

[0006] In practice, inverting fiber arrays exhibit a number of inherent defects in their manufacturing technology, introducing image artifacts that reduce the intrinsic performance of the intensifier tubes. These defects, known in English as "shear distortion," "gross distortion," "image inversion," and "chicken wire," are described in section 3.5.2 of MIL-I-49428 and are detailed below.

[0007] The inverting fiber array can introduce a significant misalignment between the input and output of the image intensifier tube. This misalignment is described in Section 3.6.14 of MIL-I-49428. This misalignment results in a lack of parallelism between the input and output fields of the monocular and a lack of parallelism between the fields seen by the right and left eyes when both monoculars are mounted on the same stand in a dual-monocular configuration. Similarly, due to the presence of the inverting fiber array, misalignment of the tube itself with the optics also introduces a lack of parallelism. When tolerable parallelism is minimal, for example, in avionics applications, optical adjustments must be made to compensate for it.

[0008] Furthermore, the fiber array can generate losses in MTF (Modulation Transfer Function) and contrast due to modulation loss near the zero frequency, which is not considered in the MTF. These losses can be significant in the field.

[0009] Luminance uniformity defects can also appear and be noticeable. These losses and uniformity defects vary in severity depending on the area of ​​the emission indicator used by the eyepiece at the tube's exit point. These losses necessitate sizing the eyepiece to utilize rays whose emission indicator diverges from the eyepiece's optical axis, thus requiring larger diameters for the eyepiece lenses. The position of the pupil within the screen space must then be controlled, which reduces the optical design flexibility of the eyepiece.

[0010] Finally, when the night vision goggles are worn by a user, the presence of inverting optical fibers adds an extra weight that is not necessarily negligible.

[0011] Another type of solution for inverting the image involves introducing a set of erecting prisms between the phosphorescent screen and the eyepiece. US patent 5,898,519, entitled "Compact Night Vision Device," describes such a solution. The eyepiece assembly 40 described in this document is reproduced on the figure 2 It features a Schmidt-Pechan prism 41 positioned between the screen 23 and the eyepiece 42, which corrects the image. French patent FR 2 721 719, entitled "Compact Night Vision Binoculars," also describes an optical combination in which one of the two eyepieces incorporates an image-rectifying assembly with two roof prisms. Prisms have the advantage, compared to the previous solution, of maintaining excellent image quality. However, prism solutions necessarily involve a folded optical axis. This folding makes them less compatible with certain night vision scope usage configurations, such as mounting them on a weapon. Furthermore, these solutions are mechanically more complex.

[0012] A final type of solution for inverting the image involves introducing an image transfer between the screen and the eyepiece itself. This solution has many advantages, provided the perceived image is of good quality. As we have seen, the fields of view of night-vision goggles' objectives and eyepieces are generally large, on the order of 40 degrees. It is therefore essential to maintain good image quality across the entire field. The optical aberration that can significantly degrade image quality is distortion, which increases, as a first approximation, with the third power of the field. Thus, it is common to observe distortions reaching 40% to 50%.

[0013] US patent 5,604,630, entitled "Night Vision Monocular with balanced optics," describes an optical solution featuring an image-carrying eyepiece. The distinctive feature of this optical architecture is that the objective lens and eyepiece incorporate identical optical subassemblies. This symmetry helps reduce system distortion. However, the choice of similar architectures for the objective lens and eyepiece presents other drawbacks.

[0014] The method for dimensioning an image-transporting eyepiece for night vision goggles according to the invention makes it possible to optimize the optical combination while allowing a distortion aberration compatible with a given resolution at the edge of the field.

[0015] More specifically, the invention relates to a night vision goggle as defined in claim 1.

[0016] Advantageously, the eyepiece includes an optical path comprising an optical splitter disposed between the first optical group and the second optical group, a fifth optical group and a display, the image of the display given by the fifth optical group and the optical splitter being confused with the imager.

[0017] Advantageously, the eyepiece includes a second channel comprising the optical splitter, a sixth optical group and a photodetector array.

[0018] The invention also has as a second object a method as defined in claim 4.

[0019] The invention also relates to a night vision scope as defined in claim 5.

[0020] The invention also relates to a method as defined in claim 6.

[0021] The invention will be better understood and other advantages will become apparent upon reading the following description, given by way of non-limiting example, and with the help of the attached figures, among which: There figure 1 represents an early night-vision goggle according to prior art; The figure 2 represents a second night-vision goggle according to the prior art; The figure 3 represents a first embodiment of a night vision goggle according to the invention; The figure 4 represents a second embodiment of a night vision scope according to the invention.

[0022] As a non-limiting example of implementation, the figure 3 represents the ocular part of a night vision goggle according to the invention. The goggle comprises a light intensifier device and an eyepiece 50, the light intensifier device comprising an imager 23. The eyepiece is an image-transport eyepiece. It comprises a first optical group 51 with a field of view arranged in front of the imager, a second optical group with an aperture 52 forming an inverted intermediate image I1 of the imager I0, a third optical group 53 arranged in the vicinity of this intermediate image, and a fourth optical group 54 forming from this intermediate image a collimated upright image at infinity visible to the observer Y. At least two diopters of the first optical group 51 or of the third optical group 53 and one diopter of the second optical group 52 have aspherical surfaces calculated so as to obtain a distortion of the upright image compatible with the resolution of the light intensifier device.

[0023] To determine aspheric surfaces, the calculation process involves the following two steps: calculation of the two third-order and fifth-order distortion coefficients of the rectified image as a function of the object field radius, the angular radius of the image field, the paraxial focal length and the resolution at the edge of the field; optimization of the aspherization coefficients of at least two diopters distributed between the first optical group and the third optical group and one diopter of the second optical group so that the calculated distortion of the rectified image corresponds to the distortion defined by the two distortion coefficients previously calculated.

[0024] The calculation of the third-order and fifth-order distortion coefficients in the first step of the process is as follows. This calculation is performed for an eyepiece and can be easily transposed to calculating the distortion coefficients of an objective lens.

[0025] To perform this calculation, we adopt the following notation: F0: Paraxial focal length of the eyepiece θ: Angle of the image field of the eyepiece θC: Half-maximum angle of the image field of the eyepiece r: Height of an image point on the imager rI: Radius of the imager K1: Third-order distortion coefficient K2: Fifth-order distortion coefficient A(θ): Derivative of the image height as a function of the angle θ A(0): Derivative of the image height at the center of the field

[0026] We note r C = F 0 .tan θ C and we have the following relationships: r = F 0 . tan θ r I = r C + K 1 . r C 3 + K 2 . r C 5 r 1 − r C r C 3 = K 1 + K 2 . r C 2 A θ = δr θ δθ = F 0 cos 2 θ . 1 + 3 . K 1 . r 2 + 5 . K 2 . r 4 A 0 = F 0 A θ A 0 = 1 cos 2 θ . 1 + 3 . K 1 . r 2 + 5 . K 2 . r 4

[0027] We note R C = A θ C A 0 This parameter corresponds to the variation in resolution at the edge of the field. If we impose this parameter RC, the distortion coefficients K1 and K2 can be deduced, and we have: K 1 = 1 2 r C 3 . − r C . 4 + R C . cos 2 θ C + 5 r 1 And K 2 = 1 2 r C 5 . r C . 2 + R C . cos 2 θ C − 3 r 1

[0028] The distortion D at the field edge can also be easily calculated. It is equal to: D = r 1 − r C r C

[0029] The assumption behind this calculation is that significant distortions, reaching several tens of percent, can be tolerated, provided that the resolution variation at the edge of the field is kept within reasonable limits so that the user retains an image of acceptable quality, albeit distorted. This type of solution is implemented, for example, in wide-angle fisheye lenses and, more recently, in panomorphic lenses.

[0030] As a first, non-limiting example of application, the dimensions of an eyepiece for a telescope according to the invention are as follows. This eyepiece has a paraxial focal length F₀ of 24 mm. The radius rI of the imager is 9 mm. The resolution of the imager of the image intensifier device is 64 lp / mm, which, given the focal length, corresponds to a resolution of 1.41 cy / mrad. The symbol "cy" means "cycle." The total field of view of the eyepiece 2θC is therefore 46.6 degrees. If we allow the resolution at the edge of the field to decrease to 1.0 cy / mrad, we deduce the value of the RC parameter, which is 0.64, and the distortion coefficients K₁ and K₂, which are respectively: K 1 = − 0.88 . E − 03 K 2 = − 3.11 . E − 06

[0031] In this example, the distortion D is 12.9%.

[0032] As a second, non-limiting example of application, the dimensions of an eyepiece for a telescope according to the invention are as follows. This eyepiece has a paraxial focal length F0 of 22 mm. The radius rI of the imager is 9 mm. The resolution of the imager of the image intensifier device is 64 lp / mm, which, given the focal length, corresponds to a resolution of 1.26 cy / mrad. The total field of view of the eyepiece 2θC is therefore 70.0 degrees. If we allow the resolution at the edge of the field to decrease to 0.33 cy / mrad, we deduce the value of the parameter RC, which is 0.35, and the distortion coefficients K1 and K2, which are respectively: K 1 = − 2.77 . E − 03 K 2 = + 4.31 . E − 06

[0033] In this example, the distortion D is 41.6%.

[0034] Once the coefficients K1 and K2 and the associated distortion curve are determined, it is then possible to determine, for each objective field of view between 0 and θC, the target value of the mean ray impact on the imager of the intensifier tube and to constrain this value during the optimization of the eyepiece's optical combination. The presence of aspheric surfaces belonging to the first optical group, near the imager, and to the third optical group allows us to approach the constrained target value for each field of view sufficiently closely. More precisely, the aspherization coefficients of at least two diopters distributed between the first and third optical groups and one diopter of the second optical group are optimized so that the calculated distortion of the rectified image corresponds to the distortion defined by the two distortion coefficients previously calculated.

[0035] Similarly, it is possible to optimize the design of a telescope objective to tolerate significant distortion while maintaining good edge-of-field resolution. In this case, the objective comprises a first optical assembly and a second optical assembly positioned near the photosensitive surface. At least two diopters of the first optical assembly and one diopter of the second optical assembly have aspherical surfaces calculated to obtain a distortion of the rectified image compatible with the resolution of the image intensifier device. The distortion coefficients in the third and fifth orders are calculated using the method described above.

[0036] Finally, it is also possible to optimize not the objective lens or eyepiece, but the diopters of the image intensifier device. Thus, the input diopter of the photosensitive surface and / or the output diopter of the imager can incorporate aspheric surfaces calculated to obtain a distortion of the rectified image compatible with the resolution of the image intensifier device.

[0037] The eyepiece architecture is well-suited to integrating an optical splitter, allowing for the injection of an additional optical path visible to the user. Indeed, the absence of an inverter fiber array at the output of the intensifier tube makes it possible to utilize rays whose emission indicator converges with respect to the optical axis, thus minimizing the diameter and optical length of the splitter compared to conventional solutions.

[0038] As an example, an architecture of this type is represented in figure 4 The 50mm eyepiece of the figure 3 includes an additional optical channel comprising an optical splitter 58 disposed between the first optical group 51 and the second optical group 52, a fifth optical group 55 and a display 57. This additional channel is symbolized by a double arrow. In the case of the figure 4 The optical splitter 58 is a splitter cube with a semi-reflective blade. It could be replaced by a simple optical blade. The image on the display produced by the fifth optical group 55 and the optical splitter 58 coincides with the imager of the image intensifier device. The user therefore sees, superimposed, an intensified image from the image intensifier device and the image on the display.

[0039] It is possible, while maintaining this architecture, to add a second additional lane as seen on the figure 4 using the unused channel of optical splitter 58. This second additional channel is symbolized by a triple arrow. On the figure 4 , this second channel includes a sixth optical group 56 and a photodetector matrix 59. The image from the light intensifier device can thus be recorded.

[0040] Another advantage of this solution is that, since the intensifier tube does not have an inverter fiber array, it can be moved along the optical axis without any loss of parallelism. This allows for a simple close focusing function coupled with simultaneous diopter adjustment, potentially useful for monocular applications of the telescope. Indeed, to move from a focus at infinity to a focus, for example, at one meter, corresponding to a convergence of one diopter, the photocathode of the intensifier tube must be moved as far away as the intensifier tube's screen must be moved closer to the optical section downstream of the tube so that the intensified object appears collimated at one meter from the eye. This is possible because the telescope's magnification is unity. The free eye and the intensified eye observe an object at a nearly identical distance, differing only by the length of the telescope.The ease of use of the monocular when focusing at close range is significantly improved.

Claims

1. Night vision goggles comprising an objective lens which introduces a field edge distortion greater than or equal to 12.9%, a light intensification device and an eyepiece (50) with a field greater than or equal to 40 degrees, the light intensification device and the eyepiece being aligned on the same axis, the light intensification device comprising an imager (23), the eyepiece being of the image transport type and comprising a first optical group (51) with a field arranged in front of the imager, a second aperture optical group (52) forming an inverted intermediate image of the imager, a third optical group (53) arranged close to this intermediate image and a fourth optical group (54) forming a rectified image from this intermediate image, where at least two dioptres distributed between the first optical group and the third optical group and a dioptre of the second optical group comprise aspherical surfaces which are calculated in order to obtain a distortion of the rectified image so that a reduction of the resolution of the rectified image brought about by the field edge distortion is less than or equal to a predetermined value as a percentage of the resolution of the imager of the intensification device.

2. Night vision goggles according to claim 1, characterized in that the eyepiece comprises an optical path which comprises an optical separator (58) arranged between the first optical group and the second optical group, a fifth optical group (55) and a display (57), the image of the display given by the fifth optical group and the optical separator being merged with the imager.

3. Night vision goggles according to claim 2, characterized in that the eyepiece comprises a second path which comprises the optical separator, a sixth optical group (56) and a photodetector matrix (59).

4. Method for sizing night vision goggles according to any one of claims 1 to 3, the eyepiece having an object field with a predetermined radius (θC), a rectified image field which corresponds to the object field with a predetermined angular radius, a predetermined paraxial focal length (F0) and a predetermined field edge resolution, characterized in that the method comprises the following steps for the calculation of the eyepiece: - calculation of the two distortion coefficients of the third order (K1) and the fifth order (K2) of the rectified image as a function of the radius of the object field, the angular radius of the image field, the paraxial focal length and the field edge resolution; - optimization of the aspherisation coefficients of at least two dioptres which are distributed between the first and the third optical group and a dioptre of the second optical group so that the calculated distortion of the rectified field edge image corresponds to the distortion defined by the two distortion coefficients calculated previously.

5. Night vision goggles comprising an objective lens which introduces a field edge distortion greater than or equal to 12.9%, a light intensification device and an eyepiece (50) with a field greater than or equal to 40 degrees, the light intensification device and the eyepiece being aligned on the same axis, the light intensification device comprising a photosensitive surface (21) and an imager (23), the eyepiece being of the image transport type and comprising a first optical field group (51) arranged in front of the imager, a second aperture optical group (52) forming an intermediate inverted image of the imager, a third optical group (53) arranged in the vicinity of this intermediate image and a fourth optical group (54) forming a rectified image from this image. wherein at least one input dioptre of the photosensitive surface and / or an output dioptre of the imager comprise aspherical surfaces which are calculated in order to obtain a distortion of the rectified image so that a reduction of the resolution of the rectified image brought about by the field edge distortion is less than or equal to a predetermined value as a percentage of the resolution of the imager of the intensification device.

6. Method for sizing night vision goggles according to claim 5, the eyepiece comprising a first optical field group (51) which is arranged in front of the imager, a second aperture optical group (52) which forms an inverted intermediate image of the imager, a third optical group (53) arranged close to this intermediate image and a fourth optical group (54) which forms a rectified image from this intermediate image, the eyepiece having an object field with a predetermined radius, a field of the rectified image corresponding to the object field with a predetermined angular radius, a predetermined paraxial focal length and a predetermined field edge resolution. characterized in that the method comprises the following steps: - calculation of the two distortion coefficients of the third order (K1) and the fifth order (K2) of the rectified image as a function of the radius of the object field, the angular radius of the image field, the paraxial focal length and the field edge resolution; - optimization of the aspherisation coefficients of at least one input dioptre of the photosensitive surface and / or an output dioptre of the imager comprise aspherical surfaces so that the calculated distortion of the rectified field edge image corresponds to the distortion defined by the two distortion coefficients calculated previously.