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
  • G02B27/0075—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 with means for altering, e.g. increasing, the depth of field or depth of focus
• • 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
  • G02B27/0012—Optical design, e.g. procedures, algorithms, optimisation routines

Definitions

• the present invention relates to a wavefront-modified imaging system and a method for increasing the depth of field of an imaging system.
• an imaging system includes a lens and an image sensor that is placed in an image plane of the lens.
• the detector is usually a matrix of photosensitive elements, also called pixels. It is connected to a storage and image processing system.
• the objective forms the image of a scene that is present in an input field of the system, and the detector makes it possible to capture this image.
• imaging systems are, for example, binoculars, cameras, digital cameras or camera phones, which can be adapted to form images from visible or infra-red radiation produced by the scene.
• variations in the separation distance between an object of the scene and the objective especially when the scene comprises several objects which are situated at distances of variable distance, with deviations of up to 100 m (m) for example for a focusing distance greater than 300 m.
• a focus of the lens can not be achieved for the entire scene.
• the length of the interval of the variations of the distance of an object, for which the image of this object remains clear, is called depth of field;
• wavefront coding TM wavefront coding
• An object of the present invention is then to provide a wavefront modification imaging system, which is less expensive and less complex to achieve than the already known systems.
• the object of the invention is a wavefront-modified imaging system, for which the wavefront modifying surface is rotational invariant.
• Another object of the invention is to mitigate as best as possible a defocusing of the system which is caused by at least one of the following causes: variable distance of objects away from the scene, thermal variations, axial chromatism and curvature of field .
• an imaging system that comprises:
• a lens that has an optical axis and a pupil
• an image detector which is placed in an image plane of the objective and which is adapted to capture an image of a scene formed by this objective
• a calculation unit which is intended to execute a digital processing of the image captured by the detector.
• the lens is further adapted to alter a wavefront of radiation passing therethrough, so that a lens response function is substantially constant over a wide range of variation of a separation distance between objects of the scene and the objective.
• the computing unit is adapted so that the processing of the image that is captured by the detector is based on data of the response function.
• the system of the invention is characterized in that the modification of the wavefront corresponds to an effect of a diopter which is located in at least part of the pupil of the objective.
• This diopter is invariant during any rotations around the optical axis of the objective and has a longitudinal offset corresponding to one of the following profiles S (u), with a maximum difference of less than 1% in absolute value relative to to this profile:
• ⁇ being a wavelength of the radiation which forms the image
• n being an index of optical refraction of the diopter for this wavelength
• S 0 (U) is a contribution to the profile of the diopter which corresponds to a constant curvature thereof.
• constant curvature is understood to mean a curvature which has a uniform value on the diopter. This value can be possibly zero. Such a constant curvature can change the position of the image plane of the lens.
• the wavefront modification diopter that is proposed in the invention has one of the profiles S (u) and is invariant during any rotations around the optical axis of the objective.
• this diopter is symmetrical of revolution, that is to say that it appears identical to itself when it is rotated by any angle around the optical axis.
• This dioptre can then be machined simply, in particular using a two-axis machining machine.
• Such a machine rotates, around the axis of symmetry of revolution, one of the optical components of the objective to be machined according to the profile S (u).
• Such a machining machine is currently available, easy to use, while allowing very precise machining. Therefore, an imaging system according to the invention can be manufactured with a cost price that is reduced. It can therefore be, in particular, an imaging system that is intended to be manufactured in large series, such as a system of individual equipment.
• the imaging system may include a pair of infrared binoculars.
• the modification of the wavefront can be at least partially provided by a surface of a lens, a mirror or a prism of the lens. It can also be performed by a phase plate that is added to the imaging system.
• the phase plate is advantageously located in the pupil of the objective, so as to modify the wavefront in a manner that is substantially identical, in the first order, for all the points of the input field. of the goal.
• the profile S (u) can be distributed over several optical components of the lens. It can also be divided between a specific phase plate and one or more optical components of the lens.
• the depth of field of the imaging system is increased.
• the invention can make it possible to increase the depth of field of the system by a factor greater than three, or even greater than five.
• the invention is particularly advantageous when the objective is of the fixed focal length type. Indeed, the fixed position of the detector compared to the objective is palliated by the increase in depth of field.
• a first advantage of the invention stems from the ability of the profiles S (u) according to the invention to reduce, in addition to defocusing which are caused by overshooting of the depth of field, some other defocusing that can be caused by variations the system operating temperature, and / or that may be caused by axial chromaticism or the field curvature of the lens.
• a second advantage of the invention lies in that the size of the imaging system is not increased compared to a similar system without wavefront modification.
• a system according to the invention is less cumbersome and less complex than an athermalized or achromatic objective system.
• the numerical aperture of the lens is not increased, so that the sensitivity of the system remains high.
• a third advantage of the invention lies in the digital processing of the captured image, which is performed by the computing unit.
• This treatment can use a deconvolution filter that is unique.
• This unique filter can be applied from each image point associated with a pixel of the detector.
• the filter can be independent to a large extent of the distance of objects that are viewed by the imaging system.
• the calculation unit can then be simpler, and the processing time of each image is short.
• the processing of each image can be performed in real time, even for image changes that are fast.
• the equivalent diopter which is located in the pupil of the objective has a longitudinal offset corresponding to one of the profiles S (u) with a maximum deviation which is less than 0.5%, or even less than 0, 1% in absolute value, compared to this profile.
• the image that is delivered by the computing unit then has an even higher sharpness, for large variations in the distance of objects away from the scene.
• the limit values that are used for the difference between the offset The longitudinal axis of the actual diopter and one of the profiles S (u), namely 1%, 0.5% and 0.1%, correspond to machining machines which have increasing precision. The best accuracy is obtained for a machine that has a control of the machining depth that is achieved with a laser.
• the diopter that is used in the invention may have a deviation from one of the theoretical profiles that corresponds to the accuracy of the machining machine that is used to manufacture it.
• the diopter may be concentric zones.
• a central zone of the diopter has the longitudinal offset of the profile S (u), with a maximum difference which is less than 1%, preferably less than 0.5%, or even less than 0.1% in absolute value. compared to this profile.
• the wavelength of the radiation which forms the image may belong to one of the following three bands: [0.4 ⁇ m; 1, 1 ⁇ m] which corresponds to the domains of visible light and light intensification, [1, 8 ⁇ m; 2.5 ⁇ m] which corresponds to the IR1 band, [3 ⁇ m; 5 ⁇ m] which corresponds to the IR2 band, and [7 ⁇ m; 13.5 ⁇ m] which corresponds to the extended IR3 band.
• a lens that has an optical axis and a pupil
• an image detector which is placed in an image plane of the objective and which is adapted to capture an image of a scene formed by this objective
• a calculation unit which is intended to execute a digital processing of the image captured by the detector.
• This process comprises the following steps:
• the method is characterized in that the modification of the wavefront corresponds to the effect of a diopter which would be placed in at least part of the pupil of the objective, which would be invariant during any rotation around the optical axis of the objective and which would have a longitudinal offset corresponding to one of the profiles S (u) described above.
• the concordance between the diopter and the profile S (u) corresponds to a maximum profile deviation which is less than 1%, preferably less than 0.5%, or even less than 0.1%.
• the depth of field of the system is substantially increased by a factor of five with respect to the same system without the wavefront modification. corresponding to the profile diopter S (u) located in the pupil.
• the adaptation of the objective to modify the wavefront may comprise a modification of at least one initial surface of a lens, a mirror or a prism of this objective.
• This surface modification of the lens, the mirror or the prism is symmetrical of revolution. It can therefore be done simply and inexpensively.
• the adaptation of the objective may comprise alternatively, or in combination, the addition of a phase plate.
• a phase plate may also be symmetrical of revolution. It is then advantageously added in the pupil of the objective.
• the adaptation of the objective can be equivalent to the effect of a diopter with concentric zones, whose central zone has the longitudinal offset of the profile S (u), with a maximum difference which is less than 1% in absolute value with respect to this profile, preferably less than 0.5%, or even less than 0.1% in absolute value.
• the invention finally proposes to use a method of increasing the depth of field of an imaging system, as described above, for a system operating at a radiation wavelength belonging to one of the three bands [0.4 ⁇ m; 1, 1 ⁇ m], [1, 8 ⁇ m; 2.5 ⁇ m], [3 ⁇ m; 5 ⁇ m] and [7 ⁇ m; 13.5 ⁇ m].
• this system may comprise a pair of infrared binoculars and / or have a fixed focal length lens.
• FIG. 1 is an operating diagram of an imaging system to which the invention can be applied;
• FIG. 2 represents a phase plate that can be used to carry out the invention
• FIG. 3 is a profile diagram for the phase plate of FIG.
• FIG. 4 illustrates an interpretation of the invention
• FIG. 5 is a validation diagram of selected profiles according to the invention.
• an imaging system 10 to which the invention is applied comprises an objective 1, a detector 2, a calculation unit 3 and a display unit 4.
• Objective 1 is represented, in a simplified manner. by a single convergent lens, but it is understood that it may have a more complex structure, in particular based on several lenses, mirrors and / or prisms.
• the detector 2, which consists of a matrix of photosensitive elements, or pixels, is superimposed on an image forming plane of the objective 1. It is perpendicular to the optical axis XX of the objective.
• the system 10 is designed to view objects that are distant from the lens 1, the detector 2 is located substantially at the focus image of objective 1, rated Fi.
• the detector 2 is electrically connected to the computing unit 3, denoted CPU, so that electrical signals which are produced by the pixels of the detector 2 can be processed numerically.
• the computing unit 3 is itself connected to the display unit 4, denoted "DISPLAY", which makes it possible to display the images captured by the detector 2 and processed by the unit 3.
• the unit 4 can be, for example, a liquid crystal display.
• the system 10 may be, for example, a pair of infrared binoculars. In this case, it may further comprise an eyepiece system 5, which is placed in front of the display unit 4.
• the position of elements of an image that is formed by the lens 1, along the axis XX, varies according to a distance D of each object of a scene that is located in front of the objective 1.
• the scene S comprises a vehicle V and a character P, the latter being closer to the objective 1 than the vehicle V.
• the objective is calibrated so that the image of the vehicle V is formed on the sensitive surface of the detector 2, at the point Fi, then the image of the character P is formed behind the sensitive surface of the detector at the point F 2 .
• the images of the vehicle V and the character P are respectively sharp and fuzzy.
• a pupil is a diaphragm which limits the opening of the objective.
• the pupil transversely limits a beam of radiation which comes from a point of the scene S and which enters the system 10. It therefore limits the brightness of the image which is formed by the objective 1.
• the pupil 1a of the lens 1 is constituted by the frame of the lens.
• the numerical aperture N denotes the quotient of the focal length of the lens 1 by the diameter of the entrance pupil 1a.
• the sensitivity of the imaging system for low radiation intensities is all the greater as the numerical aperture N is small.
• the depth of field is the width of the range of the variations of the distance of distance D of an object that is visualized, for which the image of this object which is delivered by the imaging system 10 is clear. For an imaging system without wavefront modification, it is determined by the size of the Airy task that corresponds to the image of a point, and / or by the pixel size of the detector 2. In the first case, the depth of field expressed as the interval along the XX axis in which the detector can be placed is equal to +/- 2.AN 2 , where ⁇ is the wavelength of the radiation and N is the numerical aperture N of the lens.
• the depth of field of the imaging system 10 is increased when a lens of phase 6 is added to the lens 1 (FIG. 2). This is symmetrical during any rotation around a Y-axis.
• the axis YY is therefore perpendicular to the blade 6, and cuts it at a central point marked 0.
• the blade 6 has a circular peripheral edge, with a radius which is denoted R. It furthermore has a thickness which varies in function of the radial distance to the YY axis. This radial distance is denoted by r, and u denotes the ratio r / R. In other words, u is the normalized radial distance with respect to the radius R of the blade 6.
• the blade 6 has a flat face, for example its bottom face in FIG. 2, and an upper surface with relief.
• S (u) is the profile of the diopter that constitutes the upper face of the blade 6.
• the blade 6 consists of a transparent material for the radiation which forms the image of the scene S on the detector 2.
• ⁇ denotes a wavelength of this radiation, and by n the index of refraction of the material of the blade 6 for this wavelength.
• the blade 6 is placed in the pupil 1a of the lens 1, that is to say against the single lens thereof when the lens 1 has the simplified configuration of Figure 1. It is positioned so that the axis YY of the blade 6 is superimposed on the optical axis XX of the lens 1. In order not to reduce the numerical aperture N of the imaging system 10, the radius R is at least equal to the radius of the pupil 1a.
• such an arrangement of the blade 6, which is symmetrical by rotation increases the depth of field of the imaging system 10 when the profile S (u) corresponds to one of the theoretical profiles characterized by the formula (1).
• the multiplicative factor of the increase in the depth of field of the system 10 may be greater than three, or even greater than or equal to five, depending on the amplitude of the profile S (u) and the difference between the actual profile of the the blade 6 and the theoretical profile.
• FIG. 3 is a diagram which shows the variations of one of the theoretical profiles S (u) of the formula (1), when the constant curvature term S 0 (u) is zero.
• the abscissa axis marks the normalized radial distance u, and the ordinate axis identifies the variations of S (u), that is, the theoretical variations of the thickness of the plate 6.
• u is without unit and varies between 0 and 1.
• ⁇ is the slope of the profile S (u) at the center O of the blade 6, that is to say for u equal to 0;
• S 0 (u) is a uniform term of curvature, the variations of which are superimposed on the profile S (u) of FIG. 3.
• the inventors have determined that the parameter ⁇ should be between 1.0 and 6.0, the parameter ⁇ between the values 0.42 and 0.74, and the absolute value of the parameter Ao between [1.5 ⁇ / (n -1); 7.5 ⁇ / (n-1)] to obtain an increase in the depth of field of the imaging system 10.
• the addition from blade 6 to system 10 increases the depth of field by a factor of 5 over the value for system 10 without the phase plate 6.
• the profiles S (u) which are characterized by the formula (1) have the remarkable property that the illumination along the axis XX which is produced, through the lens 1 provided with the blade 6, by a point source situated on this axis far in front of the objective 1 is constant on both sides of the image focus Fi of the objective 1. More precisely, these profiles S (u), associated with the intervals indicated for ⁇ , ⁇ , and Ao, ensure that this illumination is constant during a longitudinal displacement on the axis XX of a length less than 10- ⁇ -N 2 on either side of the focus F 1 . By way of comparison, the depth of field of the system 10 without a blade 6, brought back into the image space, corresponds to displacements on either side of the image focal point F 1 , on the axis XX, which are lower ⁇ -N 2 .
• FIG. 4 illustrates the transformation, by the lens 1 provided with the blade 6, of a plane wave which is produced by a point source (not shown) located far in front of the objective 1.
• Zi, Z 2 and Z 3 have been noted three concentric rings of the blade 6 which respectively produce cracks concentrated at the points P, F 1 and Q.
• F 1 is the focus image of the objective 1
• P and Q are each distant from F 1 of the distance 10- ⁇ -N 2 , being located in front and back of F 1 .
• the zones Z 1 , Z 2 and Z 3 must be imagined to be infinitesimal.
• the variable thickness of the blade 6, as a function of the radial distance r, locally produces a diopter inclined to the surface of the blade. This diopter directs the rays that cross the blade at the distance r from the axis XX to a point on this axis which is situated between the end points P and Q.
• the blade 6 realizes an adjustment of the point of convergence of the rays that crosses it, depending on the distance from the points of impact of these rays on the blade.
• This adjustment produces an illumination between the points P and Q which is substantially constant when the blade 6 has one of the profiles S (u) of the formula (1).
• the term A 0 u (u-1) (A u 2 + B u + C) of the profile S (u) corresponds to the thickness variation of the blade 6 which, according to the invention , makes constant the illumination produced by a point source on the optical axis XX between the points P and Q.
• the profile S (u) may further comprise the term of uniform curvature S 0 (u) which is indicated in the formula (1). Taking into account a uniform general curvature of the blade 6 has the effect of modifying, with respect to the parameter ⁇ , the value of u for which the profile S (u) is maximum. It reflects an additional effect of the lens created by the blade 6, which is added to that of the lens without blade 6.
• the impulse response function of the objective 1 provided with the blade is substantially constant, whatever the position of a source of
• the term "pulse spread function" is used to describe the distribution of the illumination produced in the plane of the sensitive surface of the detector. 2
• a point source P which belongs to the scene S and which is located at a great distance from the objective 1.
• any two points of the scene S which can be located at different positions along the axis XX and / or shifted transversely in different ways with respect to this axis, within the input field of the system 10, produce similar illumination on the detector 2.
• Figure 5 illustrates this validation, showing, for a large number of optimized profiles, the relative difference between the optimized profile and the theoretical profile.
• the abscissa axis locates the values of the normalized radial distance u
• the ordinate axis indicates, for each optimized profile S opt , the value of the quotient [S op t (u) -S ⁇ , ⁇ , Ao (u)] / S 0 ⁇ 1A0 (U). This relative difference is less than 0.4%, whatever the optimized profile obtained.
• the wavefront modification blade can be placed outside the pupil of the lens, provided that its profile is adapted to obtain an identical modification of the wavefront in the pupil.
• the inventors specify that the blade placed outside the pupil still has a symmetry of revolution;
• the modification of the wavefront according to the invention can be carried out by means of an initial optical component of the objective 1, without adding a phase plate.
• This component may be a lens, a mirror or a prism, in particular.
• the profile of a face of the component is modified in a manner that is optically equivalent to the profile S (u) located in the pupil of the objective;
• this face when the profile modification is applied to a face of an initial optical component of the objective 1, this face can be initially aspherical.
• the modification is then a additional component of the profile of this face, which corresponds to the modification of the wavefront according to the invention. Only this additional component of profile then has the symmetry of revolution;
• the profile S (u) can be applied to only a part of a phase plate or of an optical component of the objective 1, so that it has an effect only for part of the beam of radiation that enters the lens.
• the profile S (u) may be applied to a crown only of an optical component, which is centered with respect to the optical axis of the objective.
• a ring is central, that is to say that it extends continuously between the optical axis X-X and a maximum radius which is less than that of the radiation beam at the optical component;
• the profile S (u) of the blade 6 can be converted into a variation profile of the refractive index n thereof.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Lenses (AREA)
• Studio Devices (AREA)
• Measurement Of Optical Distance (AREA)
• Analysing Materials By The Use Of Radiation (AREA)
• Image Processing (AREA)

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• 2007
  • 2007-10-12 FR FR0707177A patent/FR2922324B1/fr not_active Expired - Fee Related
• 2008
  • 2008-10-10 EP EP08841316A patent/EP2198337A2/de not_active Withdrawn
  • 2008-10-10 CA CA2701151A patent/CA2701151A1/fr not_active Abandoned
  • 2008-10-10 WO PCT/FR2008/051847 patent/WO2009053634A2/fr active Application Filing
  • 2008-10-10 US US12/681,548 patent/US20100277594A1/en not_active Abandoned
• 2010
  • 2010-04-06 IL IL204840A patent/IL204840A/en not_active IP Right Cessation

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