Method for adjusting the inclination between a detector surface and an image focal plane of an optical system

Abstract

This method for adjusting the inclination between a detector and an optical system comprises the following steps: - Positioning at least one pair of rods (21; 23) in front of the optical system and the detector, with an object point (25) on each rod (21; 23) positioned at a focusing distance from the optical system; - Acquiring an image with the detector; - Detecting a point of best focus in the image for each rod (21; 23); - Realigning the detector surface so as to make the detected points of best focus coincide with the theoretical position of the object points (25) in said image for a desired inclination between the detector and the image focal plane. Figure for the abstract: Fig. 3

Description

Title of the invention: Method for adjusting the inclination between a detector surface and an image focal plane of an optical system. Technical field

[0001] The present invention relates to a method for adjusting an imaging system comprising a detector and an optical system configured to focus electromagnetic radiation onto said detector.

[0002] In particular, the present invention relates to the adjustment of the parallelism between the surface of the detector and the image focal plane of the optical system of the imaging system.

[0003] In general, the invention applies to any imaging system where it is necessary to adjust the correct alignment of the optical system with the detector. Prior techniques

[0004] An imaging system commonly comprises a detector and an optical system configured to focus electromagnetic radiation onto said detector.

[0005] Figure [Fig. 1] schematically represents an example of a cross-sectional view of an imaging system 1 belonging to the prior art and comprising an optical system 3 and a detector 5.

[0006] An optical system 3 is understood to be a set of optical elements, such as mirrors, and / or lenses, and / or diffraction gratings, that allow the path or properties of electromagnetic rays, for example light rays such as those in the visible, infrared, or ultraviolet range, to be modified. Here, the optical system is centered and rotationally symmetric about its optical axis.

[0007] Detector 5 is thus an electromagnetic radiation detector, for example a digital sensor.

[0008] Generally, a first mechanical system 7 supports the optical system 3 while a second mechanical system 9 supports the detector 5.

[0009] In the ideal case (not shown), the first mechanical system 7 and the second mechanical system 9 are aligned along a mechanical axis A. The optical axis B of the optical system 3 is also aligned with the normal direction C to the surface 11 of the detector 5, and the image focal plane 13 of the optical system 3 coincides with the surface 11 of the detector 5.

[0010] In reality, and as shown in [Fig. 1], although the first mechanical system 7 and the second mechanical system 9 are aligned along the mechanical axis A, the optical axis B of the optical system 3 and the normal direction C to the surface 11 of the detector 5 may not be aligned, for example due to a non- alignment between the respective mechanical axes A of the mechanical systems 7 and 9 and respectively of the optical axis B of the optical system 3 and of the normal direction C to the surface 11 of the detector 5.

[0011] Therefore, the image focal plane 13 of the optical system 3 cannot be confused with the surface 11 of the detector 5. This impairs the quality of the image acquired by the detector 5, which includes, for example, a single sharp line D at the intersection of the image focal plane 13 and the surface 11 of the detector 5, the rest of the image including undesired blurred areas.

[0012] The existing solution for adjusting the parallelism of the image focal plane 13 and the surface 11 of the detector 5 consists of measuring the modulation transfer function (“MTF”) of the optical system 3 and the detector 5 using a collimator which projects an object target at an infinite distance in front of said optical system 3.

[0013] To determine the angle of inclination between the image focal plane 13 and the surface 11 of the detector 5, these MTF measurements must be performed at different points in the field of view of the optical system 3, and for object targets located at a variable distance close to infinity. These measurements at varying distances allow a curve to be plotted showing the variation of MTF as a function of the distance to the optical system 3: the maximum of this curve thus indicates the distance corresponding to the best focus, and therefore the position of the detector 5 relative to the image focal plane 13.

[0014] To plot the curve described above, there are two ways to proceed. The first consists of moving the object target in the object focal plane of the collimator, the second consists of moving the detector 5 in the image space of the optical system 3.

[0015] However, these solutions are delicate and tedious, they require a large number of measures and can last for many hours.

[0016] In particular, they may involve very expensive automation solutions.

[0017] By way of example, document FR3143118 relates to a method for controlling an optronic system comprising displaying a sequence of targets on the pixels of a screen, acquiring the image of the target sequence on a sensor in an image plane, and matching the pixels of the object plane with the pixels of the image plane in order to determine a quality parameter of said optronic system. Description of the invention

[0018] The present invention therefore aims to overcome the aforementioned drawbacks and to provide a method allowing precise and simple adjustment of the inclination and alignment of the optical system with the detector of an imaging system.

[0019] The present invention relates to a method for adjusting the inclination between a detector surface and an image focal plane of an optical system, the method comprising the following steps:

[0020] - Alignment of the optical system and the detector on an optical axis of the system optical, preferably so that at least one line of the image focal plane of said optical system is positioned on the surface of the detector for a focusing distance;

[0021] - Positioning at least one pair of rods in an observation field of the optical system, for each pair the rods being positioned parallel to each other, each at an equal distance from said optical system to within 10%, preferably to within 5%, even more preferably to within 1%, angularly separated in said field of observation by an angle of at least 1% of said field of observation, preferably at least 5%, even more preferably at least 10%, the rods being further inclined with respect to a plane perpendicular to the optical axis, an object point of each rod being positioned at said focusing distance;

[0022] - Acquisition of an image by the detector;

[0023] - Detection in the image of a point of best focus for each rod; and

[0024] - Realignment of the detector surface so as to make the points of best focus detected with the theoretical position of the object points in said image for a desired inclination between the detector surface and the image focal plane of the optical system.

[0025] Thus, the implementation of the method makes it possible, using only one or two pairs of rods, to be able to adjust the inclination, in particular the adjustment of the parallelism between the focal plane of the optical system and the plane of the detector, in a very short time.

[0026] Advantageously, the optical axis of the optical system passes at an equidistance, to within 10%, preferably to within 5%, even more preferably to within 1%, from each object point for each pair of rods.

[0027] In a particular embodiment, the rod positioning step is carried out so as to make the rods appear entirely in the field of observation, each object point being located at the center of each rod, each rod having a length greater than or equal to an angle of 1% of said field of observation, preferably 5%, even more preferably 10%.

[0028] Advantageously, the positioning step includes the positioning of two pairs of rods so that there exists an orthonormal frame whose two direction axes are perpendicular to the optical axis of the optical system at the focusing distance, each direction axis passing through the two rods of one of the two pairs of rods at the object points of each rod.

[0029] In one embodiment, the detection step is performed by a sharpness and / or gradient and / or sharpness variation detection algorithm.

[0030] In a particular embodiment, the rods are made of heated and / or heated wires, the detector being sensitive to the infrared electromagnetic radiation emitted by said wires.

[0031] Advantageously, each rod has a linearly variable lateral dimension, a first end of each rod being wider than a second end of each rod and said first end being located at a greater distance from the optical system than the second end.

[0032] Preferably, the positioning step is carried out so as to position the rods at a distance from the optical system of between 10 centimeters and 100 meters, preferably between 10 centimeters and 10 meters.

[0033] In a particular embodiment, the alignment step includes the installation of at least one shim between the optical system and the detector, the method including a step of removing at least one shim after the realignment step of the detector surface.

[0034] Preferably, the desired inclination between the detector surface and the image focal plane of the optical system is a parallelization of the detector surface and the image focal plane of the optical system. Brief description of the drawings

[0035] Other objects, features and advantages of the invention will become apparent from the following description, given solely by way of non-limiting example, and made with reference to the accompanying drawings in which:

[0036] [Fig.1] which has already been mentioned, is a schematic cross-sectional representation of an imaging system whose optical system and detector are not aligned;

[0037] [Fig.2] is a schematic representation of the steps of the process according to the invention;

[0038] [Fig.3] is a schematic perspective representation of the positioning of rods in the field of view of the imaging system of [Fig. 1] for implementing the method according to the invention; and

[0039] [Fig.4] is an example of an image captured by the detector of the imaging system of [Fig.1] during the implementation of the method according to the invention.

[0040] Detailed description of at least one embodiment

[0041] The different stages of a method for adjusting the inclination between a surface 11 of a detector 5 and an image focal plane 13 of an optical system 3 have been schematically represented in [Fig.2]. For example, the detector 5 and the optical system 3 belong to the imaging system shown in [Fig.1] previously described.

[0042] Initially, a step 15 is performed to align the optical system 3 and the detector 5 with the optical axis B of the optical system 3, preferably such that at least one line D of the image focal plane 13 of said optical system 3 is positioned on the surface 11 of the detector 5 for a given focusing distance. The line D may be located outside the surface 11 of the detector 5: in this case, no line is sharp on the image formed by the detector 5. However, a variation in sharpness will be perceptible on the image, and during the implementation of the present method, such a line D will necessarily form on the surface 11 of the detector 5 when the realignment of the surface 11 of the detector 5 with the image focal plane 13 of the optical system 3 is in progress.

[0043] In particular, the focusing distance is a finite distance, for example between 10 centimeters and 100 meters, preferably between 10 centimeters and 10 meters.

[0044] The optical system 3 therefore operates in a focus-focus configuration.

[0045] In the case of an optical system 3 not including a focusing system and operating in an infinity-focus configuration, the alignment step 15 includes installing at least one shim, not shown, between the optical system 3 and the detector 5, for example between the first mechanical system 7 and the second mechanical system 9. The shim allows the detector 5 to be moved back relative to the optical system 3 and thus to be placed in a focus-focus configuration for the duration of the adjustment process. In other words, the shim forms a focusing system.

[0046] Then, a step 17 is carried out of positioning at least one pair of rods 21 in the field of observation 19 of the optical system 3.

[0047] A schematic perspective view of the imaging system 1 comprising the detector 5 and the optical system 3, as well as two pairs of rods 21 and 23 positioned in the field of observation 19 of said optical system 3, is shown in [Fig.3].

[0048] For each pair of rods 21 and / or 23, the rods are positioned parallel to each other, preferably so that the two rods of each pair are perfectly aligned. Perfectly aligned means that the ends of two parallel rods, four ends in total, virtually form a rectangular parallelogram when connected.

[0049] Each rod 21 and / or 23 is positioned equidistant from the optical system 3, to within 10%, preferably to within 5%, and even more preferably to within 1%. The distance is, for example, measured from the outermost optical element of the optical system. 3 towards the nearest portion of the optical system 3 of each rod 21 and / or 23. Alternatively, the distance is measured for example from the outermost optical element of the optical system 3 towards the middle portion of each rod 21 and / or 23, for example at the center of each rod.

[0050] In order to prevent the rods 21 and / or 23 from being positioned flush against each other, the rods 21 and / or 23 are respectively spaced apart at an angle of at least 1% of said field of view 19, preferably at least 5%, and even more preferably at least 10%. For example, for certain optical systems with a very limited field of view, for example less than 5°, the rods 21 and / or 23 may be spaced apart by an angular distance of up to 90% of the field of view.

[0051] In addition, the rods must also be inclined with respect to a plane perpendicular to the optical axis B. Preferably, the rods 21 and / or 23 are inclined by at least 1°, even more preferably by at least 5° with respect to the plane perpendicular to the optical axis B.

[0052] For example, as shown, the optical axis B follows the x-direction of the orthonormal coordinate system Oxyz, which has its origin at the outermost element of the optical system 3. Thus, the rods 21 and / or 23 must be inclined with respect to the yOz plane. Consequently, the rods 21 and / or 23 are at identically variable distances from the optical system 3.

[0053] Advantageously, a point 25 on each rod 21 and / or 23, called the object point 25, is to be positioned at the focusing distance of the optical system 3. The frame of reference y'O'z' shown in [Fig. 3] is parallel to the yOz frame of reference, the focusing distance being OO'. Thus, each object point 25 is in the y'O'z' plane. For simplicity of presentation in [Fig. 3] and to also simplify the implementation of the method, each object point 25 is placed on one of the axes O'y' or O'z'.

[0054] Therefore, the rods 21 and / or 23 are positioned at a distance from the optical system 3 of between 10 centimeters and 100 meters, preferably between 10 centimeters and 10 meters.

[0055] Advantageously, the rods 21 and / or 23 are positioned so that the optical axis B of the optical system 3 passes equidistant, to within 10%, preferably to within 5%, even more preferably to within 1%, from each object point 25 for each pair of rods.

[0056] Preferably, the rods 21 and / or 23 are positioned so as to make the rods 21 and / or 23 appear entirely in the field of observation 19.

[0057] Furthermore, the rods 21 and / or 23 are positioned so that each object point 25 is located at the center of each rod 21 and / or 23, each rod 21 and / or 23 having a length greater than or equal to an angle of 1% of the field of observation 19, preferably 5%, even more preferably 10%.

[0058] In addition, the apparent width of each rod 21 and / or 23 in the field of observation 19 is preferably less than ten times the length previously described.

[0059] In a particular embodiment, at least one rod 21 and / or 23 has a linearly variable lateral dimension (not shown) such that one end of the rod is wider than a second end of the rod, and said first end is located at a greater distance from the optical system 3 than the second end. This feature allows the detector 5 to capture an image of the rod with a constant apparent width. In other words, the linearly variable lateral dimension compensates for the distance or proximity of certain portions of the rod to the object point 25.

[0060] In another particular embodiment, the rods 21 and / or 23 are made of heated and / or heated wires, the detector being sensitive to the infrared electromagnetic radiation emitted by said wires.

[0061] In other embodiments, the rods 21 and / or 23 can be formed in a solid and opaque material such as metal, plastic, wood.

[0062] After performing step 17 of positioning the rods 21 and / or 23, a step 27 of acquiring an image or a series of images by the detector 5 is performed. For example, the detector 5 can also acquire a video stream in order to be able to adjust the inclination between the surface 11 of the detector 5 and the image focal plane 13 more quickly and in real time.

[0063] A step 29 is then carried out to detect in the acquired image a point of best focus 33 for each rod 21 and / or 23. In an ideal case, and as represented in [Fig.4], the point of best focus is a sharp point in the acquired image, but it may also be that the optimum sharpness is outside the surface 11 of the detector 5, in which case the point of best focus 33 will be on the edge of the image.

[0064] An example of an image captured by detector 5 is shown schematically in [Fig.4]. The image is acquired for a single pair of rods 21.

[0065] The two lines 31 of the image correspond to the image of the rods 21. The lines are blurry because the rods 21 are inclined with respect to a plane perpendicular to the optical axis B and only one point per rod 21, namely the point of best focus 33 on the image, is correctly in focus.

[0066] The detection step 29 is, for example, performed by an operator or by a sharpness and / or gradient detection algorithm and / or by analyzing the variation in image sharpness, particularly in cases where the best focus point 33 is not yet a sharp point, making the present method particularly robust. The best focus point 33 can then be identified on the image. Feedback to an operator can be provided graphically using a point displayed in an orthonormal coordinate system whose position changes according to the adjustments made in real time.

[0067] In the case where the imaging system 1 is correctly aligned, namely in the case where the image focal plane 13 coincides with the surface 11 of the detector 5, the best focus points 33 correspond to the image of the object points 25 on the image.

[0068] If this is not the case, it means that the imaging system 1 is misaligned.

[0069] A step 35 is then performed to realign the surface 11 of the detector 5 of in order to make the detected best focus points 33 coincide with the theoretical position of the object points 25 in the image for a desired inclination between the surface 11 of the detector 5 and the image focal plane 13 of the optical system 3.

[0070] Step 35 is for example carried out by screwing or unscrewing screws or knobs allowing the inclination of the detector 5 relative to the optical system 3 to be modified along two axes of the detector 5.

[0071] Here, the desired inclination between the surface 11 of the detector 5 and the image focal plane 13 of the optical system 3 is a parallelization of the surface 11 of the detector 5 and the image focal plane 13 of the optical system 3.

[0072] The theoretical position of the object points 25 in the image means the position of the best focus points 33 corresponding to the object points 25 for a properly aligned imaging system 1.

[0073] The implementation of the method allows, for the positioning of a single pair of rods 21, the adjustment of the inclination between the surface 11 of the detector 5 and the image focal plane 13 of the optical system 3 along a first axis of the detector 3.

[0074] In order to be able to adjust the inclination between the surface 11 of the detector 5 and the image focal plane 13 of the optical system 3 along a second axis of the detector 3, a second pair of rods 23 can then be placed so that there is an orthonormal frame, for example the y'O'z' frame, whose two director axes are perpendicular to the optical axis B of the optical system 3 at the focusing distance, each director axis passing through the two rods of one of the two pairs of rods at the object points 25 of each rod 21 and / or 23.

[0075] For example, the O'y' axis passes through the rods 21 at their object point 25 and the O'z' axis passes through the rods 23 at their object point 25. The two pairs of rods 21 and 23 are said to be positioned perpendicularly to each other.

[0076] Only two axes are needed to obtain all possible tilt configurations for detector 5, so a third pair of rods is not necessary.

[0077] In an embodiment requiring two pairs of rods 21 and 23, it is possible to first place one pair of rods 21, and then implement the process, then move the pair of rods 21 in order to reimplement the process for a different location, which amounts to having two pairs of rods 21 and 23.

[0078] Finally, in the case where a shim has been previously positioned between the optical system 3 and the detector 5, the method optionally includes a step 37 of removing the shim after the step 35 of realigning the surface 11 of the detector 5.

Claims

Demands

1. Method for adjusting the inclination between a surface (11) of a detector (5) and an image focal plane (13) of an optical system (3), characterized in that it comprises the following steps: - Alignment (step 15) of the optical system (3) and the detector (5) on an optical axis (B) of the optical system (3); - Positioning (step 17) of at least one pair of rods (21; 23) in an observation field (19) of the optical system (3), for each pair the rods (21; 23) being positioned parallel to each other, each at an equal distance from said optical system (3) within 10%, angularly separated by an angle of at least 1% of said observation field (19), the rods (21; 23) being further inclined with respect to a plane perpendicular (y'O'z') to the optical axis (B), an object point (25) of each rod (21; 23) being positioned at a focusing distance; - Acquisition (step 27) of an image by the detector (5);- Detection (step 29) in the image of a best focus point (33) for each rod (21; 23); and - Realignment (step 35) of the surface (11) of the detector (5) so as to make the detected best focus points (33) coincide with the theoretical position of the object points (25) in said image for a desired inclination between the surface (11) of the detector (5) and the image focal plane (13) of the optical system (3).;

2. A method according to claim 1, wherein the optical axis (B) of the optical system (3) passes equidistant, within 10%, from each object point (25) for each pair of rods (21; 23).

3. A method according to any one of claims 1 and 2, wherein the step (17) of positioning the rods is carried out so as to make the rods (21; 23) appear entirely in the field of observation (19), each object point (25) being located at the center of each rod (21; 23), each rod (21; 23) having a length greater than or equal to an angle of 1% of said field of observation (19).

4. A method according to any one of claims 1 to 3, wherein the positioning step (17) comprises the positioning of two pairs of rods (21; 23) so that there exists an orthonormal frame (y'O'z') whose two director axes (O'y', O'z') are perpendicular to the optical axis (B) of the optical system (3) at the focusing distance, each director axis (O'y', O'z') passing through the two rods (21; 23) of one of the two pairs of rods (21; 23) at the level of the object points (25) of each rod (21; 23).

5. A method according to any one of claims 1 to 4, wherein the detection step (29) is performed by a sharpness and / or gradient and / or sharpness variation detection algorithm.

6. A method according to any one of claims 1 to 5, wherein the rods (21; 23) are made of heated and / or heated wires, the detector (5) being sensitive to infrared electromagnetic radiation emitted by said wires.

7. A method according to any one of claims 1 to 6, wherein each rod (21; 23) has a linearly variable lateral dimension, a first end of each rod (21; 23) being wider than a second end of each rod (21; 23) and said first end being located at a greater distance from the optical system (3) than the second end.

8. A method according to any one of claims 1 to 7, wherein the positioning step (17) is carried out so as to position the rods (21; 23) at a distance from the optical system (3) of between 10 centimeters and 100 meters, preferably between 10 centimeters and 10 meters.

9. A method according to any one of claims 1 to 8, wherein the alignment step (15) comprises the installation of at least one shim between the optical system (3) and the detector (5), the method comprising a step (37) of removing the at least one shim after the realignment step (35) of the surface (11) of the detector (5).

10. A method according to any one of claims 1 to 9, wherein the desired inclination between the surface (11) of the detector (5) and the image focal plane (13) of the optical system (3) is a parallelization of the surface (11) of the detector (5) and the image focal plane (13) of the optical system (3).

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