Cassegrain-type catadioptric optical system with two lenses

Abstract

This optical system (3) comprises, aligned in this order on the same optical axis (X-X'), a first lens (7) defining a first diopter (11) and a second diopter (12), a free space (8), and a second lens (9) defining a third diopter (13) and a fourth diopter (14), the first diopter (11) comprising an outer zone (15) of refraction and a central zone (17) of reflection, the fourth diopter (14) comprising an outer part (21) of reflection and a central part (23) of refraction, so that an electromagnetic radiation (R) entering the optical system (3) through the outer zone (15) of the first diopter (11) exits through the central part (23) of the fourth diopter (14) after reflection on the outer part (21) of the fourth diopter (14) and then on the central zone (17) of the first diopter (11). Figure for the abridged version: Fig. 1

Description

Title of the invention: Cassegrain-type catadioptric optical system with two lenses Technical field

[0001] The present invention relates to optical systems for imaging.

[0002] A particularly interesting application of the invention concerns an optical system for the infrared domain, in particular the mid-infrared and the far-infrared, with a small footprint and a large geometric aperture.

[0003] Generally speaking, the invention can also be applied to any part of the visible or infrared spectrum. Previous Techniques

[0004] Existing Cassegrain-type optical systems generally operate in the visible and infrared regions of the electromagnetic spectrum and allow for the reduction of optical system size thanks to two primary and secondary mirrors.

[0005] However, existing systems are highly subject to optical aberrations, or include expensive correction plates, such as Schmidt plates, and are only designed for the observation of small fields, for example less than or equal to 8°, with low numerical apertures, for example with an aperture number greater than 3.

[0006] Furthermore, optical systems are increasingly associated with photodetector matrix type sensors that are increasingly larger, in terms of dimensions, but also in terms of the number of pixels, covering an observation field that is itself increasingly larger. For this reason, they must have an increasingly larger aperture to be bright and ensure that each pixel receives sufficient light, thus increasing optical aberrations. The modulation transfer function of such optical systems of the prior art is degraded, in particular on the axis but especially in the corners of the sensor, where said modulation transfer function is then difficult to correct.

[0007] Finally, existing systems are not compact enough to be embedded or encapsulated in certain boxes. Statement of the invention

[0008] The present invention therefore aims to overcome the aforementioned drawbacks and to provide an optical system with an improved modulation transfer function on the axis and in the field, as well as better sensitivity, all of which can be manufactured at low cost.

[0009] The present invention relates to an optical system comprising, aligned in this order on the same optical axis, a first lens, a free space, and a second lens, the first lens defining a first diopter and a second diopter, the second lens defining a third diopter and a fourth diopter, the first diopter comprising an outer zone configured to refract incident electromagnetic radiation and a central zone configured to reflect incident electromagnetic radiation, the fourth diopter comprising an outer portion configured to reflect incident electromagnetic radiation and a central portion configured to refract incident electromagnetic radiation, such that electromagnetic radiation entering the optical system through the outer zone of the first diopter exits said optical system through the central portion of the fourth diopter after being reflected in the outer portion of the fourth diopter and then in the central zone of the first diopter.

[0010] Thus, a two-lens optical system architecture makes it possible to increase the number of diopters in order to correct certain optical aberrations and, above all, to reduce the transverse dimension of the electromagnetic radiation incident on the central zone of the first diopter, so that this central zone has the smallest possible diameter, improving the aperture of the optical system as well as the modulation transfer function, in particular on the periphery of the image formed by the optical system.

[0011] Advantageously, the first diopter and / or the second diopter and / or the third diopter and / or the fourth diopter is revolution-symmetrical, preferably aspherical.

[0012] In one embodiment, the shape of the first diopter is defined by a first equation for defining the outer zone different from a second equation for defining the central zone.

[0013] In a particular embodiment, the shape of the second diopter is defined by a single defining equation.

[0014] In a particular embodiment, the shape of the third diopter is defined by a first equation defining a central portion different from a second equation defining an external portion defined by the remainder of said third diopter.

[0015] In one embodiment, the shape of the fourth diopter is defined by a first equation defining the outer part different from a second equation defining the central part.

[0016] Advantageously, the first lens and the second lens are made of the same material.

[0017] According to one embodiment, the first lens and / or the second lens is made of a material comprising chalcogenide glass, and / or ZnS, and / or ZnSe.

[0018] Advantageously, each diopter, and / or part of a diopter, and / or diopter zone, configured to refract electromagnetic radiation comprises a single-layer or multi-layer anti-reflection treatment.

[0019] Advantageously, the optical system further comprises a mounting barrel comprising a first location into which the first lens is slid, and a second location into which the second lens is slid, the second location having a larger diameter than the diameter of the first location.

[0020] According to one embodiment, the outer zone of the first diopter is configured to refract toward the second diopter electromagnetic radiation coming from outside the first lens, the second diopter being configured to refract toward the third diopter electromagnetic radiation coming from the first diopter and to refract toward the central zone of the first diopter electromagnetic radiation coming from the third diopter, the third diopter being configured to refract toward the fourth diopter electromagnetic radiation coming from the second diopter and to refract toward the second diopter electromagnetic radiation coming from the fourth diopter, the outer part of the fourth diopter being configured to reflect toward the third diopter electromagnetic radiation coming from said third diopter,the central zone of the first diopter being configured to reflect towards the second diopter electromagnetic radiation coming from said second diopter, said electromagnetic radiation reflected by the central zone of the first diopter then being refracted by the second diopter, then the third diopter then the central part of the fourth diopter. ,

[0021] The invention also relates to an imaging system comprising an optical system as defined previously, as well as an infrared sensor. Brief description of the drawings

[0022] Other aims, characteristics and advantages of the invention will appear on reading the following description, given solely by way of non-limiting example, and made with reference to the appended drawing in which:

[0023] [Fig.l] is a schematic view in longitudinal section along the optical axis of an imaging system according to the invention.

[0024] Detailed description of at least one embodiment

[0025] [Fig.l] shows schematically an imaging system 1 according to the invention.

[0026] This imaging system 1 comprises in particular an optical system 3 and a sensor 5. The optical system 3 can be described as a Cassegrain telescope.

[0027] The sensor 5 is an electromagnetic radiation sensor R, for example a sensor of the visible part of the electromagnetic spectrum and / or preferably a sensor 5 configured to detect infrared radiation, in particular in the mid and / or far infrared.

[0028] The sensor 5 preferably comprises a number of pixels greater than one million. The sensor 5 is for example a matrix of 1280 pixels by 1024 pixels, the sensor 5 having a diagonal of between 1 millimeter and 2 centimeters, preferably between 5 and 10 millimeters.

[0029] The optical system 3 can also be used as an optical unit arranged with various other optical units positioned before or after on the optical path passing through said optical system 3. In this embodiment, the optical system 3 is therefore not directly coupled to a sensor 5.

[0030] The optical system 3 generally comprises, aligned in this order on the same optical axis X-X', a first lens 7, a free space 8, and a second lens 9. It is generally configured to define a focal length of between 10 and 150 millimeters.

[0031] [Fig.l] illustrates electromagnetic radiation R propagating only on one side of the optical axis X-X' for greater readability, the same type of electromagnetic radiation R also propagating and evidently on the other side of the optical axis X-X' by symmetry effect.

[0032] The term free space 8 refers to the space between the first lens 7 and the second lens 9, the latter not being in contact with each other. The free space 8 is composed of a fluid, in particular air when the optical system 3 is used in an environment composed of air. Alternatively, when used in space, the free space 8 may be composed of a vacuum, or a gas or a fluid, in particular at low pressure. Generally speaking, the free space 8 makes it possible to create a difference in refractive index between the first lens 7 and the free space 8 on the one hand, and between the free space 8 and the second lens 9 on the other hand. The free space 8 has, for example, a thickness of between 1 and 20 millimeters.

[0033] The optical system 3 is rather intended to be used in an environment such as air. Thus, the first lens 7 defines a first diopter 11 between the air and its material as well as a second diopter 12 between its material and the air of the free space 8. Similarly, the second lens 9 defines a third diopter 13 between the air of the free space 8 and its material, as well as a fourth diopter 14 between its material and the surrounding air. As mentioned previously, the air can be replaced by any other fluid or by vacuum.

[0034] Advantageously, the first lens 7 and the second lens 9 are made of the same material.

[0035] Thus, the optical system 3 is athermalized. In other words, if heat expands one of the two lenses 7 or 9, the other will be expanded in the same way, thus reducing the disparity in behavior of the optical system 3.

[0036] According to one embodiment, the first lens 7 and / or the second lens 9 is made of a material such as chalcogenide glass, and / or ZnS, in other words zinc sulfide, and / or ZnSe, in other words zinc selenide.

[0037] These materials are particularly suitable for optical systems intended for observation in the infrared.

[0038] The first diopter 11 comprises an outer zone 15 configured to refract incident electromagnetic radiation R. The outer zone 15 of the first diopter 11 is, when using the optical system 3, the first surface of said optical system 3 to interact with electromagnetic radiation R. The latter therefore comes from the left as shown in [Fig.l]. In other words, the outer zone 15 of the first diopter 11 is configured to refract electromagnetic radiation R coming from outside the first lens 7 towards the second diopter 12.

[0039] In one embodiment, the outer zone 15 is an annular zone.

[0040] The first diopter 11 further comprises a central zone 17 configured to reflect incident electromagnetic radiation R. Electromagnetic radiation R coming from outside the optical system 3, in other words from the left of [Fig.l], does not pass through the central zone 17: on the other hand, the central zone 17 of the first diopter 11 is configured to reflect electromagnetic radiation R coming from said second diopter 12 towards the second diopter 12. The central zone 17 is comparable to a secondary mirror of a Cassegrain type telescope of the prior art.

[0041] The central zone 17 and the outer zone 15 together form the first diopter 11.

[0042] In a particular embodiment, the shape of the first diopter 11 is defined by a first equation defining the outer zone 15 different from a second equation defining the central zone 17. In particular, each of the outer zone 15 or central zone 17 can be convex or concave independently of the shape of the other.

[0043] The second diopter 12 is configured to refract towards the third diopter 13 an electromagnetic radiation R coming from the first diopter 11. It is also configured to refract towards the central zone 17 of the first diopter 11 an electromagnetic radiation R coming from the third diopter 13.

[0044] In a particular embodiment, the shape of the second diopter 12 is defined by a single defining equation. Indeed, the electromagnetic radiation R being refracted several times in different directions and at the same locations on this second diopter 12, it is preferable that the shape of said second diopter 12 is defined by a single defining equation, rays of electromagnetic radiation R passing at the same location in both directions. Preferably, the second diopter 12 is convex.

[0045] The third diopter 13 is configured to refract towards the fourth diopter 14 an electromagnetic radiation R coming from the second diopter 12. It is also configured to refract towards the second diopter 12 an electromagnetic radiation R coming from the fourth diopter 14.

[0046] Preferably, the third diopter 13 is concave in its entirety. However, optionally, a central portion 19 of the third diopter 13 may be convex.

[0047] In other words, in a particular embodiment, the shape of the third diopter 13 can be defined by a first equation for defining the central portion 19 different from a second equation for defining the outer portion defined by the remainder of said third diopter 13.

[0048] The fourth diopter 14 comprises an outer part 21 configured to reflect incident electromagnetic radiation R. In particular, the outer part 21 of the fourth diopter 14 is configured to reflect electromagnetic radiation R coming from said third diopter 13 towards the third diopter 13. This outer part 21 is comparable to a primary mirror of a Cassegrain type telescope of the prior art.

[0049] In one embodiment, the outer portion 21 is an annular portion.

[0050] The fourth diopter 14 further comprises a central portion 23 configured to refract incident electromagnetic radiation R.

[0051] The central part 23 and the outer part 21 together form the fourth diopter 14.

[0052] In a particular embodiment, and on a principle analogous to the first diopter 11, the shape of the fourth diopter 14 can be defined by two different definition equations, one for the outer part 21, and a second for the central part 23. In particular, each of the outer part 21 or central part 23 can be convex or concave independently of the shape of the other.

[0053] In other words, in a particular embodiment, the shape of the fourth diopter 14 is defined by a first equation for defining the outer part 21 different from a second equation for defining the central part 23.

[0054] Preferably, the central part 23 is convex and the outer part 21, as a reflective part, is concave.

[0055] Thus, the optical system 3 is configured so that electromagnetic radiation R entering the optical system 3 through the outer zone 15 of the first diopter 11 exits from said optical system 3 through the central part 23 of the fourth diopter 14 after being refracted by the second diopter 12, then the third diopter 13, then reflected in the outer part of the fourth diopter 14, again refracted by the third diopter 13, then the second diopter 12, then reflected in the central zone 17 of the first diopter 11 in order to be finally refracted by the second diopter 12, then the third diopter 13 and finally the central part 23 of the fourth diopter 14 as mentioned previously.

[0056] The sensor 5 can be positioned close to the central part 23 of the fourth diopter 14, on the optical axis X-X' of the optical system 3.

[0057] Advantageously, the first diopter 11 and / or the second diopter 12 and / or the third diopter 13 and / or the fourth diopter 14 is aspherical. Preferably, each diopter is aspherical.

[0058] Each diopter can be produced by machining using a diamond. This manufacturing method allows for low costs, even if each diopter is aspherical.

[0059] Alternatively, the first lens 7 and the second lens 9 may also be molded, in particular when they are made of chalcogenide glass, thus reducing manufacturing costs.

[0060] Advantageously, each diopter 12, 13, and / or diopter part 23, and / or diopter zone 15, configured to refract electromagnetic radiation R comprises a single-layer or multi-layer anti-reflection treatment.

[0061] Obviously, the central zone 17 of the first diopter 11 and the outer part 21 of the fourth diopter 14 configured to reflect electromagnetic radiation R comprise a single-layer or multi-layer reflective treatment.

[0062] Advantageously, the optical system 3 further comprises a mounting barrel 25.

[0063] The mounting barrel 25 is for example cylindrical in shape and makes it possible to house the first lens 7 and the second lens 9 without establishing contact between them.

[0064] Thus, the mounting barrel 25 comprises a first location 27 into which the first lens 7 is slid, and a second location 29 into which the second lens 9 is slid, the second location 29 having a larger diameter than the diameter of the first location 27. This latter characteristic makes it possible to easily and inexpensively slide the first lens 7 and then the second lens 9 into the mounting barrel 25. The mounting mechanism is thus greatly simplified and less expensive to implement.

[0065] Generally speaking, the optical system 3 which is the subject of the invention makes it possible to reduce the diameter of the central zone 17 of the first diopter 11 and therefore to optimize the modulation transfer function. The diameter of the central zone 17 of the first diopter 11 is for example between 10 and 20 millimeters, with a diameter of the first diopter 11 for example between 30 and 60 millimeters. Preferably, the ratio between the diameter of the central zone 17 and the diameter of the first diopter 11 is less than 0.3- The aperture number of the optical system 3 is for example between 1 and 1.4.

Claims

Claims

1. Optical system (3) comprising, aligned in this order on the same optical axis (X-X'), a first lens (7), a free space (8), and a second lens (9), the first lens (7) defining a first diopter (11) and a second diopter (12), the second lens (9) defining a third diopter (13) and a fourth diopter (14), characterized in that the first diopter (11) comprises an outer zone (15) configured to refract incident electromagnetic radiation (R) and a central zone (17) configured to reflect incident electromagnetic radiation (R), and in that the fourth diopter (14) comprises an outer part (21) configured to reflect incident electromagnetic radiation (R) and a central part (23) configured to refract incident electromagnetic radiation (R),such that electromagnetic radiation (R) entering the optical system (3) through the outer zone (15) of the first diopter (11) exits said optical system (3) through the central part (23) of the fourth diopter (14) after being reflected in the outer part (21) of the fourth diopter (14) then in the central zone (17) of the first diopter (11).,

2. Optical system (3) according to claim 1, wherein at least one of said first (11), second (12), third (13) and fourth diopter (14) is rotationally symmetrical, preferably aspherical.

3. Optical system (3) according to one of claims 1 and 2, in which the shape of the first diopter (11) is defined by a first equation for defining the outer zone (15) different from a second equation for defining the central zone (17).

4. Optical system (3) according to any one of claims 1 to 3, in which the shape of the second diopter (12) is defined by a single defining equation.

5. Optical system (3) according to any one of claims 1 to 4, in which the shape of the third diopter (13) is defined by a first equation for defining a central portion (19) different from a second equation for defining an outer portion defined by the remainder of said third diopter (13).

6. Optical system (3) according to any one of claims 1 to 5, in which the shape of the fourth diopter (14) is defined by a first definition equation of the outer part (21) which is different of a second equation defining the central part (23).

7. Optical system (3) according to any one of claims 1 to 6, wherein the first lens (7) and the second lens (9) are made of the same material.

8. Optical system (3) according to any one of claims 1 to 7, wherein the first lens (7) and / or the second lens (9) is made of a material comprising chalcogenide glass, and / or ZnS, and / or ZnSe.

9. An optical system (3) according to any one of claims 1 to 8, further comprising a mounting barrel (25) comprising a first location (27) into which the first lens (7) is slid, and a second location (29) into which the second lens (9) is slid, the second location (29) having a larger diameter than the diameter of the first location (27).

10. Optical system (3) according to any one of claims 1 to 9, wherein the outer zone (15) of the first diopter (11) is configured to refract towards the second diopter (12) electromagnetic radiation (R) coming from outside the first lens (7), the second diopter (12) being configured to refract towards the third diopter (13) electromagnetic radiation (R) coming from the first diopter (11) and to refract towards the central zone (17) of the first diopter (11) electromagnetic radiation (R) coming from the third diopter (13), the third diopter (13) being configured to refract towards the fourth diopter (14) electromagnetic radiation (R) coming from the second diopter (12) and to refract towards the second diopter (12) electromagnetic radiation (R) coming from the fourth diopter (14),the outer part (21) of the fourth diopter (14) being configured to reflect towards the third diopter (13) an electromagnetic radiation (R) coming from said third diopter (13), the central zone (17) of the first diopter (11) being configured to reflect towards the second diopter (12) an electromagnetic radiation (R) coming from said second diopter (12), said electromagnetic radiation (R) reflected by the central zone (17) of the first diopter (11) then being refracted by the second diopter (12), then the third diopter (13) then the central part (23) of the fourth diopter (14).,

11. Imaging system (1) comprising an optical system (3) according to any one of claims 1 to 10, as well as a sensor (5) in- infrared.

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