WO2024256603A1 - Wide-aperture optical systems for wide-angle detection - Google Patents

Abstract

According to one aspect, the present invention relates to an optical system having a given paraxial focal length F, the system comprising: a first lens made of alkali metal halide comprising a first paraxial focal length F₁ and a first curvature β₁, such that the modulus of F₁/F is greater than 4.5 and the modulus of beta β₁ is greater than 2, the first lens being arranged upstream of the diaphragm; a second converging lens made of alkali metal halide comprising a second paraxial focal length F ₂ and a second curvature β₂ such that F₂/F is between 0.5 and 2 and β₂ is between -1 and 3.5, the second lens being arranged downstream of the diaphragm; a third, diverging lens made of an alkali metal halide comprising a third paraxial focal length F₃ and a third curvature β₃, such that the modulus of F₃/F is greater than 2 and the modulus of β₃ is greater than 2.5, the third lens being arranged downstream of the second lens.

Description

DESCRIPTION

TITLE: Large aperture optical systems for wide-field detection Technical field of the invention

The present description relates to large aperture optical systems for wide-field detection as well as detection systems equipped with such optical systems. The present description relates more specifically to low-cost optical systems for infrared detection with very good optical performances.

State of the art

The present description concerns optical systems for infrared imaging, which have a large aperture, i.e. typically an aperture number F# less than or equal to 2. Large aperture and wide field infrared detection systems using thermal detectors of the microbolometer, thermopile or pyroelectric type are experiencing significant growth in many fields (thermography, home automation, automotive, leisure, robotics, etc.). This is due in particular to the drop in the cost of infrared detectors. For microbolometer detectors, this drop is mainly due to the reduction in the dimensions of the thermosensitive elements ("pixels") of the detector, which makes it possible to produce a greater number of detectors on the same wafer.

As a result, traditional imaging optical systems represent a significant portion of the overall cost of detection systems.

Among traditional imaging optical systems, optical systems made with one or more silicon lenses are known, see for example US20150109456 [Ref.

1] or US20190170973 [Ref. 2],

[Ref. 2] thus describes an optical system for the infrared (7.5 - 13.5 pm) comprising a first optical element and a second optical element made respectively of a first material and a second material with a high refractive index, typically greater than 2.2. Silicon is particularly interesting because it has a high refractive index, of the order of 3.4, which allows a reduction of aberrations with a limited number of lenses. In addition, the manufacture of silicon lenses can be done by photolithography techniques which are relatively low cost.

However, such techniques impose constraints on the specifications of the achievable lenses such as maximum deflection or camber. This limits the optical performance of the optical system. Furthermore, silicon is absorbent in the spectral band of the visible (below a wavelength of 1.1 pm) and is absorbent for wavelengths greater than 9 pm, for thicknesses greater than 1 mm.

Patent application US20150206909 [Ref. 3] proposes a low-cost architecture using a polyethylene-type resin lens, which can be molded.

However, polyethylene is infrared absorbent, which does not allow the use of more than one lens, with a limited thickness, typically less than 1 mm. Furthermore, the refractive index of the resins is low, generally less than 2. A low refractive index, coupled with the need to limit the thickness of the lenses and their number, greatly limits the optical quality of optical systems using this type of material, a single lens not being sufficient to guarantee satisfactory image quality.

Chalcogenide glasses are also known for the production of optical systems.

As described for example in published patent application US20200116979 [Ref. 4], chalcogenide glass lenses enable imaging in a broad infrared spectrum. Chalcogenide glasses have high refractive indices, typically greater than 2, and can be molded, which allows the realization of infrared imaging optical systems with very good optical quality.

However, the manufacture of chalcogenide glasses is delicate and involves many steps, particularly because the materials used to obtain these glasses are polluting and dangerous to handle; the production cost of such optical systems is therefore increased.

Furthermore, a simple and low-cost method is known (see [Ref. 5]) for the fabrication of plano-convex and plano-concave microlens arrays based on the use of potassium bromide (KBr) powder. The KBr microlens arrays described in [Ref. 5] are interesting in particular because they can be used in the visible and infrared. However, these microlens arrays are not suitable for large aperture and wide field imaging.

This description proposes a large aperture optical system based on the use of alkali halides including KBr, whose original architecture allows wide-field imaging with very good optical quality in a wide range of wavelengths, including in particular the near infrared and infrared (0.7 pm - 14 pm) but also the visible (0.4 pm - 0.7 pm).

Summary of the invention As used herein, the term "include" means the same as "include" or "contain", and is inclusive or open-ended and does not exclude other elements not described or shown.

Furthermore, in this description, the term “approximately” or “substantially” is synonymous with (means the same as) a lower and/or upper margin of 10%, e.g. 5%, of the respective value.

According to a first aspect, the present invention relates to an optical system having a given paraxial focal length F and a given aperture defined by an aperture number, the optical system comprising: a diaphragm configured to receive incident radiation, said diaphragm defining the aperture of the optical system; a first lens made of a first material, comprising a first paraxial focal length Fi and a first camber 0i, such that the modulus of Fi/F is greater than 4.5 and the modulus of Pi is greater than 2, said first lens being arranged upstream of the diaphragm; a second lens made of a second material, converging, comprising a second paraxial focal length F2 and a second camber P2, such that F2/F is between 0.5 and 2 and P2 is greater than -1 and less than 3.5, said second lens being arranged downstream of the diaphragm; a third lens made of a third material, comprising a third paraxial focal length F3 and a third camber P3, such that the modulus of F3/F is greater than 2 and the modulus of P3 is greater than 2, said third lens being arranged downstream of the second lens; and wherein: the first material, the second material and the third material are alkali halides.

The optical system according to the invention may comprise at least one or more other optical element(s) (blade(s), filter(s), and/or porthole(s), etc.).

At least one other lens or lenses than the first lens, the second lens and the third lens can be inserted into the system according to the invention, in particular: lenses whose effects compensate for each other (for example to create an intermediate image plane) located between the first lens and the second lens, or lenses whose effects compensate for each other (for example to create an intermediate image plane) located between the second lens and the third lens, but such embodiments are not preferred because they would harm the compactness of the optical system according to the invention.

Thus, preferably, the optical system according to the invention is a three-lens optical system comprising no more lenses than the first lens, the second lens and the third lens.

However, the optical system according to the invention can be inserted into or combined with another device or another optical architecture (detection, etc.) which comprises other lenses and/or at least one other optical element (such as a detector, a blade, a filter, and/or a porthole, etc.).

Preferably, the optical system according to the invention consists only, for its elements interacting with the incident radiation, of the diaphragm, the first lens, the second lens and the third lens.

In the present description, it will be said that a first optical element is located upstream of a second optical element when this first optical element is configured to be on the object (or scene) side relative to the second optical element in a detection system, while a first optical element is located downstream of a second optical element when this first optical element is configured to be on the image (or detector) side relative to the second optical element.

An optical lens, simply called a “lens” in the present description, is an optical element made of a material with a given refractive index, comprising an entry surface and an exit surface, at least one of the surfaces of which is not flat, and forming two diopters between the external medium and the material of which the lens is composed. The entry surface, like the exit surface, may be revolution-symmetric, spherical or aspherical, or of any shape.

In this description, a center of curvature and a radius of curvature of each surface are defined as the center and radius of the "best sphere" defined as the sphere that minimizes a deviation from said surface. The optical axis of the lens is the axis that passes through the centers of curvature of the two surfaces.

A useful surface of each surface of the lens is defined as the minimum surface that receives, in operation, all the rays that pass through the diaphragm. We speak of useful diameter when said useful surface has a circular outline. The useful diameter is then the diameter of said outline. In a thin lens approximation, the useful entrance surface and the useful exit surface are substantially merged and we can speak of the useful surface of the lens. In other cases, we can call the useful surface of the lens the smallest of the surfaces between the useful inlet surface and the useful outlet surface.

The applicant has shown that such an arrangement of three optical lenses made of alkali metal halide allows the production of low-cost, wide-field detection systems, i.e. fields greater than approximately 50°, typically between approximately 50° and approximately 120°, for operation in particular in the infrared.

Furthermore, such an optical system is panchromatic since, due to the high transparency of alkali metal halides, it allows imaging in the visible and infrared spectral bands, for wavelengths ranging from 0.4 pm to 14 pm. Thus, an optical system according to the first aspect can be used without modifications, or with a simple adjustment of the size of the diaphragm, both in the visible and in the infrared. Furthermore, such an arrangement of three optical lenses made of alkali metal halides as described in the present description allows the production of a large aperture optical system, i.e. having for example an aperture number F# less than or equal to 2.

Such an optical system exhibits very good optical quality, particularly in the infrared, with a cut-off frequency compatible with detectors with sampling steps between about 8 pm and about 17 pm. The cut-off frequency is the first zero-contrast spatial frequency of the modulation transfer function (MTF) of the optical system, free from manufacturing errors or alignment errors.

According to one or more exemplary embodiments, the paraxial focal lengths and the cambers of the lenses are chosen in the following ranges: the modulus of Fi/F is greater than 4.5 and the modulus of Pi is greater than 2.2; and/or F2/F is between 0.7 and 1.6 and P2 is greater than -1 and less than 3.1; and/or the modulus of F3/F is greater than 2 and the modulus of 3 is greater than 2.3.

The applicant has shown that the parameters thus chosen are optimized to obtain very good optical quality for many alkali halide compositions.

According to one or more exemplary embodiments, the alkalis of the alkali halides are chosen from sodium (Na), potassium (K) and rubidium (Rb).

According to one or more exemplary embodiments, the halogens of the alkali halides are chosen from chlorine (Cl), bromine (Br) and iodine (I).

According to one or more exemplary embodiments, the alkali metal halides comprise cubic crystal structures. Such alkali metal halides may be chosen, for example, from: potassium iodide (Kl), potassium bromide (KBr), sodium chloride (NaCl), potassium chloride (KC1). In particular, Kl and KBr have a refractive index greater than 1.5 at the wavelength of 10pm, which makes them particularly advantageous.

In exemplary embodiments, the first material from which the first lens is formed, the second material from which the second lens is formed, and the third material from which the third lens is formed are identical. They may also be different, all or in part.

According to one or more exemplary embodiments, the Fi/F modulus is greater than 10 and pi is less than -2.2. These parameters for the first lens are advantageous in particular when the refractive index of the first material is greater than 1.5 at a wavelength of 10 pm. These parameters are thus advantageous for example when the first material is made of Kl or KBr.

According to one or more exemplary embodiments, F2/F is between 0.7 and 1.2 and P2 is between -1 and 1.3. These parameters for the second lens are advantageous in particular when the refractive index of the second material is greater than 1.5 at a wavelength of 10 pm. These parameters are thus advantageous for example when the second material is made of Kl or KBr.

According to one or more exemplary embodiments, the modulus of F3/F is greater than 2.5 and the modulus of P3 is greater than 2.3. These parameters for the third lens are advantageous in particular when the refractive index of the third material is greater than 1.5 at a wavelength of 10 pm. These parameters are thus advantageous for example when the third material is made of Kl or KBr.

According to one or more exemplary embodiments, a central thickness of the first optical lens and/or the second optical lens and/or the third optical lens is greater than or equal to one fifth of a maximum dimension of the lens, for example a diameter of the lens when the lens has a circular outline. The central thickness is defined as the thickness of the lens measured on the optical axis of the lens. Such a condition on the central thickness allows easier handling of the lenses and makes manufacturing by molding easier.

According to a second aspect, the present invention relates to a detection system for visible and infrared imaging comprising: an imaging optical path with an optical axis and at least a first detector configured for detection in at least a first spectral band and comprising a detection surface; an optical system according to the first aspect, arranged on said imaging path, upstream of said at least one first detector.

According to one or more exemplary embodiments, said at least one first detector comprises first sensitive elements in a first spectral band and second sensitive elements in a second spectral band different from said first spectral band.

In exemplary embodiments, the first spectral band is an infrared spectral band and the first elements are heat-sensitive elements.

In exemplary embodiments, the second spectral band is a visible or near infrared spectral band.

According to one or more exemplary embodiments, the useful surface of the first lens and/or the second lens and/or the third lens is vignetted to stop the rays at the edge of the field. This makes it possible to improve the image quality at the edge of the field. This also makes it possible to reduce the optical deflection of the lenses and to reduce the angle of incidence of the rays relative to the normal to the surface, thus facilitating the production of the lenses and their alignment. The vignetting may be such that it stops rays at the edge of the field without excessively deteriorating the illumination. In exemplary embodiments, the vignetting of at least one of said useful surfaces of the lenses is such that in operation, an illumination measured at any point in the field and in the entire spectral range of use is greater than or equal to approximately 70% of the illumination measured on the optical axis.

Brief description of the figures

Other advantages and characteristics of the invention will appear on reading the detailed description of implementations and embodiments which are in no way limiting, and the following appended drawings:

Fig. 1, a schematic of a detection system comprising an example of an optical system according to the present invention;

Fig. 2, a diagram illustrating the parameters of an optical lens;

Fig. 3 A, a diagram showing a first example of an optical system according to the present invention;

Fig. 3B, curves representing the modulus of the (normalized) transfer function of the optical system illustrated in Fig. 3A as a function of spatial frequency, for different values of fields of view;

Fig. 4A, a diagram showing a second example of an optical system according to the present invention; Fig. 4B, plots representing the modulus of the (normalized) transfer function of the optical system illustrated in Fig. 4A as a function of spatial frequency, for different values of field of view;

Fig. 5A, a diagram showing a third example of an optical system according to the present invention;

Fig. 5B, plots representing the modulus of the (normalized) transfer function of the optical system illustrated in Fig. 5A as a function of spatial frequency, for different values of fields of view.

These embodiments being in no way limiting, it will be possible in particular to consider variants of the invention comprising only a selection of features described or illustrated subsequently isolated from the other features described or illustrated (even if this selection is isolated within a sentence comprising these other features), if this selection of features is sufficient to confer a technical advantage or to differentiate the invention compared to the state of the prior art. This selection comprises at least one preferably functional feature without structural details, and/or with only a part of the structural details if this part alone is sufficient to confer a technical advantage or to differentiate the invention compared to the state of the prior art.

Detailed description of the invention

Fig. 1 shows an example of a detection system 100 according to the present description and Fig. 2 illustrates the main parameters of an optical lens in an optical system. The detection system 100 comprises an imaging optical path with an optical axis A and at least a first detector schematically represented in Fig. 1 by a detection surface 180. In the present description, the term detection surface of the detector refers to a useful detection surface, i.e. the surface formed by the elementary detectors or “pixels” configured to detect radiation in the desired spectral band(s) for the detection system.

The detection system 100 further comprises an optical system 101 of optical axis A with a given paraxial focal distance F, the system comprising an arrangement with 3 optical lenses 110, 120, 130 and a diaphragm 150.

By convention in this description, the positive sense of a direction parallel to the optical axis is the sense which is directed from the scene towards the detector, in other words the sense of propagation of the light. L denotes the total size of the detection system, defined by the distance between the top of the entrance face of the first lens (surface 111) and the detection surface 180. According to the present description, the optical system 100 (Fig. 1) comprises a diaphragm 150 configured to receive incident radiation. The diaphragm defines the aperture of the optical system.

The aperture number is defined by F# = F/PE, where PE is a maximum dimension of the entrance pupil of the optical system, the entrance pupil being optically conjugated with the diaphragm by the set of optical elements located upstream of the diaphragm. The diaphragm is for example circular and PE is the diameter of the entrance pupil of the optical system.

We denote FOV as the total angular field of the detection system:

[Math 1 rmr

FOV =

Where h _max is, in the present description, a maximum dimension of the detection surface, for example a length, a width or a diagonal in the case of a rectangular detection surface.

As illustrated in Fig. 2, an optical lens 200, simply called a “lens” in the present description, is an optical element made of a material of given refractive index, comprising an entry surface 210 and an exit surface 220, at least one of which is not planar, and forming two diopters between the external medium and the material of which the lens is composed. The entry surface 210, or surface on the object side, is the surface configured to be on the scene side, and the exit surface 220, or surface on the image side, is the surface configured to be on the detector side.

The input surface 210, like the output surface, can be revolution-symmetric, spherical or aspherical, or of any shape (so-called “freeform” surface). In all cases, we can define for each surface a center of curvature Cs and a radius of curvature Rs which are respectively the center and the radius of the best sphere which minimizes a deviation from said surface.

Thus, as illustrated in Fig. 2 for the entrance face 210, it is possible to define for a surface of an optical lens, the arrow z of this surface. The arrow z is the distance measured in a direction parallel to the optical axis, at a given distance from the optical axis, between said surface and a reference plane perpendicular to the optical axis and comprising the vertex Oi of the surface. In the case of a surface with rotational symmetry, the arrow z(r) depends only on the polar coordinate r measured in the reference plane from the vertex Oi of the surface, as illustrated in Fig. 2. In a general case, we can denote z(x,y) the arrow measured from a point with Cartesian coordinates (x, y) of the reference plane, in a frame (Oi, x, y).

We define the radius Rs of the best sphere as the value which minimizes the function Z(Rs) defined by:

[Math 2]

With r _max the radius of the useful surface and:

Note that in the case of any surface, the radius Rs of the best sphere is defined as the value which minimizes the function Z(Rs) where:

[Math 5]

Z(Rs) = JJ (z(x,y) - z _s (x,y)')dxdy

And r ² = x ² + y ² .

In the example of Fig. 2, the arrow zi(r) of the input surface 210 and the arrow zsi(r) of the best sphere which minimizes the deviation from the surface 210 and which is represented diagrammatically in dotted lines by the surface 215 are shown for illustration purposes. Oi is the vertex of the input face 210. C si and Rsi are respectively the center and the radius of the best sphere 215.

The optical axis A of the lens is the axis that passes through the centers of curvature of the two surfaces

210, 220. By convention in this description, the radius of curvature Rs is oriented from the vertex to the center of curvature. If the radius of curvature is oriented towards the detector, Rs is positive, if it is oriented towards the scene, Rs is negative.

The camber of an optical lens 200 as shown in Fig. 2 is defined by the equation:

où Rsi le rayon de la meilleure sphère qui minimise l’écart avec la surface d’entrée 210 de la lentille 200 et Rs2 est le rayon de courbure de la meilleure sphère qui minimise l’écart avec la surface de sortie 220 de la lentille. [Math 6]

where Rsi is the radius of the best sphere that minimizes the deviation from the entrance surface 210 of the lens 200 and Rs2 is the radius of curvature of the best sphere that minimizes the deviation from the exit surface 220 of the lens.

The optical system 100 comprises, in addition to the diaphragm 150, a first lens 110 made of a first material, a second lens 120 made of a second material and a third lens 130 made of a third material. The first material, the second material and the third material are alkali halides.

According to the present description, the first lens 110 has a first paraxial focal length Fi and a first camber i. The modulus of Fi/F is greater than 4.5 and the modulus of Pi is greater than 2, advantageously greater than 2.2. The first lens 110 is arranged upstream of the diaphragm 150. The second lens has a second paraxial focal length F2 and a second camber 02. F2/F is between 0.5 and 2, advantageously between 0.7 and 1.6 and P2 is greater than -1 and less than 3.5, advantageously less than 3.1. The second lens 120 is arranged downstream of the diaphragm 150. The third lens 130 has a third paraxial focal length F3 and a third camber P3. The modulus of F3/F is greater than 2 and the modulus of 3 is greater than 2, advantageously greater than 2.3. The third lens is arranged downstream of the second lens.

All or at least part of the surfaces of the lenses 110, 120, 130 may be aspherical.

In an optical system 101 as shown in FIG. 1, the second lens 120 is convergent and contributes mainly to the optical power of the system, the optical power being defined as being the inverse of the focal length. Thus we have the relations: |F ₂ /F|<|F1/F| and |F ₂ /F|<|F ₃ /F|.

The applicant has shown that the choice of parameters for the lenses of the optical system which is the subject of the present description allows good image quality thanks in particular to the limitation of aberrations. For example, the camber of a lens made of a given material can be chosen to minimize spherical aberration or cancel coma aberration. The camber that minimizes spherical aberration is given by the relation: [Math 7]

Where n is the refractive index of the material from which the lens is made.

The camber that cancels the coma aberration is given by the following relation: [Math 8]

Consider for example a lens made of KBr whose refractive index is 1.52 at a wavelength of 10 pm. The camber that minimizes spherical aberration is -0.15 and the camber that cancels coma aberration is -0.09.

We can thus see that the choice of the camber P2 of the second lens 120 between -1 and 3.5 makes it possible to attenuate the coma aberration and the spherical aberration of this lens.

In exemplary embodiments, the first lens 110 and the third lens 130 may both be diverging or both converging; they may also be converging at the center and diverging at the edge or vice versa; they also have lower optical powers than that of the second optic 120. They may make it possible to compensate for the aberrations of the second optic 120 and/or to correct field aberrations, for example astigmatism.

For example, the first lens 110 and/or the third lens 130 can in a known manner help to compensate for the Petzval curvature that one seeks to minimize, as described for example in [Ref. 6],

In the case of the thin lens approximation (e.g. thickness at least less than the paraxial focal length of the lens divided by 10), the Petzval curvature C _p of a multi-lens optical system is given by the following relation: [Math 9]

Where m is the refractive index of the ^nth lens in the optical system and Fi is the paraxial focal length of the lens. In the case of thick lenses (thickness at least greater than the paraxial focal length of the lens divided by 10), the Petzval curvature C _p of an optical system with several lenses and therefore several surfaces of radii of curvature Ri, is given by the following relation:

[Math 10]

Zn' — neither n-niRt

Where m is the refractive index of the medium upstream of the ^nth surface of the optical system with radius of curvature Ri (best-sphere radius of the ^nth surface) and ni' is the refractive index of the medium downstream of the ^nth surface of the optical system with radius of curvature Ri. Thus, the field curvature can be reduced as C _p tends to 0, either by using alternating concave or convex surfaces in the case of thick lenses, or by increasing the refractive index, or by using both diverging and converging optical lenses.

Thus, for example, in a three-lens system as in the present description, the first and/or third lens can be chosen to be divergent and the second lens to be convergent.

In the case of lenses with a low refractive index, i.e. refractive indices less than 2, the field curvature can be corrected by using aspherical lenses.

For example, we assume that the deflection z(r) of each surface of each lens can be approximated by the equation:

[Math 11]

R is the radius of curvature at the vertex of the surface, k is the conicity coefficient, ai are the aspherization coefficients of order 2i. The aspherization coefficients describe the deviation of the surface from an axially symmetric quadric surface defined by the radius of curvature at the vertex R and the conicity coefficient k. In other words, if the coefficients a; are all null, then the surface is a conic section that has a symmetry of revolution about the optical axis, with R the radius of curvature measured at the vertex (where r = 0) and k the constant of the conic that determines its shape. Note that the radius of curvature measured at the vertex R is not necessarily the same as the radius Rs of the best sphere as defined previously.

To reduce the field curvature, we can choose aspherical surfaces, that is to say with non-zero aspherical terms ai.

For example, it is possible to correct the 4th order field curvature by 6th and 8th order aspherical terms.

In exemplary embodiments, it is possible to obtain a so-called “aspherical” lens which has a focal length which varies in r; in other words, the focal length, moving away from the optical axis, is no longer equal to the paraxial focal length measured at the optical axis (r close to 0). It is possible to obtain aspherical lenses with at least one surface which has oscillations.

In the present description the third lens (arranged on the detector side) can advantageously be aspherical. In the present description, the first lens can also be aspherical.

la lentille. The high aspherization of the third lens 130 can be used to adjust the focal length of this lens in the field. In particular, it makes it possible to obtain a good level of illumination throughout the field of view, for example illumination at any point in the field and in the spectral band of use at least equal to 70% of the illumination at the center of the field. Advantageously, to improve the image quality at the edge of the field, the surfaces of the lenses can be reduced relative to the useful surfaces in order to vignetize rays at the edge of the field. The vignetting can be limited by guaranteeing a level of illumination at any point in the field greater than 70° of the illumination at the center of the field. More particularly, the useful surface of the second lens can be reduced to vignetize the rays at the edge of the field. In exemplary embodiments, it will be sought to have a central thickness e of the lenses such that: [Math 12] e with <pi _being a maximum dimension of the lens, for example the diameter of

the lens.

In this way, the lenses can be molded and handled easily.

La dimension maximale h_max est par exemple la longueur, la largeur ou la diagonale de la surface de détection dans le cas d’une surface de détection rectangulaire. On limite ainsi la taille des lentilles et donc leur coût. In exemplary embodiments, the ratio between half the maximum diameter of the lenses and a maximum dimension h _max of the detection surface can validate the relationship: [Math 13]

The maximum dimension h _max is for example the length, width or diagonal of the detection surface in the case of a rectangular detection surface. This limits the size of the lenses and therefore their cost.

Lenses may include, in a known manner, a coating deposited on the surfaces to protect them from the environment and to limit the hygroscopic effects of the materials. This coating may also serve as an anti-reflective treatment.

The manufacture of an optical lens in an optical system according to the first aspect can be obtained in a manner similar to the manufacturing method described in [Ref. 5]. For example, a powder of given purity, for example of purity > 99.5, dried, is used. The powder is placed in a steel die (mold) between two pistons. The face of the pistons in contact with the powder has the desired shape to produce the lens. The mold is placed in a controlled force press, arranged in an oven. The temperature of the oven can vary between about 20°C and about 200°C. The pressure applied can vary between about 100 MPa and 1000 MPa, with a controlled variation of the pressure. Such a method makes it possible to obtain non-cracked optical lenses.

In a detection system as shown in Fig. 1, at each pixel (elementary thermosensitive surface) of a thermal detector 180 can be associated a sensitive zone in the visible or infrared spectral band. The high transparency of the materials envisaged makes it possible to manufacture panchromatic cameras using a single optical path and sensitive in several spectral bands. Diffractive surfaces can be added in a known manner in the optical system, in particular to correct chromatic aberrations.

Fig. 3A, Fig. 4A and Fig. 5A illustrate 3 exemplary embodiments of optical systems according to the present description.

In these examples, we assume that the deflection z(r) of each surface of each lens can be approximated by the equation [Math 11],

Fig. 3A shows a first example of an optical system 301 integrated into a detection system 300, the detection system comprising a detector represented by its detection surface 380. The first optical system 301 comprises a first optical lens 310, a diaphragm 350, a second optical lens 320 and a third optical lens 330.

The paraxial focal length of the 301 optical system is F = 4.7 mm, the aperture number is F# = 1.5, the total field FOV is 50°, the total footprint is 7.5 mm. In this example, the paraxial focal length of the first lens is Fi = 134 mm; the paraxial focal length of the second lens is F2 = 4.5 mm; the paraxial focal length of the third lens is F3 = -236 mm; the camber of the first lens is Pi = -12.2; the camber of the second lens is 2 = 1.0; the camber of the third lens is ₃ = - 4.7. We calculate F2/F = 0.95, Fi/F = 28.5, _F3 /F = -50.2.

Table 1 below gives the optical parameters of the optical system illustrated in Fig.3A.

[Table 1]

R corresponds to the radius of curvature at the top (in mm) of the designated surface, the thickness corresponds to the distance (in mm) measured on the optical axis, between the designated surface and the following surface; more precisely, ti is the thickness of the first lens 310, di is the distance between the exit surface 312 of the first lens and the diaphragm 350, d ₃ is the distance between the diaphragm 350 and the entrance surface 321 of the second lens, t2 is the thickness of the second lens 320, d ₃ is the distance between the exit surface 322 of the second lens and the entrance surface 331 of the third lens, t3 is the thickness of the third lens 330 and d4 is the distance between the exit surface 332 of the third lens and the detection surface of the detector 380. In the material column, the material between the designated surface and the following surface is reported. The material is air if not specified, k is the conicity coefficient of the designated surface, r _max (in mm) is half of a maximum dimension of the useful surface of said surface, in this example half of the diameter of the useful surface of said surface, and ai, 2, as, a4 are the aspherization coefficients of order 2i of the designated surface (see [Math 11]).

Fig. 3B shows the polychromatic modulation transfer function (MTF) of the optical system (normalized) calculated as a function of spatial frequency, in the wavelength range 8pm - 12pm. Curves 361, 362 correspond to the curves calculated at the diffraction limit (system not affected by aberrations), respectively in tangential and sagittal, that is to say according to two perpendicular axes in the image plane: the tangential (southern) orientation and the sagittal (radial) orientation. These curves therefore correspond to the best performances expected for the system.

Curves 363 - 368 correspond to the curves calculated for different points of the field, the field being taken on the detection surface, relative to the center of the detector positioned on the optical axis. More precisely, curve 363 is calculated at the center of the field (0 mm tangential), curve 364 is calculated at the center of the field (0 mm sagittal). Curve 365 is calculated for 1 mm tangential, curve 366 is calculated for 1 mm sagittal, curve 367 is calculated for 2.04 mm tangential and curve 368 is calculated for 2.04 mm sagittal. The fact that all the curves are very close to the curves calculated for the diffraction limit indicates that the quality of the optical system is close to the theoretical limit, for all points of the field. The detection system is therefore of very good quality.

Fig. 4A shows a second example of an optical system 401 associated with a detection system 400, the detection system comprising a detector represented by its detection surface 480. The optical system 401 comprises a first optical lens 410, a diaphragm 450, a second optical lens 420 and a third optical lens 430.

The paraxial focal length of the 401 optical system is F = 3.4 mm, the aperture number is F# = 1.5, the field is 90°, the total size is 6.3 mm.

In this example, the paraxial focal length of the first lens is Fi = -138 mm; the paraxial focal length of the second lens is F2 = 2.5 mm; the paraxial focal length of the third lens is F3 = -15 mm; the camber of the first lens is Pi = -8.8; the camber of the second lens is 2 = -0.44; the camber of the third lens is 3 = 5.3. Furthermore, we calculate: F2/F = 0.74, Fi/F = -40.6, F3/F = -4.4.

Table 2 below gives the optical parameters of the optical system illustrated in Fig.4A.

[Table 2]

R corresponds to the radius of curvature (in mm) of the designated surface, the thickness corresponds to the distance (in mm) measured on the optical axis, between the designated surface and the following surface; more precisely, ti is the thickness of the first lens 410, di is the distance between the exit surface 412 of the first lens and the diaphragm 450, d2 is the distance between the diaphragm 450 and the entrance surface 421 of the second lens, t2 is the thickness of the second lens 420, ds is the distance between the exit surface 422 of the second lens and the entrance surface 431 of the third lens, t3 is the thickness of the third lens 430 and d4 is the distance between the exit surface 432 of the third lens and the detection surface of the detector 480. The material designates the material between the designated surface and the following surface. The material is air if not specified, k is the conicity coefficient of the designated surface, r _ma x (in mm) is the radius of the optical surface, and oti, a.2, 0.3, oi4 are the 2i-order aspherization coefficients of the designated surface (see [Math 11]). Fig. 4B shows the polychromatic modulation transfer function (MTF) of the optical system (normalized) calculated as a function of spatial frequency, in the wavelength range 8pm - 12pm.

Curves 461, 462 correspond to the curves calculated for the diffraction limit, respectively in tangential and sagittal. These curves therefore correspond to the best performances expected for the system.

Curves 463 - 468 correspond to the curves calculated for different points of the field, the field being taken on the detection surface, relative to the center of the detector positioned on the optical axis. More precisely, curve 463 is calculated at the center of the field (0 mm tangential), curve 464 is calculated at the center of the field (0 mm sagittal). Curve 465 is calculated for 1 mm tangential, curve 466 is calculated for 1 mm sagittal, curve 467 is calculated for 2.04 mm tangential and curve 468 is calculated for 2.04 mm sagittal. Here again, the curves are very close to the curves calculated for the diffraction limit. This indicates that the quality of the optical system is close to the theoretical limit, for all points of the field, and with an even larger FOV than in the previous example.

Fig. 5A shows a third example of an optical system 501 integrated into a detection system 500, the detection system comprising a detector represented by its detection surface 580. The first optical system 501 comprises a first lens optical lens 510, a diaphragm 550, a second optical lens 520 and a third optical lens 530.

The paraxial focal length of the 501 optical system is F = 2.8 mm, the aperture number is F# = 1.5, the field is 120°, the total size is 6.1 mm.

In this example, the paraxial focal length of the first lens is Fi = -39 mm; the paraxial focal length of the second lens is F2 = 2.4 mm; the paraxial focal length of the third lens is F3 = -54 mm; the camber of the first lens is Pi = -7.5; the camber of the second lens is 2 = -0.29; the camber of the third lens is 3 = 5.8. Furthermore, we calculate: F2/F = 0.86, Fi/F = -13.9, F3/F = -19.3.

Table 3 below gives the optical parameters of the optical system illustrated in Fig.5A.

[Table 3]

R is the radius of curvature (in mm) of the designated surface, the thickness is the distance (in mm) measured on the optical axis, between the designated surface and the next surface; more precisely, ti is the thickness of the first lens 510, di is the distance between the exit surface 512 of the first lens and the diaphragm 550, d2 is the distance between the diaphragm 550 and the entrance surface 521 of the second lens, t2 is the thickness of the second lens 520, di is the distance between the exit surface 522 of the second lens and the entrance surface 531 of the third lens, t3 is the thickness of the third lens 530 and d4 is the distance between the exit surface 532 of the third lens and the detection surface of the detector 580. The material designates the material between the designated surface and the next surface. The material is air if not specified, k is the conicity coefficient of the designated surface, r _max (in mm) is the radius of the optical surface, and ai, 2, as, 4 are the aspherization coefficients of order 2i of the designated surface (see [Math 11]). Fig. 5B shows the polychromatic modulation transfer function (MTF) of the optical system (normalized) calculated as a function of spatial frequency, in the wavelength range 8pm - 12pm.

Curves 561, 562 correspond to the curves calculated for the diffraction limit, respectively in tangential and sagittal. These curves therefore correspond to the best performances expected for the system.

Curves 563 - 568 correspond to the curves calculated for different points of the field, the field being taken on the detection surface, relative to the center of the detector positioned on the optical axis. More precisely, curve 563 is calculated at the center of the field (0 mm tangential), curve 564 is calculated at the center of the field (0 mm sagittal). Curve 565 is calculated for 1 mm tangential, curve 566 is calculated for 1 mm sagittal, curve 567 is calculated for 2.04 mm tangential and curve 568 is calculated for 2.04 mm sagittal. Here again, the curves are very close to the curves calculated for the diffraction limit, only curve 567 deviates from the curves at the diffraction limit, the system remaining very satisfactory with regard to its large field and its large aperture.

Of course, the invention is not limited to the examples which have just been described and numerous arrangements can be made to these examples without departing from the scope of the invention. Although described through a certain number of exemplary embodiments, the optical systems according to the present description include different variants, modifications and improvements which will be obvious to those skilled in the art, it being understood that these different variants, modifications and improvements are part of the scope of the invention as defined by the claims which follow.

References

Ref. 1: US20150109456

Ref. 2: US20190170973

Ref. 3: US20150206909

Ref 4: US20200116979

Ref. 5: Florence de la Barrière et al. 'Fabrication of concave and convex potassium bromide lens arrays by compression molding', Applied Optics, Vol. 51, No. 21, 20 July 2012

Ref. 6: José Sasiân, "Field curvature aberration," Proc. SPIE 9293, International Optical

Design Conference 2014, 929322 (17 December 2014); https://doi.Org/10.l 117/12.2075938

Claims

1. An optical system (101) having a given paraxial focal length F and a given aperture defined by an aperture number (F#), the optical system comprising: a diaphragm configured to receive incident radiation, said diaphragm defining the aperture of the optical system; a first lens made of a first material, comprising a first paraxial focal length Fi and a first camber Pi, such that the modulus of Fi/F is greater than 4.5 and the modulus of Pi is greater than 2, said first lens being arranged upstream of the diaphragm; a second lens made of a second material, converging, comprising a second paraxial focal length F2 and a second camber P2, such that F2/F is between 0.5 and 2 and P2 is between -1 and 3.5, said second lens being arranged downstream of the diaphragm; a third lens made of a third material comprising a third paraxial focal length F3 and a third camber P3, such that the modulus of F3/F is greater than 2 and the modulus of P3 is greater than 2, said third lens being arranged downstream of the second lens; and wherein: the first material, the second material and the third material are alkali halides. 2. Optical system according to claim 1, wherein: the modulus of Fi/F is greater than 10 and Pi is less than -2.2; and/or F2/F is between 0.7 and 1.2 and P2 is between -1 and 1.3; and/or the modulus of F3/F is greater than 2.5 and the modulus of P3 is greater than 2.3. 3. Optical system according to any one of the preceding claims, in which the alkalis of the alkali halides are chosen from sodium (Na), potassium (K) and rubidium (Rb). 4. Optical system according to any one of the preceding claims, in which the halogens of the alkali metal halides are chosen from chlorine (Cl), bromine (Br) and iodine (I). 5. Optical system according to any one of the preceding claims, in which the alkali halides are of cubic crystalline structures. 6. Optical system according to any one of the preceding claims, in which a central thickness of the first optical lens and/or the second optical lens and/or the third optical lens is greater than or equal to one fifth of a maximum dimension of said lens. 7. Detection system (100) for visible and infrared imaging comprising: an imaging optical path with an optical axis and at least one first detector configured for detection in at least one first spectral band and comprising a detection surface (180); an optical system (101) according to any one of the preceding claims, arranged on said imaging path, upstream of said at least one first detector. 8. Detection system according to claim 7, wherein: said at least one first detector comprises first sensitive elements in a first spectral band and second sensitive elements in a second spectral band different from said first spectral band. 9. A detection system according to claim 8, wherein the first spectral band is an infrared spectral band and the first elements are thermosensitive elements. 10. Detection system according to any one of claims 7 to 9, in which at least one of the useful surfaces of said first lens, second lens, third lens is vignetted to stop, in operation, rays at the edge of the field, the vignetting of at least one of said useful surfaces of the lenses being such that an illumination measured at any point of the field remains greater than or equal to approximately 70% of the illumination measured on the optical axis.