Catoptric light focusing and collection system adapted for multispectral microscopy by direct imaging

Abstract

The invention relates to a catoptric focusing and collection optical system with an optical axis Δ and focal point F1, comprising a first pair of axisymmetric reflecting mirrors (S1, S2) about the optical axis Δ, adapted to focus a collimated incident light beam at the focal point F1. Each of the mirrors in the first pair (S1, S2) has a correction zone (S1c, S2c; S3c, S4c) for the radial profile, exhibiting a predefined continuous variation of eccentricity, the focal point(s) and the focal axis remaining identical, such that the light rays of a beam emitted from the focal point F1 have a regular radial distribution at the output of the optical system (1). Figure for the abstract: Fig. 2A

Description

Title of the invention: Catoptric light focusing and collection system adapted for multispectral microscopy by direct imaging technical field

[0001] The field of the invention is that of optical systems for focusing and collecting light, of the catoptric and axisymmetric type around the optical axis. The invention applies in particular to multispectral microscopy by direct imaging, but also to confocal microscopy, Raman microscopy, fluorescence or photoluminescence microscopy, and to all forms of microscopy. PREVIOUS STATE OF THE ART

[0002] Optical objectives fall into two categories. On the one hand, there are dioptric systems where light rays undergo refraction at the surfaces they pass through. These systems are mainly composed of a series of transparent lenses of different shapes and refractive indices, thus minimizing the aberrations inherent in refraction phenomena. Among these aberrations, chromatic aberrations, i.e., different focusing points depending on the wavelength, are particularly restrictive insofar as they limit the chromatic range of applications in optical spectroscopy.

[0003] On the other hand, there are catoptric systems where light rays undergo reflections on reflective surfaces. These have the advantage of being intrinsically achromatic in terms of focusing.

[0004] Catoptric lenses can advantageously be axisymmetric and comprise mirrors whose surfaces are generated by curves of revolution around the optical axis. The generatrix of the mirror surface can, in particular, be a conic section (parabola, ellipse, hyperbola) in any radial plane passing through the optical axis, which is also the axis of symmetry: this is then referred to as the radial conical profile of the mirrors. The reflecting surface of the mirrors is then obtained by the rotational symmetry of the conic curve around the optical axis of the lens.

[0005] These objectives having a radial conical profile can be subclassified into two types: centered objectives and off-axis objectives, depending on whether the foci of the conics are aligned or not on the optical axis which coincides with the axis of revolution.

[0006] Figure 1A illustrates an example of a centered axisymmetric catoptric system. It comprises a first mirror SI with an elliptical radial profile and foci F1 and F2. The foci F1 and F2 are located on the optical axis (axis of revolution), and the focus Fl corresponds to the focal point of the optical system. This first mirror SI radially surrounds a second mirror S2, with a parabolic radial profile and sharing the same focus F2. Thus, a beam of light emitted from the focal point Fl is collected by mirror SI, which reflects it towards mirror S2, which then reflects it in the +ez direction in a collimated manner. An example of a centered axisymmetric catoptric system is the Schwarzschild lens. Note that this is an optical system with infinite conjugation, meaning that the light rays from a point source are reflected to infinity (the outgoing light rays are all parallel and collimated).

[0007] However, although centered catoptric systems offer very good performance in direct imaging with a wide field of view (i.e., the image plane field maintaining focus), they suffer from obstruction problems due to the presence of reflective elements (here, mirror S2) along the optical axis. This obstruction can become prohibitive when the signals are weak and / or predominantly located along the optical axis.

[0008] One way to circumvent the obstruction problem is to use an off-axis axisymmetric catoptric system, an example of which is shown in [Fig. 1B] (here also with infinite conjugation). It comprises a first mirror S1 with an elliptical radial profile and foci F1 and F2. Here, only the focus F1 is located on the optical axis (axis of revolution), while the focus F2 is located at a finite distance from the optical axis. The focus F1 is located at the focal point of the optical system. This first mirror S1 is radially surrounded by a second mirror S2 with a parabolic radial profile and focus F2. In its volumetric form, the foci F2 are distributed on a circle obtained by rotational symmetry. Thus, a light beam emitted from the focal point F1 is collected by the mirror S1, which reflects it towards the mirror S2, which then reflects it in a collimated manner along the +ez direction.

[0009] Document WO2022 / 043460A1 describes another example of an off-axis axisymmetric catoptric system, adapted to focus an illuminating light beam onto an object placed at the focal plane, and to collect a light beam from the illuminated object. Its structural configuration allows for a large numerical aperture, a small footprint, and a working distance (focal length) that can be adapted to the desired applications.

[0010] However, off-axis catoptric systems, while eliminating the obstruction problem, exhibit geometric aberrations, particularly coma aberration, which greatly reduces the field of view. The field of view is significantly degraded compared to centered systems. Therefore, it is not feasible to use such optical systems as they stand for direct imaging microscopy applications (full-field imaging). Description of the invention

[0011] The invention aims to remedy at least in part the disadvantages of the prior art, and more particularly to propose a catoptric, axisymmetric and off-axis optical focusing and collection system, exhibiting minimized geometric aberrations and an improved field of view so that it can be used in particular in direct imaging by taking advantage of both the achromatic properties of catoptric systems and the non-obstruction of off-axis systems.

[0012] For this purpose, the object of the invention is an optical system for focusing and catoptric collection, and of an optical axis A and focal point Fl, comprising a first pair of axisymmetric reflecting mirrors around the optical axis A, adapted to focus at the focal point Fl a collimated incident light beam, of which: a first mirror, of radial profile defined by at least one elliptical generatrix with foci Fl and F2 and focal axis AF1, the focus Fl being located at the focal point, and adapted to collect light beams coming from the focal plane; and a second mirror, of radial profile defined by at least one conical generatrix with focus F2 and focal axis AF2, radially surrounding the first mirror and adapted to receive a light beam reflected by the first mirror.

[0013] The optical system may optionally include a second pair of axisymmetric reflecting mirrors around the optical axis A, of which: a third mirror, with a radial profile defined by at least one conical generatrix with focus F3 and focal axis AF3, located above the second mirror with respect to the focal plane, and adapted to receive a light beam reflected by the second mirror; and a fourth mirror, with a radial profile defined by at least one conical generatrix with focus F4 different from F3 and focal axis AF4, located above the first mirror with respect to the focal plane, and surrounded radially by the third mirror and adapted to receive a light beam reflected by the third mirror.

[0014] According to the invention, the mirrors of the first pair, or where applicable, those of the second pair, each have a radial profile correction zone having a predefined continuous variation of eccentricity, the focal point(s) and the focal axis remaining identical, such that the light rays of a light beam emitted from the focal point Fl have a regular radial distribution at the output of the optical system.

[0015] Some preferred but not limiting aspects of this optical system are the following.

[0016] The radial profile of the mirrors in the correction zone can be a function sigmoidal.

[0017] The mirrors of the first pair may each include a correction zone.

[0018] The radial profile of the first mirror in the correction zone can be defined by elliptical generatrices.

[0019] The radial profile of the second mirror in the correction zone can be defined by hyperbolic generators.

[0020] The first and second mirrors may be of the metallic reflection type.

[0021] The optical system may include an additional reflective and axisymmetric mirror about the optical axis A, adapted to receive a light beam reflected by the second mirror and to reflect it in a collimated manner.

[0022] The optical system may include the third mirror whose radial profile is defined by the same conical generatrix, and the fourth mirror whose radial profile is defined by the same conical generatrix.

[0023] The third and fourth mirrors may be of the vitreous reflection type.

[0024] The mirrors of the second pair may each include a correction zone.

[0025] The first mirror may have a radial profile defined by the same generatrix elliptical, and the second mirror can present a radial profile defined by the same parabolic generatrix.

[0026] The optical system may include a semi-reflective blade located on the optical axis A above the fourth mirror, adapted to transmit, respectively reflect, a first light beam from an optical source towards the fourth mirror, and to reflect, respectively transmit, a second light beam from an object located at the focal point Fl and reflected by the fourth mirror, towards an imager.

[0027] The optical system may include a shaping optical block, adapted to receive a first light beam from an optical source, then collimated and parallel to the optical axis, to make it annular around the optical axis A and transmit it in the direction of the fourth mirror; a reflecting optical block, adapted to receive a second light beam from an object located at the focal point Fl reflected by the fourth mirror and to focus it on an imager. Brief description of the drawings

[0028] Other aspects, objects, advantages and features of the invention will become more apparent upon reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the accompanying drawings in which:

[0029] [Fig.1A], already described, is a schematic and partial cross-sectional view of an infinite conjugation, catoptric, axisymmetric, centered optical system, according to an example of the prior art;

[0030] The [Fig.1B], already described, is a schematic and partial cross-sectional view of an infinite conjugation, catoptric, axisymmetric and off-axis type optical system, according to an example of the prior art;

[0031] [Fig.2A] is a schematic and partial cross-sectional view of an optical, catoptric, axisymmetric system according to a first embodiment where the first and second mirrors each have a correction zone;

[0032] [Fig.2B] illustrates the optical system of [Fig.2A] where only certain light rays emitted from the focal point are shown, in the case of uncorrected mirrors (without correction zones) and in the case of corrected mirrors (with correction zones);

[0033] [Fig.3] illustrates an example of variations in thickness gap of the correction zones of the first and second mirrors of the optical system according to the first embodiment;

[0034] [Fig.4A] is a schematic and partial cross-sectional view of an optical system according to a variant of the first embodiment allowing the pupil diameter to be reduced while maintaining the radial distribution and magnification, here with 3 reflection surfaces by advantageously adding a third flat surface S3;

[0035] [Fig.4B] is a schematic and partial cross-sectional view of an optical system according to another variant of the first embodiment allowing the pupil diameter to be reduced while maintaining the radial distribution and magnification, here with 4 reflection surfaces by advantageously adding two plane surfaces S3 and S4;

[0036] [Fig.4C] is a schematic and partial cross-sectional view of an optical system according to another variant of the first embodiment allowing the pupil diameter to be reduced by increasing the magnification but keeping the radial distribution, here with 6 reflection surfaces by advantageously adding two centered axisymmetric surfaces S3 and S4 and two planar surfaces S5 and S6;

[0037] [Fig.5A] is a schematic and partial cross-sectional view of an optical system according to a second embodiment where the correction zone is transferred to the third and fourth mirrors;

[0038] [Fig.5B] illustrates an example of variations in thickness gap ôe of the third and fourth mirrors in the correction zones, for an optical system according to the second embodiment;

[0039] [Fig.6] illustrates an example of variation in the width of the angular distribution of rays originating from an emission point located in the focal plane and transmitted by the optical system of [Fig.5A], as a function of its transverse distance from the emission point to the optical axis;

[0040] Fig. 7A illustrates an object placed at the focal plane of the optical system, here a resolution target;

[0041] [Fig.7B] illustrates the image of the resolution target, in the case of using the optical system according to an example of the prior art;

[0042] [Fig.7C] illustrates the image of the resolution target, in the case of using the optical system according to [Fig.5A] which results in a widening of the field of vision;

[0043] Fig. 8A illustrates an example of a modulation transfer function associated with the optical system according to a prior art example, in the case of a resolution target which is progressively moved away from the optical axis;

[0044] [Fig.8B] illustrates an example of a modulation transfer function associated with the optical system according to [Fig.5A], in the case of a resolution target which is also progressively moved away from the optical axis;

[0045] [Fig.9A] is a schematic and partial cross-sectional view of an optical system according to a variant of the second embodiment;

[0046] [Fig.9B] is a schematic and partial cross-sectional view of an optical system according to another variant of the second embodiment;

[0047] [Fig.9C] is a schematic and partial cross-sectional view of an optical system according to another variant of the second embodiment.

[0048] DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0049] In the figures and in the following description, the same reference numerals represent identical or similar elements. Furthermore, the various elements are not drawn to scale in order to enhance the clarity of the figures. Moreover, the different embodiments and variants are not mutually exclusive and may be combined. Unless otherwise indicated, the terms "approximately," "about," and "in the order of" mean within 10%, and preferably within 5%. Furthermore, the terms "between ... and ..." and equivalents mean that the limits are inclusive, unless otherwise stated.

[0050] Fig. 1B, already briefly described, is a cross-sectional view of an optical focusing and collection system, which is catoptric and axisymmetric about its optical axis A.

[0051] A three-dimensional orthogonal direct frame (er, e0, ez) in cylindrical coordinates is defined here and for the remainder of the description, where the vector ez is centered on the optical axis A of the optical system 1 and directed from the focal point Fl towards the optical system. In the remainder of the description, the terms "lower" and "upper" are understood to refer to an increasing positioning as one moves away from the focal point Fl along the direction +ez. "r" is defined as the radial distance from the optical axis of an outgoing light ray emitted by the point source Fl with an azimuthal angle e.

[0052] Let us first recall that a conic with directrix D, focus F, and eccentricity e is the set of points M in a Euclidean plane satisfying: d(M,F) = exd(M,D), where d(M,F) and d(M,D) are the distances from point M, respectively, to the focus F and to the directrix D. The conic is an ellipse when e<l, une parabole lorsque e=l, et une hyperbole lorsque e> l. Furthermore, the line perpendicular to the directrix D and passing through the focus F is the focal axis AF of the conic.

[0053] As previously stated, the first mirror SI has a conical radial profile defined by an elliptical generatrix Glref, with foci Fl and F2, eccentricity elref, and focal axis AFL. The radial profile is carried by the single elliptical generatrix G1 ref, meaning that its entire reflective surface coincides with the elliptical generatrix Glref. The focus Fl is located at the focal point of the optical system, and the focus F2 is located on a plane that coincides with the focal plane but is laterally offset from the optical axis A. It is arranged to collect the beams emitted from the focal point FL. Since the mirror SI is carried by the single generatrix Glref, the eccentricity remains constant and equal to elref at every point of the mirror SI, and therefore along a curvilinear abscissa si along the radial profile of the mirror SI from bottom to top.

[0054] The second mirror S2 radially surrounds mirror SL II, which has a conical radial profile defined by the same parabolic generatrix G2ref, with eccentricity e2ref, focus F2, and focal axis AF2. It is arranged to receive the rays of the light beam originating from the focal point Fl and reflected by mirror SL. The light beam reflected by mirror S2 is collimated, meaning that the rays are substantially parallel to each other (and here to the optical axis A since they originate from the focal point Fl). The eccentricity remains constant and equal to e2ref at every point of mirror S2, and therefore along a curvilinear abscissa s2 along the radial profile of mirror S2 from bottom to top.

[0055] The optical system comprises a lateral retaining frame 10, a peripheral block 11 whose outer surface forms the mirror S2 and which is assembled to the lateral frame 10, a central block 12 whose outer surface forms the mirror SI, and a transparent flat plate 13 to which the central block 12 is assembled. Also, the rays of a light beam emitted from the focal point Fl are collected by the mirror SI and reflected towards the mirror S2. They are then reflected by the mirror S2, and then transmitted through the transparent plate.

[0056] However, it appears that the geometric aberration of this optical system results in the rays of the outgoing light beam, emitted from the focal point Fl, exhibiting a radial distribution dr / de that is not constant. In other words, two adjacent rays emitted from the same source point with a slight azimuth difference de are radially separated by a distance dr that varies according to the average azimuth e. Thus, Light rays emitted regularly at an angle ε = θexi with respect to the optical axis A exit the optical system at a radial position θ. The distance dr = θ + θ - θ is not constant regardless of the index i considered. It appears here that the radial distribution dr / de (ray density) decreases as one moves radially away from the optical axis A.

[0057] Indeed, if we compare [Fig. 1A] and [Fig. 1B], the outgoing light rays, even though they are parallel to each other, do not have the same radial distribution or the same pupil diameter. The radial distribution dr / de is the function where a light ray emitted by the point source Fl with an azimuthal angle e emerges from the optical system at a distance r from the optical axis. In particular, in [Fig. 1A], the function dr / de is constant, whereas in [Fig. 1B], the function dr / de decreases monotonically as e increases. Furthermore, the pupil diameter is defined as the radius rmax of the beam at emax. Finally, the magnification of the objective is defined by the value of Tmax - rmin, where rmin is the distance from the optical axis of the beam emitted at emin. We will see later that adding at least a 3rd reflective surface allows us to reduce rmin and rmax by the same amount.

[0058] The aim is therefore to reduce this geometric aberration by slightly modifying the SI and S2 surfaces via the correction zones described later, in order to recover the regular radial distribution (constant dr / de) of the rays of the outgoing light beam originating from the focal point Fl, as observed in the case of centered axisymmetric optical systems. By doing so, the optical system then presents a larger field of view, notably allowing its use in direct imaging microscopy. The invention thus consists of slightly deforming the SI and S2 surfaces from their initial conical shape (i.e., the reference generators Glref and G2ref) to change the dr / de function of the rays emitted by the point source in the azimuthal direction e, while maintaining the fact that the outgoing beams remain collimated (or parallel) along the optical axis.

[0059] In general, the invention relates to an optical focusing and collection system 1 that can be used for direct imaging microscopy as well as for all forms of multispectral microscopy.

[0060] Note that direct imaging (or full-field imaging) is a technique where the entire object is illuminated simultaneously, and the image is captured all at once on a matrix photodetector (imager). It differs from other techniques where the image is constructed point by point or line by line via scanning.

[0061] For this purpose, the optical system 1 is catoptric, and has an optical axis A and a focal point FL. In other words, it is adapted to focus a beam at the focal point Fl. The incoming light is collimated (from top to bottom, i.e., along the -ez direction). It is, in turn, adapted to collect an outgoing light beam from the focal plane, and in particular from the focal point Fl, and to transmit it while exhibiting minimized geometric aberrations, especially with regard to coma aberration. Furthermore, the light beams emitted from the focal plane outside the focal point are collimated: the optical system therefore presents an improved field of view compared to the prior art, particularly that of [Fig. 1B].

[0062] The optical system 1 comprises a first pair of reflecting mirrors SI, S2, axisymmetric about the optical axis A, and adapted to focus the incoming light beam at the focal point Fl. The first mirror S1 has a radial profile defined by at least one elliptical generatrix G1 with foci Fl and F2 and focal axis AF1, the focus Fl being located at the focal point. It is adapted to collect light beams originating from the focal plane. The second mirror S2 has a radial profile defined by at least one conical generatrix G2 with focus F2 and focal axis AF2. It radially surrounds the first mirror SI and is adapted to receive a light beam reflected by the mirror SL.

[0063] The optical system may also include a second pair of axisymmetric reflecting mirrors S3, S4 about the optical axis A. The third mirror S3 has a radial profile defined by at least one conical generatrix G3 with focus F3 and focal axis AF3. It is located above mirror S2 with respect to the focal plane and is adapted to receive a light beam reflected by the second mirror S2. The fourth mirror S4 has a radial profile defined by at least one conical generatrix G4 with focus F4 different from F3 and focal axis AF4. It is located above mirror S1 with respect to the focal plane and is radially surrounded by mirror S3 and is adapted to receive a light beam reflected by mirror S3.

[0064] According to the invention, the mirrors of the first pair SI, S2 (in the first embodiment), or where applicable those of the second pair S3, S4 (in the second embodiment), each comprise a radial profile correction zone having a predefined continuous variation of eccentricity, the focus(s) and the focal axis remaining identical, such that the light rays of a light beam emitted from the focal point Fl have a regular radial distribution at the output of the optical system 1.

[0065] In other words, each point of the radial profile correction zone is carried by a conical generatrix with an eccentricity different from that of the generatrices of the neighboring points, but with identical focus(s) and focal axis.

[0066] Thus, unlike the optical system of [Fig. 1B], the mirrors in question (mirrors S1 and S2 or mirrors S3 and S4) have a radial profile defined by a set N+1 of conical generatrices {G,}h):\ , with N>1, of eccentricity e;, but which retain the same focus(s) and the same focal axis. The generators G; are different in terms of eccentricity e; in the correction zone.

[0067] If we consider a curvilinear abscissa s traversing the mirror in question from bottom to top (along the direction +ez), we obtain a continuous variation of eccentricity {ej i-0:N} going from an initial value ei=0 to a final value ei=N. At the output, the rays of the light beam exiting and coming from the focal point Fl exhibit a regular radial distribution {rCji}i=0:N, that is to say that Vie[0 ; N], drc>i = rc>i+i - rc>i is constant for two rays emitted with the same azimuthal difference of for any mean azimuth e. Here, q is the radial position of the ray i of the beam exiting the optical system 1 and emitted from the focal point Fl with the angle e;. We denote r; the radial position of the outgoing ray when the mirror does not have a correction zone (as in [Fig.1B]), and rci the radial position of the same outgoing ray when the mirror has the correction zone.

[0068] It should also be noted that the two extreme light rays, that is to say those having the outgoing radial position r^o and ri=N, are respectively emitted from the focal point with the angles 8,-0=8,,,,,,=0° and ei=N=emax=arcsin(ON), where ON is the numerical aperture of the optical system 1. The angle e is the angle formed by the ray emitted from the focal point Fl measured with respect to the optical axis.

[0069] Figure 2A is a schematic and partial cross-sectional view of an optical system 1 according to the first embodiment, where mirrors S1 and S2 each have a correction zone Sic, S2c of their radial profile allowing the light rays from the point source to be redistributed to obtain the radial distribution of a centered system where the function dr / de is constant. Figure 2B reproduces Figure 2A and illustrates rays of the outgoing light beam associated with uncorrected mirrors (no correction zones), as well as the same rays associated with corrected mirrors (with correction zones), these corrected rays being then translated laterally while maintaining parallelism with the optical axis.

[0070] In this example, only the rays of a light beam emitted from the focal point FL are shown. However, the optical system 1 is also adapted to transmit light beams emitted from a point on the focal plane, but laterally offset from the optical axis A. The optical system 1 is thus adapted to focus a collimated incoming light beam onto the focal point FL, and to collect light beams emitted at different points on the focal plane and transmit them collimated and with a regular radial distribution. It can be associated with an optical source for illuminating an object located in the focal plane and on the optical axis A, and with an imager for imaging the illuminated object. The optical source and the imager are described with reference to Figures 9A and 9B.

[0071] The optical system 1 is similar to that of [Fig. 1B], in that it comprises two reflecting mirrors S1 and S2 assembled to a retaining frame 10 and a transparent flat plate 13. These mirrors S1 and S2 are reflective, that is, their reflectivity is on the order of 90%, or even 95%, or even substantially equal to 100%, and their absorption and transmission rates are negligible. They are axisymmetric about the optical axis A: each has an axis of symmetry that is coaxial with the optical axis A of the optical system 1.

[0072] The mirror S1 has a radial profile defined by several elliptical conical generatrices {G1} with foci Fl and F2 and focal axis AF1, the focus Fl being located at the focal point. More precisely, the radial profile of the mirror S1 includes a correction zone Sic, where each point is carried by several elliptical conical generatrices {G1} with eccentricities different from those of the generatrices of neighboring points, but with identical foci Fl and F2 and focal axis AF1.

[0073] The mirror S2 has a radial profile defined by several elliptical conical generatrices {G2;} with focus F2 and focal axis AF2. More precisely, the radial profile of the mirror S2 has a correction zone S2c, where each point is carried by an elliptical conical generatrice {G2,} with eccentricity e2; different from that of the generatrices of the neighboring points, but with the same focus F2 and focal axis AF2.

[0074] Thus, as illustrated in [Fig.2B], a ray of index i is emitted from the focal point Fl with an azimuthal angle e;=(emax- emin)xi / N+emin, is reflected by the corrected mirror Sic (Slc= SI + ôel), where ôel is a sigmoid-shaped thickness variation whose dependence on the curvilinear abscissa s is shown in [Fig.3], and is then reflected by the corrected mirror S2c (S2c= S2 + ôe2) in the correction zone, so that the emergent beams are collimated and have a homogeneous radial distribution (i.e. with dr / de constant).

[0075] A method used to make the radial distribution dr / de constant in the off-axis axisymmetric optical system is presented here, in other words to determine the correction zones of mirrors S1 and S2.

[0076] Step 1: We assume that the light rays exiting the optical system are all collimated along the Oz axis, but that the radial distribution dr / de is not constant as a function of e. Furthermore, we assume for the moment that the generators SI and S2 are fixed. We then measure the radial positions (distance to the optical axis) rmin and rmax of the extreme light rays between emin = 0° and emax = arcsin(ON) (where ON is the numerical aperture).

[0077] Step 2: The solid angle of the rays emitted by the source point at Fl is regularly subdivided into N+l (on the order of 100) between emin = 0° and emax = arcsin(ON). This yields a list of rays emitted by Fl, each with a different azimuthal angle e;= (emax - £min)xi / N + emin, where i is an integer index between 0 and N. For each index, the radial translation necessary to make the radial distribution dr / de constant is calculated. For example, we choose to ensure that dr / de = (rmax - rmin) / (emax - emin) regardless of the value of e. Thus, for a given index i, rc>i = (rmax - rmin)xi / N + rmin, and there then exists a unique translation value ôr corresponding to this index i that allows us to correct the geometric aberrations. Thus, we translate parallel to the optical axis A the ray from its initial uncorrected position q to its corrected position rc>i (deterministic action) on the curve S2, assuming that the reflection point at the curvilinear coordinate sh on SI remains fixed. This defines two new angles of reflection: ôOi = 0'i - 0i and ô02 = 0'2 - 02 respectively at the curvilinear abscissa sh on the surface SI, and sc2; = s2; + ôr on the surface S2.0i and 02 are the angles of reflection on mirrors S1 and S2.

[0078] Step 3: From this information, the slope correction for the two conics can be calculated as a function of the curvilinear coordinate s. The associated contour modification is calculated by deforming the generatrix along its normal by integration. The aim is to obtain a corrected local slope of Δ01 / 2 and Δ02 / 2 respectively at the curvilinear abscissa s1 on the surface S1 and sc2 = s2 + Δr on the surface S2. Thus, S'1(s) = S1(s) + Δel(s), n, where s is a curvilinear abscissa, Δel(s) = Δ01 / 2 ds, and n is a unit normal vector. The same calculation is repeated to obtain S'2. The deformations have the shape of a sigmoid curve, as shown in [Fig. 3].

[0079] Step 4: The final step of the iteration consists of correcting the fact that the beams emanating from S'2 are not collimated with each other. The path of the light rays is recalculated using the new curves S'1 and S'2. The modification of the angle Δ02 required to make the beam collimated at each index i without changing the curvilinear abscissa sc2 on S2 is evaluated. From this information, the curve S'2 is calculated by integrating the angle correction over the curvilinear abscissa, as in step 3.

[0080] Following steps 1 to 4, a calculation of the light ray tracing shows that the width of the dr / de distribution has decreased while maintaining a collimated beam. The process can therefore be repeated by starting again at step 1. This asymptotically approaches a uniform distribution.

[0081] The algorithm converges quite quickly. After about 10 iterations, an error of less than 0.01% is reached. The effects of the corrections are visible on curves 2A and 2B, where curves S1 and S2 before correction are the dashed curves and curves S1 and S2 after 5 iterations are the solid line curves.

[0082] Figure 3 illustrates the variation, along the curvilinear abscissa s (si for mirror S1 and s2 for mirror S2), of the thickness difference ôe of each of the mirrors S1 and S2 of the optical system 1 of Figure 2A, compared with the reference generatrices Glref and G2ref. Thus, the thickness difference is denoted: ôe = ec1 - eref, where eref is the eccentricity of the reference generatrice Glref or G2ref, and eC1 is the eccentricity of the generatrice of the point considered with index i in the correction zone. In this example, the order of magnitude of the eccentricity variations is at most 3% of the lateral size.

[0083] Figures 4A to 4C are schematic and partial cross-sectional views of optical systems 1 according to variants of the first embodiment. The purpose of these embodiments is to reduce the pupil diameter by bringing the collimated beams closer to the optical axis.

[0084] Initially, we consider a system 1 where we add right mirrors.

[0085] Fig. 4A illustrates an optical system 1, where the mirror SI is an external surface of a central block 12, and where the mirror S2 is an external surface of a peripheral block 11 assembled to the retaining frame 10. A transparent central block 18 is assembled to the retaining frame 10, and the block 12 of the mirror SI is assembled to a lower face of the central block 18. The latter includes an internal face which forms a third mirror S3.

[0086] Thus, the light beam emitted from the focal point Fl is reflected by mirror SI towards mirror S2, then by mirror S2 towards mirror S3. It enters the central block 18 through a first face and exits through a second face whose normal is parallel to the collimated beam in order to avoid refraction phenomena, and re-enters the central block 18 through a third face (parallel to the first two) to then be reflected by total internal reflection by mirror S3 along the +ez direction. It exits the central block 18 through a top face orthogonal to the optical axis A.

[0087] The material of the upper central block 18 is transparent and has a refractive index greater than or equal to 1.5. It should be noted that there is no diffraction phenomenon when the rays pass through the central block 18, since they are incident along the normal to the interfaces. They then undergo total internal reflection on S3 to be collimated along the optical axis. This makes it possible to reduce the pupil diameter of the off-axis axisymmetric catoptric system with infinite conjugation by using only 3 mirrors: a gain in simplicity and efficiency (each mirror reflection is a source of absorption).

[0088] Figure 4B illustrates an optical system 1 in which mirrors S1 and S2 of Figure 2A are found, but which also includes a second pair of reflecting mirrors S3 and S4. Mirror S3 is arranged above mirror S2 and radially surrounds mirror S4, which is located above mirror SL. Thus, a beam of light from the focal point Fl is reflected by mirror S1 towards mirror S2, and then is reflected by mirror S2 towards mirror S3, then is reflected by mirror S3 towards mirror S4, then is finally reflected by mirror S4 along a direction parallel to the optical axis A.

[0089] In this example, the space between mirrors SI and S2 is a gas or a vacuum: mirrors SI and S2 are external surfaces of blocks 11 and 12. On the other hand, the space between mirrors S3 and S4 is a transparent material: mirrors S3 and S4 are internal surfaces of a transparent upper block 14. This is assembled to an upper face of the transparent flat plate 13.

[0090] Also, after being reflected by mirror S2, the light beam enters a transparent flat plate 13 through a lower flat face (orthogonal to the optical axis A), then enters the upper block 14, and is subsequently reflected by total internal reflection by mirrors S3 and S4. It finally exits this upper block 14 through an upper flat surface also orthogonal to the optical axis A.

[0091] A lateral space is present between the retaining frame 10 and a peripheral edge of the upper block 14. Thus, an annular beam of illumination can propagate in this space to enter directly into the flat plate 13, and then be reflected by the mirror S2 and then by the mirror SI to be focused at the focal point Fl. This limits the crossing of the incoming illumination beam and the outgoing response beam (originating from the illuminated focal point Fl).

[0092] Figure 4C illustrates an optical system 1 where, as in Figure 4B, mirrors S1 and S2 are external surfaces of assembled blocks 11, 12, one to the retaining frame 10, and the other to the lower face of the flat plate 13. A mirror S3 is an external surface of a block 15 assembled to the retaining frame 10, and a mirror S4 is an external surface of a block 16 assembled to the upper face of the flat plate 13 and located on the optical axis A. A transparent central block 17 is located above mirror S4 and is centered on the optical axis, and is assembled to the block 15 of mirror S3. Mirrors S3 and S4 are here axisymmetric surfaces centered using a parabola-ellipse pair as in Figure 1A. This central block 17 has a lateral surface that forms a mirror S5, and a central surface that forms a mirror S6.

[0093] Thus, the light beam emitted from the focal point Fl is reflected by mirror S1 towards mirror S2, then by mirror S2 towards mirror S3. It then passes through the flat plate 13 and exits to be reflected by mirror S3 towards mirror S4. After reflection by mirror S4 along the +ez direction, the light beam enters the central block 17 through a lower face orthogonal to the optical axis A. It is then reflected by mirror S5 and then by mirror S6 along the +ez direction. It exits the central block 17 through an upper face orthogonal to the optical axis A.

[0094] There is a lateral gap between the central block 17 and the block 15 of mirror S3. Thus, an annular beam of illumination can propagate through this gap to impact directly the mirror S4 at the level of a peripheral edge of block 16, therefore without crossing the central block 17. The beam is reflected by mirror S4, then S3, then S2 then SI, and is focused at the focal point Fl. Here too, the crossing of the incoming illumination beam and the outgoing response beam (coming from the illuminated focal point Fl) is limited.

[0095] In a second step, the pair of mirrors S3 and S4 are used to make the corrections of geometric aberrations.

[0096] Fig. 5A is a schematic and partial cross-sectional view of an optical system 1 according to a second embodiment, where the mirrors S3 and S4 each have a correction zone S3c, S4c of their radial profile.

[0097] The lens's large numerical aperture and achromaticity are also taken advantage of to add dark-field object illumination to the device. Generally speaking, the optical system 1 is adapted to collect the light beam emitted by the object O, here illuminated by an optical source 2, and to transmit it to the imager 3.1, 3.2 with limited geometric aberrations. The object O is placed in the focal plane of the optical system 1, and is here centered on the optical axis A where the focal point FL is located.

[0098] The optical source 2 is adapted here to illuminate the object. It can consist of at least one light-emitting diode (LED) or at least one laser diode. The diode(s) can be combined, if necessary, with optics (not shown) for shaping the light beam so as to illuminate the entire desired area of ​​the object. The illuminating light beam can be polychromatic (e.g., white light) or monochromatic. The optical source 2 emits white light and can directly illuminate the object without first passing through the optical system 1. Alternatively, the illuminating light beam can pass through the optical system.

[0099] The imager can be formed of an imaging optic 3.1 and a matrix photodetector 3.2. The imaging optic receives the light beams from the illuminated object, transmitted collimated by the optical system 1, and focuses them onto the matrix photodetector, which can be, for example, a CMOS sensor. An image of the object is thus acquired.

[0100] The optical system 1 comprises a pair of mirrors SI and S2 identical or similar to those of [Fig. 1B], i.e., they do not have correction zones. Mirror SI has a radial profile defined by the same elliptical generatrix G1 with foci Fl and F2 and focal axis AF1, the focus Fl being located at the focal point. Mirror S2 has a radial profile defined by the same parabolic generatrix G2ref with focus F2 and focal axis AF2, radially surrounding mirror SI and adapted to receive a light beam reflected by mirror SL

[0101] The optical system 1 also includes a second pair of reflective mirrors S3 and S4, axisymmetric about the optical axis A. Mirror S3 has a radial profile defined by several conical generatrices {G3}i=0:N with focus F3 and focal axis AF3. It is located above mirror S2 with respect to the focal plane and is adapted to receive a light beam reflected by mirror S2. Mirror S4 has a radial profile defined by several conical generatrices [G4, with focus F4 different from F3 and focal axis AF4. It is located above the first mirror S1 with respect to the focal plane, is radially surrounded by mirror S3, and is adapted to receive a light beam reflected by mirror S3.

[0102] In other words, each of the mirrors S3, S4 has a correction zone S3c, S4c of the radial profile having a predefined continuous variation of eccentricity, the focus and the focal axis remaining identical, such that the light rays of a light beam emitted from the focal point Fl have at the output of the optical system 1 a regular radial distribution {rCji}i=0:N using the same algorithm as that presented previously.

[0103] Preferably, mirrors SI and S2 are made so that the reflection is metallic type, and mirrors S3 and S4 are made so that the reflection is vitreous type.

[0104] Mirror S1 has the shape of a non-truncated cone with its apex oriented towards the focal point FL2. It is adapted to receive the light beams from the object and reflect them towards mirror S2. Mirror S2 is radially farther from the optical axis A than mirror S1, which it radially surrounds. It is superimposed on and located below mirror S3. It is frustoconical. Mirror S3 is radially farther from the optical axis A than mirror S4, which it radially surrounds. It is superimposed on and located above mirror S2. It is frustoconical. Mirror S4 has the shape of a non-truncated cone with its apex oriented in a direction opposite to the focal point FL2. It is located above mirror S1.

[0105] In operation, the optical source 2 emits the light beam that illuminates the surface of the object O to be imaged, here directly, that is, without being transmitted through the optical system 1. In return, each point on the illuminated surface of the object O emits a light beam (for example, by reflection, fluorescence, Raman effect, etc.) towards the optical system 1. The light beams are then collected and reflected by the mirror S1, and then successively by mirrors S2, S3, and S4, which reflect them towards the imager. The rays of the light beam originating from the focal point exhibit a regular radial distribution at the output. Furthermore, the rays of the light beam originating from the same point on the focal plane, whether located on or off the optical axis, are parallel to each other: the light beam is therefore collimated. An image of the object can therefore be obtained, provided that the coma aberration is reduced.

[0106] Fig. 5B illustrates an example of variation of the thickness gap ôe of each of the mirrors S3 and S4 of the optical system 1 of Fig. 5A in comparison with the reference generators G3ref and G4ref.

[0107] The curvilinear abscissas uS3 and uS4 follow the radial profiles of the mirrors S3 and S4, and are oriented from bottom to top, i.e. along the +ez direction.

[0108] The ordinate is the thickness difference: εe = eCji - eref where eref is the eccentricity of the reference generatrix G3ref or G4ref, and ecji is the eccentricity of the generatrix of the considered point with index i in the correction zone S3c, S4c. In this example, the order of magnitude of the eccentricity variations is at most equal to 1% of the lateral size. The eccentricity variations are monotonic and strictly increasing.

[0109] By way of example, the variations in the thickness gap ôe correspond here to sigmoids, that is to say, they correspond to a mathematical function of the type:

[0110] [Math.l] ôe = a ( 1 4- À \ exp^ ! 1 + b

[0111] where a, b, and X are predefined scalars of the mathematical function and Pn(u) is a polynomial of order n in the variable ôe = (x - x0) / o. The variable x here represents the curvilinear abscissa s defined in [Fig. 2B], and the variable x0 represents the abscissa at index 0. The variable o is an adjustable scale parameter. Thus, it can be splines such as non-uniform rational B-splines (NURBS). Note, however, that other functions can be used, such as sinusoidal functions, among others.

[0112] Fig. 6 illustrates an example of angular distribution of rays emitted from an emission point of the object to be imaged located in the focal plane, as a function of the radial distance re to the optical axis A.

[0113] In one case, an optical system 1 is considered according to the second embodiment, where mirrors S1 and S2 do not have correction zones, while mirrors S3 and S4 do. The angular distribution is compared to that associated with an optical system where none of the mirrors S1 to S4 has a correction zone.

[0114] In both cases (mirrors S3 and S4 with and without correction), the variation of the angular distribution is a monotonically increasing function that passes through the origin. The variation of the angular distribution of the rays emitted from a point in the focal plane as a function of the radial distance re has a slope of the order of 650prad / pm with mirrors S3 and S4 without correction, whereas it is only around 150prad / pm with mirrors S3 and S4 with correction.

[0115] Thus, the angular distribution for an emission point 2pm from the optical axis is on the order of 1350prad with mirrors S3 and S4 with correction, whereas it is only on the order of 250prad with mirrors S3 and S4 without correction. Similarly, for an emission point 6pm from the optical axis, the angular distribution of the rays is on the order of 4000prad with mirrors S3 and S4 with correction, whereas it is on the order of 900prad with mirrors S3 and S4 without correction.

[0116] Generally speaking, the output light beams (originating from the object and reflected by the mirror S4) are said to be collimated when the variation of the angular distribution of the rays as a function of re is less than or equal to 200prad / pm. Thus, collimated output light beams have an angular distribution less than or equal to 1000prad when they originate from a point on the focal plane 5pm away from the optical axis, or when they have an angular distribution less than or equal to 2000prad for a distance re of 10pm from the optical axis. Indeed, as shown below in relation to [Fig. 7A] to 7C, the rays are then sufficiently parallel to each other (collimated) so that the optical system 1 can perform direct imaging of the illuminated object.

[0117] Thus, the fact that at least one pair of mirrors in the optical system, and here mirrors S3 and S4, are configured so that the rays of an outgoing light beam from the focal point have a regular radial distribution, means that the rays of a light beam originating from a point on the focal plane spaced from the optical axis are collimated. This opens up the possible application of optical system 1 to direct imaging.

[0118] Furthermore, the incoming light beam, collimated and parallel to the optical axis, is focused at the focal point. Thus, achromatism and confocal properties are preserved, allowing this same optical system 1 to be used for confocal microscopy and spectroscopy. This avoids correcting coma by introducing a corrective diopter placed inside the optical system. Indeed, such a diopter would degrade the achromatic nature of the optical system.

[0119] Figure 7A illustrates an object placed in the focal plane of the optical system 1, whose image is to be acquired. The object corresponds to a so-called resolution target, which allows for the evaluation, in particular, of the geometric aberrations of the optical system 1, and therefore its direct imaging capabilities.

[0120] The resolution target is circular with a radius of 5 pm, and centered on the optical axis A. It consists of a central zone with a radius of 0.5 pm formed by alternating quarter-disks of white and quarter-disks of black, and a peripheral zone of 4.5 pm of a regular alternation around the optical axis A of white angular sectors and black angular sectors.

[0121] Figure 7B illustrates the image of the target illuminated by a polychromatic RGB light beam composed of three wavelengths (606 nm, 535 nm, 465 nm) when using an optical system 1 having rectilinear (uncorrected) reflective surfaces A21 and A22. It appears that the central area, whose size is on the order of the illumination wavelength, is barely distinguishable. In contrast, the patterns in the peripheral area are indistinguishable. Thus, coma aberration causes a degradation of spatial resolution, which makes it impossible to distinguish the patterns of the resolution target.

[0122] Figure 7C illustrates the image of the target, always illuminated by a polychromatic RGB light beam composed of three wavelengths (606 nm, 535 nm, 465 nm), when using the optical system according to the invention, where the reflective surfaces 21 and 22 correspond to those of Figure 3. It appears that the central area is perfectly distinguishable, as are the patterns of the peripheral area. Indeed, by collimating the outgoing light beams, in particular those off the optical axis A, the coma aberration is corrected, thus allowing the optical system to be used for direct imaging.

[0123] The modulation transfer function (MTF) allows the performance of the optical system to be evaluated in terms of geometric aberrations and in particular coma.

[0124] Accordingly, Figures 8A and 8B illustrate examples of MTF at 550 nm, for an optical system with S3 and S4 mirrors without correction ([Fig. 8A]), and for an optical system with S3 and S4 mirrors with correction ([Fig. 8B]). In each case, the MTF is evaluated considering a resolution target centered, or more or less offset with respect to the optical axis A.

[0125] Figure 8A shows that the MTF of a prior art optical system, for a centered target, is degraded compared to the theoretical MTF. Moreover, it degrades significantly as the target is moved further from the optical axis A.

[0126] In contrast, in the case of [Fig. 8B], which corresponds to an optical system according to the invention (here according to the second embodiment), the MTF for a centered target is virtually identical to the theoretical MTF. Moreover, as the target is moved further from the optical axis A, the MTF is significantly improved compared to that of the prior art. Thus, for a target located 2 pm from the optical axis and for patterns with a spatial frequency of 0.2 (dimensionless value), the MTF is equal to 0.6, whereas it is only 0.2 in the case of the prior art optical system. Similarly, for a target located 4 pm from the optical axis and for a spatial frequency of 0.4 for the patterns, the MTF is approximately 0.2, whereas it is almost zero in the case of the prior art optical system.

[0127] These curves therefore clearly show the significant improvement of the optical system according to the invention in terms of coma correction, and therefore of the ability to perform direct imaging, whereas this is not the case with the optical system according to the prior art.

[0128] Figures 9A, 9B and 9C illustrate optical systems 1 according to variants of the second embodiment.

[0129] In the example of [Fig.9A], the optical system 1 is similar or identical to that of [Fig.5A], and is used here in the context of direct imaging.

[0130] The illuminating light beam emitted by the optical source 2 is transmitted by the optical system 1 and illuminates a surface of the object O. For this to occur, the optical source 2 is located on the same side as the imager 3 with respect to the optical system, and not, as in [Fig. 5A], on the opposite side. The optical source can be one or more light sources (light-emitting diodes, sodium lamp, etc.).

[0131] The illuminating light beam is emitted towards the optical system 1, then passes through a semi-reflective plate 4 (here a semi-reflective cube). The illuminating light beam is then reflected by mirror S4, mirror S3, mirror S2, and finally by mirror S4, which directs it towards the object. The object is thus illuminated over a large area.

[0132] The object thus illuminated emits a return light beam, for example by reflection, fluorescence, Raman effect or other, towards the optical system 1. This light beam comes from different points on its surface located at the focal plane, on and off optical axis A. These light beams are collected and reflected by the mirror SI, then by the mirrors S2, S3 and then S4.

[0133] The light beam from the focal point Fl is transmitted by the optical system 1 in a collimated manner and with a regular radial distribution. Furthermore, each light beam from a point in the focal plane is transmitted in a collimated manner.

[0134] The return light beams are then reflected by the semi-reflecting cube 4 towards the imager 3. They pass through the imaging optics which focuses each light beam at a point of the matrix photodetector.

[0135] Thus, the optical system 1 allows the illumination light beam to be transmitted and the return light beams to be collected. It therefore enables direct imaging with the good performance described above.

[0136] In the example of [Fig.9B], the optical system 1 is also similar or identical to that of [Fig.5A], and is used here in the context of confocal microscopy or spectroscopy.

[0137] Here, the illuminating light beam is shaped and transmitted by the optical system 1 to be focused at the focal point Fl where the object to be analyzed is located. For this purpose, The optical system 1 comprises a shaping optical block 20, and a referring optical block 30.

[0138] Preferably, the optical source 2 emits a monochromatic light beam, so as to limit chromatic aberrations associated with certain dioptric elements of the optical source (in particular a collimator). The illuminating light beam is preferably solid and collimated. In other words, it has a continuous angular intensity distribution (e.g., Gaussian) and is thus distinguished from annular (i.e., hollow) light beams. Moreover, since it is collimated, the rays of the illuminating light beam are parallel to each other. The optical source 2 may include a collimator (not shown) when the light beam is slightly divergent, as for example in the case of a light-emitting diode.

[0139] The optical shaping block 20 is adapted to shape the illuminating light beam into an annular shape around the optical axis A and to transmit it towards the reflective surface S4. This optical block 20 is also catoptric. It comprises a central conical mirror 21, radially surrounded by a peripheral frustoconical mirror 22. These two mirrors 21 and 22 are coaxial with each other and with the optical axis A. Each has a straight (linear) radial profile, but it can be concave or convex.

[0140] The optical block 30 is adapted to transmit the return light beam to the photodetector. This optical block 30 is also catoptric. It comprises a central mirror 31, located and centered on the optical axis A. It is positioned between mirror 21 and mirror S4. In particular, it is located in the space radially delimited by the excitation light beam transmitted by mirror 22. It reflects the return light beam in a radial direction, towards mirror 32, which focuses the return light beam onto the photodetector. This mirror 32 may be absent, and mirror 31 may itself be a focusing mirror.

[0141] The fact that the optical system 1 includes optical blocks 20, 30 allows the working distance to be adapted and the numerical aperture increased if necessary, depending on the desired applications, while segregating the optical paths of the illuminating light beam and the responding light beam.

[0142] In the example of [Fig.9C], optical system 1 is a variant of that of [Fig.4A]. It differs essentially in that mirrors S3 and S4 each have a correction zone, whereas mirrors SI and S2 are uncorrected.

[0143] The optical system 1 comprises a peripheral block 11, the outer surface of which forms the mirror S2. It is assembled to a retaining frame 10. It also comprises a central block 12, the outer surface of which forms the mirror SL. A transparent block 14 is assembled to the retaining frame 10 and secures the central block 12. This transparent block 14 has an inner upper surface which forms mirror S3 and an inner lower surface which forms mirror S4.

[0144] The transparent block 14 has a flat lower surface located between the retaining frame 10 and the central block 12, and a flat peripheral upper surface located between the retaining frame 10 and the mirror S3, and a flat central upper surface surrounded by the mirror S3.

[0145] Thus, mirrors SI and S2 are of the metallic reflection type, and mirrors S3 and S4 are of the total internal reflection type (glass reflection).

[0146] In this configuration, the optical system 1 has a configuration where the incoming light beam does not cross the outgoing light beams inside the transparent block 14. This makes it possible to reduce in particular the parasitic fluorescence.

[0147] Thus, an annular illuminating beam of light is incident on the upper peripheral surface and passes through the transparent block 14, and emerges through the lower flat surface, to then be reflected by the mirror S2 and then by the mirror SI which focuses it on the object at the focal point Fl.

[0148] In return, a beam of light is emitted and is collected and reflected by mirror S1 and then by mirror S2. It then enters the transparent block 14 through the lower surface, then is reflected by mirror S3 and then mirror S4, and finally exits through the central upper surface.

[0149] The outgoing light beam is therefore transmitted by the optical system 1 and exhibits a regular radial distribution at the output. The outgoing light beams and those originating from other points of the object are collimated by the optical system 1. Furthermore, this configuration, where the incoming beam is not reflected by mirrors S3 and S4, and does not intersect the outgoing light beams inside the transparent block 14, improves the performance of the optical system 1.

[0150] Specific embodiments have just been described. Various variants and modifications will be apparent to those skilled in the art.

Claims

1. Demands Optical system (1) for focusing and collecting, catoptric, and of an optical axis A and focal point Fl, • comprising a first pair of axisymmetric reflecting mirrors (SI, S2) about the optical axis A, adapted to focus a collimated incident light beam at the focal point Fl, of which: • a first mirror (S 1), with a radial profile defined by at least one elliptical generatrix (Gl) with foci Fl and F2 and focal axis AF1, the focus Fl being located at the focal point, and adapted to collect light beams coming from the focal plane; • a second mirror (S2), with a radial profile defined by at least one conical generatrix (G2) with focus F2 and focal axis AF2, radially surrounding the first mirror (SI) and adapted to receive a light beam reflected by the first mirror (SI); • possibly comprising a second pair of axisymmetric reflecting mirrors (S3, S4) around the optical axis A, of which: • a third mirror (S3), with a radial profile defined by at least one conical generatrix (G3) with focus F3 and focal axis AF3, located above the second mirror (S2) with respect to the focal plane, and adapted to receive a light beam reflected by the second mirror (S2); • a fourth mirror (S4), with a radial profile defined by at least one conical generatrix (G4) with a focus F4 different from F3 and a focal axis AF4, located above the first mirror (SI) with respect to the focal plane, and surrounded radially by the third mirror (S3) and adapted to receive a light beam reflected by the third mirror (S3); • characterized in that the mirrors of the first pair (S1, S2), or where applicable, those of the second pair (S3, S4), each have a correction zone (S1, S2c; S3c, S4c) of the radial profile exhibiting a predefined continuous variation of eccentricity, the focus(s) and the focal axis remaining identical, such that the light rays of a light beam emitted from the focal point Fl exhibit at the output of the optical system (1) a regular radial distribution.

2. Optical system (1) according to claim 1, wherein the radial profile of the mirrors in the correction zone is a sigmoidal function.

3. Optical system (1) according to claim 1 or 2, wherein the mirrors of the first pair (SI, S2) each have a correction zone (Sic, S2c).

4. Optical system (1) according to claim 3, wherein the radial profile of the first mirror (SI) in the correction zone (Sic) is defined by elliptical generatrices.

5. Optical system (1) according to claim 3 or 4, wherein the radial profile of the second mirror (S2) in the correction zone (S2c) is defined by hyperbolic generators.

6. Optical system (1) any one of claims 1 to 5, wherein the first and second mirrors are of the metallic reflection type.

7. Optical system (1) any one of claims 1 to 6, comprising an additional reflective and axisymmetric mirror (S3) about the optical axis A, adapted to receive a light beam reflected by the second mirror (S2) and to reflect it in a collimated manner.

8. Optical system (1) any one of claims 1 to 6, comprising the third mirror (S3) whose radial profile is defined by the same conical generatrix, and the fourth mirror (S4) whose radial profile is defined by the same conical generatrix.

9. Optical system (1) claim 8, wherein the third and fourth mirrors are of the glassy reflection type.

10. Optical system (1) according to claim 8 or 9, wherein the mirrors of the second pair (S3, S4) each have a correction zone (S3c, S4c).

11. Optical system (1) according to claim 10, wherein the first mirror has a radial profile defined by the same generatrix elliptical, and the second mirror has a radial profile defined by the same parabolic generatrix.

12. Optical system (1) according to any one of claims 1 to 11, comprising a semi-reflective blade (4) located on the optical axis A above the fourth mirror (S4), adapted to transmit, respectively reflect, a first light beam from an optical source (2) towards the fourth mirror (S4), and to reflect, respectively transmit, a second light beam from an object located at the focal point Fl and reflected by the fourth mirror (S4), towards an imager (3).

13. Optical system (1) according to any one of claims 1 to 11, comprising: a shaping optical block (20), adapted to receive a first light beam from an optical source (2), then collimated and parallel to the optical axis, to make it annular around the optical axis A and transmit it in the direction of the fourth mirror (S4); a reflecting optical block (30), adapted to receive a second light beam from an object located at the focal point Fl reflected by the fourth mirror (S4) and to focus it on an imager (3).

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