Patent Description:
The present invention relates to an illumination optical device comprising a collimator. In particular, the invention refers to an illumination device or projector that can be used to obtain an Optical Ground Support Equipment (OGSE) capable of testing, calibrating and/or verifying the optical characteristics of an electro-optical device. The present invention finds advantageous but not exclusive application in the aerospace field.

As is known, an electro-optical device may be subjected to one or more test and verification steps before the actual use on field of the electro-optical device itself, especially in applications where high reliability of the electro-optical device is required.

For example, in the aerospace field, Optical Ground Support Equipment (OGSE) is known to be used to verify the optical performance of various electro-optical devices during specific Assembly Integration and Test (AIT) or Assembly and Integration Test (AIT) activities.

One of the key parameters that is analysed during the testing activities of an electro-optical device, later identified as a Unity Under Test (UUT), is the stray light generated by the unit under test when subjected to an optical stimulus.

The stray light is generally defined as a light noise component that is generated by an optical device, e.g. formed by light that is propagated following an optical path different from a desired optical path.

Analysing the stray light generated by a unit under test thus allows to verify the optical performance of the unit under test.

<FIG> shows a system <NUM> comprising a known illumination device or illuminator <NUM> and a unit under test <NUM> having an optical architecture 5A and an optical detector 5B.

The illuminator <NUM> comprises one or more light sources and a collimator (not shown here) and is configured to generate a light beam <NUM> that propagates towards the unit under test <NUM>.

The light beam <NUM> comprises a collimated portion <NUM>, which constitutes a desired collimated optical stimulus for the unit under test <NUM>, and a stray portion <NUM>, which constitutes a non-collimated noise light radiation.

The stray portion <NUM> constitutes a signature of the illuminator <NUM> and is generated by imperfections inside the illuminator <NUM> itself, in particular of the respective collimator.

The optical architecture 5A receives the optical beam <NUM> and generates a device light beam <NUM>, focused on the detector 5B.

The optical architecture 5A receives and propagates both the collimated component <NUM> and the stray component <NUM> of the optical beam <NUM>.

Consequently, the device light beam <NUM> comprises both a primary portion <NUM> and an illuminator stray portion <NUM>.

The primary portion <NUM> forms the desired optical signal output by the optical architecture 5A, i.e. the optical signal generated starting from the collimated portion <NUM> of the optical beam <NUM>. The illuminator stray portion <NUM> forms a noise signal generated starting from the stray portion <NUM> of the optical beam <NUM>, and thus dependent on the optical properties of the illuminator <NUM>.

In addition, the optical architecture 5A may generate a device stray light component <NUM>, starting from the collimated portion <NUM> of the optical beam <NUM>, due to imperfections of the optical elements, e.g. mirrors, lenses, etc. that form the optical architecture 5A. Consequently, the device optical beam <NUM> also comprises a device stray portion <NUM>, generated starting from the device stray light component <NUM>.

In practice, when the optical detector 5B receives the device optical beam <NUM>, it detects the primary portion <NUM> thereof, the illuminator stray portion <NUM> and the device stray portion <NUM>.

The presence of the illuminator stray portion <NUM> in the device optical beam <NUM> makes it difficult, if not impossible, to isolate the device stray portion <NUM>, or to isolate the noise contribution introduced by the unit under test <NUM>.

Therefore, the fact that the illuminator <NUM> generates a stray portion of light does not allow to isolate the noise contribution introduced by the unit under test <NUM>, i.e. it does not allow to reliably verify the performance of the optical unit under test <NUM>, during the testing steps. Relevant prior art can be found in <NPL>; <CIT>; <NPL>.

Aim of the present invention is to overcome the disadvantages of the prior art.

According to the present invention there is thus provided an illumination optical device comprising a collimator, as defined in the appended claims.

To better understand the present invention preferred embodiments thereof will be now described, for merely exemplary and non-limiting purposes, with reference to the appended drawings, wherein:.

The following description is provided to enable a person skilled in the art to use and realize the invention. Various modifications to the embodiments will be apparent to those skilled in the art, without departing from the scope of protection of the claimed invention. Consequently, the present invention is not intended to be limited to the embodiments shown, but the widest protection scope is to be accorded consistently with the principles and features described below and defined in the appended claims.

Unless otherwise specified, all scientific and technical terms used below have the same meaning as commonly understood by a person skilled in the ordinary art. In the event of any conflict, this description, including the definitions provided, shall be binding. In addition, the examples are illustrative only and are not intended to be limiting.

The terminology used below is intended to describe particular embodiments and is not to be construed as limiting the purpose of the present disclosure.

<FIG> show an illumination device, hereinafter referred to as an illuminator <NUM>, comprising an extended light source <NUM>, a collimator <NUM> having an input focal plane F<NUM> and an output focal plane F<NUM> and defining a collimator optical path <NUM>, and an aperture or pupil <NUM>, mutually optically coupled.

The extended light source <NUM> is of a known type, e.g. an LED source or any other type of known source, of monochromatic or polychromatic type, configured to generate a light radiation <NUM>, schematically represented by arrows in <FIG>, propagating from the extended light source <NUM>. For example, the light radiation <NUM> comprises electromagnetic radiation in the frequency range between infrared and ultraviolet.

The light radiation <NUM> is formed by a primary light beam <NUM> (<FIG>), propagating along directions intercepting the quaternary mirror 70D so as to follow the collimator optical path <NUM>, and a secondary light beam <NUM> (<FIG>), propagating along directions not intercepting the quaternary mirror 70D.

In detail, the extended light source <NUM> is arranged in the input focal plane F<NUM> of the collimator <NUM> and the aperture <NUM> is arranged in the output focal plane F<NUM>, so that the primary light beam <NUM> is collimated, by the collimator <NUM>, in an output beam <NUM> having a flat wavefront on the aperture <NUM>, as schematically shown in <FIG>.

The extended light source <NUM> is represented in <FIG> by three mutually distinct point sources and hereinafter referred to as first point source 58A, second point source 58B and third point source 58C.

The point sources 58A-58C each emit a respective optical beam 66A-66C having a spherical wavefront, the propagation of which along the collimator optical path <NUM> is schematically shown in <FIG>. <FIG> shows in detail the propagation of the optical beam 66B of the second point source 58B along the collimator optical path <NUM>.

The three point sources 58A-58C may be sources equal to or different from one another, for example each having a respective frequency or they may each represent a respective pixel of the extended light source <NUM>.

In detail, the first and third point source 58A, 58C are arranged at two end points of the extended light source <NUM>. The second point source 58B is arranged at a midpoint of the extended light source <NUM>.

The collimator <NUM> is a collimator of the Schiefspiegler type, i.e. comprising a plurality of mirrors <NUM> arranged off-axis and mutually tilted so as to form an unobstructed optical path.

In detail, as described in Patent Application <CIT>, the collimator <NUM> comprises four mirrors: a primary mirror 70A, a secondary mirror 70B, a tertiary mirror 70C, and a quaternary mirror 70D, each having a respective reflective surface <NUM> and a respective support surface <NUM>.

The mirrors 70A-70D are preferably spherical mirrors, each concave or convex.

The mirrors 70A-70D each have a respective focal ratio, which may be chosen depending on the specific design of the collimator <NUM>.

<FIG> shows by way of example a cross-section of the primary mirror 70A, here of concave type, whose reflective surface <NUM> has a radius of curvature R<NUM> with respect to a centre O defined on the support surface <NUM>.

The primary mirror 70A has a vertex <NUM>, on the reflective surface <NUM>, formed by the intersection between the reflective surface <NUM> and a mirror axis S passing through the centre O and perpendicular to the support surface <NUM>.

Similar to what has been described for the primary mirror 70A, the secondary mirror 70B, the tertiary mirror 70C and the quaternary mirror 70D also have a respective radius of curvature R<NUM>, R<NUM>, R<NUM> and a respective vertex <NUM> defined by a respective mirror axis S.

Again with reference to <FIG>, the collimator optical path <NUM> of the collimator <NUM> comprises, in succession, the extended light source <NUM> (here arranged in the input focal plane F<NUM>), the quaternary mirror 70D, the tertiary mirror 70C, the secondary mirror 70B, the primary mirror 70A and the aperture <NUM> (here arranged in the output focal plane F<NUM>).

In detail, the collimator optical path <NUM> has a collimator optical axis <NUM>, indicated by a succession of arrows in <FIG>, comprising a first stretch <NUM>, a second stretch <NUM>, a third stretch <NUM>, a fourth stretch <NUM> and a fifth stretch <NUM>.

The first stretch <NUM> of the collimator optical axis <NUM> extends between the second point source 58B and the vertex <NUM> of the quaternary mirror 70D. The second stretch <NUM> of the collimator optical axis <NUM> extends between the vertex <NUM> of the quaternary mirror 70D and the vertex <NUM> of the tertiary mirror 70C. The third stretch <NUM> of the collimator optical axis <NUM> extends between the vertex <NUM> of the tertiary mirror 70C and the vertex <NUM> of the secondary mirror 70B. The fourth stretch <NUM> of the collimator optical axis <NUM> extends between the vertex <NUM> of the secondary mirror 70B and the vertex <NUM> of the primary mirror 70A. The fifth stretch <NUM> of the collimator optical axis <NUM> extends between the vertex <NUM> of the primary mirror 70A and a point of the aperture <NUM>, in particular a midpoint M of the aperture <NUM>.

In practice, with reference to the collimator optical axis <NUM>, the first stretch <NUM> defines a distance e<NUM> between the extended light source <NUM> and the quaternary mirror 70D, the second stretch <NUM> defines a distance e<NUM> between the quaternary mirror 70D and the tertiary mirror 70C, the third stretch <NUM> defines a distance e<NUM> between the tertiary mirror 70C and the secondary mirror 70B, and the fourth stretch <NUM> defines a distance e<NUM> between the secondary mirror 70B and the primary mirror 70A.

The mirrors 70A-70D are each arranged rotated with respect to the collimator optical axis <NUM> by a respective tilt angle.

In detail, the mirror axis S of the primary mirror 70A forms a tilt angle α<NUM> with respect to the fifth stretch <NUM> of the collimator optical axis <NUM>. The mirror axis S of the secondary mirror 70B forms a tilt angle α<NUM> with respect to the fourth stretch <NUM> of the collimator optical axis <NUM>. The mirror axis S of the tertiary mirror 70C forms a tilt angle α<NUM> with respect to the third stretch <NUM> of the collimator optical axis <NUM>. The mirror axis S of the quaternary mirror 70D forms a tilt angle α<NUM> with respect to the second stretch <NUM> of the collimator optical axis <NUM>.

The mirror axis S of the quaternary mirror 70D and the mirror axis S of the primary mirror 70A form between them an angle comprised between <NUM>° and <NUM>°, in particular approximately of <NUM>°. In this way, in use, it is possible to avoid that the main light beam <NUM>, after impinging on the quaternary mirror 70D, is reflected towards the primary mirror 70A.

Further, the first stretch <NUM> of the collimator optical axis <NUM> forms a tilt angle αS with a source axis Ss perpendicular to the focal plane F of the collimator <NUM> and passing through the second point source 58B.

The distance between the mirrors 70A-70D, the tilt angle and the radius of curvature of the mirrors 70A-70D may be determined, at the design stage, as a function of a desired effective focal length of the collimator <NUM> and/or of a frequency range of the light radiation <NUM> generated by the extended light source <NUM>.

Additionally or alternatively, the distance between the mirrors 70A-70D, the tilt angle and the radius of curvature of the mirrors 70A-70D may be chosen, at the design stage, so as to eliminate, in the first approximation, the aberrations of the collimator <NUM>.

For example, the mirrors 70A-70D may be designed to reduce the aberrations of coma C and astigmatism A, as described in Patent Application <CIT>.

For example, the tilt angle α<NUM> of the tertiary mirror 70C is lower than <NUM>°.

For example, the distance e<NUM> between the primary mirror 70A and the secondary mirror 70B is comprised between a factor <NUM> and <NUM> of the focal length of the primary mirror 70A.

For example, the primary mirror 70A may be a concave spherical mirror and the secondary mirror 70B a convex spherical mirror.

For example, the quaternary mirror 70D may be a flat mirror, i.e. mathematically representable as a spherical mirror having infinite radius of curvature R<NUM>.

In addition, the specific three-dimensional arrangement of the mirrors 70A-70D may be chosen at the design stage depending on the specific application of the collimator <NUM>. For example, as shown in the embodiment of <FIG> and described below, the mirrors 70A-70D may be mounted on a single optical working plane, so that the collimator optical axis <NUM> lies on a single plane.

However, the mirrors 70A-70D may have a different three-dimensional arrangement.

The collimator <NUM> further comprises a plurality of baffles 100A-100E, hereinafter individually referred to as first baffle 100A, second baffle 100B, third baffle 100C, fourth baffle 100D, and fifth baffle 100E, each configured to prevent a reflection of an incident light beam.

The baffles 100A-100E may each be of an opaque material, e.g., a thin metal plate, configured to absorb an incident light radiation.

In this embodiment, the first and the fourth baffle 100A, 100D are formed as one piece; the second, the third, and the fifth baffle 100B, 100C, 100E are distinct from each other. However, the baffles 100A-100E may form portions of a single absorbent body, as shown for example in <FIG>.

With reference to <FIG>, the first baffle 100A has a first end 103A and a second end 103B and is arranged between the extended light source <NUM> and the primary mirror 70A, so as to form a first shadow region 105A of the secondary light beam <NUM> of the extended light source <NUM>.

In the top view of <FIG>, the first shadow region 105A is delimited by a first boundary ray 106A and a second boundary ray 106B, and is an umbra region of the extended light source <NUM>.

The first boundary ray 106A belongs to the secondary light beam <NUM> and is a ray generated by the first point source 58A and passing contiguous to the first end 103A of the first baffle 100A. The second boundary ray 106B belongs to the secondary light beam <NUM> and is a ray generated by the third point source 58C and passing contiguous to the second end 103B of the first baffle 100A.

In practice, each ray of the secondary light beam <NUM> propagating between the first and the second boundary ray 106A, 106B impinges on the first baffle 100A and is absorbed by it.

The primary mirror 70A, in particular the respective reflective surface <NUM>, is arranged in the first shadow region 105A.

Consequently, in use, the first baffle 100A ensures that no portion of the secondary light beam <NUM> of the extended light source <NUM> directly impinges on the reflective surface <NUM> of the primary mirror 70A.

With reference to the top view in <FIG>, the second baffle 100B extends between a first end 107A and a second end 107B and is arranged between the extended light source <NUM> and the tertiary mirror 70C, so as to form a second shadow region 105B of the secondary light beam <NUM> of the extended light source <NUM>.

The second shadow region 105B is delimited by a first boundary ray 108A and a second boundary ray 108B, and is an umbra region of the extended light source <NUM>.

The first boundary ray 108A belongs to the secondary light beam <NUM> and is a ray generated by the first point source 58A and passing contiguous to the first end 107A of the second baffle 100B. The second boundary ray 108B belongs to the secondary light beam <NUM> and is a ray generated by the third point source 58C and passing contiguous to the second end 107B of the second baffle 100B.

In practice, each ray of the secondary light beam <NUM> propagating between the first and the second boundary ray 107A, 107B impinges on the second baffle 100B and is absorbed by it.

The tertiary mirror 70C, in particular the respective reflective surface <NUM>, is arranged in the second shadow region 105B.

Consequently, in use, the second baffle 100B ensures that no portion of the secondary light beam <NUM> directly impinges on the reflective surface <NUM> of the tertiary mirror 70C.

With reference to the top view of <FIG>, the third baffle 100C extends between a first end 111A and a second end 111B and is arranged between the extended light source <NUM> and the secondary mirror 70B, so as to form a third shadow region 105C of the secondary light beam <NUM> of the extended light source <NUM>.

The third shadow region 105C is delimited by a first boundary ray 110A and a second boundary ray 110B, and is an umbra region of the extended light source <NUM>.

The first boundary ray 110A belongs to the secondary light beam <NUM> and is a ray generated by the first point source 58A and passing contiguous to the first end 111A of the third baffle 100C. The second boundary ray 110B belongs to the secondary light beam <NUM> and is a ray generated by the third point source 58C and passing contiguous to the second end 111B of the third baffle 100C.

In practice, each ray of the secondary light beam <NUM> propagating between the first and the second boundary ray 110A, 110B impinges on the third baffle 100C and is absorbed by it.

The secondary mirror 70B, in particular the respective reflective surface <NUM>, is partially arranged in the third shadow region 105C.

In detail, the second boundary ray 110B of the third shadow region 105C impinges, in use, on the reflective surface <NUM> of the secondary mirror 70B.

The secondary mirror 70B and the quaternary mirror 70D, in particular the respective mirror axes, are mutually tilted of an angle comprised between <NUM>° and <NUM>°, in particular approximately of <NUM>°.

The secondary mirror <NUM>, the quaternary mirror 70D, and the fourth baffle 100D form a trap optical path <NUM>.

The secondary mirror 70B reflects the second boundary ray 110B and forms a first reflected ray <NUM> incident on the reflective surface <NUM> of the quaternary mirror 70D. The quaternary mirror 70D reflects the first reflected ray <NUM> and forms a second reflected ray <NUM> incident on the fourth baffle 100D.

In practice, the secondary mirror 70B is partially shaded with respect to the extended light source <NUM>, so that part of the secondary light beam <NUM> incident directly on the reflective surface <NUM> of the secondary mirror 70B follows the trap optical path <NUM> and is then absorbed by the fourth baffle 100D.

With reference to the top view of <FIG>, the fifth baffle 100E extends between a first end 121A and a second end 121B and is arranged between the secondary mirror 70B and the quaternary mirror 70D.

The fifth baffle 100E forms a fourth shadow region 105D both with respect to the extended light source <NUM> and with respect to rays reflected by the tertiary mirror 70C that are external to the collimator optical path <NUM>, i.e. with respect to rays reflected by the tertiary mirror 70C that are not directed towards the secondary mirror 70B.

In detail, the fourth shadow region 105D is delimited by a first boundary ray 120A passing contiguous to the first end 121A of the fifth baffle 100E and by a second boundary ray 120B passing contiguous to the second end 121B of the fifth baffle 100E, and is an umbra region of the extended light source <NUM> and of the tertiary mirror 70C.

The aperture <NUM> is arranged in the fourth shadow region 105D.

In practice, each light ray, identified by way of example by an arrow <NUM> in <FIG>, of the secondary light beam <NUM> that is generated by the extended light source <NUM>, passes between the quaternary mirror 70D and the secondary mirror 70B, and is directed towards the aperture <NUM>, impinges on the fifth baffle 100E. Similarly, each light ray, identified by way of example by an arrow <NUM> in <FIG>, which is reflected by the tertiary mirror 70C, passes between the quaternary mirror 70D and the secondary mirror 70B, and is directed toward the aperture <NUM> impinges on the fifth baffle 100E.

In practice, the baffles 100A-100E and the mutual arrangement of the quaternary mirror 70D and of the primary mirror 70A ensure that only the light rays generated by the extended light source <NUM> and following the collimator optical path <NUM> (i.e., the primary light beam <NUM> of the light radiation <NUM> of <FIG>) reach the aperture <NUM>. In practice, the collimator <NUM> has low intrinsic noise.

In other words, the stray light, that is, the light rays that do not follow the collimator optical path <NUM>, generated inside the illuminator <NUM>, does not reach the aperture <NUM>.

Consequently, the illuminator <NUM> outputs, through the pupil <NUM>, only the collimated output beam <NUM>, which followed the collimator optical path <NUM>.

The illuminator <NUM> therefore has a low signature. In other words, the illuminator <NUM> has a low intrinsic noise.

<FIG> shows a different embodiment of the present illuminator, here indicated by <NUM>.

In particular, the illuminator <NUM> has a same structure as the one of the illuminator <NUM> of <FIG>, except for the differences discussed below. Thus, common elements are indicated by the same reference numerals and the illuminator <NUM> will be described only with reference to the differences compared to the illuminator <NUM>.

In detail, the illuminator <NUM> comprises the extended light source <NUM>, the aperture <NUM> and a collimator, here denoted by <NUM>.

The collimator <NUM> comprises, also in this embodiment, the mirrors 70A-70D, the first baffle 100A, the second baffle 100B, the third baffle 100C, the fourth baffle 100D, and an absorbent structure 200F including the fifth baffle 100E.

The absorbent structure 200F further comprises two encapsulation portions 201A, 201B.

The encapsulation portion 201A forms a single piece with the fifth baffle 100E.

The encapsulation portions 201A, 201B laterally delimit the aperture <NUM> and the final stretch of the collimator optical path <NUM>, i.e. the fifth stretch <NUM> of the collimator optical axis <NUM>.

In practice, the encapsulation portions 201A, 201B form further portions for absorbing the stray light of the illuminator <NUM>; they thus contribute to further decreasing the signature of the collimator <NUM>.

<FIG> shows a perspective view of a further embodiment of the present illuminator, here indicated by <NUM>.

In detail, the illuminator <NUM> is formed by the extended light source <NUM>, the aperture <NUM> (shown only schematically in <FIG>) and by a collimator, here indicated by <NUM> and comprising the mirrors 70A-70D.

The mirrors 70A-70D and the extended light source <NUM> are mounted on a working optical plane <NUM> such that the collimator optical axis <NUM> lies in a single plane parallel to the working optical plane <NUM>.

The collimator <NUM> also comprises here the first baffle 100A, the second baffle 100B, and the fourth baffle 100D.

The third baffle and the fifth baffle are shown in transparency in <FIG>, for clarity's sake.

In this embodiment, the first baffle 100A, the second baffle 100B and the fourth baffle 100D are mutually contiguous to portions forming a single absorbent body <NUM> having a through cavity <NUM>.

The first, the second, and the third stretch <NUM>, <NUM>, and <NUM> of the collimator optical axis <NUM> pass through the through cavity <NUM>.

The illuminators <NUM>, <NUM>, <NUM> may be used to obtain a system <NUM> for testing the stray light of a unit under test <NUM>, as shown in <FIG>.

For simplicity's sake, in the following, reference will be made only to the illuminator <NUM>. However, the same considerations also apply to the illuminators <NUM> and <NUM>.

For example, the illuminator <NUM> may be used to obtain an Optical Ground Support Equipment (OGSE) capable of testing, calibrating, and/or verifying the optical characteristics, in particular the optical characteristics of stray light, of the unit under test <NUM>, during specific Assembly Integration and Test (AIT) activities or Assembly and Integration Test (AIT) activities.

The unit under test <NUM> can be any electro-optical device, e.g. of the objective, telescope or afocal type, and is formed by an optical architecture 305A and an optical detector 305B.

The optical architecture 305A is configured to receive the output beam <NUM> output from the aperture <NUM> of the illuminator <NUM>, and generate a device light beam <NUM>, focused on the optical detector 305B.

In use, the optical architecture 305A may generate stray light <NUM>, starting from the propagation of the output beam <NUM> received by the illuminator <NUM>.

Consequently, the device light beam <NUM> comprises both a primary portion <NUM>, forming the desired optical signal in output from the optical architecture 305A, and a device stray portion <NUM>, generated by the stray light <NUM>.

The optical detector 305B receives the device light beam <NUM> and generates a corresponding detection signal, e.g., an electrical signal.

The fact that the illuminator <NUM> outputs only the desired optical stimulus for the unit under test <NUM>, i.e. in this case a collimated beam without an uncollimated stray component, makes it possible to distinguish, in the detection signal, the stray portion <NUM> of the device light beam <NUM> generated by the unit under test <NUM> itself.

Consequently, thanks to the illuminator <NUM>, the optical properties of the stray light of the unit under test <NUM> can be effectively tested.

Finally, it is evident that modifications and variations can be made to the collimator and to the illuminator described and shown herein, without departing from the scope of the present invention.

The extended light source <NUM> may be generically any non-point light source, for example a distributed light source.

For example, one or more of the mirrors 70A-70D may be aspherical. For example, as described in patent application <CIT>, an eccentricity can be introduced to one or more of the spherical mirrors 70A-70D, in particular to the primary mirror 70A and the mirror 70B, in order to reduce the aberrations of the respective collimator.

Claim 1:
An illumination optical device (<NUM>; <NUM>; <NUM>) comprising:
- a collimator (<NUM>; <NUM>; <NUM>);
- a light source (<NUM>); and
- an aperture (<NUM>),
wherein the collimator comprises a primary mirror (70A), a secondary mirror (70B), a tertiary mirror (70C) and a quaternary mirror (70D) mutually tilted so as to form a collimator optical path (<NUM>) extending between the light source and the aperture and formed, in succession, by the light source, the quaternary mirror, the tertiary mirror, the secondary mirror, the primary mirror and the aperture,
wherein the light source (<NUM>) is configured to emit light rays along respective directions, said light rays forming:
- a main light beam (<NUM>) including light rays (66A, 66B, 66C) propagating along directions intercepting the quaternary mirror (70D) so as to follow the collimator optical path (<NUM>); and
- a secondary light beam (<NUM>) including light rays (106A, 106B, 108A, 108B, 110A, 110B, <NUM>) propagating along directions not intercepting the quaternary mirror (70D);
the illumination optical device being characterised in that the collimator comprises a shielding structure (100A-100E; 200F; <NUM>) configured to prevent illumination of the aperture by the secondary light beam,
wherein the primary mirror (70A) and the quaternary mirror (70D) are mutually tilted of an angle comprised between <NUM>° and <NUM>°.