Spectral separation component without a visible ghost image

Abstract

A spectral splitting component is provided, having two faces, a planar front face comprising a dichroic treatment and a back face. It is intended to be placed downstream of a convergent objective. The back face is convex and forms a cylindrical surface defined by a generatrix of fixed direction moving perpendicularly along a circular arc comprising two ends, the plane passing through these two ends and parallel to the generatrix of the cylindrical surface forming a dihedral with the plane of the front face, the generatrix of the cylindrical surface being parallel to the edge of the dihedral.

Claims

1. A spectral splitting component having two faces, a planar front face comprising a dichroic treatment and a back face, wherein the back face is convex and forms a cylindrical surface defined by a generatrix of fixed direction moving perpendicularly along a circular arc comprising two ends, a plane passing through these two ends and parallel to the generatrix of the cylindrical surface forming a dihedral with the plane of the front face, the generatrix of the cylindrical surface being parallel to the edge of the dihedral.

2. The spectral splitting component as claimed in claim 1, wherein the back face is given an antireflection treatment.

3. The spectral splitting component as claimed in claim 1, characterized in that the dichroic treatment is able to transmit wavelengths comprised between 3 μm and 5 μm.

4. The spectral splitting component as claimed in claim 3, wherein the dichroic treatment is able to reflect wavelengths shorter than 3 μm.

5. The spectral splitting component as claimed in claim 1, wherein the component is circular and of diameter D and has a thickness e such that D/e is comprised between 14 and 36.

6. The spectral splitting component as claimed in claim 1, wherein the angle of the dihedral is comprised between 0.4 mrd and 2 mrd.

7. The spectral splitting component as claimed in claim 1, wherein the circular arc has a radius of curvature comprised between 10 m and 100 m.

8. The spectral splitting component as claimed in claim 1, wherein the component is made of YAG or Si or aluminum on/nitride (Al.sub.23O.sub.27N.sub.5) or Spinel or MgO or ZnS or ZnSe or Ge or GaAs.

9. A mono-pupil multispectral optronic system intended to form an image of an object, comprising on its optical axis: a convergent objective; a spectral splitting component as claimed in claim 1, inclined to the optical axis at a preset angle of inclination; and a matrix-array detector; wherein, the optical axis of the system taking the form of a zigzag line with an optical axis incident on the spectral splitting component and an optical axis refracted by the component, the component is inclined to the incident optical axis about the axis parallel to the generatrix of the cylindrical surface passing through the intersection of the incident optical axis with the planar front face, and the angle of inclination is such that the back face of the component is less inclined to the incident optical axis than the front face, so that, the image comprising a main image taking account of diffraction and a parasitic image formed by double reflection in the spectral splitting component, the parasitic image is shifted back under the diffraction spot of the main image.

10. The mono-pupil multispectral optronic system as claimed in claim 9, characterized in that the angle of inclination is comprised between 30° and 45°.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages of the invention will become apparent on reading the following detailed description, which is given by way of nonlimiting example and with reference to the appended drawings, in which:

(2) FIGS. 1a-1d, which have already been described, schematically show a dichroic plate designed for convergent beams, said plate being inclined to the optical axis, with the intermediate focal plane on which the main image forms (FIG. 1a) and the intermediate focal plane on which the parasitic image forms (FIG. 1b); and the results obtained for a system equipped with such a plate (PDP standing for prismatic dichroic plate): a graph (FIG. 1c) detailing the distribution of illumination in the intermediate focal plane for the geometric image of the source, the image of the source taking account of diffraction, and the parasitic image of the source; and (FIG. 1d) the weighted MWIR MTF calculated at the centre of the transmission field, and the diffraction limit;

(3) FIGS. 2a and 2b schematically show an example of a spectral splitting component according to the invention, seen in cross section (FIG. 2a) and en perspective (FIG. 2b);

(4) FIG. 3 schematically shows an example of a spectral splitting component designed for convergent beams inclined to the optical axis of the system, according to the invention;

(5) FIGS. 4a-c illustrate the results obtained for a system equipped with a first example of a spectral splitting component (CPDP standing for cylindrical prismatic dichroic plate) according to the invention, taking the form of a weighted MWIR MTF calculated at the centre of the transmission field compared to the diffraction limit (FIG. 4a); and of graphs giving the distribution of illumination in the intermediate focal plane for the geometric image, the image taking account of diffraction and the parasitic image of a disc of 0.5 mm diameter (FIG. 4b) and 0.05 mm diameter (FIG. 4c);

(6) FIGS. 5a-5c illustrate the results obtained for a system equipped with a second example of a spectral splitting component (CPDP standing for cylindrical prismatic dichroic plate) according to the invention, taking the form of a weighted MWIR MTF calculated at the centre of the transmission field compared to the diffraction limit (FIG. 5a); and of graphs giving the distribution of illumination in the focal plane for the geometric image, the image taking account of diffraction and the parasitic image of a disc of 0.5 mm diameter (FIG. 5b) and 0.05 mm diameter (FIG. 5c);

(7) FIGS. 6a-6c illustrate the results obtained for a system equipped with a third example of a spectral splitting component (CPDP standing for cylindrical prismatic dichroic plate) according to the invention, taking the form of a weighted MWIR MTF calculated at the centre of the transmission field compared to the diffraction limit (FIG. 6a); and of graphs giving the distribution of illumination in the focal plane for the geometric image, the image taking account of diffraction and the parasitic image of a disc of 0.5 mm diameter (FIG. 6b) and 0.05 mm diameter (FIG. 6c).

(8) From one figure to another, the same elements have been given the same references.

DETAILED DESCRIPTION

(9) An example spectral splitting component 10 according to the invention is described with regard to FIGS. 2a and 2b (which are not to scale).

(10) It comprises a planar spectral splitting (or dichroic) front surface 11 and a convex cylindrical back surface 12′.

(11) The dichroic treatment is typically able to transmit wavelengths comprised between 3 μm and 5 μm and optionally able to reflect wavelengths shorter than 3 μm or longer than 8 μm.

(12) The cylindrical surface 12′ is defined by a generatrix of fixed direction moving perpendicularly along a circular arc 12 (very accentuated in these figures) having two ends. The plane 13′ passing through the two ends of the circular arc and parallel to the generatrix of the cylindrical surface forms a dihedral with the plane of the front face 11 (this dihedral being very accentuated in these figures), the generatrix of the cylindrical surface being parallel to the edge 14′ of the dihedral. For practical reasons, the apex of the dihedral is generally removed.

(13) The average thickness “e” of the spectral splitting component is as small as possible, within reasonable limits, i.e. so that the component keeps its shape during fitting and operation, and typically has a diameter:thickness ratio comprised between 14 and 36. In FIG. 2b, the front face 11 of the spectral splitting component 10 is rectangular in order to make the figure easier to understand, but the invention is not limited to this case; it may notably be circular and of diameter D.

(14) The angle α of the dihedral is adjusted so as to drown the parasitic image under the diffraction spot of the transmitted main image, this amounting to orienting the prism in the wrong direction from the point of view of aberrations. It is not necessary to superpose rigorously the parasitic image and the direct image in order to mask it. This allows the aberrations introduced by the prism to be limited, which aberrations are then more easily compensated for by the cylinder.

(15) The aberrations induced by the prism are corrected by the convex cylindrical back face.

(16) Generally, the angle α of the prism and the radius of curvature of the cylindrical back face (=radius of the circular arc 12) depend on the geometric extent of the MWIR beam, on the angle of inclination of the component to the optical axis of the system in which it is placed, on its refractive index, on its thickness, on the position of the pupil and on the position of the component relative to the focal plane. The larger the angle of inclination, the more difficult it becomes to find a satisfactory compromise between the quality of the transmitted wavefront and the concealment of the parasitic image.

(17) The two parameters—prismaticity and radius of curvature of the circular arc—are determined experimentally or optimized using an optical design software package such as the software packages ZEMAX™ or Code V™. The angle of the dihedral is typically comprised between 0.4 mrd and 2 mrd; the circular arc typically has a radius of curvature comprised between 10 m and 100 m.

(18) The back face 12′ is advantageously given an antireflection treatment.

(19) Preferably, the spectral splitting component is made of a material having a high refractive index, typically higher than 1.65, and is sufficiently stiff not to deform during fitting and operation. Specifically, a high index promotes the alignment of the parasitic image and the main image, and limits aberrations; a high stiffness allows the thickness of the component to be decreased, thereby naturally bringing the parasitic image and the main image further into alignment and attenuating aberrations.

(20) Thus, logical candidate materials are for example silicon (Si), YAG (Y.sub.3Al.sub.5O.sub.12), spinel (MgAl.sub.2O.sub.4), Al.sub.23O.sub.27N.sub.5 designated by the commercial trade name ALON™, and magnesium oxide (MgO). Materials of lower stiffness, such as zinc sulfide (ZnS), zinc selenide (ZnSe), germanium (Ge), or gallium arsenide (GaAs) may nevertheless be used, with suitable thicknesses.

(21) A mono-pupil multispectral optronic system using a matrix-array sensor and comprising a spectral splitting component such as described above will now be considered. The spectral splitting component 10 is inclined at an angle of 36° to the optical axis Oz of the system about the axis parallel to the generatrix of the cylindrical surface 12′ passing through the intersection of the incident optical axis with the planar front face 11. The apex of the dihedral points upward (into the quadrant YoZ where Y and Z are positive) if the component deflects downward as shown in FIG. 3. It is located downstream of a convergent frontal multispectral objective that is assumed to be perfect (not shown in the figures) and that, in the MWIR band, works with an aperture of F/4 and an image field of 9.6 mm×7.2 mm, i.e. a diagonal of 12 mm. The pupil 15 is located about 220 mm upstream of the focal plane 2 of the objective, and the component 10 at about 130 mm.

(22) In addition, in the MWIR, the parasitic reflectance of the front face 11 is assumed to be 7% and that of the back face 12′ 1%, this being accessible with this type of component.

(23) Obviously, this type of component does not introduce any aberration into the reflected beam. Therefore, to within manufacturing tolerances, the reflection MTF may be considered not to be degraded.

(24) According to a first example embodiment, the spectral splitting component is made of YAG and has an average thickness “e” of 1.7 mm. The angle α of the dihedral is about 0.06° (=1.05 mrad). The back face 12′ is convex and cylindrical with a circular arc radius of about 38 m.

(25) The weighted polychromatic MTF of the system equipped with this component, calculated in transmission in the band 3.4 μm-4.2 μm, is chromatism limited and is very close to the diffraction limit, as FIG. 4a shows for the centre of the field. This is true everywhere in the field, meaning that the spectral splitting component does not significantly degrade the optical MTF of the system at any point in the field. The offset between the barycenter of the parasitic image and the direct (or main) image is about 410 μm and this is enough to place the parasitic image under the diffraction of the direct image. The two FIGS. 4b and 4c illustrate this, for images of a disc of 0.5 mm diameter and for images of a disc of 0.05 mm diameter, respectively.

(26) According to a second example embodiment, the spectral splitting component is made of silicon and has an average thickness of 3 mm. The angle α of the dihedral is about 0.04° (=0.7 mrad). The back face 12′ is convex and cylindrical with a circular arc radius of about 83 m.

(27) As in the preceding example, the weighted MTF of the system equipped with this component, calculated in the band 3.4 μm-4.8 μm in transmission, is very close to the diffraction limit, as shown in FIG. 5a for the centre of the field; and the same applies to everywhere in the field.

(28) The offset between the barycenter of the parasitic image and the direct (or main) image is about 86 μm and this is more than enough to place the parasitic image under the diffraction of the direct image. The two FIGS. 5b and 5c illustrate this, for images of discs of 0.5 mm diameter and of 0.05 mm diameter, respectively.

(29) According to a third example embodiment, the spectral splitting component is made of zinc sulfide and has an average thickness of 3.3 mm. The angle α of the dihedral is about 0.08° (=1.4 mrad). The back face 12′ is convex and cylindrical with a circular arc radius of about 34 m.

(30) As in the preceding example, the weighted MTF of the system equipped with this component, calculated in the band 3.4 μm-4.8 μm in transmission, is very close to the diffraction limit, as shown in FIG. 6a for the centre of the field; and the same applies to everywhere in the field.

(31) The offset between the barycenter of the parasitic image and the direct image is about 450 μm and this is more than enough to place the parasitic image under the diffraction of the direct image. The two FIGS. 6b and 6c illustrate this, for images of discs of 0.5 mm diameter and of 0.05 mm diameter, respectively.