EP0587810B1

EP0587810B1 - Process for the production of lenses with a variable refraction index

Info

Publication number: EP0587810B1
Application number: EP92917326A
Authority: EP
Inventors: David Harrison; Masahiro Fujimoto; Gerhard Maier
Original assignee: Thomson Multimedia SA
Current assignee: Technicolor SA
Priority date: 1991-06-03
Filing date: 1992-05-25
Publication date: 1996-08-14
Anticipated expiration: 2012-05-25
Also published as: ATE141445T1; CA2109936A1; ES2090677T3; DE69212829D1; WO1992022846A3; JPH06507735A; KR940701589A; WO1992022846A2; DE69212829T2; EP0587810A1

Abstract

This invention presents a process for the production of a lens with a given variable refraction index by fixing together several parts, at least some of them have a variable refraction index. The realization of the desired refraction index of the parts can be achieved by compressing dielectric material of a first shape into its final shape or by a doping or a mixing process respectively. Outer parts, which extend to the outer surface of the lens to be produced, have a pyramid-like shape with a pointed or truncated apex respectively. In the first case the apexes of the outer parts meet in the symmetry point of said lens; in the latter case a center part is used which is located around said symmetry point. It is also possible that some of the outer parts used have pointed apexes and the rest of them has got truncated apexes. In this case a center part used is easy to fix near the symmetry point of the lens. If dielectric material is taken, the lens to be produced can preferably be used as part of microwave antenna system.

Description

The present invention relates to a lens with a variable refraction index, e.g. a Luneburg-type lens, which might be used as part of a microwave antenna system, and an appropriate process for the production of said lens.
It is known, e.g. from US 4 288 337, that lenses of the said art can be used as radar reflectors or, as is known from E. F. Buckley; "Stepped-Index Luneburg Lenses"; Electronic Design, April, 13, 1960, as part of an antenna sytem.
As Buckley has described in said article, it is a known process for the production of Luneburg lenses to use a hemisperical shell construction with a given number of layers.
According to said US patent the layers for the fabrication of Luneburg and Eaton-Lippmann lenses for microwave applications can be produced by dielectrics. In such cases the relation between the relative dielectric constant E and the refraction index n is ${n =E}^{1/2.}$
A mixed dielectric can be obtained by mixing expanded particles selected from the group consisting of expanded polystorols, expanded polyethylenes, expanded polyurethanes, glass balloons and silica balloons, with metal-coated particles consisting of said expanded particles, surfaces of which have been coated with a thin film selected from the group of chromium, aluminium, copper, nickel, gold, silver, and magnesium in proper proportions to obtain a desired dielectric constant then forming the same to the desired shape by the use of a binder.
As M. A. Mitchel et al. described in the article "A multiple-beam multiple frequency spherical Lens Antenna System providing hemispherical Coverage"; 6. Int. Conference on Antennas and Propagation (ICAP), 1989, Part 1; pp. 394 -398, for a dielectric material, such as polystyrene, the relation between its relative dielectric constant E and its density d may be expressed by ${E = 0.4 ∗ E}_{0} ^{(d/do)} {+ 0.6 ∗ (1 + E}_{0} - D(d/do)),$
where

E₀: is the relative dielectric constant of the unexpanded dielectric material, and
do: is the density of the unexpanded dielectric material.

Using the relation (2), Mitchel et al. produced shells for a shell-construction of a Luneburg lens.
By the mentioned methods for the fabrication of lenses it is just possible to approximate the variation of the refraction index required, which is dependent on the dielectric conslant. No practical scheme for smoothly varying the refractive index has been achieved.
By using shells with different dielectric constants and thereby with different refraction indexes, reflection losses occur, by which power is reflected from the dielectric boundaries.
In the US patent 3 470 561, which presents a spherical Luneburg Lens, is mentioned that the variation of the refraction index n as a function of polystyrene density d (in Lbs./cu.ft.) is given by ${n = (1 + 0.02d)}^{0.5} .$
Another method for the variation of the refraction index n, which is presented by said US patent, is achieved by means of a variably loaded artificial dielectric medium.
There it is also mentioned to fabricate a plurality of substantially identical orange-slice shaped wedges which collectively form the Luneburg lens to be produced.
The fabrication of said orange-slice shaped wedges is quite costly.
Moreover US-A-3 133 285 discloses a spherical luneberg lens composed of a plurality of pyramidal sectors each having a graded dielectric constant and a four-edge base.
It is an object of the invention, to present a lens with a given variable refraction index, which allows a more accurate representation of the required profile of the refraction index by using parts, which can be produced in an easy way.
This can be realized by a lens according to claim 1 or by a process according to the first process claim, respectively.
According to the present invention a lens to be produced, having a symmetry point and a variable refraction index n, is built by a number of parts, which have a given variation of the refraction index n in such a manner, that the lens to be produced has got the given variable refraction index. The shapes of said parts collectively form the shape of said lens. Most of the parts have the same shape, i.e. they are pyramid-like.
The kind of the symmetry point depends on the shape of the lens to be produced. If the lens is spherical, the symmetry point may be identical with the center point of the sphere. If the lens is hemispherical, then the symmetry point may be the center point of that sphere, which can be built by two of said hemispherical lenses.
If the lens is cylindrical, the symmetry "point" may be the center line of the cylinder.
The invention has the advantage, that the aperture efficiency of an antenna system, including the lens to be produced, is increased compared to the use of a shell-type lens. This is achieved by avoiding or reducing power reflections from dielectric boundaries, which normally occur at shell interfaces.
It is another advantage of the invention, that phase of colliminated rays at feed points of an antenna system is more exact, whereby aperture efficiency can be increased.
By avoiding or reducing electromagnetic fields inside the lens, which are caused to propagate tangentially to shell surfaces causing surface waves to be set up, aperture efficiency can also be increased.
By using identical parts, i.e. pyramid-like shaded, which might have a shape extending from the symmetry point of the respective lens to its outer surface, the invention has the advantage, that an identical process is used to produce each part which collectively constitute the lens.
By using different kinds of parts, whereby one of said parts, which may be built of sub-parts, is located around the symmetry point, and the other parts extend from the outer surface of the first part to the outer surface of the lens to be produced, a sharp angle at the apex of a pyramid-like shape can be avoided.
The present invention will be better understood with the aid of the following description of preferred embodiments and accompanying drawings, wherein

Fig. 1: shows a pyramid shaped part, which is used for the production of a lens according to a first embodiment,
Fig. 2: shows a top view of the lens constructed of parts like shown in fig. 1,
Fig. 3: shows in principle a production process of the part of fig. 1,
Fig. 4: shows in principle another production method for a lens according to a second embodiment.

In the following description means and details with the same function or meaning which are used in several figures have got the same reference numbers and if they are explained once, they will only be explained in the further description as far as it is necessary for the understanding of the present invention.
According to the first embodiment of the invention parts like the one 10 shown in fig. 1 are used. Several of them collectively form a lens to be produced. The base 11 of the part 10, which has five edges 12,..., 16 might be flat or rounded.
Other edges 17,..., 21 run from the base 11 to the apex 22 of the pyramid shaped part 10. Its not shown height h is defined by the shortest distance between the base 11 and the apex 22.
Fig. 2 shows a top view of a lens 23 to be produced by a number of parts 10, 10a,..., 10e, which have all the same shape and the same composition like the part 10. Although one can see in fig. 2 only six identical parts 10,..., 10e with a five-edge base 11, it is necessary to take twelve of them for the constitution of the spherical lens 23.
In this embodiment the apexes 22 of the twelve parts meet in the symmetry point of the lens 23. The bases 11 of the twelve parts collectively form the outer surface of the lens 23.
In the further description a production process for the part 10 is in principle described. As all the twelve parts, which collectively form the lens 23 are identical, the same process can be taken for the other shown parts 10a,..., 10e and for the not shown parts.
The material, the part 10 is built of, is dielectric, such as polysterene, and in such case the relation between the refraction index n and the dielectric constant E is according to formula (1).
The refraction index n(r) and the relative dielectric constant E(r) vary for a Luneburg lens, having not shown focal points at its outer surface for parallel waves, with radius according to ${n(r) = (1 - (r/r}_{0})^{2})^{0.5},$
${E(r) = 1 - (r/r}_{0})^{2}$
where r is the actual radius and r₀ is the radius of the sperical lens 23.
The shape of the part 10, in this embodiment pyramid-like, may be formed by compressing the expanded dielectric material, such as polysterene, at an elevated temperature.
To achieve the necessary dielectric constant variation along the hight h, a greater amount of forming pressure would be applied at the apex 22 than at the base 11 of the part 10.
As the dependence of the relative dielectric constant E with density may be expressed by formula (2), an uncompressed shape 25, as shown in fig. 3 may be taken to achieve the final pyramid-type shape of the part 10. The uncompressed shape 25 may be achieved e.g. by shaping or cutting.
Another preferred embodiment is in principle indicated in fig. 4. Different kinds of parts are used to build the lens 23.
A center part 26, which may be created of sub-parts, has a spherical shape and is located around the symmetry point 24 (see fig. 2). In this embodiment one needs twelve outer parts, which have a pyramid-like shape as the truncated part 27 are identical to eachother, and extend from the outer surface of the center part 26 to the outer surface of the lens 23.
It may be mentioned that the outer parts with the shape of the truncated part 27 may have flat or rounded edges at their bases. In the first case the spherical shape of the lens 23 is only approximated.
In a version of this embodiment, the center part 26 may have a homogeneous dielectric constant, e.g. in that case that the size of the center part 26 is so small, that a deviation of desired ray trajectories may be negligible.
For another version of this embodiment, the center part 26 has not a spherical shape but any other, e.g. one with flat planes whereby the number of said planes is identical with the number of the outer parts.
It is also possible that some of the outer parts used have more or less pointed apexes and the rest of them has got truncated apexes. In this case a center part used with an appropriate shape can be fixed more easily.
Versions of the preferred embodiments may contain at least one of the following variations:

By using appropriate pyramid-like shaped parts, like the part 10 or the truncated part 27 lenses with non-spherical shapes may be produced,
the realization of the profile of the refraction index n may be achieved by other processes, e.g. by doping, mixing or thermal processes, whereby metal-coated particles may be used,
the boundaries of the single parts 10, 26, 27 may be connected together or linked with eachother, e.g. by an appropriate adhesive or melting process,
by using an appropriate material, the lens to be produced may be able to refract other electromagnetic waves, such as visible or infrared light,
the lens to be produced may have any desired relationship between the dielectric constant E or the refraction index respectively and the normalized radius r/r₀, e.g. in that way, that the focal point for parallel waves is inside or outside of the surface of the lens,
by using a material with an appropriate refraction index even lenses, which are able to refract any other waves, e.g. acoustic waves, may be produced,
instead of the pyramid-like shaped parts 10 or 27 respectively with a five-edge base, other pyramid-like shaped parts can be used, e.g. 36 of then with a three-edge base, to build a spherical lense,
for the construction of a needed lens, which is just a part of a sperical lens (hemispherical, quartersperical, or the like), a spherical lens may be cut, whereby only one assembling process is necessary for the production of two or more needed lenses, or other kinds of parts can be used, which collectively form a non-spherical lens.

This invention presents a lens with a given variable refraction index by bringing together several parts, which themselves have preferably a variable refraction index, and an according process for the production of said lens.
The realization of the desired refraction index of the parts can be achieved by compressing dielectric material of a first shape into its final shape or by mixing dielectric materials, which may be metal-coated.
Outer parts, which extend to the outer surface of the lens to be produced, have a pyramid-like shape with a pointed or truncated apex respectively. In the first case the apexes of the outer parts meet in the symmetry point of said lens; in the latter case a center part is used which is located around said symmetry point.
It is also possible that some of the outer parts used have pointed apexes and the rest of them has got truncated apexes. In this case a center part used is easy to fix near the symmetry point of the lens.
Compared with lenses, produced according to known processes, lenses produced by the process according to the invention have on one hand the advantage that the aperture efficiency is increased by avoiding reflections from the dielectric boundaries and by avoiding electromagnetic fields inside the lens caused to propagate tangentially to the shell surfaces causing surface waves to be set up.
Additionally the phase of collminated rays at feed points is more exact, also leading to an increased apperture efficiency.
On the other hand the parts that collectively form the lens can be produced easily.
If identical parts are used, the same process steps for their manufacturing can be used.
If dielectric material is taken, the lens to be produced can prefarably used as part of a microwave antenna system.

Claims

Lens (23) with a symmetry point (24) and a given variable refraction index (n(r)), said lens (23) having
- parts (10, 26, 27) with respective refraction indexes, shapes of said parts (10, 26, 27) collectively form the shape of said lens (23), where said parts (10, 26, 27) include

- outer parts (10, 27), which extend to the outer surface of the lens (23) and have a variable refraction index (n) and a pyramid-like shape,
characterized in that said outer parts have a five-edge (pentagonal) base or a three-edge (triangular) base.
Lens (23) according to claim 1, characterized in that the outer parts (10, 27) have a shape, which extends from the symmetry point (24) of said lens (23) to its (outer) surface.
Lens (23) according to claim 1, characterized in that a center part (26) is located around the symmetry point (24) and one or more of the outer parts (27) extend from the (outer) surface of the center part (26) to the (outer) surface of said lens (23).
Lens (23) according to one of the claims 1 to 3, characterized in that said parts (10, 26, 27) are formed of dielectric material and the desired refraction index (n) of one or more of said parts (10, 26, 27) is at least partially realized by a variation of density (d) of dielectric material.
Lens (23) according to claim 4, characterized in that the variation of density (d) is realized by compressing an uncompressed block of dielectric material with a first given shape (25) into its final shape (10, 27).
Lens (23) according to one of the claims 1 to 5, characterized in that it is a Luneburg type lens.
Lens (23) according to one of the claims 1 to 6, characterized in that it has a spherical shape and includes twelve outer parts (10) with five-edge (pentagonal) bases or thirtysix parts (10) with three-edge (triangular) bases.
Lens (23) having a hemi-spherical shape, a quarterspherical shape, or the like, characterized in that it is produced out of a lens according to one of the claims 1 to 7 by cutting, sawing, or the like.
Process for the production of a lens (23) having a symmetry point (24) and a given variable refraction index (n(r)), with the steps of
- manufacturing of parts (10, 26, 27) with respective refraction indexes and shapes in such a manner that said parts (10, 26, 27) collectively form the shape of said lens (23), where at least some of said parts (10, 26, 27) are outer parts (10, 27), which have a variable refraction index (n) and a pyramid-like shape and are assembled in such a manner that they extend to the outer surface of the lens (23),
characterized in that said outer parts (10, 27) are such produced that they have a five-edge (pentagonal) base or a three-edge (triangular) base.