Lighting module for a motor vehicle

Abstract

Lighting module for a motor vehicle. Combination device (10, 100) of at least two light sources for a motor vehicle lighting module, characterized in that it comprises at least two light sources (11, 14; 31, 32) of substantially the same colors, which are single-mode or multi-mode laser diodes, in that said at least two light sources (11, 14; 31, 32) are capable of emitting a respective light beam (1, 4; 51, 52) of different polarization, and in that it comprises a polarization beam splitter (43) arranged so as to superimpose two light beams (1, 4; 51, 52) which would be emitted respectively by the at least two light sources (11, 14; 31, 32) to generate a single output lighting beam (5; 55). Figure for the abstract: Fig. 1

Description

Title of the invention: Lighting module for a motor vehicle

[0001] The invention relates to a lighting module and a lighting device for a motor vehicle. The invention also relates to a motor vehicle equipped with such a lighting device. Finally, the invention relates to a lighting method for a motor vehicle.

[0002] A motor vehicle lighting device, particularly for front and rear lights, is equipped with one or more lighting modules arranged in a housing closed by a glass cover so as to produce one or more lighting and / or signaling beams at the output of the lighting device. In simplified terms, a lighting module in the housing includes, in particular, a light source, for example, one or more light-emitting diodes (LEDs), which emit a light beam, an optical system for projecting the light beam comprising one or more lenses and, where applicable, an optical element, for example, a reflector, for directing the light rays from the light sources to form the output light beam of the optical system. Lighting devices are commonly used to illuminate the road at night.It is also useful to be able to use lighting devices during the day, for illumination purposes, or more often for signaling. However, existing lighting devices are insufficient for daytime use, particularly on sunny days. Often, light generated by a device designed for nighttime use is virtually invisible during the day.

[0003] The object of the invention is to provide a lighting and / or signaling solution suitable for daytime use, even on a sunny day.

[0004] To this end, the invention relates to a device for combining at least two light sources for a motor vehicle lighting module, characterized in that it comprises at least two light sources of substantially the same colors, which are single-mode or multi-mode laser diodes, in that said at least two light sources are capable of emitting a respective light beam of different polarization, and in that it comprises a polarization beam splitter arranged so as to superimpose two light beams which would be emitted respectively by the at least two light sources to generate a single output lighting beam.

[0005] The at least two light sources may be capable of directly emitting a respective light beam of different polarization or may comprise at at least one half-wave plate capable of changing the polarity of at least one light beam emitted by one of at least two light sources so as to obtain at least two light beams of different polarization.

[0006] The combining device may include at least three light sources of substantially the same colors, which are single-mode laser diodes, the first two light sources may be capable of emitting a beam of the same first polarity and of offset wavelength, may include at least one first dichroic mirror, and optionally a mirror, so as to be able to superimpose the two light beams from the first two light sources into a single light beam, which is oriented towards the polarization beam splitter so as to be able to be superimposed on a light beam from at least a third light source, of second polarity different from the first polarity.

[0007] The combining device may include at least four light sources of substantially the same colors, which are single-mode laser diodes, the two second light sources may be capable of emitting a beam of the same second polarity and of offset wavelength, may include at least one second dichroic mirror, and optionally a mirror, so as to be able to superimpose the two light beams from the two second light sources into a single light beam, which is oriented towards the polarization beam splitter so as to be able to be superimposed on a light beam from the first two light sources, of first polarity different from the second polarity.

[0008] The said wavelength(s) may be shifted by at least 10 nm, or even shifted by a wavelength between 10 and 50 nm, or even between 10 and 30 nm.

[0009] The combining device may include at least two light sources which are multimode laser diodes, and may include cylindrical lenses associated respectively with each of the at least two light sources capable of resizing the light beam emitted respectively by each light source to form an output lighting beam comprising the same dimension in two perpendicular directions, vertical and horizontal.

[0010] The combination device may include between 2 and 8 light sources which are single-mode laser diodes or may include between 2 and 4 light sources which are multi-mode laser diodes.

[0011] The invention also relates to a lighting module for a motor vehicle, characterized in that it comprises at least three combination devices as described above, respectively capable of emitting a red, green, and blue beam of light, and may include a device for superposition capable of superimposing three beams of different colour light emitted from the output of said at least three combining devices.

[0012] The automotive vehicle lighting module may include a scanning device to illuminate a wider area.

[0013] The invention also relates to a lighting device for a motor vehicle, in particular at the front and / or rear of a motor vehicle, characterized in that it comprises at least one lighting module as described above.

[0014] The invention also relates to a motor vehicle characterized in that it is equipped with a lighting module as described above or a lighting device as described above.

[0015] The invention also relates to a method of lighting for a motor vehicle using at least one lighting device, characterized in that it comprises the following steps:

[0016] - Generate at least a first beam of light by at least a first source luminous which is a single-mode or multi-mode laser diode, of first polarization,

[0017] - Generate at least a second light beam by at least a second a light source, of essentially the same color as the first light source, which is a single-mode or multi-mode laser diode, with a second polarization different from the first polarization,

[0018] - Superimpose said two light beams by means of a separator polarization beam.

[0019] The method for lighting a motor vehicle may include the following steps: - Generate at least three light beams from at least three light sources of substantially the same color, which are single-mode laser diodes, including at least two light beams with slightly different wavelengths, in particular by at least 10 nm, and including at least two light beams of different polarities, - Superimpose the said at least two light beams of slightly offset wavelengths by means of a dichroic mirror and optionally a mirror.

[0020] These objects, features and advantages of the present invention will be described in detail in the following description of a particular embodiment, given by way of non-limiting example, with reference to the accompanying figures, among which:

[0021] Fig. 1 schematically illustrates a device for combining several light sources for a lighting module according to a first embodiment of the invention.

[0022] Fig. 2 schematically illustrates a device for combining several light sources for a lighting module according to a second embodiment of the invention.

[0023] According to embodiments of the invention, a lighting module first comprises several light sources, each of which is a laser diode. Furthermore, the lighting modules comprise at least light sources of different colors, including at least red light sources, at least green light sources, and at least blue light sources, the beams of which are intended to be superimposed, at least partially, but preferably perfectly, in order to form a white light beam, according to a lighting approach designated by the acronym RGB (red-green-blue).

[0024] According to the first embodiment of the invention, the lighting module uses single-mode laser diode light sources. This choice makes it possible to form a small beam of light, which allows for precise overall illumination because it is based on small pixels, thus contributing to lighting accuracy. However, a single-mode laser diode emits a beam of light with insufficient power for good daytime visibility.

[0025] For this reason, the lighting module uses several light sources of the same color, so as to add up their respective powers and achieve a combined beam whose power is compatible with the objective of daytime use.

[0026] Figure 1 illustrates a schematic diagram of a combining device 10 of four light sources 11 to 14 of substantially the same given color according to this first embodiment. These light sources are four single-mode laser diodes. The function of this combining device 10 is to superimpose at the output the four light beams 1 to 4 respectively emitted by each of the four light sources 11 to 14, so as to form a single output illumination beam 5 of the given color whose power is substantially equal to the sum of the powers of each light source 11 to 14.

[0027] According to this embodiment, the four light sources 11 to 14 are chosen as follows: - The first two light sources 11 and 12 emit a p-polarized light beam, while the second two light sources 13 and 14 emit an s-polarized light beam; - On the other hand, the wavelengths of the first two light sources 11,12 are very slightly shifted, by a shift with no visible or little effect on the color, so that they emit a light beam of essentially the same color. For example, this shift may be 10 nm; Similarly, the wavelengths of the second two light sources 13 and 14 are very slightly offset, by an offset that has little or no visible effect on the color. According to one embodiment, the wavelengths of light sources 11 and 14 can be identical, as can the wavelengths of light sources 12 and 13.

[0028] In all cases, the wavelength shifts are offset by a wavelength of at least 10 nm, or even by a shift between 10 and 50 nm or between 10 and 30 nm. For example, for the color red, the wavelengths are advantageously chosen in the range from 620 to 650 nm. For the color green, the wavelengths are advantageously chosen in the range from 515 to 545 nm. For the color blue, the wavelengths are advantageously chosen in the range from 515 to 545 nm. As mentioned previously, this shift is chosen to be large enough to allow a different effect of the two light beams concerned on the same dichroic mirror, while being small enough that the two light beams visually retain the same color, or that their difference is not or only slightly perceptible to the naked eye in terms of color.Under these conditions, we will use the expression "approximately the same color" to refer to both two beams of light that are strictly identical or slightly different in wavelength, as explained above.

[0029] The light source combination device 10 according to this first embodiment further comprises two mirrors 21, 24, two dichroic mirrors 22, 23, and a polarized beam splitter 25. The operation of this light source combination device will now be explained.

[0030] A first light beam 1 emitted by the first light source 11 is reflected by a first mirror 21. The angle of incidence of the first light beam 1 on the mirror 21 is close to 45 degrees, so that the reflected beam has an angle of 90 degrees relative to the incident beam. This reflected beam is then directed towards a first dichroic mirror 22, with an angle of incidence close to 45 degrees. The second light source 12 emits a second light beam 2 which likewise reaches the first dichroic mirror 22 at the same point. The wavelengths of the two light beams 1 and 2 are shifted as explained previously, and the first dichroic mirror 22 is chosen so that the first light beam 1 is totally reflected while the second light beam 2 is totally transmitted.The arrangement is such that the two light beams 1, 2 reach the first dichroic mirror 22 at the same point, at an angle of incidence of approximately 45 degrees, so that the two light beams 1, 2 are finally superimposed at the exit of the first dichroic mirror 22. This first arrangement therefore allows the addition of the first and . second light beams 1, 2 into a single light beam 6. Note that this light beam 6 is p-polarized, like the two light beams 1, 2.

[0031] A similar arrangement allows the third light beam 3 and the fourth light beam 4 to be superimposed into a single light beam 7. As a note, this light beam 7 is s-polarized, like the two light beams 3, 4.

[0032] The two light beams 6, 7 then propagate towards the polarization beam splitter 25, which has a different effect on the two beams 6, 7 due to their different polarizations, in particular their perpendicular polarization. These two light beams 6, 7 reach the polarization beam splitter 25 at an angle of incidence forming a 90-degree angle between them, so that one of the light beams is totally transmitted without deviation while the other is totally reflected. The geometry of the assembly allows the two light beams 6, 7 to be superimposed at the output of the polarization beam splitter 25.

[0033] Finally, the light source combination device 10, comprising light sources 11, 12, 13, 14, generates an output light beam 5 formed by the superposition of the light beams 1, 2, 3, 4 emitted respectively by the four light sources 11, 12, 13, 14, all of the same color. As a result, this output light beam 5 offers the advantages of the precision of each individual light beam 1, 2, 3, 4, while also exhibiting a high power output, between 500 and 800 mW, suitable for daytime use. In an alternative embodiment, if multimode diodes are used instead of single-mode diodes, it is possible to achieve a power output of between 4 and 5 W.

[0034] According to this first embodiment, a lighting module comprises at least three combining devices 10 as described above, associated respectively with red, green, and blue light sources, the output beams 5 of which, according to these three colors, are intended to be superimposed, at least partially, but preferably perfectly, in order to form a high-power white light beam. Such a lighting module therefore further comprises a device for superimposing the three output beams 5 of different colors. Such a superimposing device may, for example, comprise a mirror and two dichroic mirrors, implementing a principle similar to that explained with the mirror 21 and the dichroic mirror 22.

[0035] Variations of the first embodiment can be envisioned. In particular, it is possible to use a different number of single-mode laser diodes for each primary color than the four shown in [Fig. 1]. Preferably, at least three single-mode laser diodes can be used: in such a three-diode variant, the combining device can be simplified and a dichroic mirror eliminated, for example by removing the light source 13 and the mirror dichroic 23 of [Fig. 1]. Alternatively, it is possible to use more than four light sources for a primary color, for example, 5, 6, 7, or 8 light sources, by adding an associated dichroic mirror for each additional light source and choosing shifted wavelengths to achieve superposition of all the light beams, according to the principle described above. Alternatively, only two single-mode laser diodes could be used, in which case the two dichroic mirrors can be omitted. Furthermore, mirrors 21 and 24 are optional, depending on the arrangement of the various light sources 11, 12, 13, and 14. The objective is to direct the light beams from these light sources, at least in pairs, onto the same dichroic mirror.

[0036] According to yet another embodiment, there may be a different number of light sources for each primary color. Indeed, a red laser diode is inherently limited in flux and exhibits a greater geometric extent and divergence than a blue or green laser diode. For this reason, an embodiment may include a larger number of red light sources than green or blue light sources. To take into account the specific characteristics of the three primary colors, an embodiment may distribute the total number of light sources according to the following ratio: 50% red, 30% green, and 20% blue.

[0037] A second embodiment consists of using multimode laser diodes for all or part of the light sources, and not only single-mode laser diodes as in the first embodiment. A multimode laser diode has the advantage of having a significantly higher illumination power than a single-mode laser diode. On the other hand, it has the disadvantage that its laser beam has a substantially rectangular shape with a first axis along which the beam is narrow and a second perpendicular axis along which the beam is wider. Indeed, one of the characteristics of a multimode laser diode is that it generates less coherent light than a single-mode laser diode, and is not homogeneous in all directions.

[0038] For this reason, when using a multimode laser diode, the combining device includes a focusing system whose function is to modify the aforementioned non-homogeneous geometry to obtain a homogeneous lighting beam, comprising the same dimension in the two aforementioned perpendicular axes.

[0039] Figure 2 thus represents a schematic diagram of a combining device 100 of two light sources 31, 32 of the same given color according to a second embodiment, which are two multimode laser diodes. The function of this combining device 100 is to superimpose the two light beams 51, 52 at the output. respectively emitted by each of the two light sources 31, 32, so as to form a single output lighting beam 55 of the given color, the power of which is substantially equal to the sum of the powers of each light source 31, 32.

[0040] According to this second embodiment, the first light source 31 emits a polarized s light beam, while the second light source 32 emits a polarized p light beam.

[0041] The light source combining device 100 according to this first embodiment further comprises three cylindrical lenses 41, 42, 45, and a polarized beam splitter 43. The operation of this light source combining device 100 will now be explained.

[0042] A first light beam 51 emitted by the first light source 31 is reflected by the polarization beam splitter 43, such that the reflected beam is at an angle of 90 degrees relative to the incident beam. The second light source 32 emits a second light beam 52, which likewise reaches the polarization beam splitter 43, so that it is totally transmitted. The geometry of the assembly allows the two light beams 51 and 52 to be superimposed at the output of the polarization beam splitter 43.

[0043] Furthermore, each light beam 51, 52 emanating from each respective light source 31, 32 passes through a first respective cylindrical lens 41, 42. These first cylindrical lenses 41, 42 have no effect on the light beams in a horizontal plane, but only in a vertical plane. Conversely, a third cylindrical lens 45 arranged downstream of the polarization beam splitter 43 straightens the output lighting beam 55, composed of the two light beams 51, 52, along the horizontal direction.

[0044] Finally, through this transformation due to the combined effect of the three cylindrical lenses 41, 42, 43, the resulting output illumination beam 55 has the same dimensions in all directions, or a chosen dimension in each direction. This ultimately makes it possible to obtain a spot projected onto the road comprising the same dimensions, or a chosen dimension, in the horizontal and vertical directions. The combining device 100 thus makes it possible to achieve such an objective using illumination from multimode laser diodes.

[0045] Finally, the light source combination device 100 31, 32 makes it possible to generate an output lighting beam 55 formed by the superposition of the light beams 51, 52 generated respectively by the two light sources 31, 32, all of the same color. As a result, this output lighting beam 55 has the advantage of high power, between 2000 and 3000 mW, suitable for daytime use.

[0046] This combination device 100 according to this second embodiment is advantageously used for red light sources 31, 32, due to the specific characteristics of red illumination explained above. Alternatively, a combination device 100 could comprise more than two multimode laser diode light sources, in particular between 2 and 4, or could even combine multimode and single-mode laser diodes.

[0047] Alternatively, the cylindrical lenses could be arranged differently. There could be two cylindrical lenses per light source upstream of the polarization beam splitter 43, or two cylindrical lenses downstream of the polarization beam splitter 43.

[0048] According to this second embodiment, a lighting module comprises at least three combining devices, associated respectively with red, green, and blue light sources, whose output beams of these three colors are intended to be superimposed, at least partially, but preferably perfectly, to form a high-power white light beam. Such a lighting module therefore further comprises a device for superimposing the three output beams of different colors. Such a superimposing device may, for example, comprise a mirror and two dichroic mirrors, implementing a principle similar to that explained with the mirror and the dichroic mirror.

[0049] As mentioned previously, a lighting module may advantageously include a combining device 100 according to [Fig. 2] for the color red, and two combining devices 10 according to [Fig. 1] for the colors green and blue. Alternatively, all other combinations of the two embodiments described above are conceivable.

[0050] In all embodiments, to obtain the aforementioned polarizations of the different light beams, a first approach consists of choosing laser diodes that intrinsically generate directly cross-polarizations s and p. Alternatively, a second approach consists of using laser diodes of the same polarization, and adding one or more half-wave plates in order to modify the polarization of certain emitted light beams.

[0051] In all cases, a white output lighting beam can then be generated by superimposing the output lighting beams of each combination device, respectively for at least the three aforementioned primary colors, to finally be emitted by a lighting device of a motor vehicle, for example at the front or rear of a motor vehicle, to fulfill any signaling and / or lighting function.

[0052] In all cases, a lighting module comprises at least three combining devices as described above, associated respectively with red, green, and blue light sources, whose output beams of these three colors are intended to be superimposed, at least partially, but preferably perfectly, to form a high-power white light beam. Such a lighting module therefore further comprises a device for superimposing the three output beams of different colors. Such a superimposing device may, for example, include a mirror and optionally two dichroic mirrors, implementing a principle similar to that explained with mirror 21 and dichroic mirror 22.Furthermore, the combination devices are designed so that their respective output lighting beams have the same beam geometry for each color, to allow for proper overlap.

[0053] Furthermore, to illuminate a wide area with the output beam of a lighting module according to the invention, a scanning device can be used. This scanning device can, for example, be based on MEMS (Micro-Electro-Mechanical-Systems) elements, comprising one or more micromirrors that reflect the output beam. These micromirrors are, for example, rotated at least once around a first axis, which generates the scanning in a first direction, and advantageously rotated about at least once around a second axis, in particular perpendicular to the first axis, to produce a scanning in two directions. This approach makes it possible to enlarge the output beam, which can then be used by any other optical means of a lighting device to finally be projected out of the lighting device.

[0054] Furthermore, the invention also relates to a lighting module, which includes at least one combining device as described above. It also relates to a motor vehicle lighting device that includes at least one such lighting module and / or at least one light source combining device according to the invention. It also relates to a motor vehicle, which includes at least one lighting module and / or at least one light source combining device as described above.

[0055] Finally, the invention also relates to a method of lighting for a motor vehicle using at least one lighting device, characterized in that it comprises the following steps:

[0056] - Generate at least one first light beam 6, 51 by at least one first a light source, particularly of red color, which is a single-mode or multi-mode laser diode, of first polarization

[0057] - Generate at least a second light beam 7, 52 by at least a second a light source, particularly of red color, which is a single-mode or multi-mode laser diode, with a second polarization different from the first polarization,

[0058] - superimpose said two light beams 6, 51; 7, 52 by means of a polarization beam splitter 43.

[0059] According to one variant of the method, the method may comprise the following steps: - Generating at least three light beams 1, 2, 4 of substantially the same color by at least three single-mode laser diodes, of which at least two light beams have slightly different wavelengths, in particular of at least 10 nm, - Superimpose the said at least two light beams 1, 2 of wavelengths slightly offset by an optional mirror 21 and at least one dichroic mirror 22.

[0060] According to one variant of the method, the method may include the following step:

[0061] - Focusing each of the at least two light beams 51, 52 emitted by Two multimode laser diodes are connected by two cylindrical lenses 41 and 45 to obtain at least one output illumination beam 55 of substantially the same size or of a size chosen along the horizontal and vertical axes. Furthermore, the at least one output illumination beam 55 has substantially the same divergence along the horizontal and vertical axes.

[0062] The lighting method may further comprise the following steps:

[0063] - Superimpose three output light beams of different colors to obtain a beam of white light,

[0064] - Optionally, increase the area covered by the light beam white by a scanning device.

Claims

1.

2.

3. Demands Combination device (10, 100) of at least two light sources for motor vehicle lighting module, characterized in that it comprises at least two light sources (11, 14; 31, 32) of substantially the same colors, which are single-mode or multi-mode laser diodes, in that said at least two light sources (11, 14; 31, 32) are capable of emitting a respective light beam (1, 4; 51, 52) of different polarization, and in that it comprises a polarization beam splitter (43) arranged so as to superimpose two light beams (1, 4; 51, 52) which would be emitted respectively by the at least two light sources (11, 14; 31, 32) to generate a single output lighting beam (5; 55). Combination device (10, 100) according to the preceding claim, characterized in that the at least two light sources (11, 14; 31, 32) are capable of directly emitting a respective light beam (1, 4; 51, 52) of different polarization or in that it comprises at least one half-wave plate capable of modifying the polarization of at least one light beam (1, 31) emitted by one of the at least two light sources (11, 14; 31, 32) so as to obtain at least two light beams (1, 4; 51, 52) of different polarization. Combination device (10, 100) according to any one of the preceding claims, characterized in that it comprises at least three light sources (11, 12, 14) of substantially the same colors, which are single-mode laser diodes, in that the first two light sources (11, 12) are capable of emitting a beam of the same first polarity and of offset wavelength, in that it comprises at least one first dichroic mirror (22), and optionally a mirror (21), so as to be able to superimpose the two light beams (1,2) from the first two light sources (11, 12) into a single light beam (6), which is oriented towards the polarization beam splitter (43) so as to be able to be superimposed on a light beam (7) from at least a third light source (14), of second polarity different from the first polarity.

4. Combining device (10, 100) according to the preceding claim, characterized in that it comprises at least four light sources (11, 12, 13, 14) of substantially the same colors, which are single-mode laser diodes, in that the two second light sources (13, 14) are capable of emitting a beam of the same second polarity and of offset wavelength, in that it comprises at least one second dichroic mirror (23), and optionally a mirror (24), so as to be able to superimpose the two light beams (3, 4) from the two second light sources (13, 14) into a single light beam (7), which is oriented towards the polarization beam splitter (43) so as to be able to be superimposed on a light beam (6) from the first two light sources (11, 12), of first polarity different from the second polarity.

5. Combining device (10, 100) according to claim 3 or 4, characterized in that said shifted wavelength(s) are shifted by at least 10 nm, or even are shifted by a wavelength between 10 and 50 nm, or even between 10 and 30 nm.

6. Combining device (100) according to claim 1 or 2, characterized in that it comprises at least two light sources (31, 32) which are multimode laser diodes, and in that it comprises cylindrical lenses (41, 42, 45) associated respectively with each of the at least two light sources (31, 32) capable of resizing the light beam emitted respectively by each light source (31, 32) to form an output lighting beam (55) comprising the same dimension in two perpendicular directions, vertical and horizontal.

7. Combination device (10, 100) according to any one of the preceding claims, characterized in that it comprises between 2 and 8 light sources (11, 12, 13, 14) which are single-mode laser diodes or in that it comprises between 2 and 4 light sources (31, 32) which are multi-mode laser diodes.

8. A lighting module (1) for a motor vehicle, characterized in that it comprises at least three combining devices (10, 100) according to any one of the preceding claims, respectively capable of emitting a red, green, and blue light beam, and in that it comprises a superposition device capable of superimposing three light beams of different colour emitted at the output of said at least three combination devices (10, 100).

9. Lighting module (1) for motor vehicle according to the preceding claim, characterized in that it comprises a scanning device for illuminating an enlarged area.

10. Lighting device for a motor vehicle, in particular at the front and / or rear of a motor vehicle, characterized in that it comprises at least one lighting module according to one of claims 8 or 9.

11. Motor vehicle characterized in that it is equipped with a lighting module according to one of claims 8 or 9 or with a lighting device according to the preceding claim.

12. A method for lighting a motor vehicle from at least one lighting device, characterized in that it comprises the following steps: - Generating at least one first light beam (6, 51) by at least one first light source (11, 31) which is a single-mode or multi-mode laser diode, of first polarization, - Generating at least one second light beam (7, 52) by at least one second light source (14, 32), of substantially the same color as the first light source (11, 31), which is a single-mode or multi-mode laser diode, of second polarization different from the first polarization, - Superimposing said two light beams (6, 51; 7, 52) by means of a polarization beam splitter (43).

13. A motor vehicle lighting method according to the preceding claim, characterized in that it comprises the following steps: - Generating at least three light beams (1, 2, 4) by at least three light sources (11, 12, 14) of substantially the same color, which are single-mode laser diodes, of which at least two light beams (11, 12) have slightly different wavelengths, in particular of at least 10 nm, and of which at least two light beams (11, 14) have different polarities, - Superimposing said at least two light beams (11, 12) of slightly different wavelengths via a dichroic mirror (22) and optionally a mirror (21).

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