WO2017017267A1

WO2017017267A1 - Device for emitting a light beam intended to produce an image, and corresponding display

Info

Publication number: WO2017017267A1
Application number: PCT/EP2016/068224
Authority: WO
Inventors: Paul DEBA
Original assignee: Valeo Comfort And Driving Assistance
Priority date: 2015-07-29
Filing date: 2016-07-29
Publication date: 2017-02-02
Also published as: FR3039658B1; FR3039658A1

Abstract

The invention relates to a device (2) for emitting a light beam (50) intended for producing an image, said device comprising at least one source (20, 20', 20") emitting a light beam (23, 23', 23"), and further comprising attenuation means (40) situated downstream of said source (20, 20', 20"), which means are designed to vary the optical power of the light beam (50) by modifying an orientation of a polarization of the light beam (50) in relation to a polarization element (42). According to the invention, the device further comprises an optical component (30) which is located upstream of the attenuation means (40), has a flat boundary (31) and is arranged such that the light beam (50) is refracted on said flat boundary (31) substantially at Brewster's angle of incidence. The invention also relates to a display (1) comprising an emission device (2) of this type.

Description

Device for emitting a light beam intended to form an image and associated display

TECHNICAL FIELD TO WHICH THE INVENTION RELATES The present invention relates to a device for emitting a light beam 5 intended to form an image, and a display device using such a device.

It relates more particularly to a device for emitting a light beam intended to form an image, comprising at least one source emitting a light beam, and attenuation means configured to vary the optical power of the light beam by modifying the light beam. an orientation of a polarization of the light beam with respect to a polarization element.

The invention applies particularly advantageously in a head-up display for a motor vehicle.

BACKGROUND

For the driver of a motor vehicle, it is particularly comfortable to be able to view additional information relating to the operation of the vehicle, the state of the traffic, or the like, without having to look away from the road facing the road. vehicle.

It is known for this purpose to equip a motor vehicle with a so-called head-up display device. Such a device uses a partial reflection on an element situated in front of the driver, for example the windshield of the vehicle, to project an image comprising the information to be displayed, so that it is superimposed visually on the environment facing the vehicle. .

In order for the information thus displayed to be sufficiently bright to be correctly displayed, without risk of dazzling the driver, it is necessary to adapt the brightness of the projected image to the brightness of the external environment of the vehicle, which varies strongly between day driving and night driving or in a tunnel.

Thus, it is desirable to be able to vary the brightness of such a projected image in a ratio of the order of 1000.

To control the brightness of such a projected image, it is known from FR 2 993 675 a device comprising a liquid crystal cell followed by a polarizing filter. This device comprises at least one light source producing a light beam, of partially rectilinear polarization, intended to form the image to be projected. The orientation of the polarization of this beam is modified by the liquid crystal cell, which causes, after crossing the polarizing filter, a change in the optical power of the light beam, in this case attenuation. The value of the corresponding attenuation coefficient can be adjusted by controlling the liquid crystal cell.

OBJECT OF THE INVENTION

In this context, the present invention proposes a device for emitting a light beam intended to form an image, as defined in the introduction, further comprising an optical component:

- located upstream of the attenuation means,

having a plane diopter separating a propagation medium from the light beam and a material in which said optical component is formed, the optical index of this material being larger than the optical index of said propagation medium, and

arranged so that said light beam is reflected on this plane dioptre substantially at the incidence of Brewster.

The polarization of said light beam is made more rectilinear by this reflection on the optical component, substantially at Brewster incidence, whereby the power of this beam can be modified, by said attenuation means, to advantageously large proportions.

Indeed, for such attenuation means, the ratio between the maximum power and the minimum power that can be imposed on the light beam is even greater than the light beam has a purely rectilinear polarization.

Thus, the power of the light beam intended to form an image, produced by this emission device, can be modified in a particularly large ratio, sufficient to adapt the brightness of the projected image to the large variations in brightness between a daytime driving. and night driving, and this even if the source or sources of this emission device initially produce a beam whose polarization is only partially straight. In particular, a source based on a laser diode, and which is therefore very bright and inexpensive, can thus advantageously be used in such a device.

Other non-limiting and advantageous features of the transmission device according to the invention are the following:

said light beam has a polarization substantially rectilinear upstream of the optical component;

the orientation of said substantially rectilinear polarization is substantially perpendicular to an incidence plane containing the principal ray of the incident light beam on the diopter of said optical component and a direction perpendicular to this diopter;

the transmission device comprises means for indexing the orientation of said substantially rectilinear polarization, and said optical component is arranged according to said indexing;

the beam emitted by said source is collimated;

said source comprises a laser diode;

the value of the optical index of the material in which said optical component is formed is greater than 2;

said optical component is made of silicon;

said optical component is a prism;

said optical component is a blade with substantially parallel faces;

the polarizing element has a direction of passing polarization, and transmits only a component of the polarization light beam oriented parallel to said direction of passing polarization;

said modification of an orientation of a polarization of the light beam is carried out by a liquid crystal cell;

the transmission device comprises at least one other source emitting another beam, said light beam intended to form an image being obtained by combining the beams emitted respectively by each of the sources of the transmission device;

said combination is performed upstream of said attenuation means;

said combination is performed upstream of said optical component. The invention also proposes a display, in particular a head-up display, comprising a device for emitting a light beam as described above, and an imaging system adapted to form an image from said light beam.

In this display, it is expected that an element of said imaging system may be placed upstream of said attenuation means.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT The following description with reference to the accompanying drawings, given as non-limiting examples, will make it clear what the invention consists of and how it can be achieved.

In the accompanying drawings:

FIG. 1 is a first schematic representation of a display according to the invention, comprising a device for emitting a light beam intended to form an image,

FIG. 2 is a second schematic representation of the display of FIG. 1, and

FIG. 3 schematically represents a reflection coefficient of the optical power of a light beam, reflected on a diopter, as a function of an angle of incidence of this beam on this diopter.

Figures 1 and 2 show schematically the main elements of a display 1, here a head-up display for a motor vehicle.

Such a display head 1 includes:

an emission device 2 designed to generate a light beam 50 adapted to form an image (as explained hereinafter), and

an imaging system 3 adapted to form an image from said light beam 50 and adapted to project this image into the field of vision of the driver of the vehicle.

The imaging system 3 comprises for example a scanning module, here a movable mirror 4, and a diffuser 7.

The light beam 50 generated by the emission device 2 is reflected here on the mobile mirror 4.

The orientation of this mobile mirror 4 is controllable, so that the light beam 50, once reflected by the movable mirror 4, sweeps the rear face of the diffuser 7.

The diffuser 7 thus emits on the front face another light beam representing an image to be displayed (this other beam having at each point a slight angular spread due to the action of the diffuser 7).

This image is projected into the field of view of the driver by means in particular of a reflecting mirror 5, and a semi-transparent plate 6 located between the windshield 8 of the vehicle and the driver.

In this case, the reflecting mirror 5 is arranged so as to send back light beam emitted by the diffuser 7 towards the semitransparent blade 6.

Alternatively, the reflecting mirror could return the light beam emitted by the diffuser directly on the windshield (in which case the semitransparent blade is omitted).

The emission device 2 of this light beam 50 comprises at least one source 20, 20 ', 20 "emitting a beam, here a beam of the laser type 23, 23', 23", said device 2 being configured here to form said light beam 50 for forming an image from said laser beam 23, 23 ', 23 ".

Here, the emission device 2 of this light beam 50 comprises three such sources 20, 20 ', 20 ", each of these sources emitting for example a laser beam 23, 23', 23" substantially monochromatic.

Preferably, the laser beams 23, 23 ', 23 "emitted respectively by the three aforementioned sources 20, 20', 20" have a color respectively corresponding to a red, a green, and a blue.

Each of said sources 20, 20 ', 20 "here comprises a laser diode 21, 21', 21" followed by a collimation device 22, 22 ', 22 ", such as a lens, for example an aspherical lens, or such as a multi-lens collimation lens.

This collimation device 22, 22 ', 22 "corrects the possible angular dispersion of the beam 24, 24', 24" produced by said laser diode 21, 21 ', 21 ", so that the laser beam 23, 23', 23" finally emitted by this source 20, 20 ', 20 "is substantially collimated and, otherwise formulated, this laser beam 23, 23', 23" is composed of light rays that are substantially parallel to each other.

Here, the residual angular dispersion of each of these laser beams

23, 23 ', 23 "is less than 4 degrees.

As a variant, this residual angular dispersion is, for example, less than 10 degrees.

According to another variant, each of said sources comprises, instead of a laser diode, a device of another type producing a beam of the laser type, substantially collimated.

According to yet another variant, each of said sources comprises a device, for example a light emitting diode, of a beam of another type than a laser beam. Each of said laser beams 23, 23 ', 23 "here has a partially rectilinear polarization.

More particularly, each of these beams has here an elliptical polarization.

In general, it is recalled that any laser beam or light beam can be decomposed into two components, each polarized rectilinearly, these two rectilinear polarizations being oriented perpendicular to one another.

One of these two components, called main, has a polarization (rectilinear) whose orientation, among the various possible orientations, is that for which the optical power of this main component is maximum.

The other of these two components, called residual, has a polarization (rectilinear) whose orientation, among the different possible orientations, is that for which the optical power of this residual component is minimal.

The polarization extinction rate of any laser beam or light beam corresponds to the ratio between:

the optical power of the principal component, polarized rectilinearly, of this beam,

- divided by the optical power of the residual component of this beam.

Here, the polarization extinction ratio of each of said laser beams 23, 23 ', 23 "has values greater than approximately 10, under current operating conditions of the emission device 2, it can in particular reach a value of the order of 10.

Alternatively, this polarization extinction rate may reach values of less than about 10.

Here, the main components of said laser beams 23, 23 ', 23 "have rectilinear polarizations oriented substantially parallel to each other.

Advantageously, the emission device 2 of said light beam 50 comprises indexing means (not shown) of the orientation common to these rectilinear polarizations. Here, these indexing means comprise for example, made on each of the laser diodes 20, 20 ', 20 ", at least one rib or flat.

Here, the laser beams 23, 23 ', 23 "emitted respectively by the three sources 20, 20', 20" are combined, for example using semi-reflecting elements 25, 25 ', 25 "such as mirrors. dichroic, intercepting these laser beams 23, 23 ', 23 "to combine them to form the light beam 50 for forming an image.

The light beam 50 obtained by combining these laser beams 23, 23 ', 23 "thus also has a partially rectilinear polarization, More precisely, the TEP polarization extinction ratio of this light beam 50 has here values greater than 10. approximately, it can in particular reach a value of the order of 10.

On the other hand, this light beam 50 is substantially collimated. More precisely, here it also exhibits a residual angular dispersion of less than 4 degrees.

Here, this light beam 50 is also polychromatic.

The optical power of this light beam can be controlled by attenuation means 40 of this emission device 2.

These attenuation means 40, located downstream of said sources 20, 20 ', 20 ", are configured to vary the optical power of said light beam 50, by changing a polarization orientation of this light beam 50 relative to to a polarization element 42.

This polarization element 42 here has a direction of passing polarization, and transmits only a component of the light beam 50 corresponding to a rectilinear polarization oriented parallel to said passing polarization direction.

The polarization element 42 is for example a polarizing film or a thin film polarizer.

Said modification of an orientation of a polarization of this light beam 50 is carried out here by means of a liquid crystal cell 41 traversed by this beam.

This cell 41 makes it possible to modify the orientation of the rectilinear polarization of the main component of this light beam 50, as a function of the value of a control voltage V. The orientation of the rectilinear polarization the residual component of this light beam 50 is also modified at the crossing of this cell.

Such a change in the orientation of the rectilinear polarization of the main component of this light beam 50 causes, after passing through the polarizing element 42, a change in the optical power of this beam.

The optical power of the light beam 50 at the output of the transmission device 2, that is to say here just after the polarization element 42, can thus be modified, via the value of the control voltage V, between :

a maximum power substantially equal to the optical power of the main component of the light beam 50 just before it passes through said attenuation means 40, and

a minimum power substantially equal to the optical power of the residual component of the light beam 50 just before it passes through said attenuation means 40.

The ratio between the maximum power and the minimum power thus imparted to the light beam 50 is therefore substantially equal to the polarization extinction rate TEP 'of this light beam just before it passes through said attenuation means 40.

In a variant, said attenuation means comprise only the polarization element, in this case a polarizing film, mounted to rotate about an axis perpendicular to the mean plane of this polarization element. In the context of this variant, the power of the light beam delivered by the emission device is controlled by a rotation of this polarization element about said axis.

According to another variant, said attenuation means comprise any device known to those skilled in the art, adapted to vary the optical power of said light beam by changing an orientation of a polarization of this light beam relative to the light beam. polarization element.

Remarkably, the emission device 2 of the light beam 50 intended to form an image comprises an optical component 30:

located upstream of the attenuation means 40,

having a diopter 31 plane separating a propagation medium 90 from the light beam 50 and a material 32 in which said component 30 is formed, the optical index Nm of this material 32 being larger than the optical index Np said propagation medium 90, and

arranged so that said light beam 50 is reflected on this dioptre 31 plane substantially at the incidence of Brewster.

This optical component 30 is here formed of silicon. As a variant, it is formed in another material, for example in a material having an optical index greater than 2 in the visible radiation range.

This optical component 30 is here a prism, one of whose faces constitutes said diopter 31 plane. In a variant, this optical component is a plate with substantially parallel faces, one of these faces then constituting said plane diopter.

The propagation medium 90 of the light beam 50 is constituted by air, and its optical index Np thus has a value equal to 1: Np = 1.

Here, the light beam 50, formed by combining the laser beams 23, 23 ', 23 "emitted by the three sources 20, 20', 20" of the emission device 2, is reflected by the diopter 31 of this optical component 30. towards the attenuation means 40.

On its path between these sources 20, 20 ', 20 "and the attenuation means 40, this light beam thus comprises an incident beam 60 on this dioptre 31, and a beam reflected 70 therefrom.

In FIG. 2, the incident 60 and reflected 70 beams are schematically represented by the corresponding principal ray (or "chief ray"), that is to say by the radius corresponding to the average direction of the this beam.

The main beam of this incident beam 60 and a direction 33 perpendicular to the diopter 31 are both contained in a plane, said plane of incidence.

For this incident beam 60 and this reflected beam 70, we define:

a so-called P-type polarization, corresponding to a rectilinear polarization oriented in a direction 61, 71 parallel to this plane of incidence, and

a so-called type S polarization, corresponding to a rectilinear polarization oriented in a direction 62, perpendicular to this plane of incidence.

Here, the optical component 30 is arranged in such a way that said plane of incidence, defined by the reflection of the light beam 50 on this optical component 30, is perpendicular to the direction of the rectilinear polarization of the main component of the light beam 50 incident on the diopter 31 of this component 30.

Preferably, the optical component 30 is arranged for this taking into account the aforementioned indexing means, which is recalled here that they identify the orientation of the linear polarization of the main components of said laser beams 23, 23 ', 23 ", and here, that the orientation of the rectilinear polarization of the main component of the light beam 50.

Thus, before thinking about the optical component 30:

the main component of the light beam 50, polarized rectilinearly, is oriented perpendicular to this plane of incidence: its polarization is of type S, while

the residual component of the light beam 50, polarized rectilinearly, is oriented parallel to this plane of incidence: its polarization is of type P.

The angle of incidence AGL, formed between:

the direction 33 perpendicular to the diopter 31 and

the main beam of the incident beam 60

is equal to the Brewster angle AGBR associated with this diopter 31:

AGL = AGBR (Fl)

it is recalled that it is determined by the following formula F2:

AGBR = arctan (Nm / Np) (F2)

Since this angle of incidence AGL is advantageously equal to the Brewster angle AGBR associated with this dioptre 31, the power reflection coefficient of the component of the incident beam 60 having a P-type polarization, in this case the component residual of this beam, is zero or at least particularly close to 0.

The polarization extinction rate TEP 'of the light beam 50 after reflection on this dioptre 31 is therefore higher than the PET before reflection.

The optical component 30 thus plays a role of linear polarization purifier.

Thus increasing the polarization extinction rate of the light beam 50 is particularly advantageous, since it increases by the same ratio between the maximum and minimum powers that can be obtained for the light beam 50 after passing through the attenuation means 40. The incident beam 60, which is reminded that it has a slight residual angular dispersion, comprises a plurality of light rays each having a direction that can deviate slightly from the direction of the main beam of this beam 60.

The value of the angle of incidence A1 on the diopter 31 thus has, on all these light rays, a slight dispersion around the Brewster angle.

This dispersion takes into account in particular the residual angular dispersion of the incident beam 60 and possible tolerances or misalignments of the optical component 30.

More particularly, the value of the angle of incidence A1 on the diopter 31 varies here, over all these light rays, by plus or minus 2 degrees around the Brewster angle AGBR.

Reflection coefficients RP, RS are plotted in FIG. 3, respectively in solid and dashed lines, as a function of angle of incidence A1 of such a light beam, expressed in degrees, for a reflection on the diopter 31, when the light beam 50 is substantially monochromatic, with an average wavelength λλ equal to 445 nm.

For such a radiation of wavelength λ1, the value of the optical index Nm of the material forming the optical component 30, here of silicon, is equal to 4.7; the value of the Brewster angle AGBR associated with this dioptre 31 is then equal to 78 degrees.

For such a light beam, the reflection coefficients RP and RS correspond, respectively for a P-type polarization and for an S-type polarization, to the ratio of the optical power after reflection, divided by the optical power before reflection.

In the example corresponding to FIG. 3, when this angle of incidence Al is equal to the Brewster angle AGBR, the reflection coefficient RP corresponding to a polarization of type P is zero, while that RS corresponding to a polarization type S is about 83%.

The power variations of the light beam 50, introduced by the reflection on the diopter 31, are described:

by a mean reflection coefficient RPM for a P-type polarization, equal to the average of the associated reflection coefficients RP respectively to each of the light rays that includes the incident beam 60, for a P-type polarization, and

by a mean reflection coefficient RSM for a type S polarization, equal to the average of the reflection coefficients RS respectively associated with each of the light rays constituting the incident beam 60, for a type S polarization.

The polarization purification obtained by means of the optical component 30 is described here by a purification factor FPUR equal to the ratio of this average reflection coefficient RSM for a type S polarization, divided by the average reflection coefficient RPM in average power for a polarization type P, for this same beam:

FPUR = RSM / RPM (F3)

Here, given the value of the dispersion of the angle of incidence A1 on all the light rays that the incident beam 60 comprises, the value of the purification factor FPUR is substantially equal to 350, for a substantially monochromatic light beam 50. , of average wavelength λλ equal to 445 nm.

Here, since the polarization of the main component of the incident beam 60 is an S-type polarization, the polarization extinction rate TEP 'of the light beam 50 after reflection on this dioptre 31 is equal to its polarization extinction rate PET. before reflection multiplied by the purification factor FPUR:

PET = FPUR. PET (F4)

When the light beam 50 is polychromatic, the optical component 30 is here arranged so that the value of the incidence angle AGL of the main beam of the incident beam 60 is equal to the value of the Brewster angle AGBR corresponding to the value the optical index Nm of said material 32 at the average wavelength ληι of the light beam 50.

For wavelengths whose values are between 445 nanometers and 650 nanometers, the value of the optical index Nm of the material 32, here silicon, varies respectively between 4.7 and 3.85 and, consequently, the corresponding Brewster's AGBR angle value varies between 78 degrees and 75 degrees respectively. This variation of the value of the Brewster angle AGBR with the wavelength is here reduced, so that the purification factor FPUR has a value which is advantageously high, as well for a component of the light beam 50 whose length of wave is equal to said average wavelength λιτι, that for another component of this light beam 50 having a different wavelength.

Furthermore, the optical index Nm of the material 32 forming the optical component 30 here has advantageously large values for a radiation of the visible range.

With this arrangement, the reflection coefficient RS for an S-type polarization has high values in the vicinity of the Brewster angle AGBR.

Thus, the optical component 30 purifies the polarization of the light beam 50, making it more rectilinear, without causing significant power loss for the main component of this beam 50.

For example, in the case of FIG. 3, the reflection coefficient RS for a type S bias is 83% at the Brewster angle AGBR, so that the power loss for the main component of the light beam 50 is only 17% during this reflection.

Such an optical component is moreover advantageously simple and robust. It supports without damaging high surface powers, especially surface powers which would deteriorate for example a polarized stretched polymer film.

On the other hand, its cost is competitive with other polarizing devices, especially when made of silicon.

Finally, a very high purification factor can be obtained by means of this device, even if its alignment has slight defects, of the order of degree, as has been illustrated above. The mounting of this component 30 is therefore simple and inexpensive.

In the embodiment shown schematically in Figures 1 and 2, it is recalled that the optical component 30 is located in the path of the light beam 50, after the combination of the laser beams 23, 23 ', 23 "produced by the three sources 20, 20 ', 20 ".

In a variant not shown, the device for transmitting a beam An image forming light source comprises three sources, and three optical components such as that described above.

In the context of this variant, the polarization of each of the laser beams, emitted by one of these sources, is purified by means of one of these optical components, placed at the output of the corresponding source, and this before combining these three laser beams to form said light beam.

In another variant not shown, the device for emitting a light beam intended to form an image comprises a source only.

According to yet another variant, not shown, the device for transmitting a light beam intended to form an image comprises one or more reflecting mirrors for modifying the direction of said light beam.

In the exemplary embodiment of a display shown diagrammatically in FIGS. 1 and 2, the imaging system 3 adapted to form an image from said light beam 50 is located downstream of the attenuation means 40.

Alternatively, at least part of the components of said imaging system is located upstream of the attenuation means.

In general, said attenuation means can be placed at any position along said light beam, after reflection of this light beam on the optical component.

Claims

1. Apparatus for transmitting (2) a light beam (50) for forming an image, said device comprising at least one source (20, 20 ', 20 ") emitting a light beam (23, 23', 23") , and comprising attenuation means (40), located downstream of said source (20, 20 ', 20 "), configured to vary the optical power of the light beam (50) by changing an orientation of a polarization of this light beam (50) with respect to a polarization element (42),

characterized in that said device further comprises an optical component (30):

located upstream of the attenuation means (40),

having a diopter plane separating a propagation medium from the light beam and a material in which said optical component is formed, the optical index (Nm) of this material being greater than the optical index (Np) of said propagation medium, and

- arranged so that said light beam (50) is reflected on this dioptre (31) plane substantially at the incidence of Brewster.

2. Device according to claim 1, wherein said light beam (50) has a substantially straight polarization upstream of the optical component (30).

3. Device according to claim 2, wherein the orientation of said substantially rectilinear polarization is substantially perpendicular to an incidence plane containing the main ray of the light beam (50) incident on the diopter (31) of said optical component (30). and a direction perpendicular (33) to this dioptre (31).

4. Device according to one of claims 2 and 3, comprising means for indexing the orientation of said substantially rectilinear polarization, and wherein said optical component (30) is arranged according to said indexing.

5. Device according to one of claims 1 to 4, wherein the beam

(23, 23 ', 23 ") emitted by said source (20, 20', 20") is collimated.

6. Device according to one of claims 1 to 5, wherein said source (20, 20 ', 20 ") comprises a laser diode.

7. Device according to one of claims 1 to 6, wherein the value of the optical index (Nm) of the material (31) in which said optical component (30) is formed is greater than 2.

8. Device according to one of claims 1 to 7, wherein said optical component (30) is formed of silicon.

9. Device according to one of claims 1 to 8, wherein said optical component (30) is a prism.

10. Device according to one of claims 1 to 9, wherein the polarizing element (42) has a direction of passing polarization, and transmits only a component of the polarization light beam (50) oriented parallel to said direction of polarization passing .

1 1. Device according to one of claims 1 to 10, wherein said modification of an orientation of a polarization of the light beam (50) is performed by a cell (41) liquid crystal.

12. Device according to one of claims 1 to 1 1, comprising at least one other source (20, 20 ', 20 ") emitting another beam (23, 23', 23"), said light beam (50) intended forming an image being obtained by combining the beams (23, 23 ', 23 ") emitted respectively by each of the sources (20, 20', 20") of the transmitting device (2).

The device of claim 12, wherein said combination is performed upstream of said attenuating means (40).

14. Device according to one of claims 12 or 13, wherein said combination is performed upstream of said optical component (30).

15. Display (1), in particular head-up display, comprising a device (2) for transmitting (2) a light beam (50) according to one of claims 1 to 14, and an imaging system (3) adapted to forming an image from said light beam (50).

The display of claim 15, wherein an element of said imaging system (3) is placed upstream of said attenuating means (40).