WO2014080018A1

WO2014080018A1 - Optical polarization modulator having liquid crystals

Info

Publication number: WO2014080018A1
Application number: PCT/EP2013/074645
Authority: WO
Inventors: Stephen Palmer
Original assignee: Volfoni R&D
Priority date: 2012-11-26
Filing date: 2013-11-25
Publication date: 2014-05-30
Also published as: FR2998682A1; FR2998682B1

Abstract

The invention relates to an optical polarization modulator comprising a cell having liquid crystals configured to selectively polarize a luminous flux, including: two transparent substrates facing each other; a layer of liquid crystals arranged between the two substrates, two electrodes (36) arranged between the two substrates and capable of applying an electrical field to the liquid crystals in order to switch their optical state. At least one of the two electrodes is shaped such as to have a plurality of strips (40) that are electrically insulated from one another over the length thereof, the length (L) of each one of said strips being greater than ten times the width (W) of said strip, each one of said strips being electrically connected by an end (44) to said other strips, and said end being electrically connected to an electrical energizing circuit.

Description

OPTICAL POLARIZATION MODULATOR WITH CRYSTALS

LIQUID

The invention relates to an optical polarization modulator. The invention also relates to a stereoscopic image display comprising this optical polarization modulator.

Optical polarization modulators, used in stereoscopic image display systems, are known to selectively polarize a luminous flux produced by an image projector.

US patent application 2006/0291053 A1 (ROBINSON ET AL.) Discloses such a polarization modulator. This polarization modulator comprises a liquid crystal cell. This liquid crystal cell includes:

two transparent substrates vis-à-vis;

a layer of liquid crystals disposed between the two substrates;

two electrodes, arranged between the two substrates, these electrodes being able to apply an electric field to the liquid crystals to switch their optical state, when a difference in electrical potential is applied between these two electrodes. Generally, each electrode has an electrically conductive thin layer deposited on the respective substrate.

Typically, during operation, this polarization modulator is placed immediately in front of an output lens of an image projector. Image projectors used for stereoscopic image projection, such as movie projectors, are generally configured to produce a luminous flux with high luminous power. Thus, the polarization modulator receiving this luminous flux undergoes a significant heating during its operation, because of its proximity to such a projector.

It is known that this heating tends to cause migration to the liquid crystal layer of impurities present inside the liquid crystal cell. These impurities are generally electrically conductive particles, such as ionic species from alignment elements of the liquid crystal layer. This migration is, furthermore, facilitated by the low switching frequency and the high value of the electric field used to modify the optical state of the liquid crystal layer. These values are generally chosen to ensure a satisfactory switching speed of the liquid crystal cell.

When these impurities are present inside the liquid crystal layer with a high concentration, there is a significant risk of occurrence of a leakage current between the two electrodes of the liquid crystal cell. This leakage current generally travels through structural defects incorporated in the liquid crystal cell. When this leakage current exceeds a threshold value, it can be formed, between the two electrodes, an electric arc that leads to irreversible destruction of the polarization modulator.

There is therefore a need for an optical polarization modulator comprising a liquid crystal cell having increased resistance against leakage electric currents that may occur between electrodes of this liquid crystal cell, without degrading optical switching performance. The invention thus relates to a stereoscopic image display device according to claim 1.

Embodiments of the invention may have one or more features of claims 2 to 12.

The use of an electrode shaped to present the strips makes it possible, in comparison with an electrode distributed uniformly over an entire face of the substrate, to increase the local resistance of the electrode to a leakage current, while maintaining a same external resistance value of the electrode. The increase of the local resistance makes it possible to limit the value of a leakage current flowing in the electrode, so as to limit the risk of forming an arc. The preservation of the external resistance makes it possible to maintain the dynamic switching properties of the liquid crystal cell. Thus, the risk of occurrence of arcing is reduced, while maintaining a high switching speed. Other characteristics and advantages of the invention will emerge clearly from the description which is given hereinafter, by way of indication and in no way limitative, with reference to the appended drawings, in which:

FIG. 1 schematically illustrates a stereoscopic image display, comprising an optical polarization modulator;

FIG. 2 schematically illustrates, in a front view, the optical polarization modulator of FIG. 1;

FIG. 3 schematically illustrates, in cross-section, a liquid crystal cell of the optical polarization modulator of FIG. 2;

FIG. 4 schematically illustrates an electrode of the liquid crystal cell of FIG. 3;

FIG. 5 schematically illustrates, in a perspective view, the spatial arrangement, in the liquid crystal cell of FIG. 3, of two electrodes identical to the electrode of FIG. 4;

FIG. 6 schematically illustrates another embodiment of the electrode of FIG. 4. The invention provides an optical polarization modulator comprising a liquid crystal cell of which at least one electrode is shaped so as to limit the risk of malfunction of the liquid crystal cell during operation of the modulator.

In the rest of this description, the features and functions well known to those skilled in the art are not described in detail.

FIG. 1 represents a set 2 for viewing stereoscopic images. This set 2 comprises a display 4 stereoscopic images and passive stereoscopic glasses 6.

The display 4 comprises:

an image projector 8;

a modulator 10 of optical polarization;

a control device 1 2;

-a screen 14.

The glasses 6 are configured to filter an optically polarized light flux in first and second optical polarizations.

The projector 8 is configured to produce a luminous flux 116 comprising a video sequence of stereoscopic images. The projector 8 is here a DLP projector (for Digital Light Processing in English). This stream 1 6 is here transmitted through an output lens 1 8 of the projector 8.

This video sequence comprises two sub-sequences of images temporally multiplexed alternately. Each of these two sub-sequences is configured to be observed by an eye, respectively, left and right, of an observer provided with glasses 6.

For this purpose, the modulator 10 is configured to selectively apply, to the two image subsequences, respectively, the first and second optical polarizations. To do this, the modulator 1 0 comprises a liquid crystal cell, configured to switch between two optically distinct states. Here, the modulator 1 0 is placed the objective 1 8, so as to selectively modify the optical polarization of the stream 1 6.

The device 12 is programmed to ensure the synchronization between the modulator 1 0 and the projector 8, so that each of the sub-sequences of images produced by the projector 8 is polarized by only one or the other first or second optical polarizations. This device 1 2 is here programmed to control the modulator 1 0 according to a synchronization signal received from the output interface of the projector 8.

The screen 14 is configured to display the images produced by the projector 8 while maintaining the optical polarization of these images. This screen 14 is thus configured to reflect the flow 1 6. For this purpose, this screen 14 is, for example, a screen coated with a metallic material, such as silver. FIG. 2 shows in greater detail an example of the modulator 10. This modulator 10 here comprises two liquid crystal cells 20 and 22. Each of these cells 20 and 22 is configured to switch, in response to a control signal, between two states. distinct optics:

a so-called active state, in which the cells are subjected to an electric field inducing, a predefined polarization direction, on the light passing through these cells, and

a so-called passive state in which the cells are not subjected to any electric field, inducing a different direction of polarization (generally at an angle of 90 °) than that of the active state.

These cells 20 and 22 are parallel to each other and aligned along a direction 24 of propagation of the flow 16. These cells 20 and 22 are here identical, also, for simplicity, only the cell 20 will be described in FIG. detail.

FIG. 3 represents in more detail the cell 20. This cell 20 comprises:

two transparent substrates 30, 32, essentially parallel to each other;

a layer of liquid crystals 34, disposed between the substrates 30 and

32;

two electrodes 36, 38, arranged between the substrates 30 and 32.

The substrates 30, 32 are for example made of glass. These substrates 30 and 32 delimit here the cell 20.

The layer 34 is able to change its optical state when this layer 34 is subjected to an electric field. In this example, it is this change in optical state that leads to the switching of the cell 20 between the two optical states. This layer 34 extends here between the substrates 30, 32, parallel to these substrates 30 and 32. The layer 34 is here a nematic liquid crystal layer.

The electrodes 36 and 38 are configured to apply an electric field to the layer 34 when an electric potential difference is applied between these two electrodes 36 and 38. For example, the electrodes 36 and 38 are configured to apply an electric field perpendicular to the plane in which the layer 34 extends, when electric polarizations are applied to these electrodes 36 and 38 by an electrical excitation circuit, such as a voltage source.

The electrodes 36 and 38 extend here inside the cell 20, on either side of the layer 34, parallel to the substrates 30 and 32. These electrodes 36 and 38 are here made of an electrically conductive material having transparency for visible light. For example, electrodes 36 and 38 comprise a thin layer of an alloy of indium tin oxide ("indium tin oxide" in the English language). In this description, a material is said to be electrically conductive if it has, at a temperature of 20 ° C., an electrical resistivity of less than or equal to 10 ^-4 Ω.

The cell 20 here advantageously comprises alignment layers

37 and 39. The layer 37 extends between the substrate 30 and the electrode 36. The layer

39 extends between the substrate 32 and the electrode 38.

Figure 4 shows in more detail the electrode 36. This electrode 36 is shaped to have a plurality of bands and, preferably, more than ten bands. Each band extends, in the plane of the electrode 36, between two ends opposite to each other. The length of each band is greater than ten times, or fifteen times, or twenty times the width of this band. These strips are electrically insulated from each other along their length. Each band is electrically connected to the other bands by one end of this band. This end is itself electrically connected to the electrical excitation circuit.

In this example, the strips are identical to each other, are periodically spaced from each other of the same spacing, and extend in directions parallel to each other. Also, for simplicity, only one band 40 will be described in detail.

Here, the electrode 36 has a shape that can be inscribed in a rectangular shape. The electrode 36 covers here more than 80% or more than 90% of the surface of the substrate 30 (the periphery of this substrate 30, here rectangular in shape, is shown in broken lines in Figure 4). The electrode 36 comprises here fourteen bands.

This strip 40 is here electrically connected to a rectilinear edge 42 of the electrode 36 by an end 44 (shown in dotted lines in FIG. 4) of the strip 40. The strip 40 here has a rectangular shape, and extends, in the plane of the electrode 36, in a direction perpendicular to the edge 42. The band 40 is electrically connected to the other bands of the electrode 36 only via this edge 42. This edge 42 is here electrically connected to the electrical excitation circuit. The band

40 is further electrically insulated from the contiguous strips along its longitudinal sides 46 and 48. The strip 40 has a width W greater than or equal to 500 μm or 1 mm and preferably greater than or equal to 2 mm. The length L of this band 40 is greater than or equal to ten times, or fifteen times, or twenty times this width W. Here, the length L is greater than or equal to 20mm or 50mm or 1 00mm. These dimensions are advantageously chosen so that the area of the strip 40 is less than ten times or twenty times or fifty times the total area of the electrode 36. The strip 40 is separated from the contiguous strips of the electrode 36 by a spacing of width E. This spacing E is here defined as being the smallest distance, measured in the plane of the electrode 36, separating a 46 or 48 side of the band 40 on a contiguous side of a band immediately adjacent to this band 40. Advantageously, this spacing E is greater than or equal to the wavelength of the radiation forming the stream 1 6, so as to limit the risk of occurrence of optical diffraction phenomena that could be caused by the arrangement of the parallel strips of the electrode 36. Here, the spacing E is greater than or equal to 2μιη or 5μιτι and, preferably, greater than or equal to 10μιτι.

This conformation of the electrode 36 makes it possible to limit the magnitude of a local leakage electric current flowing between the electrodes 36 and 38, and thus to reduce the risk of occurrence of an electric arc between these electrodes 36 and 38, without degrading the switching performance of the cell 20 between its two optical states.

Indeed, the circulation of such a leakage current between the electrodes is typically permitted by the undesired presence of an electrically conductive fault between these two electrodes. This defect is in direct electrical contact with the two electrodes. The leakage current thus circulates through this fault and in the two electrodes. In general, the conformation of the electrodes causes the region of the electrode with which this defect is in contact to have an electrical resistance of the same order of magnitude as the total resistance of the electrode. This is for example the case of electrodes of rectangular shape. However, these electrodes typically each have a low total electrical resistance, for example less than 100Ω. Due to this low resistance value, the leakage current can reach important values.

However, a low total electrode resistance value is desirable for the liquid crystal cells to exhibit satisfactory optical switching performance between their two optical states. On the contrary, a too high value of this total resistance tends to increase the time required to apply an electric field to the layer 34, which limits the switching frequency of the cell 20 and therefore degrades the optical performance of the modulator 10.

The spatial conformation of the electrode 36 allows both:

to obtain a low total resistance, because of the strips electrically connected in parallel, which makes it possible not to degrade the switching speed between the two optical states of the cell 20;

-to ensure that a fault that would be in electrical contact with the electrode 36 has a high probability of being in electrical contact only with a region of this electrode 36, the resistance through this region being high, which limits the intensity of a leakage current flowing through this defect. The electrical resistance is here called high if it is at least twenty times or fifty times greater than the total resistance of the electrode.

In this example, such a defect present in the layer 34 would be found:

-is in contact with a gap separating two bands of the electrode 36, so with a part of the cell 20 not being electrically conductive,

or with a band of the electrode 36. The dimensions of this band are then chosen so as to have a high electrical resistance for an electric current flowing in the plane of this band.

For example, in the case of a rectangular-shaped electrode 150mm long and 270mm wide, formed of a material having a sheet resistance ("sheet resistance" in English) R _D equal to 100Ω / α, the total electrical resistance of this electrode, for an electric current flowing in the direction of the length of this electrode, is equal to (1 50/270) * 1 00 = 55.5 Ω. The layer resistance of the material is here defined as being equal to the resistivity of the material divided by the thickness of this material, this thickness being uniform and lower than the other dimensions of the material.

In the case where the electrode 36 is formed of a material having a layer resistance R _D equal to 1 00Ω / α, the width W of the strips 40 is equal to 2 mm and the number and dimensions of the strips 40 are chosen so that the electrode 36 has the same area as the rectangular electrode of 1 50 mm long and 270 mm wide, then:

the band 40 has a resistance equal to L * R _D / W = (1 50/2) * 100 = 7.5 kΩ;

the total resistance of the electrode 36 is equal to (150/270) * 100 = 55.5 Ω.

Thus, the conformation of the electrode 36 makes it possible to maintain a low total resistance, while having a high local resistance to limit the value of the leakage current. The risk of damaging the cell 20 and therefore the modulator 10 are reduced, without degrading the operating performance of the modulator 1 0. FIG. 5 shows in more detail the arrangement of the electrodes 36 and

38 in the cell 20. For simplicity, the detail of the cell 20 is not illustrated and the distance between these two electrodes 36 and 38 is enlarged. The layer 34 (not shown in this figure) extends in the zone 50 located between the electrodes 36 and 38 (and defined by the dashed lines in FIG. 5). In this example, the electrode 38 is identical to the electrode 36. The electrodes 36 and 38 are arranged facing each other. The strips of the electrode 36 extend in the same direction as the strips of the electrode 38. The strips of the electrode 36 are substantially aligned opposite the strips of the electrode 38. The electrodes 36 and 38 are placed head to tail relative to the 'other. The borders of the electrodes 36 and 38 extend in directions parallel to each other.

With this arrangement of the two electrodes 36 and 38, the risk of occurrence of a leakage current between these two electrodes 36 and 38 can be reduced. FIG. 6 represents an electrode 60 adapted to be used in place of one and / or the other of the electrodes 36 and 38. This electrode 60 is identical to the electrode 36, except that it comprises a second edge 62. This edge 62 electrically connects the strips to each other on their respective end opposite the edge 42.

Many other embodiments are possible.

The projector 8 may be something other than a DLP projector. For example, the projector 8 is a liquid crystal projector (LCD).

The optical polarizations of the cells 20 and 22 are not necessarily linear. For example, these polarizations are circular.

The cells 20 and 22 are not necessarily identical. For example, the respective electrodes of these cells 20 and 22 may have different shapes.

The layer 34 may comprise a liquid crystal other than nematic. For example, layer 34 is a cholesteric liquid crystal.

Alternatively, the electrodes 36 and 38 are formed of another material. For example, these electrodes 36 and 38 comprise a thin layer of a transparent conductive oxide, such as tin-doped cadmium oxide or aluminum-doped zinc oxide. These electrodes 36 and 38 may not be formed of the same material.

The electrodes 36 and 38 may be different. For example, only the electrode 36 is shaped to present the strips 40. In this case, the electrode 38 does not have the strips and extends, for example, uniformly with a rectangular shape.

The electrodes 36 and 38 may be placed differently relative to one another within the cell 20. For example, the electrodes 36 and 38 may be placed in parallel planes, but not be aligned one by the other. relative to the other, i.e. the respective strips of these electrodes 36 and 38 extend in distinct directions.

The strips of the same electrode 36 are not necessarily identical to each other. These bands may not be parallel to each other. Alignment layers 37 and 39 may be omitted.

The strips 40 are not necessarily rectilinear, but may have different shapes, such as a curve, or a slot.

The ratio of areas between the band 40 and the electrode 36 may be different.

The strips of the electrode 36 may not be spaced apart by the same spacing value E. The spacing E may be equal to the wavelengths, or even less than these wavelengths if the bands of the electrode 36 are not parallel to each other.

The border 42 may have another shape. This border 42 can also be omitted. In this case, the strips of the electrode are separated from each other. These strips are then electrically connected to each other by means of wire links, each formed on one end of the respective strip, and / or by means of the electrical excitation circuit.

The edge 42 does not necessarily electrically connect together all the bands of the electrode 36. For example, the border 42 connects only two of these bands. The electrode 36 then comprises another edge, distinct from this edge 42, which electrically connects the other bands of the electrode 36 and also one of the bands to which the edge 42 is connected.

Claims

1. Display (4) of stereoscopic images, comprising:

an image projector (8) configured to produce a luminous flux comprising a video sequence of stereoscopic images, this video sequence including two temporally multiplexed image subsequences, each of these image sub-sequences being respectively intended to be viewed by an eye different from an observer equipped with passive stereoscopic glasses (6) configured to filter the luminous flux according to first and second optical polarizations;

an optical polarization modulator (10) configured to selectively apply the first and second optical polarizations, respectively, to both image sub-sequences;

an optical polarization optically reflective optically reflecting screen (14) configured to display the image sub-sequences produced by the image projector and optically polarized by the optical polarization modulator;

the optical polarization modulator (10), comprising a liquid crystal cell (20) configured to selectively polarize a light flux (16), said liquid crystal cell including:

- two substrates (30, 32) transparent vis-à-vis;

a layer (34) of liquid crystals disposed between the two substrates;

two electrodes (36, 38), arranged between the two substrates, these electrodes being able to apply an electric field to the liquid crystals to switch their optical state, when a difference of electric potential is applied between these two electrodes;

characterized in that at least one of the two electrodes is shaped to have a plurality of strips (40) electrically insulated from each other along their length, the length (L) of each of said strips being greater than ten times the width (W) of this band, each of said strips being electrically connected at one end (44) to the other of said strips, said end being electrically connected to an electrical driving circuit.

2. Display (4) according to claim 1, wherein said strips are parallel to each other.

A display (4) according to any one of the preceding claims, wherein each of said strips is further electrically connected by a second end to the other of said strips, said second end being electrically connected to the electrical drive circuit.

4. Display (4) according to any one of the preceding claims, wherein the total area of said electrode is at least equal to ten times the area of each of said strips.

5. Display (4) according to any one of the preceding claims, wherein the strips have the same geometry.

6. Display (4) according to any one of the preceding claims, wherein the width of the spacing (E) between two adjacent strips is greater than the wavelength of the radiation forming the luminous flux.

7. Display (4) according to claim 6, wherein the width of the spacing (E) between each of said bands is greater than or equal to 2μιη.

8. Display (4) according to any one of the preceding claims, wherein each of said strips has a length (L) greater than or equal to 20mm.

9. Display (4) according to any one of the preceding claims, wherein each of said strips has a width (W) greater than or equal to 500μιτι.

The display (4) according to any one of the preceding claims, wherein each of the two electrodes (36, 38) is shaped to present said plurality of strips.

1 1. A display (4) according to claim 10, wherein the strips of the two electrodes extend along directions parallel to each other.

12. Display (4) according to claim 1 1, wherein the respective strips of the two electrodes are superimposed in projection along a normal to one of said electrodes.