EP1690130A1

EP1690130A1 - Display device with a white-optimizing bistable nematic screen and method for the definition of said device

Info

Publication number: EP1690130A1
Application number: EP04805550A
Authority: EP
Inventors: Cécile Joubert; Daniel Stoenescu; Alexandre Carton;; Patrice Davi
Original assignee: Nemoptic SA
Current assignee: France Brevets SAS
Priority date: 2003-11-28
Filing date: 2004-11-25
Publication date: 2006-08-16
Also published as: CN100541303C; TW200527039A; US7532275B2; TWI406028B; JP2007512561A; CN1997934A; FR2863061A1; US20070103619A1; KR101265335B1; KR20060115906A; FR2863061B1; JP5230859B2; WO2005054940A1

Abstract

The invention relates to a nematic liquid crystal display having two stable states, without an electric field, obtained by anchoring break, characterized in that it comprises two polarizers (10, 40), one(10) of which is placed on the observer side, the other (40) being placed on the opposite side of the liquid crystal cell whereby the latter is at least partially reflecting in order to define a reflective mode or transflective mode. The orientation of the two polarizers (10, 40) is offset by a value which is equal to the rotatory power of the cell at approximately +/- 30°.

Description

FIELD OF THE INVENTION The present invention relates to the field of liquid crystal display devices and more precisely to the optical configuration of a nematic display. bistable operating in an optical mode which optimizes the white state of this display. The present invention applies in particular to displays of the reflective type. However, it is not strictly limited to this application. The invention indeed also relates to transflective type displays. OBJECT OF THE INVENTION A general aim of the present invention is to improve the white state of a bistable display device, that is to say more precisely to obtain a reflectance of the white state of very good quality in terms of luminance and colorimetry across the whole viewing angle. STATE OF THE ART Bistable liquid crystal display switching between two textures differing by a twist of 180 °

The type of bistable liquid crystal display considered in this invention is a display that switches between two textures, stable with no electric field applied (hence its bistability), different from each other by an angle of π. For one of the textures, the angle φu between the directors of the liquid crystal molecules on the two surfaces of the cell is of the order of 0 to ± 30 °. The molecules remain almost parallel to each other, and we will call this texture U. The second texture T has a twist angle φ _τ = φu ± π • The molecules in the texture T rotate around 180 ° (± 30 °) between the two cell surfaces. To date, two displays using this principle have been described.

The nematic liquid crystal is chiralised so as to have a spontaneous pitch p ₀ close to four times the thickness d of the cell, for equalize the energies of the two textures. The ratio between the thickness d of the cell and the spontaneous step p ₀ , ie d / p ₀ , is therefore approximately equal to 0.25 +/- 0.1, preferably 0.25 ± 0.05. Without an electric field, these are the minimum energy states. Document [1] describes a display which switches between the two textures U and T by applying an electric field pulse of precise shape. This display is based on a break in the anchoring of the liquid crystal molecule on one of the so-called zenith alignment surfaces (documents [2] and [3]), that is to say that the molecule is lifted by the field electric before falling back on one side or the other, thus allowing the obtaining of the two textures U and T. In this case, the structure of the electrodes necessary for the application of the field is standard, identical to that used for the TN or STN type liquid crystal displays. This display is generally called BiNem ^® . ^"

Document [4] describes a display which also uses an anchoring break and a particular type of electrode (called "comb shaped electrodes"), making it possible to obtain a component of the lateral electric field, ie parallel to the substrate. The switching between the two textures is carried out in this case by an effect qualified by the author as a break in the azimuth anchoring (document [5]).

The switching method is not essential for the present invention. Indeed whatever the switching mode (zenith or azimuthal anchoring break), the textures of the liquid crystal molecules are the same, the switching taking place between two twisted textures, one slightly twisted at an angle φu, called U, and the other strongly twisted at an angle φ _τ = φu ± π called T. And the optical behavior of the display depends only on the textures of the liquid crystal molecules. Optical modes of such displays

The optical mode requiring white optimization is mainly the reflective mode, which uses ambient light as light source. In this case, what is sought is to obtain a screen which most resembles black ink on white paper. There are two types of reflective mode: a mode that uses two polarizers and a mirror. This mirror is generally diffusing. One of the polarizers is placed on the side of the observer. It is transmissive. The second polarizer and the mirror are instead placed on the back of the display, opposite the observer. The rear polarizer and the mirror can be confused. Usually the mirror (and the rear polarizer) are outside the cell, which creates an additional image shifted by twice the distance between the mirror and the cell. To remove this image, it would be necessary to integrate inside the cell the rear polarizer and the mirror. However, integration of the polarizer is not possible on an industrial scale to date. This mode uses a cell delay of the same order of magnitude as a display operating in a transmitted mode, between λ / 3 and λ / 2. a mode using a single polarizer (transmissive) on the observer side and a mirror placed on the back of the cell. The advantage of this mode is that one can integrate the mirror inside the cell. In this case the two images coincide, there is no longer a spurious image.

The reflective mode studied in detail in the literature is the mode operating with a polarizer and a mirror. For example, documents [4], [6] and [7] calculate optimal modes for a linear input polarizer. Document [8] studies this reflective mode using a circular type input polarizer.

This mode, when the mirror is integrated in the cell, involves a complication of the manufacturing process because it is necessary to integrate in the cell a reflecting and diffusing function. In addition, the delay value Δnd giving the best optical results is of the order of λ / 4, λ representing a wavelength of the visible spectrum, which is approximately two times lower than the delay value adapted in the case. a mode with two polarizers. A reduction of a factor two of the delay can in theory be obtained by decreasing either the value of the Δn of the crystal liquid is the thickness of the cell. The thickness recommended for a bistable display as described above is however already small, of the order of 2 microns, compared to that of a standard liquid crystal display of TN or STN type (around 4 to 5 microns) and halving this value would lead to serious complications in the cell manufacturing process. A reduction in the optical anisotropy of the liquid crystal mixture while retaining the electrical anisotropy necessary for breaking the zenithal anchoring is also difficult to achieve. For these reasons, the objective of the present invention is to optimize a reflective mode with two polarizers, making it possible to use commercial components and not entailing any substantial modification of the manufacturing process and of the optical delay Δnd, compared with a cell operating in transmissive mode. In addition, recent polarizer technologies (document [9]) make it possible to produce polarizers inside the cell (of the transmissive or reflective type), which would make it possible to eliminate the parallax effect of this mode, at the cost of course a modification of the cell manufacturing technology. Document [10] describes the main lines of a reflective mode with two polarizers without specifying the exact configuration and the method of calculation of such a configuration. BASIS OF THE INVENTION The above object is achieved within the framework of the present invention by means of a nematic liquid crystal display device having two stable states, without electric field, obtained by anchoring breakage, characterized in that it has two polarizers, one placed on the observer side, the other on the opposite side of the liquid crystal cell and the latter being at least partially reflective to define a reflective or transflective mode, the orientation of the two polarizers being offset by '' a value equal to the rotational power of the cell to within +/- 30 °. According to other advantageous features of the invention: . the two stable states correspond to two textures of liquid crystal molecules whose torsion differs from 150 ° to 180 ° in absolute value. . the orientation of the polarizer placed opposite the observer, relative to the nematic director on the associated face of the cell, being included in the range comprising the range + or - (10 ° to 80 °) while the orientation of the polarizer placed on the side of the observer, relative to the same nematic director reference, is included in the range comprising the range of + or - (10 ° to 80 °) for an optical delay Δnd of the order of 250+ 70nm. . for an optical delay Δnd = 250 +/- 70 nm, in the case of a levogyre liquid crystal, the orientation of the polarizer placed opposite the observer, relative to the nematic director on the associated face of the cell , is in the range [-75 °; -30 °] U [10 °; 65 °], while the orientation of the polarizer placed on the side of the observer, relative to the same reference of nematic director, is included in the range [-60 °; -10 °] U [30 °; 80 °], and in the case of a dextrorotatory liquid crystal, the orientation of the polarizer placed opposite the observer, relative to the nematic director on the associated face of the cell, is in the range [-65 °; -10 °] U [30 °; 75 °], while the orientation of the polarizer placed on the side of the observer, with respect to the same nematic director reference, is included in the range [-80 °; -30 °] U [10 °; 60 °],

. for an optical delay Δnd = 280 +/- 30 nm, in the case of a levogyre liquid crystal and for brushing directions making between them between 15 ° and 30 °, the orientation of the polarizer placed opposite to the the observer, relative to the nematic director on the associated face of the cell, is included in the range [-65 °; -30 °] U [25 °; 65 °] preferentially [-50 °; -40 °] U [40 °; 50 °], while the orientation of the polarizer placed on the side of the observer, with respect to the same nematic director reference, is included in the range [-40 °; - 10 °] U [50 °; 80 °] preferably [-25 °; -10 °] U [64 °; 80 °], and in the case of a dextrorotatory liquid crystal, for brushing directions making between them between -15 ° and -30 °, the orientation of the polarizer placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-65 °; -25 °] U [30 °; 65 °] preferentially [-50 °; -40 °] U [40 °; 50 °], while the orientation of the polarizer placed on the side of the observer, relative to the same reference of nematic director, is included in the range [-80 °; -50 °] U [10 °; 40 °] preferably [-80 °; -64 °] U [10 °; 25 °]

. for an optical delay Δnd = 220 +/- 30 nm, in the case of a levogyre liquid crystal and for brushing directions making between them between 10 ° and 20 °, the orientation of the polarizer placed opposite the the observer, relative to the nematic director on the associated face of the cell, is included in the range [-60 °; -30 °] U [25 °; 65 °] preferentially [-50 °; -40 °] U [40 °; 50 °], while the orientation of the polarizer placed on the side of the observer, with respect to the same nematic director reference, is included in the range [-40 °; - 20 °] U [50 °; 70 °] preferably [-35 °; -25 °] U [55 °; 65 °], and in the case of a dextrorotatory liquid crystal, for brushing directions making between them between -10 ° and -20 °, the orientation of the polarizer placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-65 °; -25 °] U [30 °; 60 °] preferably [-50 °; -40 °] U [40 °; 50 °], while the orientation of the polarizer placed on the side of the observer, with respect to the same nematic director reference, is included in the range [-70 °; -50 °] U [20 °; 40 °] preferably [-65 °; -55 °] U [25 °; 35 °]

. for an optical delay Δnd = 280 +/- 30 nm, in the case of a levogyre liquid crystal and for brushing directions making between them between 0 ° and +/- 5 °, the orientation of the polarizer placed at the opposite of the observer, relative to the nematic director on the associated face of the cell, is included in the range [-75 °; -35 °] U [15 °; 55 °] preferably [-60 °; -50 °] U [30 °; 40 °], while the orientation of the polarizer placed on the side of the observer, with respect to the same reference of nematic director, is included in the range [-50 °; - 20 °] U [40 °; 70 °] preferably [-40 °; -30 °] U [50 °; 60 °], and in the case of a dextrorotatory liquid crystal, for brushing directions making between them between 0 ° and +/- 5 °, the orientation of the polarizer placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-55 °; - 15 °] U [35 °; 75 °] preferentially [-40 °; -30 °] U [50 °; 60 °], while the orientation of the polarizer placed on the side of the observer, relative to the same reference of nematic director, is included in the range [-70 °; -40 °] U [20 °; 50 °] preferably [-60 °; -50 °] U [30 °; 40 °]

. for an optical delay Δnd = 220 +/- 30 nm, in the case of a levogyre liquid crystal and for brushing directions making between them between 0 ° and +/- 5 °, the orientation of the polarizer placed at the opposite of the observer, relative to the nematic director on the associated face of the cell, is included in the range [-65 °; -40 °] U [20 °; 60 °] preferably [-55 °; -45 °] U [35 °; 45 °], while the orientation of the polarizer placed on the side of the observer, relative to the same reference of nematic director, is included in the range [-60 °; - 20 °] U [30 °; 70 °] preferably [-45 °; -35 °] U [45 °; 55 °], and in the case of a dextrorotatory liquid crystal, for brushing directions making between them between 0 ° and +/- 5 °, the orientation of the polarizer placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-60 °; - 20 °] U [40 °; 65 °] preferentially [-45 °; -35 °] U [45 °; 55 °], while the orientation of the polarizer placed on the side of the observer, with respect to the same nematic director reference, is included in the range [-70 °; -30 °] U [20 °; 60 °] preferably [-55 °; -45 °] U [35 °; 45 °],. for an optical delay Δnd = 220 +/- 30 nm, in the case of a levogyre liquid crystal and for brushing directions making between them between 0 ° and +/- 5 °, the orientation of the polarizer placed at the opposite of the observer, relative to the nematic director on the associated face of the cell, is included in the range [-80 °; -50 °] U [10 °; 60 °], while that the orientation of the polarizer placed on the side of the observer, with respect to the same nematic director reference, is included in the range [-50 °; -25 °] U [45 °; 65 °], and in the case of a dextrorotatory liquid crystal, for brushing directions making between them between 0 ° and +/- 5 °, the orientation of the polarizer placed at l the opposite of the observer, relative to the nematic director on the associated face of the cell, is included in the range [-60 °; -10 °] U [50 °; 80 °], while the orientation of the polarizer placed on the side of the observer, with respect to the same nematic director reference, is included in the range [-65 °; -45 °] U [25 °; 50 °],

. for an optical delay Δnd = 280 +/- 30 nm, in the case of a levogyre liquid crystal and for brushing directions making between them between 5 ° and 30 °, taking into account an elastic stall between 1 ° and 12 °, the orientation of the polarizer placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-65 °; -45 °] U [25 °; 55 °], while the orientation of the polarizer placed on the side of the observer, with respect to the same nematic director reference, is included in the range [-40 °; -10 °] U [50 °; 80 °], and in the case of a dextrorotatory liquid crystal, for brushing directions between them between -5 ° and 30 °, taking into account an elastic stall between 1 ° and 12 °, the orientation of the polarizer placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-55 °; -25 °] U [45 °; 65 °], while the orientation of the polarizer placed on the side of the observer, with respect to the same nematic director reference, is included in the range [-80 °; -50 °] U [10 °; 40 °],. for an optical delay Δnd = 220 +/- 30 nm, in the case of a levogyre liquid crystal and for brushing directions making between them between 5 ° and 15 °, taking into account an elastic stall between 1 ° and 12 °, the orientation of the polarizer placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-65 °; -40 °] U [25 °; 50 °] preferably [- 60 °; -45 °] U [30 °; 45 °], while the orientation of the polarizer placed from side of the observer, relative to the same nematic director reference, is in the range [-50 °; -20 °] U [40 °; 65 °] preferably [-40 °; -30 °] U [48 °; 60 °], and in the case of a dextrorotatory liquid crystal, for brushing directions between them between -5 ° and -15 °, taking into account an elastic stall between 1 ° and 12 °, the orientation of the polarizer placed opposite the observer, with respect to the nematic director on the associated face of the cell, is included in the range [-50 °; -25 °] U [40 °; 65 °] preferentially [- 45 °; -30 °] U [45 °; 60 °], while the orientation of the polarizer placed on the side of the observer, relative to the same reference of nematic director, is included in the range [-65 °; -40 °] U [20 °; 50 °] preferably [-60 °; -48 °] U [30 °; 40 °]

. for an optical delay Δnd = 250 +/- 70 nm, in the case of a levogyre liquid crystal and for brushing directions making between them between 0 ° and +/- 5 °, taking into account an elastic stall between 1 ° and 12 °, the orientation of the polarizer placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-70 °; -40 °] U [20 °; 55 °], while the orientation of the polarizer placed on the side of the observer, relative to the same reference of nematic director, is included in the range [-60 °; -25 °] U [30 °; 65 °], and in the case of a dextrorotatory liquid crystal, for brushing directions between them between 0 ° and +/- 5 °, taking into account an elastic detachment between 1 ° and 12 °, the orientation of the polarizer placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-55 °; -20 °] U [40 °; 70 °], while the orientation of the polarizer placed on the side of the observer, with respect to the same nematic director reference, is included in the range [-65 °; -30 °] U [25 °; 60 °]. The present invention also provides a method for optimizing the orientation of two polarizers in a nematic liquid crystal display device having two stable states by anchoring breakage, one of the polarizers being placed on the observer side while the other polarizer is placed on the opposite side of the liquid crystal cell and the latter being at least partially reflective to define a reflective or transflective mode, characterized in that it comprises the steps consisting in:

- calculate the rotary power PR using a formula exploiting the optical delay Δnd, the torsion φ and the wavelength λ,

- set the orientation A of the output polarizer equal to P + PR, P representing the orientation of the polarizer on the side opposite to the observer and PR the rotary power,

- find the values of P which give the lowest resulting transmission value for the torsion φ increased by π and

- deduce A.

Other characteristics, aims and advantages of the present invention will appear on reading the detailed description which follows, and with reference to the appended drawings, given by way of nonlimiting examples and in which:

FIG. 1 schematically represents a liquid crystal cell according to the present invention and defines the angles used in the rest of the description,

FIG. 2 schematically represents the angles ψ and w characteristic of an elliptical polarization,

- Figure 3 shows the optical transmission of the configuration (Δnd = 275nm, φopt = -156 °) described in Table 1 as a function of the wavelength, respectively for the white state in Figure 3a and for the black state in FIG. 3b, FIG. 4 schematically represents the brushing directions on the analyzer and polarizer side and the orientations of the liquid crystal molecules on the alignment layers for the case of an infinitely strong azimuth anchoring,

- Figure 5 represents the optical transmission of the configuration (Δnd = 230nm, φ imposed on φu = 0 ° and φT = -180 °) described in table 2 according to the wavelength, respectively for the white state in FIG. 5a and for the black state in FIG. 5b, and - Figure 6 shows schematically a view similar to Figure 4 for the case of a finished azimuth anchoring on the cell plate located on the side of the analyzer. DETAILED DESCRIPTION OF THE INVENTION The inventors propose an approach which makes it possible to obtain an optical mode with two preferentially reflective polarizers, for a cell [Φu; Φτ optimized to obtain a very good overall optical quality of the white state and taking into account certain industrial constraints. The inventors also propose to apply this approach to a real cell, that is to say taking into account an azimuth anchoring known as “finite” (that is to say not infinitely strong) for example on one of the alignment layers . In this case, the two textures differ by a different angle from π. Of course, the present invention also applies to the case of a display in which the azimuth anchoring is finished on the two alignment layers, that is to say on the two faces of the display.

When trying to optimize a reflective mode, the ideal is to approach the quality of black ink on white paper. The paramount parameter is therefore the quality of the white, characterized by its luminance and its color on the whole of the half plan of vision of the screen.

We will therefore choose for white the texture of better overall optical quality. Texture T has a lower wavelength dispersion and greater optical stability in the viewing angle than texture U, and will therefore be chosen for white.

To obtain a correct black with the texture U, it is necessary to have a delay Δnd of the cell not too far from λ / 2, λ representing a wavelength of the visible spectrum. CALCULATION PROCESS TO OPTIMIZE THE REFLECTIVE MODE OF A CELL [φ; φ ± π]

The approach proposed by the inventors consists in calculating the optimum optical mode giving the best white for several values of delay Δnd. The delay value finally chosen will depend on the quality compromise of the desired white-contrast.

The orientations of the polarizers which maximize white and minimize black in "round trip" are the same as those which _/ -maximize white and minimize black in "one way". On the other hand, if Tas is the optical transmission of the cell in “one way”, T _R = Tas ² is the transmission in “round trip” which makes it possible to calculate the performance of the reflective display (Luminance, contrast, color). The black, therefore the contrast, is very clearly improved during a round trip through the cell compared to a one way.

Optical characteristics of a liquid crystal cell

There is shown schematically in FIG. 1 appended a liquid crystal display cell according to the present invention comprising: an analyzer polarizer 10 on the observer side,

two plates 20, 30 confining the nematic liquid crystal molecules separated by a distance d, and

- A polarizer 40 associated with a reflector, arranged on the rear of the display, ie opposite the observer. In appended FIG. 1, the arrow 50 schematically represents the path of the light rays, the section 52 representing the outward part (of Taller optical transmission) of the path of the light rays which passes through the analyzer 10 and the cell materialized by the plates 20, 30 before reaching the polarizer 40, while the section 54 represents the return part (Tretour optical transmission) of the path of the rays reflected on the polarizer 40.

As previously indicated, the transmission Tréflectif = Taller x Tretour = T ² return, because Taller = Tretour We have shown in Figure 1 an orthonormal reference x ', y', z ', whose directions x', y 'define a perpendicular plane to the direction of propagation of the light rays and z 'is parallel to this direction of propagation. The nematic director of the molecules on the plate 20, (that is to say the direction of anchoring on this plate 20) is referenced 22. The nematic director on the plate 30 is referenced 32.

The anchors on the plates 20 and 30 are adapted to allow switching of the nematic liquid crystal molecules between two respectively stable states U and T, which differ between them by a twist of the order of π, by application of applied electrical signals on electrodes provided on the plates 20 and 30 according to the known methods described in the aforementioned documents. Such a liquid crystal cell is characterized by:

- its delay Δnd, product of the difference of the indices Δn of the liquid crystal by the thickness d of the cell

-its twist φ

- The angles P and A made respectively by the rear polarizer 40 and the front polarizer / analyzer 10 with a fixed reference (which according to Figure 1 arbitrarily coincides with the axis x ').

In the case of a reflective cell, the light which illuminates the screen is the ambient light situated on the side of the observer. The polarizer 40 of FIG. 1 is therefore of the reflective type and the output polarizer 10 (observer side) is of the transmissive type.

The angles which the polarizer 40 and the analyzer 10 make respectively with the director of the liquid crystal located on the same side are called α and β. Let φ _P and φ _{A be} the angles made by the directors 22, 32 on the two plates 20 and 30 with respect to the axis x 'of the coordinate system x' y 'z ¹ , then: P = α + φ _P and A = β + φ _A

The value of the torsion of the liquid crystal cell is obtained by making the difference between the director 22 of the liquid crystal on one of the faces 20 of the cell and the director 32 on the other face 30: φ ≈ φ _{A -}

For a simplification of the notations, we will take the director 32 of the liquid crystal on the polarizer side 40 according to x 'that is φ _P = 0 and ψ _A = φ, hence: P = α and A = β + φ. Calculation of the transmission of a twisted liquid crystal cell

The analytical form of the optical transmission of a liquid crystal cell as a function of the parameters Δnd, φ, P (or α) and A (or β), is given in numerous publications (document 7 or 8 for example), for a polarized light.

We consider here the transmission of a simple “return”, which we will call Trs, insofar as as indicated previously, the orientations of the polarizers which maximize the white are the same for a single path and for a round trip. Document [8] for example gives the following formula used by the inventors:

Trs {φ, λ) = cos ² (a + β) - cos ² tan Y + tan2 / [1] The formula [1] can also be obtained as a function of A and P instead of α and β.

Polarization characteristics after crossing the liquid crystal layer

The inventors use the Poincaré formalism, which describes the different possible polarization states as well as the evolution of this during its propagation in the cell by a trace on a sphere called Pointcaré sphere (documents [9] or [10 ]).

This very powerful tool for those who can see in three-dimensional space allows a better understanding of the optical effect of the liquid crystal cell for the strongly twisted texture (twist of the order of π).

The main result obtained by the inventors using this tool is that the texture T (torsion φ _τ of the order of π) is, for cell delays

Δnd less than or equal to λ / 2 (λ wavelength of the visible spectrum), equivalent to an almost perfect PR rotary power. This means that whatever the angle of the input polarization, the output polarization Pout is weakly elliptical (almost linear) and the long axis of this ellipse forms an angle PR with respect to the input polarization. The output polarization Pout, a priori arbitrary therefore elliptical, can be characterized by 2 angles ψ and ω. Ψ is the angle made by the major axis of the ellipse with x 'and ω characterizes the ellipticity of the polarization (see figure 2).

We have (for the single return journey) Ψ = P + PR

Thanks to the Poincaré sphere it is also possible to obtain the analytical form of PR and ω: sin 2 <2? = 2sin sin (cos 2αcos77sinA ^" + sin2û; cos) [4]

with cos77 = - 'X The formula [3] is valid as a first approximation, when X is close to π, which is always true in the cases treated in the context of the present invention.

The cancellation of the ellipticity ω is equivalent to obtaining at the cell output a linear polarization, that is to say a configuration where it is possible to obtain with the analyzer 10 a perfect black or white.

The fact of being able to predict thanks to the formula [3] the value of the rotary power PR, makes it possible to calculate the angle ψ of the output polarization Pout. The configuration for A (that is to say the orientation of the analyzer 10) giving the best white for a given P (P representing the orientation of the polarizer 40) is A parallel to ψ. let A = P + PR [5]

The condition which cancels the ellipticity ω is X = π. Is :

So when λ is fixed, there is a relationship between φ and Δnd allowing to obtain a linear Pout output polarization. We will call φ _opt the value of φ calculated with formula [6] for a given delay.

Calculation of the optimal configuration - Case where the parameter φ is free

The optimal configuration is calculated for a cell delay Δnd and a given wavelength λ.

The formula [6] allows to calculate the value of φ optimal φ _opt for the delay Δnd chosen. White optimization

From the fixed values of Δnd, φ _opt and λ, we calculate with the formula

[3] the value of the rotary power PR.

To obtain an optimum white it is necessary to place the output analyzer 10, or,

A, parallel to Pout (formula [5]): let A = P + PR

Search for the best black A (or β) is replaced by its value as a function of P (or α) in the formula of the transmission T _Re f _e ictιf = Trs ² with Trs as defined by the formula [1], with φu = φ _τ + π. The only remaining variable is P (or α). We are looking for the value of P (or α) which gives the lowest value of

Very ² . Once P is determined, the value of A is obtained using the formula [5].

Calculation examples - Case where the parameter φ is free Table 1 describes the optimal reflective optical configurations for different delay values equal to λ / 2 in the visible spectrum range (400nm for blue to 620nm for red). The parameter φ is chosen equal to φ _opt for each delay value. To estimate the performances of these configurations and compare them with each other, also appear in table 1 the value of the normalized luminances of the black state and of the white state calculated on the whole of the visible spectrum, as well as the contrast CR ratio. of the two luminances. Standardized luminances are calculated as follows: with T _R (λ) optical transmission of the liquid crystal cell back and forth, y (λ) sensitivity of the eye and s (λ) spectrum of the illumination source, which we assume constant and equal to 1 (spectrum called " dish "). The x, y color coordinates of white (CIE system) are also calculated, which are very important for the visual rendering of the screen. Remember that perfectly neutral white has the coordinates (0.333; 0.333). For all the delays Δnd considered in table 1, the white is very close to 1, because the texture T works optimally, the output polarization being linear for the case φ _op . The color of the white remains close to the theoretical white (0.333; 0.333) for all cases. However, there is a "yellowing" when the delay increases. For this measurement, the values corresponding to a delay Δnd of less than 230nm are preferred.

Black is obtained with the texture U. It is therefore optimized when it has a delay of λ / 2 for the wavelength where the eye is most sensitive. We find in Table 1 the best theoretical contrast value for the 275 nm delay, which corresponds to λ / 2 for 550 nm, peak of sensitivity of the eye. The contrast value for 275 nm is very high, due to the double pass through the cell. This double pass also makes it possible to obtain good contrast values (> 50) over a large range of delays. A satisfactory configuration for contrast is in theory a delay of 275 nm, but a smaller delay (230 nm or 200 nm) makes it possible to obtain a more neutral colorimetric white and would therefore be preferred, as indicated above. The delay chosen will depend on the compromise desired. All of these configurations have a priori acceptable performance for a screen in reflective mode. By way of illustration, the optical transmissions T _R of the white and black states corresponding to the 275 nm delay of table 1 are given in FIG. 3 as a function of the wavelength.

Realization of the liquid crystal cell - case of the infinitely strong azimuth anchoring

When the azimuth anchoring of the liquid crystal molecule on the alignment layer is infinitely strong, the director 22, 32 of the liquid crystal molecule on each face 20, 30 of the cell is determined by the brushing direction of the alignment layer (of polyimide chemical type for example) used on this face. Indeed for an infinitely strong azimuth anchoring, the director of the liquid crystal aligns parallel to the brushing direction (see Figure 4). In this case, a precise value of φ is obtained by fixing the brushing directions of the two alignment layers on the production machine of the display, such that they make an angle φ between them.

Calculation of the optimal configuration - Case where the parameter φ * is imposed

The brushing directions of the cell can be imposed for example by the industrial process. In this case, the calculation of the best configuration for the polarizer 40 or P and the analyzer 10 or A according to the best white criterion is carried out as follows: Optimization of the white

From the fixed values of Δnd, φι _mp0 sé and λ, we calculate with the formula [3] the value of the rotary power PR. To obtain an optimum white, A is necessary parallel to Pout (formula [5]): either A = P + PR Search for the best black A (or β) is replaced by its value as a function of P (or α) in the formula of the transmission T _R = Trs ² , with φu = φ _τ + π. The only remaining variable is P (or α). We are looking for the value of P (or α) which gives the lowest value of Trs ² . Once P is determined, the value of A is obtained using the formula [5]. Calculation examples - Case where the parameter φ * is imposed

Table 2 describes the optimal reflective optical configurations for different values of the delay close to λ / 2. The parameter φimp _dared is chosen such that _φ τ = -180 ° φu = 0 °. The performances of the reflective mode for φ = 0 ° are close to those obtained for φ _opt . The white is slightly less good (around 5%) because the texture T is not at the optimum, but the output polarization remains very slightly elliptical, which guarantees a good level of white. The more the delay decreases, the more one approaches the condition φ _opt (φ _opt approaches -180 ° when the delay decreases). For 200 nm, the white is very close to 1. We find the yellowing effect again when the delay increases.

The black is produced by a texture U without twist which acts, as before, like a birefringent blade. The maximum value of the contrast corresponds as previously to a delay of 275 nm, but the contrast remains high for the other delays due to the double pass.

The best configuration for contrast is a delay of 275 nm, but a smaller delay (230nm or 200 nm) makes it possible to obtain a more neutral colorimetric white. The delay chosen depends on the compromise desired.

All of these configurations have acceptable performance for a screen in reflective mode. By way of illustration, the optical transmissions T _R of the white and black states corresponding to the 230 nm delay of table 2 are given in FIG. 5 as a function of the wavelength.

Improvement of the colorimetric neutrality of the white A solution to improve the colorimetric neutrality of the white is to adjust the polarizers 10 or A and 40 or P to angles similar but different from those calculated by the method described previously. In this case, the condition on the rotary power which guarantees the brightest white is no longer strictly observed. In fact we agree to lose on the luminance of white to improve the colorimetry. An example is given in table 2 'for a delay of 230 nm and an imposed φ such as for table 2 (φ _{ιm osé} is chosen such that φ _τ = -180 ° φu = 0 °). It is noted that for the angles of P and A of table 2 ', the contrast has been divided by 2 compared to the calculated solution of table 2, but remains acceptable. The color of the white has been changed, it is slightly bluish and closer to the theoretical white. Transflective mode

All the calculations described above can be applied to the intermediate optical mode called transflective, for which the rear polarizer 40 or P is transflective, that is to say partially reflecting: part of the polarized light is transmitted, the other part is thoughtful. This allows the screen to operate either in transmissive mode when it is backlit, or in reflective mode using ambient light as a light source when not backlit. The optimization of white is identical for the reflective and transflective modes, but for the latter the quality of black is low when it operates in transmissive mode (black is not optimized since the light operates, a "one way" through the cell instead of a "round trip" for the reflective mode). REAL CASE OF A FINISHED AZIMUTHAL ANCHOR

When the azimuthal anchoring is finished (not infinitely strong), the elastic forces which are exerted on the molecules close to the surface due to the chiral doping of the liquid crystal mixture, cause these molecules to "unhook", ie the director 22, 32 of the liquid crystal is no longer strictly parallel to the brushing direction, but offset by an angle DE called “elastic stall”. To simplify the presentation of the invention on this point, it is assumed that a single anchoring layer has a finite azimuth anchoring, the other layer having an infinitely strong azimuth anchoring. The dropout goes in the direction of decreasing the absolute value of the torsion of low value φu, which becomes for example φu - DE for φu> 0, and of decreasing the absolute value of the torsion of high value φ _τ , which becomes for example φ _τ + DE for φ _τ <0 (see figure 6). We call φ * the angle that the brushing directions make between them. Due to the dropout we have: φ _j , - (fa = -π + 2DE

The elastic stall is directly linked to the azimuth anchoring force characterized by its length of extrapolation Laz according to the relationship:

2d

A finished azimuth anchor typically has Laz in the range of 100 to

200 nm, or DE between a few degrees and about 15 °. The parameter DE is a physical parameter measurable experimentally, therefore assumed to be known.

For the following examples we choose ED values of 5 ° and

10 °.

Case where the parameter ψ * is free We will calculate the optimal values of φ * according to the dropout DE, as well as the corresponding optical configuration.

For each delay Δnd we call φ _opt the optimal value of the torsion as defined by the formula [6].

The effective value of the strong twist φ _τ is (see Figure 6): φ _τ = -π + φ * + DE * = π + φ _opt - DE

The rotary power PR is calculated for (Δnd, φ _opt ) using the formula [3], then the relationship between A and P is injected into the formula of T _R for φu = φ * - DE, and the we search numerically for the values of P which minimize the transmission T _R.

The optimal optical configurations are calculated for the three delay values 310nm, 275nm and 230nm, and for the 2 values of DE 5 ° and 10 °, respectively in Tables 3, 4 and 5. It is noted on these tables that in the presence elastic stall, it is possible to optimize the brushing directions (φ *) to obtain good performance of the reflective mode: the white is for all cases very close to 1. The highest contrast corresponds as before to a delay of 275 nm, but it remains acceptable for the other delay values considered. All of these configurations have acceptable performance for a screen in reflective mode. Case where the parameter φ * is imposed

For practical reasons of industrial process or of proper functioning of the display, the brushing direction of the cells φ * can be imposed, typically at 0 ° for example.

The effective value of φ _{τ is} worth in this case (cf figure 6): φ _τ = -π + φ ^' + DE

The rotary power PR is calculated for the corresponding value of φ _τ , then as before the value of A as a function of P arising from formula 5 is injected into the formula of T _R for φu = φ * - DE, and we finds the values of P which minimize the transmission T _R. The results for the 3 previous delay values are given respectively in Tables 6, 7 and 8. It can be seen in these tables that in the presence of elastic dropout and with φ * imposed at 0 °, it is possible to obtain good performance of the reflective mode: the white is in all cases very close to 1. The highest contrast corresponds as before to a delay of 275 nm, but it remains acceptable for the other delay values considered. All of these configurations have acceptable performance for a screen in reflective mode.

From these calculated configurations, it is of course possible to modify the colorimetry of the white at the cost of a decrease in contrast by adjusting the angles of A and P as described in the case of strong azimuth anchoring.

The calculations described in this paragraph also apply to the transflective mode.

Table 1: optimal configurations for different delay values,

Table 2: optimal configurations for different delay values, with φ imposed.

Table 2 ': “modified” configuration to improve the colorimetry of white, delay 230 nm and φ imposed.

Table 3: example of solutions for different elastic dropout values for (Δnd = 310 nm; φ _opt = -149 °; λ = 550 nm), φ * libre

Table 4: example of solutions for different elastic dropout values for (Δnd = 275 nm; φ _opt = - 156 °; λ = 550 nm), φ * libre

Table 5: example of solutions for different elastic stall values for (Δnd = 230 nm; 163.5 °; λ = 550 nm), φ * free

Table 6: example of solutions for different elastic stall values for (Δnd = 310 nm; λ = 550 nm) with φ * imposed at 0 °.

Table 7: example of solutions for different values of elastic dropout for (Δnd = 275 nm; λ = 550 nm) with φ * imposed at 0 °.

Table 8: example of solutions for different elastic stall values for (Δnd = 230 nm; λ = 550 nm) with φ * imposed at 0 °. VARIANTS OF THE INVENTION The foregoing description relates to a levogyre liquid crystal. The invention is of course valid when the liquid crystal is dextrorotatory. In this case, the equivalent configurations are obtained by reversing the signs of φU and φT. The optimal orientations of the polarizers 40 and analyzer 10 are obtained by reversing the signs of P and A. As an example, we give the dextrorotatory tables corresponding to tables 2 ', 3, 5, 6 and 8 of the levorotatory case respectively in Tables 9, 10, 11, 12 and 13:

Table 9 (ex 2 '): “modified” configuration to improve the colorimetry of white, delay 230 nm and φ imposed - case of a dextrorotatory liquid crystal.

Table 10 (ex 3): example of solutions for different elastic dropout values for (Δnd = 310 nm; φ _op = -149 °; λ = 550 nm), φ * free - case of a dextrorotatory liquid crystal

Table 11 (ex 5): example of solutions for different elastic dropout values for (Δnd = 230 nm; 163.5 °; λ = 550 nm), φ * free - case of a dextrorotatory liquid crystal

Table 12 (ex 6): example of solutions for different elastic stall values for (Δnd = 310 nm; λ = 550 nm) with φ * imposed at 0 ° - case of a dextrorotatory liquid crystal.

Table 13 (ex 8): example of solutions for different elastic stall values for (Δnd = 230 nm; λ = 550 nm) with φ * imposed at 0 ° - case of a dextrorotatory liquid crystal. REFERENCES Document [1]: patent FR 2740894 Document [2]: "Fast bistable nematic display using monostable surface anchoring switching" Proceeding SID 1997, p41-44 Document [3]: "recent improvements of bistable nematic displays switched by anchoring breaking" SPIE vol.3015 (1997), pδl-69 Document [4]: US patent 2003/0076455 Document [5]: "Dynamic flow, broken surface anchoring, and switching bistability in three-terminal twisted nematic liquid crystal displays"

Journal of Applied Physics, vol 90, n ° 6, p 3121-3123 (2001) Document [6]: patent FR 2808891 Document [7]: "optical optimization of surface controlled bistable twisted nematic liquid crystal displays" Proceeding SID 2000, p 241-243 Document [8]: patent FR 2808890 Document [9]: "Thin Crystal film polarizers and retarders", Proc of SPIE vol 4658, p 79-90, 2002 Document [10]: "reflective bistable nematic displays (BiNem ^® ) fabricated by standard manufacturing equipment ", Journal of the SID 11/1, 2003, 217-224

Claims

1. Nematic liquid crystal display device having two stable states, without electric field, obtained by anchoring breakage, characterized in that it has two polarizers (10, 40), one (10) placed on the side observer, the other (40) on the opposite face of the liquid crystal cell and the latter being at least partially reflecting to define a reflective or transflective mode, the orientation of the two polarizers (10, 40) being offset by value equal to the rotary power of the cell to within +/- 30 °.

2. Device according to claim 1, characterized in that. the orientation of the polarizer (40) placed opposite the observer, relative to the nematic director on the associated face of the cell, being included in the range comprising the range + or - (10 ° to 80 °) while that the orientation of the polarizer (10) placed on the side of the observer, relative to the same nematic director reference, is included in the range comprising the range of + or - (10 ° to 80 °) for an optical delay Δnd of the order of 250+ 70nm.

3. Device according to one of claims 1 or 2, characterized in that for an optical delay Δnd = 250 +/- 70 nm, in the case of a levogyre liquid crystal, the orientation of the polarizer (40) placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-75 °; -30 °] U [10 °; 65 °], while the orientation of the polarizer (10) placed on the side of the observer, with respect to the same reference of nematic director, is included in the range [-60 °; -10 °] U [30 °; 80 °], and in the case of a dextrorotatory liquid crystal, the orientation of the polarizer (40) placed opposite the observer, relative to the nematic director on the face associated with the cell, is in the range [-65 °; -10 °] U [30 °; 75 °], while the orientation of the polarizer (10) placed on the side of the observer, relative to the same reference of nematic director, is included in the range [-80 °; -30 °] U [10 °; 60 °].

4. Device according to one of claims 1 to 3, characterized in that for an optical delay Δnd = 280 +/- 30 nm, in the case of a levogyre liquid crystal and for brushing directions forming between them 15 ° and 30 °, the orientation of the polarizer (40) placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-65 °; -30 °] U [25 °; 65 °] preferentially [-50 °; -40 °] U [40 °; 50 °], while the orientation of the polarizer (10) placed on the side of the observer, relative to the same reference of nematic director, is included in the range [-40 °; -10 °] U [50 °; 80 °] preferably [-25 °; -10 °] U [64 °; 80 °], and in the case of a dextrorotatory liquid crystal, for brushing directions making between them between -15 ° and -30 °, the orientation of the polarizer (40) placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-65 °; -25 °] U [30 °; 65 °] preferentially [-50 °; -40 °] U [40 °; 50 °], while the orientation of the polarizer (10) placed on the side of the observer, with respect to the same reference of nematic director, is included in the range [-80 °; -50 °] U [10 °; 40 °] preferably [-80 °; -64 °] U [10 °; 25 °].

5. Device according to one of claims 1 to 3, characterized in that for an optical delay Δnd = 220 +/- 30 nm, in the case of a levogyre liquid crystal and for brushing directions forming between them 10 ° and 20 °, the orientation of the polarizer (40) placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-60 °; -30 °] U [25 °; 65 °] preferentially [-50 °; -40 °] U [40 °; 50 °], while the orientation of the polarizer (10) placed on the side of the observer, relative to the same reference of nematic director, is included in the range [-40 °; -20 °] U [50 °; 70 °] preferably [-35 °; -25 °] U [55 °; 65 °], and in the case of a dextrorotatory liquid crystal, for brushing directions making between them between -10 ° and -20 °, the orientation of the polarizer (40) placed opposite the observer, relative to the nematic director on the associated face of the cell, is in the range [-65 °; -25 °] U [30 °; 60 °] preferably [-50 °; -40 °] U [40 °; 50 °], while the orientation of the polarizer (10) placed on the side of the observer, with respect to the same reference of nematic director, is included in the range [-70 °; -50 °] U [20 °; 40 °] preferably [-65 °; -55 °] U [25 °; 35 °].

6. Device according to one of claims 1 to 3, characterized in that for an optical delay Δnd = 280 +/- 30 nm, in the case of a levogyre liquid crystal and for brushing directions forming between them 0 ° and +/- 5 °, the orientation of the polarizer (40) placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-75 °; -35 °] U [15 °; 55 °] preferably [-60 °; -50 °] U [30 °; 40 °], while the orientation of the polarizer (10) placed on the side of the observer, relative to the same reference of nematic director, is included in the range [-50 °; -20 °] U [40 °; 70 °] preferably [-40 °; -30 °] U [50 °; 60 °], and in the case of a dextrorotatory liquid crystal, for brushing directions making between them between 0 ° and +/- 5 °, the orientation of the polarizer (40) placed opposite the observer , with respect to the nematic director on the associated face of the cell, is included in the range [-55 °; -15 °] U [35 °; 75 °] preferentially [-40 °; -30 °] U [50 °; 60 °], while the orientation of the polarizer (10) placed on the side of the observer, with respect to the same reference of nematic director, is included in the range [-70 °; -40 °] U [20 °; 50 °] preferably [-60 °; -50 °] U [30 °; 40 °].

7. Device according to one of claims 1 to 3, characterized in that for an optical delay Δnd = 220 +/- 30 nm, in the case of a levogyre liquid crystal and for brushing directions forming between them 0 ° and +/- 5 °, the orientation of the polarizer (40) placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-65 °; -40 °] U [20 °; 60 °] preferably [-55 °; -45 °] U [35 °; 45 °], while the orientation of the polarizer (10) placed on the side of the observer, with respect to the same reference of nematic director, is included in the range [-60 °; -20 °] U [30 °; 70 °] preferably [-45 °; -35 °] U [45 °; 55 °], and in the case of a dextrorotatory liquid crystal, for brushing directions making between them between 0 ° and +/- 5 °, the orientation of the polarizer (40) placed opposite the observer , with respect to the nematic director on the associated face of the cell, is included in the range [-60 °; -20 °] U [40 °; 65 °] preferentially [-45 °; -35 °] U [45 °; 55 °], while the orientation of the polarizer (10) placed on the side of the observer, with respect to the same reference of nematic director, is included in the range [-70 °; -30 °] U [20 °; 60 °] preferably [-55 °; -45 °] U [35 °; 45 °].

8. Device according to one of claims 1 to 3, characterized in that for an optical delay Δnd = 220 +/- 30 nm, in the case of a levogyre liquid crystal and for brushing directions forming between them 0 ° and +/- 5 °, the orientation of the polarizer (40) placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-80 °; -50 °] U [10 °; 60 °], while the orientation of the polarizer (10) placed on the side of the observer, with respect to the same reference of nematic director, is included in the range [-50 °; -25 °] U [45 °; 65 °], and in the case of a dextrorotatory liquid crystal, for brushing directions making between them between 0 ° and +/- 5 °, the orientation of the polarizer (40) placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-60 °; -10 °] U [50 °; 80 °], while the orientation of the polarizer (10) placed on the side of the observer, with respect to the same reference of nematic director, is included in the range [-65 °; -45 °] U [25 °; 50 °].

9. Device according to one of claims 1 to 3, characterized in that for an optical delay Δnd = 280 +/- 30 nm, in the case of a levogyre liquid crystal and for brushing directions forming between them 5 ° and 30 °, taking into account an elastic detachment between 1 ° and 12 °, the orientation of the polarizer (40) placed opposite the observer, relative to the nematic director on the associated face of the cell , is in the range [-65 °; -45 °] U [25 °; 55 °], while that the orientation of the polarizer (10) placed on the side of the observer, with respect to the same reference of nematic director, is included in the range [-40 °; -10 °] U [50 °; 80 °], and in the case of a dextrorotatory liquid crystal, for brushing directions between them between -5 ° and 30 °, taking into account an elastic stall between 1 ° and 12 °, the orientation of the polarizer (40) placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-55 °; -25 °] U [45 °; 65 °], while the orientation of the polarizer (10) placed on the side of the observer, with respect to the same reference of nematic director, is included in the range [-80 °; -50 °] U [10 °; 40 °].

10. Device according to one of claims 1 to 3, characterized in that for an optical delay Δnd = 220 +/- 30 nm, in the case of a levogyre liquid crystal and for brushing directions forming between them 5 ° and 15 °, taking into account an elastic dropout between 1 ° and 12 °, the orientation of the polarizer (40) placed opposite the observer, relative to the nematic director on the associated face of the cell , is in the range [-65 °; -40 °] U [25 °; 50 °] preferably [-60 °; -45 °] U [30 °; 45 °], while the orientation of the polarizer (10) placed on the side of the observer, relative to the same reference of nematic director, is included in the range [-50 °; - 20 °] U [40 °; 65 °] preferably [-40 °; -30 °] U [48 °; 60 °], and in the case of a dextrorotatory liquid crystal, for brushing directions between them between -5 ° and -15 °, taking into account an elastic stall between 1 ° and 12 °, the orientation of the polarizer (40) placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-50 °; -25 °] U [40 °; 65 °] preferentially [-45 °; -30 °] U [45 °; 60 °], while the orientation of the polarizer (10) placed on the side of the observer, relative to the same reference of nematic director, is included in the range [-65 °; -40 °] U [20 °; 50 °] preferably [-60 °; -48 °] U [30 °; 40 °].

11. Device according to one of claims 1 to 3, characterized in that for an optical delay Δnd = 250 +/- 70 nm, in the case of a levogyre liquid crystal and for brushing directions forming between them 0 ° and +/- 5 °, taking into account an elastic stall between 1 ° and 12 °, the orientation of the polarizer (40) placed opposite the observer, relative to the nematic director on the associated face of the cell, is in the range [-70 °; -40 °] U [20 °; 55 °], while the orientation of the polarizer (10) placed on the side of the observer, with respect to the same reference of nematic director, is included in the range [-60 °; -25 °] U [30 °; 65 °], and in the case of a dextrorotatory liquid crystal, for brushing directions between them between 0 ° and +/- 5 °, taking into account an elastic detachment between 1 ° and 12 °, the orientation of the polarizer (40) placed opposite the observer, relative to the nematic director on the associated face of the cell, is included in the range [-55 °; -20 °] U [40 °; 70 °], while the orientation of the polarizer (10) placed on the side of the observer, with respect to the same reference of nematic director, is included in the range [-65 °; -30 °] U [25 °; 60 °].

12. Device according to one of claims 1 to 11, characterized in that the ratio between the thickness d of the cell and the spontaneous step p ₀ of the liquid crystal molecules, is approximately equal to 0.25 +/- 0.1, preferably 0.25 ± 0.05.

13. Device according to one of claims 1 to 12, characterized in that the two stable states correspond to two textures of liquid crystal molecules whose torsion differs from 150 ° to 180 ° in absolute value.

14. Device according to one of claims 1 to 13, characterized in that the passing state is obtained with the texture of greatest torsion.

15. Method for optimizing the orientation of two polarizers

(10, 40) in a nematic liquid crystal display device having two stable states, without electric field, obtained by anchoring break, one of the polarizers (10) being placed side partially reflecting to define a reflective or transflective mode, characterized in that it comprises the steps consisting in orienting the two polarizers (10, 40) so that they are offset by a value equal to the rotational power of the cell to +/- 30 °.

16. Method according to claim 15, characterized in that it comprises the steps consisting in:

- fix the orientation A of the output polarizer (10) equal to P + PR, P representing the orientation of the polarizer (40) on the side opposite to the observer and PR the rotary power,

- deduce A.

17. Method according to claim 16, characterized in that the rotary power PR is calculated on the basis of the relation:

18. Method according to one of claims 16 or 17, characterized in that the value of the transmission is defined by the relation:

Tτs (φ, λ) = cos ² (+ β) - cos ² X cos 2α cos 2β t tanA ^r - tan2o: l tan 2β X x tan X +

[1].

19. Method according to one of claims 16 to 18, characterized in that the rotary power PR is calculated on the basis of an optimal torsional value φopt determined on the basis of the relationship:

20. Method according to one of claims 16 to 19, characterized in that the rotary power PR is calculated on the basis of a torsion value imposed by the azimuthal anchoring.

21. Method according to one of claims 16 to 20, characterized in that it comprises a step of adapting the angles of the polarizers to improve the colorimetric neutrality of the white obtained.

22. Method according to one of claims 16 to 21, characterized in that the rotary power PR is calculated on the basis of a torsion value which incorporates a stall (DE) resulting from a finite azimuth anchoring.