Display device with a white-optimizing bistable nematic screen and method for the definition of said device
View Patent ↗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°.
1. Nematic liquid crystal display device having two stable states, without an electric field, that are obtained by anchoring break, characterized by the fact that it comprises two polarizers ( 10 , 40 ), the first polarizer ( 10 ) being placed on the side of the observer, the other polarizer ( 40 ) being placed on the opposite side of the liquid crystal cell and said other polarizer being at least partially reflecting in order to define a reflective or transflective mode, the orientation of the two polarizers ( 10 , 40 ) being shifted by a value equal to the rotatory power of the cell at approximately +/−30°.
2. Device according to claim 1 , characterized by the fact that the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, being comprised within the range containing the sub-range + or − (10° to 80°) whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range comprising the sub-range from + or − (10° to 80°) for an optical delay Δnd of the order of 250±70 nm.
3. Device according to claim 1 , characterized by the fact that for an optical delay Δnd=250+/−70 nm, in the case of a levo-rotatory liquid crystal, the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, is comprised within the range [−75°; −30°] U [10°; 65°], whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range [−60°; −10°] U [30°; 80°], and in the case of a dextro-rotatory liquid crystal, the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, is comprised within the range [−65°; −10°] U [30°; 75°], whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range [−80°; −30°] U [10°; 60°].
4. Device according to claim 1 , characterized by the fact that for an optical delay Δnd=280+/−30 nm, in the case of a levo-rotatory liquid crystal and for brushing directions forming between themselves between 15° and 30°, the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, is comprised within the range [−65°; −30°] U [25°; 65°] preferentially [−50°; −40°] U [40°; 50°], whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range [−40°; −10°] U [50°; 80°] preferentially [−25°; −10°] U [64°; 80°], and in the case of a dextro-rotatory liquid crystal, for brushing directions forming between themselves between −15° and −30°, the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, is comprised within the range [−65°; −25°] U [30°; 65°] preferentially [−50°; −40°] U [40°; 50°], whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range [−80°; −50°] U [10°; 40°] preferentially [−80°; −64°] U [10°; 25°].
5. Device according to claim 1 , characterized by the fact that for an optical delay Δnd=220+/−30 nm, in the case of a levo-rotatory liquid crystal and for brushing directions forming between themselves between 10° and 20°, the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, is comprised within the range [−60°; −30°] U [25°; 65°] preferentially [−50°; −40°] U [40°; 50°], whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range [−40°; −20°] U [50°; 70°] preferentially [−35°; −25°] U [55°; 65°], and in the case of a dextro-rotatory liquid crystal, for brushing directions forming between themselves between −10° and −20°, the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, is comprised within the range [−65°; −25°] U [30°; 60°] preferentially [−50°; −40°] U [40°; 50°], whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range [−70°; −50°] U [20°; 40°] preferentially [−65°; −55°] U [25°; 35°].
6. Device according to claim 1 , characterized by the fact that for an optical delay Δnd=280+/−30 nm, in the case of a levo-rotatory liquid crystal and for brushing directions forming between themselves between 0° and +/−5°, the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, is comprised within the range [−75°; −35°] U [15°; 55°] preferentially [−60°; −50°] U [30°; 40°], whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range [−50°; −20°] U [40°; 70°] preferentially [−40°; −30°] U [50°; 60°], and in the case of a dextro-rotatory liquid crystal, for brushing directions forming between themselves between 0° and +/−5°, the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, is comprised within the range [−55°; −15°] U [35°; 75°] preferentially [−40°; −30°] U [50°; 60°], whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range [−70°; −40°] U [20°; 50°] preferentially [−60°; −50°] U [30°; 40°].
7. Device according to claim 1 , characterized by the fact that for an optical delay Δnd=220+/−30 nm, in the case of a levo-rotatory liquid crystal and for brushing directions forming between themselves between 0 and +/−5°, the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, is comprised within the range [−65°; −40°] U [20°; 60°] preferentially [−55°; −45°] U [35°; 45°], whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range [−60°; −20°] U [30°; 70°] preferentially [−45°; −35°] U [45°; 55°], and in the case of a dextro-rotatory liquid crystal, for brushing directions forming between themselves between 0° and +/−5°, the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, is comprised within the range [−60°; −20°] U [40°; 65°] preferentially [−45°; −35°] U [45°; 55°], whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range [−70°; −30°] U [20°; 60°] preferentially [−55°; −45°] U [35°; 45°].
8. Device according to claim 1 , characterized by the fact that for an optical delay Δnd=220+/−30 nm, in the case of a levo-rotatory liquid crystal and for brushing directions forming between themselves between 0° and +/−5°, the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, is comprised within the range [−80°; −50°] U [10°; 60°] whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range [−50°; −25°] U [45°; 65°], and in the case of a dextro-rotatory liquid crystal, for brushing directions forming between themselves between 0° and +/−5°, the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, is comprised within the range [−60°; −10°] U [50°; 80°], whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range [−65°; −45°] U [25°; 50°].
9. Device according to claim 1 , characterized by the fact that for an optical delay Δnd=280+/−30 nm, in the case of a levo-rotatory liquid crystal and for brushing directions forming between themselves between 5° and 30°, taking account of an elastic uncoupling between 1° and 12°, the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, is comprised within the range [−65°; −45°] U [25°; 55°], whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range [−40°; −10°] U [50°; 80°], and in the case of a dextro-rotatory liquid crystal, for brushing directions forming between themselves between −5° and 30°, taking account of an elastic uncoupling between 1° and 12°, the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, is comprised within the range [−55°; −25°] U [45°; 65°], whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range [−80°; −50°] U [10°; 40°].
10. Device according to claim 1 , characterized by the fact that for an optical delay Δnd=220+/−30 nm, in the case of a levo-rotatory liquid crystal and for brushing directions forming between themselves between 5° and 15°, taking account of an elastic uncoupling between 1° and 12°, the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, is comprised within the range [−65°; −40°] U [25°; 50°] preferentially [−60°; −45°] U [30°; 45°], whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range [−50°; −20°] U [40°; 65°] preferentially [−40°; −30°] U [48°; 60°], and in the case of a dextro-rotatory liquid crystal, for brushing directions forming between themselves between −5° and −15°, taking account of an elastic uncoupling between 1° and 12°, the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, is comprised within the range [−50°; −25°] U [40°; 65°] preferentially [−45°; −30°] U [45°; 60°], whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range [−65°; −40°] U [20°; 50°] preferentially [−60°; −48°] U [30°; 40°].
11. Device according to claim 1 , characterized by the fact that for an optical delay Δnd=250+/−70 nm, in the case of a levo-rotatory liquid crystal and for brushing directions forming between themselves between 0° and +/−5°, taking account of an elastic uncoupling between 1° and 12°, the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, is comprised within the range [−70°; −40°] U [20°; 55°], whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range [−60°; −25°] U [30°; 65°], and in the case of a dextro-rotatory liquid crystal, for brushing directions forming between themselves between 0° and +/−5°, taking account of an elastic uncoupling between 1° and 12°, the orientation of the polarizer ( 40 ) placed on the opposite side with respect to the observer, as referring to the nematic director on the associated face of the cell, is comprised within the range [−55°; −20°] U [40°; 70°], whilst the orientation of the polarizer ( 10 ) placed on the side of the observer, as referring to the same nematic director reference, is comprised within the range [−65°; −30°] U [25°; 60°].
12. Device according to claim 1 , characterized by the fact that the ratio between the thickness d of the cell and the spontaneous pitch p o of the liquid crystal molecules is approximately equal to 0.25+/−0.1, preferably 0.25±0.05.
13. Device according to claim 1 , characterized by the fact that the two stable states correspond to two textures of liquid crystal molecules the twist of which differs from 150° to 180° in absolute values.
14. Device according to claim 1 , characterized by the fact that the passing-state is obtained with the texture with the strongest twist.
15. Method for the optimization of the orientation of two polarizers ( 10 , 40 ) in a nematic liquid crystal display device presenting two stable states, without an electric field, that are obtained by anchoring break, one of the polarizers ( 10 ) being placed on the partially reflecting side in order to define a reflective or transflective mode, characterized by the fact that it comprises the steps consisting of orientating the two polarizers ( 10 , 40 ) such that they are shifted by a value equal to the rotatory power of the cell at approximately +/−30°.
16. Method according to claim 15 , characterized by the fact that it comprises the steps consisting of:
calculating the rotatory power PR using a formula which utilizes the optical delay Δnd, the twist Φ and the wavelength λ,
fixing the orientation A of the output polarizer ( 10 ) equal to P+PR, P representing the orientation of the polarizer ( 40 ) on the opposite side with respect to the observer and PR the rotatory power,
researching the values of P which produce the lowest resultant transmission value for the twist Φ increased by λ and
deducing A from it.
17. Method according to claim 16 , characterized by the fact that the rotatory power PR is calculated on the basis of the relationship:
PR
≅
ϕ
-
arc
tg
(
ϕ
X
tgX
)
with
[
3
]
X
(
ϕ
,
λ
)
=
ϕ
2
+
(
π
Δ
nd
λ
)
2
.
[
2
]
18. Method according to claim 16 , characterized by the fact that the transmission value is defined by the relationship:
Trs
(
ϕ
,
λ
)
=
cos
2
(
α
+
β
)
-
cos
2
X
cos
2
α
cos
2
β
[
ϕ
X
tan
X
-
tan
2
α
]
[
ϕ
X
tan
X
+
tan
2
β
]
.
[
1
]
19. Method according to claim 16 , characterized by the fact that the rotatory power PR is calculated on the basis of an optimal twist value Φopt determined on the basis of the relationship:
ϕ
opt
=
π
1
-
(
Δ
nd
λ
0
)
2
.
[
6
]
20. Method according to claim 16 , characterized by the fact that the rotatory power PR is calculated on the basis of a twist value imposed by the azimuthal anchoring.
21. Method according to claim 16 , characterized by the fact that it comprises a step of adapting the angles of the polarizers in order to improve the colorimetric neutrality of the white obtained.
22. Method according to claim 16 , characterized by the fact that the rotatory power PR is calculated on the basis of a twist value which integrates an uncoupling (DE) resulting from a finite azimuthal anchoring.