VARIABLE CONTRAST, WIDE VIEWING ANGLE LCD LIGHT-SWITCHING FILTER

• US 20120002121A1
• Filed: 08/12/2011
• Published: 01/05/2012
• Est. Priority Date: 05/20/2005
• Status: Active Grant

• Current Assignee: Institute Jozef Stefan
• Original Assignee: Institute Jozef Stefan
• Inventors: Bazec, Matej, Pirs, Silvija, Marin, Bojan, PIRS, Janez, Urankar, Bernarda, Ponikvar, Dusan
• Granted Patent: US 8,542,334 B2
• Time in Patent Office: Days
• US Class Current: 349/14
• CPC Class Codes: A61F 9/067 with variable transmission
• CPC Class Codes: G02F 1/133531 characterised by the arrangement of polariser or analyser axes
• CPC Class Codes: G02F 1/133634 the refractive index Nz perpendicular to the element surface being different from in-plane refractive indices Nx and Ny, e.g. biaxial or with normal optical axis
• CPC Class Codes: G02F 1/13471 in which all the liquid crystal cells or layers remain transparent, e.g. FLC, ECB, DAP, HAN, TN, STN, SBE-LC cells G02F1/13475 takes precedence
• CPC Class Codes: G02F 1/1397 the twist being substantially higher than 90°, e.g. STN-, SBE-, OMI-LC cells
• CPC Class Codes: G02F 2202/40 Materials having a particular birefringence, retardation
• CPC Class Codes: G02F 2413/02 Number of plates being 2

Abstract

A new concept of the LCD light-switching optical filter allowing for very high, electrically controlled, continuously variable light attenuation (up to ≧100,000) optimized/symmetrized for the light incidence along the normal to the LCD plane is proposed. The new LCD light-switching optical filter also exhibits very low light attenuation dependence for the oblique incidence of light within a limited cone of angles off the normal incidence direction in compliance with the international safety and quality regulation EN 379 for personal protection equipment. According to the invention the problem is solved by specific, novel modification of the general principle of highly twisted nematic LCDs, allowing for the adaptation of the light transmission/driving voltage characteristics to specific requirements of the driving electronics as well as “symmetrizing” the overall optical birefringent properties, which in turn results in a high degree of their angular compensation.

Claims

1. High contrast, wide viewing angle LCD light switching filter comprising two electrically controlled LC-cells (LCD1, LCD2) angularly compensated by two birefringent layers (R1, R2), adjacent to the either side of the said LC-cells (LCD1, respectively LCD2) and positioned between two mutually crossed pairs of polarizing films (P₁, A₁ and P₂, A₂), each of the said LC-cells (LCD1, LCD2) having a thin liquid crystal molecular layer (d^a+d^b+e) of highly twisted liquid crystal enclosed between two boundary glass plates (1a^a, 1a^b), the said plates (1a^a, 1a^b) being on an inner side covered by transparent electrodes (1b^a, 1b^b) and alignment layers (1c^a, 1c^b), characterized in

that the alignment layers (1c^a, 1c^b) on the transparent electrodes (1b^a, 1b^b) covering the boundary glass plates (1a^a, 1a^b) of both LC-cells (LCD1, LCD2) are oriented in such a way that in an optically open state of the LCD light switching filter, when no driving electric voltage (V=0) is applied to the transparent electrodes (1b^a, 1b^b), the liquid crystal molecular layer (d^a+d^b+e) adopts a highly twisted molecular orientation with twist angles Φ

between 120°

to 240° and

that the two birefringent layers (R1 respectively R2), adjacent to the said LC-cells (LCD1 respectively LCD2) are negative birefringent c-plates having a nominal negative birefringence with their optic axis oriented perpendicularly to the LCD light switching filter plane and exhibiting intrinsic positive in-plane birefringence aligned in-plane of the LCD light switching filter, said intrinsic positive birefringence being less than 10% of the nominal negative-birefringence, and

that the two crossed pairs of polarizing films (P₁, A₁ and P₂, A₂) are oriented with respect to LC molecular alignment surface directions (1r^a, 1r^b respectively 2r^a, 2r^b) of each of the LC cells (LCD1, LCD2) in such a way that the light transmission axes of a first (P₁, A₁) of said pairs of polarizing films are aligned within a small angle α

along symmetry axes (S¹₁, S²₁) of the LC-molecular alignment surface directions (1r^a, 1r^b) in a first LC-cell (LCD1) and that the light transmission axes of a second (P₂, A₂) of said pairs of polarizing films are aligned within a small angle a along symmetry axes (S¹₂, S²₂) of the LC-molecular alignment surface directions (2r^a, 2r^b) in a second LC-cell (LCD2), wherein said angle a is smaller than 15 degrees, allowing for the expansion of an overall viewing angle of the LCD light switching filter.

2. High contrast, wide viewing angle LCD light switching filter according to claim 1, characterized in

that the LC-cells (LCD 1, LCD2) provide a light transmission/driving voltage characteristics of high light transmission at driving voltages lower than a threshold voltage V_th followed by a steep decay to low light transmission with increasing driving voltages as a first regime and a further less inclined decay of light transmission beginning at a limiting driving voltage V_LS with further increasing driving voltages as a second, “

low-slope”

regime (LS) and

that the LCD light switching filter is adapted to be operated in the second regime of the light transmission/driving voltage characteristics of the LC-cells (LCD1, LCD2) in an “

optically-closed”

state, wherein

the light attenuation is varied only within the second regime being always greater than 10.

3. High contrast, wide viewing angle LCD light switching filter according to claim 2, characterized in

that the twist angle Φ

of the said highly twisted liquid crystal in the LC-cells (LCD1, LCD2) is selected to adjust the slope of the light transmission/driving voltage characteristics of the LC-cells (LCD1, LCD2) in the “

optically-closed”

state and

that the angle α

of the relative alignment of the said two crossed pairs of polarizing films (P₁, A₁ and P₂, A₂) with respect to the LC molecular alignment surface directions (1r^a, 1r^b respectively 2r^a, 2r^b) is selected to optimize a required amplitude of electric driving signals to best fit with a driving electronics in particular with an available driving battery, wherein

the twist angle Φ

is varied between 210° and

240° and

the relative alignment angle α

between 2° and

15°

.

4. High contrast, wide viewing angle LCD light switching filter according to one of claims 1, 2 and 3 characterized in

that a concentration of a chiral dopant in the said liquid crystal molecular layer (d^a+d^b+e) is selected to further optimize the slope of the light transmission vs. voltage characteristics in the said second regime (LS), wherein

said concentration of the said chiral dopant is in the range between concentrations causing 5% and concentrations causing 120% of the twist angle Φ

of the molecular orientation within the said liquid crystal molecular layer (d^a+d^b+e), when no driving voltage (V) is applied to it.

5. High contrast, wide viewing angle LCD light switching filter according to claim 1 or 2, characterized in

that angle α

between the said first pair of crossed polarizing films (P₁, A₁) and the said symmetry axes (S¹₁, S²₁) in said first LC-cell (LCD1) and between the said second pair of crossed polarizing films (P₂, A₂) and the said symmetry axes (S¹₂, S²₂) in the said second LC-cell (LCD2) is adjusted to maximize the overall viewing angle, while keeping the light attenuation within a cone of +/−

15 degrees around the normal to the LCD light switching filter such that even at a maximum light attenuation of 100,000 the said light attenuation varies less than for a factor 1.6 around the average value within the said 15 degree cone angle and

that the transmission axis of a light output polarizing film (A₁) of said first (P₁, A₁) of said pairs of polarizing films of the first LC-cell (LCD 1) is aligned to be parallel within a small angle t to the transmission axis of a light input polarizing film (P₂) of said second (P₂, A₂) of said pairs of polarizing films of the second LC-cell (LCD2), wherein

the said angle β

is smaller or equal to 15 degrees.

6. High contrast, wide viewing angle LCD light switching filter according to claim 1 or 2, characterized in that the said two LC-cells (LCD1, LCD2) have a complementary configuration, in which all technical parameters are identical except for the liquid crystal molecular alignment directions (2r^a, 2r^b) of the second LC-cell (LCD2), which are swapped for 90 degrees with respect to the first LC-cell (LCD1), so that the symmetry axis (S¹₂) through an acute angle between the molecular alignment directions (2r^a, 2r^b) of the second LC-cell (LCD2) is perpendicular to the symmetry axis (S¹_i) through an acute angle between the molecular alignment directions (1r^a, 1r^b) of the first LC-cell (LCD1).

7. High contrast, wide viewing angle LCD light switching filter according to claim 6, characterized in

that a first (R1) of the negative-birefringent layers (R1, R2) is arranged between the either one of the said boundary glass plates (1a^a, 1a^b) of the first LC-cell (LCD1) and the adjacent polarizing film (P₁) or (A₁) of said first (P₁, A₁) of said pairs of polarizing films and a second (R2) of the negative-birefringent layers (R1, R2) is arranged between the either one of the said boundary glass plates (2a^a, 2a^b) of the second LC-cell (LCD2) and the adjacent polarizing film (P₂) or (A₂) of the second (P₂, A₂) of said pairs of polarizing films and

that the negative-birefringent layers (R1, R2), an optical axis of which is perpendicular to a LCD light switching filter plane, exhibit the same negative optical retardation Δ

n_R⁻

×

d_R⁻

, where Δ

n_R⁻

is the difference between the refractive indices of the ordinary and extraordinary light ray in the said negative-birefringent layers (R1, R2) and d_R⁻

is the thickness of the said negative-birefringent layers (R1, R2), and

that the negative-birefringent layers (R1 respectively R2) are further oriented relatively to the pairs of crossed polarizing films (P₁, A₁ respectively P₂, A₂) so that said intrinsic positive in-plane birefringence of the said layers is aligned with either one of the adjacent crossed polarizers (P₁, Arespectively P₂, A₂) and

that an absolute value of a negative optical retardation Δ

n_R⁻

×

d_R⁻

of the said negative-birefringent layers (R1, R2) is selected to be equal to the difference between an absolute value of a positive optical retardation Δ

n_e⁺×

d_e⁺ of a homeotropically aligned part of the liquid crystal molecular layer (e) in said LC-cells (LCD1, LCD2) and an absolute value of a negative optical retardation Δ

n_p⁻

×

d_p⁻

of the said pairs of polarizing films (P₁, A₁, P₂, A₂) , where

Δ

n_e⁺ is the difference between the refractive indices of the ordinary and extraordinary light ray in the said homeotropically aligned part of the liquid crystal molecular layer (e), (d_e⁺) is the thickness of a said homeotropically aligned liquid crystal molecular layer (e) and Δ

n_p⁻

is the difference between the refractive indices of the ordinary and extraordinary light ray in the polarizing films (P₁, A₁, P₂, A₂) while d_p⁻

is the thickness of the said polarizing films (P₁, A₁, P₂, A₂).

8. High contrast, wide viewing angle LCD light switching filter according to claim 6, characterized in

that said birefringent layers (R1, R2) are negative c-plate birefringent films (R1, R2) with optical retardation values being in the range from 200 to 500 nm and

are additionally oriented with respect to the said LC molecular alignment surface directions (1r^a, 1r^b respectively 2r^a, 2r^b) in such a way that the intrinsic positive in-plane birefringence of the negative c-plate birefringent films (R1 respectively R2) are substantially orthogonal to the residual positive in-plane birefringence of the LC layers in the LC-cells (LCD 1 respectively LCD2) in their optically closed state and which are parallel to the LC molecular alignment symmetry axes (S¹₁, S²₁), wherein

an angle γ

between the intrinsic positive in-plane birefringence of the negative c-plate birefringent films and the residual positive in-plane birefringence of the LC layers in the LC-cells is smaller than 15°

.

9. High contrast, wide viewing angle LCD light switching filter according to claim 8 characterized in

that a light input polarizing film (P₁) of a first (P₁, A₁) of said pairs of polarizing films for the first LC-cell (LCD1) is perpendicular within a small angle a to the symmetry axis (S¹₁) through an acute angle between the LC molecular alignment directions (1r^a, 1r^b) of the first LC-cell (LCD1) and a light input polarizing film (P₂) of a second (P₂, A₂) of said pairs of polarizing films for the second LC-cell (LCD2) is perpendicular within a small angle α

to the symmetry axis (S¹₂) through an acute angle between the liquid crystal molecular alignment directions (2r^a, 2r^b) of the second LC-cell (LCD2), the said angle a being smaller than 15 degrees and that a first (R1) of the negative-birefringent layers (R1, R2) is arranged between the light output boundary glass plate (1a^b) of the first LC-cell (LCD1) and the adjacent light output polarizing film (A₁) and a second (R2) of the negative-birefringent layers (R1, R2) is arranged between the light output boundary glass plate (2a^b) of the second LC-cell (LCD2) and the adjacent light output polarizing film (A₂) and

that both said negative-birefringent layers (R1, R2), the optic axes of which are perpendicular to a LCD light switching filter plane, exhibit the same negative optical retardation Δ

n_R⁻

×

d_R⁻

, where Δ

n_R⁻

is the difference between the refractive indices of the ordinary and extraordinary light ray in the said negative-birefringent layers (R1, R2) and d_R⁻

is the thickness of the said negative-birefringent layers (R1, R2), and

that an absolute value of the negative optical retardation Δ

n_R⁻

×

d_R⁻

of said negative-birefringent layers (R1, R2) is selected to be smaller than the difference between an absolute value of a positive optical retardation Δ

n_e⁺×

d_e⁺ of a homeotropically aligned part of the liquid crystal molecular layer (e) in the LC-cells (LCD1, LCD2) and an absolute value of a negative optical retardation Δ

n_p⁻

×

d_p⁻

of the said pairs of polarizing films (P₁, A₁, P₂, A₂) so that a resulting positive optical retardation (Δ

n_e⁺×

d_e⁺)−

(Δ

n_p⁻

×

d_p⁻

)−

(Δ

n_R⁻

×

d_R⁻

) in the direction of the normal to the LCD light switching filter, in combination with an in-plane residual positive optical retardation of boundary parts of the liquid crystal molecular layer (d^a, d^b) in both LC-cells (LCD1, LCD2) allows for rather efficient additional compensation of an angular dependence of the pairs of crossed polarizing films (P₁, A₁ and P₂, A₂), where

Δ

n_e⁺ is the difference between the refractive indices of the ordinary and extraordinary light ray in the homeotropically aligned part of the liquid crystal molecular layer (e), d_e⁺ is the thickness of said homeotropically aligned part of the liquid crystal molecular layer (e) and Δ

n_p⁻

is the difference between the refractive indices of the ordinary and extraordinary light ray in the polarizing films (P₁, A₁, P₂, A₂) while d_p⁻

is the thickness of the said polarizing films (P₁, A₁, P₂, A₂).

10. High contrast, wide viewing angle LCD light switching filter according to claim 9, characterized in that the thickness d_R⁻

of the said negative-birefringent layers (R1, R2) is selected to be such that the absolute value of their negative optical retardation Δ

n_R⁻

×

d_R⁻

is at least 100 nm but not more than 250 nm smaller than the difference between the absolute value of the positive optical retardation Δ

n_e⁺x d_e⁺ of the homeotropically aligned part of the liquid crystal molecular layer (e) in the LC-cells (LCD1, LCD2) and an absolute value of the negative optical retardation Δ

n_e⁺×

d_e⁺ of the said pairs of polarizing films (P₁, A₁, P₂, A₂).

11. High contrast, wide viewing angle LCD light switching filter according to claim 1 or 2, characterized in that the said two LC-cells (LCD1, LCD2) exhibit also a further complementary configuration, in which all technical parameters are identical for both complementary parts except for

liquid crystal (LC) molecular alignment directions (2r^a, 2r^b) of the second LC-cell (LCD2), which are swapped with respect to the first LC-cell (LCD 1) in such a way that the symmetry axis (S¹₂) through an acute angle between the molecular alignment directions (2r^a, 2r^b) of the second LC-cell (LCD2) is perpendicular to the symmetry axis (S¹₁) through an acute angle between the molecular alignment directions (1r^a, 1r^b) of the first LC-cell (LCD1) and

liquid crystal (LC) chiral doping, which is changed in such a way that the LC molecular twist (Φ

) in the first LC-cell (LCD 1) adopts a reverse chirality as the LC molecular twist (Φ

) in the second LC-cell (LCD2), e.g. LC-cell (LCD1) exhibiting a left handed twist of the LC molecular orientation and LC-cell (LCD2) exhibiting a right handed twist in the electrically non-driven state or vice versa.

12. High contrast, wide viewing angle LCD light switching filter according to claim 11, characterized in that the two LCD light shutters constituting the LCD light switching filter, comprising first crossed polarizing filters (P₁, A₁), first LC-cell (LCD1), first negative-birefringent layer (R1) respectively second crossed polarizing filters (P₂, A₂), second LC-cell (LCD2), second negative-birefringent layer (R2) are constructed to adopt a “

mirror image”

of one another such that;

LC alignment in the first LC-cell (LCD1) exhibits the same alignment twist angle (Φ

) but opposite chirality as the LC alignment in the second LC-cell (LCD2) and LC alignment symmetry axis (S¹₁) in the first LC-cell (LCD1) is substantially perpendicular within a small angle β

≦

15°

to the LC alignment symmetry axis (S¹₂) in the second LC-cell (LCD2) and

relative angle (α

₁) between the LC alignment symmetry axes of the first LC-cell (LCD1) with the adjacent crossed polarizers (P1, A1) has the opposite sign (+/−

) with respect to the relative angle (α

₂) between the LC alignment symmetry axes of the second LC-cell (LCD2) with the adjacent crossed polarizers (P2, A2) and

relative alignment within a small angle γ

₁≦

12°

of the intrinsic positive in-plane birefringence of the negative-birefringent layer (R1) with the LC alignment symmetry axis (S²₁) of the first LC-cell (LCD1) has the opposite sign (+/−

) with respect to the relative alignment within a small angle γ

<sub>2b ≦

12°</sub>

of the intrinsic positive in-plane birefringence of the negative-birefringent layer (R2) with the LC alignment symmetry axis (S²₂) of the second LC-cell (LCD2).

13. High contrast, wide viewing angle LCD light switching filter according to claim 12 characterized in

that a light input polarizing film (P₁) of a first (P₁, A₁) of said pairs of polarizing films for the first LC-cell (LCD1) is perpendicular within a small angle (+α

) to the symmetry axis (S¹₁) through an acute angle between the LC molecular alignment directions (1r^a, 1r^b) of the first LC-cell (LCD1) and a light input polarizing film (P₂) of a second (P₂, A₂) of the said pairs of polarizing films for the second LC-cell (LCD2) is perpendicular within a small angle (−

α

) to the symmetry axis (S¹₂) through an acute angle between the liquid crystal molecular alignment directions (2r^a, 2r^b) of the second LC-cell (LCD2), the said angle (α

) being smaller than 15 degrees and

that a first (R1) of said negative-birefringent layers (R1, R2) is arranged between the light output boundary glass plate (1a^b) of the first LC-cell (LCD 1) and the adjacent light output polarizing film (A₁) and a second (R2) of said negative-birefringent layers (R1, R2) is arranged between the light output boundary glass plate (2a^b) of the second LC-cell (LCD2) and the adjacent light output polarizing film (A₂) and

that both said negative-birefringent layers (R1, R2), the optic axes of which are perpendicular to a LCD light switching filter plane, exhibit the same negative optical retardation Δ

n_R⁻

×

d_R⁻

, where Δ

n_R⁻

is the difference between the refractive indices of the ordinary and extraordinary light ray in the said negative-birefringent layers (R1, R2) and dR is the thickness of the said negative-birefringent layers (R1, R2), and

that an absolute value of the negative optical retardation Δ

n_R⁻

×

d_R⁻

of said negative-birefringent layers (R1, R2) is selected to be smaller than the difference between an absolute value of a positive optical retardation An_e⁺×

d_e⁺ of a homeotropically aligned part of the liquid crystal molecular layer (e) in the LC-cells (LCD1, LCD2) and an absolute value of a negative optical retardation Δ

n_p⁻

×

d_p⁻

of the said pairs of polarizing films (P₁, A₁, P₂, A₂) so that a resulting positive optical retardation (Δ

n_e⁺×

d_e⁺)−

(Δ

n_p⁻

×

d_p⁻

)−

(Δ

n_R⁻

×

d_R⁻

) in the direction of the normal to the LCD light switching filter, in combination with an in-plane residual positive optical retardation of boundary parts of the liquid crystal molecular layer (d^a, d^b) in both LC-cells (LCD1, LCD2) allows for rather efficient additional compensation of an angular dependence of the pairs of crossed polarizing films (P₁, A₁ and P₂, A₂), where

Δ

n_e⁺ is the difference between the refractive indices of the ordinary and extraordinary light ray in the homeotropically aligned part of the liquid crystal molecular layer (e), d_e⁺ is the thickness of said homeotropically aligned part of the liquid crystal molecular layer (e) and Δ

n_p⁻

is the difference between the refractive indices of the ordinary and extraordinary light ray in the polarizing films (P₁, A₁, P₂, A₂) while d_p⁻

is the thickness of the said polarizing films (P₁, A₁, P₂, A₂).

14. High contrast, wide viewing angle LCD light switching filter according to claim 13, characterized in that the thickness dR of the said negative-birefringent layers (R1, R2) is selected to be such that the absolute value of their negative optical retardation Δ

n_R⁻

×

d_R⁻

is at least 100 nm but not more than 250 nm smaller than the difference between the absolute value of the positive optical retardation Δ

n_e⁺×

d_e⁺ of the homeotropically aligned part of the liquid crystal molecular layer (e) in the LC-cells (LCD1, LCD2) and an absolute value of the negative optical retardation Δ

n_p⁻

×

d_p⁻

of the said pairs of polarizing films (P₁, A₁, P₂, A₂).