CA2006244C

CA2006244C - Liquid crystal display device

Info

Publication number: CA2006244C
Application number: CA002006244A
Authority: CA
Inventors: Haruhiro Matino; Toshihiro Ueki
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1988-12-21
Filing date: 1989-12-20
Publication date: 1993-08-31
Anticipated expiration: 2009-12-20
Also published as: DE68915414T2; CA2006244A1; EP0375269B1; JPH02166419A; EP0375269A2; US5121235A; EP0375269A3; DE68915414D1

Abstract

ABSTRACT

The disclosure describes the provision of a liquid crystal display device which permits of multi-gradation display merely he the application and non-application of a voltage to a liquid crystal without increasing an area of one display unit and without the necessity for fine adjustment of the voltage applied to the liquid crystal of each pixel.

Description

2~al6~:4~

LIQUID CRYSTAL DISPLAY DEVICE

The present invention relates to a liquid crystal device allowing multi-gradation display.

PRIOR ART

A liq~lid crystal display device is disclosed in JA
Published Unexamined Patent Application (PUPA) No.
58-220185, in which one display unit is constituted by four pixels 101, 102, 104, and 108 with area ratios of 1 : 2 : 4 : 8, allowing 16 gradation (gray scale) display by the combination of the selection and non-selection of the foux pixel~.
Also, since the light transmission factor of a liquid crystal varies when a voltage applied to the liguid crystal is varied, it is considered theoretically po~sible to obtain a required gradation by varying a voltage applied to a liquid crystal.

PROBLEMS TO BE SOLVED BY THE INVENTION

The prior art in said PIJPA No. 220185 ha~ to form a pixel 108 having an area eight times that of the pixel 101, which is the smalle~t area, has a limit for decrea~ing the area of one display unit, and can not be, therefore, appl:ied to a liquid crystal display device having high resolution and a large area.

2~06~4 Since change in the light transmission factor of a liguid crystal is very large for a change in a voltage applied to a liquid crystal, it is necessary to provide electronic circuits operating with very high pre~ision in order to obtain many gradations by adjusting a voltage applied to a liquid crystal, which are very difficult -to attain.
An object of the present invention is to provide a liquid crystal display device which allow~ multi-gradation display to be attained by only selection of application and non-application of a voltage to a liquid crystal without needing to increase an area of one display unit and without needing to finely adjust the value of voltage applied to the liquid crystal of each pixel.

MEANS FOR SOLVING THE PROBLEMS

In a liquid crystal display device in which the application of a voltage to a li~uid crystal for each pi~el is controlled, in accordance with the present invention, one display unit is constltuted with n (n is a positive integer) adjacent pixels;
a light transmi~sion ~actor control layer for controlling the liyht transmission factor of the pixel i~
provided for each pixel; and the ratio between light transmission factors of light transmission factor control layers of n pixels in one 2(~0~i~49L

display unit is set as ollows: 2 : 21 : 22 2n-whereby a 2n gradation display is allowed.

BRIEF DESCRIPTION OF THE DRAWINGS:

Fig. 1 is a perspective view showing one embodiment of a liquid cry~tal display device according to the present invention;
Fig. 2 is a diagram illustrating the relation between display units of the liguid crystal display device shown in Fig. 1 and pixels; ~ A
Fig. 3 is a diagram illustrating the fact that the combinations of the light transmission and interception ;
of four pixels in one display unit of the liguid crystal : .
display device shown in Fig. 1 allows 16 gradations to be obtained; ~
Fig. 4 is a graph showing the relation between a RPM . :
and a light transmission factor in the case where a reguired light transmission factor is obtained by rotative painting;
Fig. 5 is a diagram illustrating the relation between a display unit of another embodiment according to the present invention and pixels;
Fig. 6 is a diagram illustrating the fact that the combinations of the light transmission and interception ..
of the three pixels in the display unit shown in Fig. 5 allow~ eight graclation~ to be obtained; and `

: .

, .... .... .. ... . ~ ... . .. .. .

Z~ 4~

Fig. 7 is a diagram illustrating the relation between the display unit and the pixeil according to the prior art.

EMBOD I MENT

Eig. 1 shows an embodiment of a lic~uid crystal di~play clevice according to the present invention which realizes 16 gradations. In the embodiment, a display unit DU of a licluid crystal display device is made to be a square, and the clisplay unit DU is composed of four pixels Tl, T2, T3, and T4 located at positions of four squares formed by dividing the scluare into four egual parts as shown in Fig. 2.
Re~erring to Fig. 1, regions corre~ponding to the pixels Tl, T2, T3, and T4 in Fig. 2 in a transparent glass substr~te 2, what i~ called the TFT array subskrate, are provided with transparent pixel electrodes 11, 12, 13, and 14 made of ingium tin oxide (hereinafter called IT0). The pixel electrodes 11, 12, 13, and 14 are respectively connected to source electrodes 20 of thin-film transistors (hereinafter called a~Si TFT) 16 macle of amorphous silicon, each of which controls the application of a voltage to a liquid crystal in the pixel region. The a-Si TE'T 16 is connected at its gate electrode (not ~hown) to an address wiring 22, and at its drain electrode 18 to a data wiring 23. Al#o, a liquid crystal orientation film (not shown) i~ provided ranging over the whole c)f the substrate 2 insicle the pixel electrode 12.

JA9-8~o?g 4 A transparent glass substrate 4, which i5 called the counter substrate, is disposed so as to be opposite to the TFT array substrate 2. Inside the counter substrate 4 (that is, on the side toward the TFT array substrate 2), light transmission factor control layers 31, 32, 33, and 34 are provided. The light transmission factor control layer~ 31, 32, 33, and 34 are disposed in the regions of the pixel Tl, T2, T3, and T4 shown in Fig. 2, respectively, and the ratio between light transmission ~actors of the layers is set to 2 ~ : 23, that is, ~ : 2 : 4 : 8.
In5ide the light transmission factor control layers 31, 32, 33, and 34 (that is, on the side toward the TFT
array substrate3, a common electrode 36 is provided all over the whole surface of the counter substrate 4. Inside the common electrode 36 a liquid crystal orientation film (not shown) is provided over the whole surface of the substrate 4. A liquid crystal 3 is filled between the TFT
array substrate 2 and the counter substrate 4, i more correctly ~peaking, between an orientation film provided on the pixel electrodes 11, 12, 13, and 14 and an orientation film provided on the common electrode 36. At the rear of the TFT array substrate 2 i~ provided a back illumination source 46, and outside the TFT array ~ubstrate 2 (that i 9, on the s.ide toward the back lllumination source 46) is provided a polarizing plate 40.
Between the bacX illumi.nation source 46 and the polarizing plate 40 is a diffusion plate 44 ~or diffusing light.

:.
~ .

" . . . . .. .. .

21)C)~iZ~L4 Also, outside the counter electrode 4 (that is, on the opposite side of the TFT array substrate 2) is provided a polarizing plate 42.
Upon the application of a gate pulse to the address wiring 22, the a-Si TFT 16 on the wiring 22 is placed in the ON state, and a data voltage on the data wiring 23 at that time i9 applied to the liquid crystal 3. Assuming that the directions of the polarizing plates 40 and 42 of the liquid crystal display device ~hown in Fig. 1 are parallel with each other and the molecular axis of tha liquid crystal is twistecl by 9O0 in a state that no voltage is applied to the licluid crystal 3, when the data voltag0 is applied to the liquid crystal 3, the molecular axis of the licluid crystal is made parallel and light passes through the lic~id crystal. Assuming that the directions of the polarizing plates 40 and 42 of the licluid crystal display device in Fig. 1 are perpendicular with each other and the molecular axis of the liquid crystal is twisted by 9O0 in a stake that no voltage is applied to the liquid crystal 3, when no voltage is applied to the liquid crystal, the liquid crystal allows light to pass through it, and on the contrary, when a voltage is applied to the liquid crystal, the molecular axes of the lic~id crystal are parallel with each other and the liquicl crystal intercept~ light. In both cases, the ON-OFF contxol of the a-Si TFT 16 allows the light tran~mission and interception thrc)ugh the liquid crystal of each pixel to be controlled.

JA9-88~029 6 In this way, the application of voltages to the pixels Tl, T2, T3, and T4 can be independently controlled by the a-Si TFT 16, and the light transmission and interception can be also independently controlled.
Accordingly, sixteen different combinations of light transmission and interception in the pixels Tl, T2, T3, ~, and T4 are available, as shown in Fig. 3.
~i Fig. 3(a) shows a state in which all of the pixels Tl, T2, T3, and T4 intercept light. The amount of light transmission of the whole display unit DU in this case is as follows, as a relative value:

. o = O

l + 2 + 4 + 8 15 .
! Fig. 3(b) shows a state in which the pixel Tl allowslight to pass through it, and the pixels T2, T3, and T4 intercept light. The amount of light transmission of the whole display unit DU in this case is as follows, as a relative value:

___ _ .

3 l ~ 2 ~ 4 + 8 15 ~
.~ ,.
Fig. 3(c) shows a state in whi~h the pixel T2 allows light to pass through it and the pixels Tl, T3, and T4 JA9-88~029 7 ~

,:
~.

~o~
intercept light The amount of light transmission of the whole display unit DU in this case is a~i follows, as a relative value:

2 = 2 ' ;-1 + 2 + 4 + 8 15 Fig. 3(d) shows a state in which the pixel T3 allowslight to pass through it and the pixels Tl, T2, and T4 intercept light. An amount of light transmission of the whole display unit DU in this case is as follows, as a ;.~ ~
relative value: ~ -4 = 4 1 + 2 + 4 + 8 15 ::

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Fig. 3(e) shows a state in which the pixel T4 allows light to pass through it and the pixels Tl, T2, and T3 , i ,.... ...
intercept llght. The amount of light transmission of the whole display unit DU in this case is as follows, as a relative value:

. j .
8 = 8 .... _ ..... _ :, 1 + 2 + 4 + 8 15 .

Fig. 3(f) shows a state in which the pixels T1 and T2 allow light to pass through them and the pixels T3 and T4 intercept light. The amount of light transmission of the whole display unit DU in this case is as follows, as a relative value:

1 + 2 = 3 1 ~ 2 + 4 ~ 8 15 Fig. 3(g) shows a state in which the pixels T2 and T3 allow light to pass through them and the pixels Tl and T4 intercept light. The amount of light transmission of the whole display unit DU in this case is as follows, as a relative value:

2 + 4 = 6 1 ~ 2 + 4 + B 15 Fig. 3(h) shows a state in which the pixels T3 and T4 allow light to pass through them and the pixels Tl and T2 intercept light. The amount of light transmission of the whole display unit DU in this case i9 as follows, as a relative value:

~o~

4 ~ 8 - 12 1 ~ 2 ~ 4 + 8 15 Fig. 3(i) shows a state in which the pixels T1 and T3 allow light to pass through thlem and the pixels T2 and T4 intercept light. The amount of light tra~smis~ion of the whole display unit DU in this case is as follows, as a relative value:

1 ~ ~ - 5 ~ ~
'.:
1 + 2 ~ 4 + 8 15 -~ -Fig 3(j) shows a state in which the pixels T2 and T4 allow light to pass through them and the pixels Tl and T3 ~ ;
intercept light. The amount of light transmission of the whole display unit DU in this case is as follows, as a relative value: , 2 ~ 8 = lO

1 + 2 ~ 4 ~ ~ 15 Fig. 3tk) shows a state in which the pixels T1 and T4 allow light to pass through them and the pixels T2 and T3 intercept light. The amount of light transmlssion of .:
JA9-88-029 10 ~

. , .

~0~62~4 the whole display unit DU in this case is as follows, as a relative value:

1 + 8 = 9 1 + 2 ~ 4 ~ 8 lS

Fig. 3(l) shows a state in which the pixels T2, T3, and T4 allow light to pass through them and the pixel Tl intercepts light. The amount of light transmission of the whole display unit DU in this case is as follows, as a relative value:

2 ~ 4 + 8 = 14 _ ~ . .
1 1 2 ~ 4 + 8 15 Fig. 3(m) shows a state in which th~ pixels T1, T3, and T4 allow light to pass through them and the pixel T2 intercepts light. The amount of light transmission of the whole display unit DU in this case is as follows, as a :~

relative value:

~ + 4 ~ 8 = 13 :

1 ~ 2 + 4 ~ ~ 15 .~:

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Fig. 3(n) shows a state in which the pixels T1, T2, and T4 allow light to pass throuqh them and the pixel T3 intercepts light. The amount of light transmission of the whole display unit DU i~ this case is as follows, as a relative value:

1 + 2 ~ 8 = 11 1 ~ 2 + 4 + 8 15 .. ............. .
Fig. 3~o) shows a state in which the pixels T1, T2, and T3 allow light to pass through them and the pixel T4 intercepts light. The amount of light transmission of the whole display unit DU in this case i5 as follows, as a relative value:

l + 2 + 4 = 7 _ 1 ~ 2 + 4 ~ 8 15 Fig. 3(p) shows a state in which all the pixels T1, T2, T3, and T4 allow light to pass through them. The amount of light transmission of the whole display unit DU
in this case is as follows, as a relative value:

1 ~ 2 + ~ + 8 - 15 1 ~ 2 + 4 + 8 15 JA9-88~029 12 . . .

- ; ! ,i ; ~ ~ , :

As apparent from the above description, if the ratio of the light transmission factors of the pixels Tl, T2, T3, and T4 is left to be 1 : 2 : 4 : 8, the combination of the light transmission and interception of the pixels Tl, T2, T3, and T4 allows the relative amount of light transmissions of 0/15, 1/15, 2/15, 3/15, 4/15, 5/15, 6/15, 7/15, 8/15, 9/15, 10/15, 11/15, 12/15, 13/15, 14/15, and 15/15 to be obtained, which allows 16 gradations to be attained.
In order to make the ratio of the light transmission factors of the pixels T1, T2, T3, and T4 be 1 : 2 : 4 :
8, the light transmission factors of the light transmission factor control layers 31, 32, 33, and 34 need only to be, for example, 12.5%, 25.0%, 50.0%, and 100~.
This can be realized, for example, by controlling an amount of dispersion of carbon black into the acrylic resin. For example, a standard solution (hereinafter called a STD solution) produced by adding a photopolymerization initiator and carbon black to acrylic resin and performing viscosity adjustment, and dilution thereof is rotatively applied with painting at speeds of 520 RPM and 900 RPM, allowing the light transmission factors o 12.5% and 25.0% to be obtained. When the STD
solution diluted :in such a manner that the carbon black component amount:ing to 30% is rotated at the speed of 650 RPM, the light transmissi.on factor of 50.0% can be obtained.

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Fig. 4 shows the relationship between the revolutions per minute and a light transmission factor in the ca~e where a required light transmission factor is obtained by rotative painting. As described abov2, if resin components are provided with photosensitivity by adding a photo polymerization initiator, ,patterns can be obtained by only development. Resin components having no ~ photosensitivity is painted thereon with a photoresist, ! applied with patterning by use of a conventional technique, and then can be processed with etching.
Fig. 5 shows an example of an arrangement of pixels in the case where one display unit is composed of three , adjacent pixels. Three square-shaped pixels Sl, S2, and I S3 located at each apex of a inverse regular triangle compose one display unit, and the ratio of its light transmission factors is " '. ,.'.

¦ that is, ~ , ';.

~ 2 : 4.
'''';' In the case of this arrangement of pixels, eight :
combinations of the light transmission and interception of the pixels Sl, S2, and S3 are available, as shown in , Fig. 6.

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.: ' ~ 0~24~

Fig. 6(a) shows a state in which all of the pixels Sl, S2, and S3 intercept light. The amount of light transmission of the display unil: DU in this case is as follows, as a relative value:

o = o .

~ 1 + 2 ~ 4 7 :..

1, Fig. 6~b) shows a state in which the pixel Sl allows light to pass through it and the pixels S2 and S3 intercept light. The amount of light transmission of the display unit DU in this case is as follows, as a relative ¦ -value: -' = 1 l ~ 2 + 4 7 Fig. 6(c) shows a state in which the pixel S2 allows light to pass through it and the pixels Sl and S3 ¦ intercept light. The amount of light transmission of the i display unit DU in this case is as follows, as a relative valu~:
2 = 2 1 ~ 2 + 4 7 z~

Fig. 6~d) shows a state in which the pixel S3 alloms light to pass through it and the pixels S1 and S2 intercept light. The amount of light transmission of the display unit DU in this case is as follows, as a relative value:

4 = 4 1 + 2 + 4 7 . ~ . .

Fig. 6(e) shows a state in which the pixels S1 and -S2 allow light to pass through them and the pixel S3 intercepts light. The amount of light transmission of the display unit DU in this case is as follows, as a relative value:
: . ~
1 + 2 = 3 ; " - .
1 ~ 2 ~ 4 7 . .

Fig. 6(f) ~hows a state in which the pixels S1 and ~ ~;
S3 allow light to pass through them and the pixel S2 intercepts light. The amount of light transmission of the display unit DU in this case is as follows, as a relative value:

6~44 1 -~ 4 = 5 1 + 2 + 4 7 Fig. 6(g) shows a state in which the pixels S2 and S3 allow light to pass through them and the pixel S1 intercepts li~ht. The amount of light transmission of the display unit DU in this case i3 as follows, as a relative value:

4 = 6 ,."':
1 ~ 2 + 4 7 .:

Fig. 6~h) shows a state in which all the pixels Sl, S2, and S3 allow light to pass through them. The amount -of light transmission of the display unit DU in this case is as follows, as a relative value:

1 ~ 2 + 4 = 7 _ 1 + 2 ~ 4 7 In this way, even if three pixels compose one diæplay unit, if the ratio of the light transmission factors of the three pixels is made to be 1 : 2 : 4, the combinations of the light transmission and interception of the three pixels allows the relative amount of the light `:'.' ,~

,~ '; :

... .. ... . .. . . .. . . . .. . . . .. . ..

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transmissions of 0/7, l/7, 2/7, 3/7, 4/7, 5/7, 6/7 and 7/7 to be obtained, which allows eight gradations to be attained.
The above-mentioned embodiments are those in the cases where one display unit is composed of four and three pixels. The present invention is not limited by the number of these pixels, and can be applied to the case where one display unit is composed of n (n is a positive integer) pixels. In other words, since a pixel for forming one display unit can not be any value except the two values of ON (light transmission) and OEF (light interception), when the ratio of the light transmission .
factors of these pixels is decided in accordance with two exponential functions (2x : x = O, 1, 2, the multi-gradations become most linear. Accordingly, the ration of light transmission factors of n pixels needs only to be 2 2l 22... 2n-In the above-mentioned embodiment, although a TFT is employed as a switching element which controls the application of voltages to liquid crystals in a pixel domain, the present invention is not limited by this, but, for example, MIM (metal - insulator metal) can be employed.
Al~o, the present invention can be applied not only to the above-mentioned so-called active matrix type liquid crystal, but also to a simple matrix type liquid crystal.
The reason is that in a simple matrix type, the application of voltage~ to pixels in the region where an ~ . . .. .. .. .

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X electrode and a Y electrode intersect is selected to control light transmission and :interception for liquid crystals in the pixel region concerned.
Also, in the above-mentioned embodiment, a light transmission factor control layer is provided on the side of the counter electrode 4, it may be provided on the pixel electrodes on the substrate on the side of the switching elements such as TFT. `
The present invention allows a multi-gradation display without the necessity of enlarging an area in one display unit, or of finely adjusting voltages applied to :
the liguid crystals in each pixel.

. ,

Claims

1. A liquid crystal display device comprising:
a plurality of display units, composed of n adjacent pixels, where n is a positive integer, the pixels in each display unit being substantially coplanar so as to define a first plane;
means for applying a control voltage to each of said pixels;
a light transmission control layer having a light transmission factor for each pixel for controlling a relative amount of light transmission from the pixel so that light provided by successive pixels in said display unit is reduced with respect to light provided by a previous pixel, said control layer being disposed in a second plane parallel to and adjacent said first plane; and a ratio between the light transmission factors of the control layers for the n pixels in one display unit being 20 : 21 : 22 : ... 2n-1.

2. A display unit of claim 1 comprising:
a first pixel including a first light transmission control layer having a first light transmission factor for controlling a relative amount of light transmission from said first pixel;
a second pixel including a second light transmission control layer having a second light transmission factory approximately 2 times that of said first light transmission factor;
a third pixel including a third light transmission control layer having a third light transmission factor approximately 4 times that of said first light transmission factor.

3. A display unit according to claim 2 further comprising:
a fourth pixel including a fourth light transmission control layer having a fourth light transmission factor approximately eight times said first light transmission factor.

4. A display unit according to claim 1 comprising:
a first pixel including a first light transmission control layer having a light transmission factor of approximately 100 percent;
a second pixel including a second light transmission control layer having a relative light transmission factor of approximately 50 percent;
a third pixel including a third light transmission control layer having a light transmission factor of approximately 25 percent.

5. A display unit according to claim 4 further comprising:
a fourth pixel including a second light transmission control layer having a relative light transmission factor of approximately 12.5 percent.

6. The liquid crystal display device of claim 1 wherein the pixels in each display unit are of substantially equal area.

7. A liquid crystal device comprising:
a plurality of display units;
each of said display units including n adjacent pixels where n is a positive integer, the pixels in each display unit being substantially coplanar so as to define a first plane;
each of said pixels including a respective light transmission control layer for controlling a relative amount of light transmission from one of said pixels, said respective control layers being disposed in a second plane parallel to and adjacent said first plane, wherein each respective light transmission layer transmits light with a respective transmission factor, and wherein said respective light transmission factors are successive powers of a predetermined ratio.

8. The liquid crystal display device of claim 7, wherein the pixels in each display unit are of substantially equal area.

9. The liquid crystal display device of claim 7, wherein the pixels in each display unit are substantially coplanar.

10. A liquid crystal display device according to claim 7 wherein the respective light transmission factors are all different from one another.

11. A liquid crystal display device according to claim 7 wherein the light transmission factors have ratios of 20 :
21 : 22 : 23 : ... 2n-1.

12. A display unit comprising:
n substantially coplanar pixels disposed in a first plane;
a respective light transmission control layer for each pixel disposed in a second plane adjacent and parallel to said first plane;
a first pixel including a first light transmission control layer having a relative light transmission factor of approximately 1;
a second pixel including a second light transmission control layer having a relative light transmission factor of approximately 2; and a third pixel including a third light transmission control layer having a relative light transmission factor of approximately 4.

13. A display unit according to claim 12 further comprising:
a fourth pixel including a fourth light transmission control layer having a relative light transmission factor of approximately 8.

14. The display unit of claim 12 wherein the pixels in each display unit are of substantially equal area.

15. A liquid crystal display comprising:
a plurality of display units, each display unit including n pixels, the pixels in each display unit being substantially coplanar so as to define a first plane;

said pixels each including a respective light transmission control layer disposed in a second plane parallel to and adjacent said first plane, each said respective control layer having a predetermined light transmission factor for controlling a relative amount of light transmission from a pixel;
a first of said pixels in one of said display units including a light transmission control layer which is more transmissive than said light transmission control layers of said other pixels in said one display unit; and a second of said pixels in said one display unit including a light transmission control layer which is less transmissive than said light transmission control layer of any of said other pixels in said one display unit;
said light transmission control layer of said first pixel having a transmission factor 2n-1 greater than said transmission control layer of said second pixel.

16. The liquid crystal display device of claim 9, wherein the pixels in each display unit are of substantially equal area.