Ferroelectric liquid crystal displays with digital greyscale

• US 5,905,482 A
• Filed: 10/29/1996
• Issued: 05/18/1999
• Est. Priority Date: 04/11/1994
• Status: Expired due to Term

• Current Assignee: Cufer Asset Ltd LLC (Intellectual Ventures LLC)
• Original Assignee: The secretary of state for defence in her Britannic Majesty's Government of the United Kingdom of Great Britain And Northern Ireland
• Inventors: Graham, Alastair, Hughes, Jonathan Rennie, Towler, Michael John, Raynes, Edward Peter
• Primary Examiner(s): NGUYEN, CHANH DUY
• Application Number:
  US08/722,062
• Time in Patent Office: 931 Days
• Field of Search: 345/87, 345/88, 345/89, 345/92, 345/94, 345/96, 345/97, 345/99, 349/33, 349/34, 349/36, 349/39, 349/42
• US Class Current: 345/89
• CPC Class Codes: G09G 2310/061 for resetting or blanking
• CPC Class Codes: G09G 2320/0626 for control of overall brightness
• CPC Class Codes: G09G 2360/144 the light being ambient light
• CPC Class Codes: G09G 3/2018 by time modulation using two or more time intervals
• CPC Class Codes: G09G 3/2074 using sub-pixels
• CPC Class Codes: G09G 3/3629 using liquid crystals having memory effects, e.g. ferroelectric liquid crystals
• CPC Class Codes: G09G 3/364 with use of subpixels

Abstract

The invention provides a ferroelectric liquid crystal display with uniformly spaced greyscale levels. The invention uses a bistable ferroelectric liquid crystal display formed by a layer of chiral smectic liquid crystal material between two cell walls. The walls carry e.g. line and column electrodes to give an x,y matrix of addressable pixels, and are surface treated to provide bistable operation. Each pixel may be divided into subpixels thereby giving spatial weighting for greyscale. Temporal weighting of greyscale is obtained by switching a pixel to a dark state for time T1 and a light state for time T2. When T1 and T2 are not equal, four different greyscales are obtainable; i.e. dark, dark grey, light grey, and light. The present invention provides a required uniform spacing of greyscale levels by addressing each pixel two or more times in one frame time. Each pixel is blanked then strobed, two or more times in each frame time; the relative times between blanking and strobing, at least four different time periods, are varied to give the desired greyscale levels. The temporal and spatial weighting may be combined to increase the number of obtainable greyscales. Further, the relative intensity between adjacent subpixels may be adjusted to vary the apparent size of the smallest subpixel; this is useful when subpixel size is near to manufacturing limits.

Claims

1. A method of multiplex addressing a bistable liquid crystal display formed by the intersections of an m set of electrodes and an n set of electrodes across a layer of smectic liquid crystal material to provide an mxn matrix of addressable pixels comprising the steps of:

generating m and n waveforms for applying to the m, n electrodes, such waveforms comprising voltage pulses of various dc amplitude and sign;

applying an m-waveform to each electrode in the m set of electrodes in a sequence whilst applying appropriate one of two n-waveforms to the n set of electrodes to address each pixel along a given m electrode into a required state;

addressing each pixel at least two times in a given frame time, the addressing being by application of a blanking waveform followed or preceded by a strobe waveform in combination with one of two data waveforms, the time between application of blanking and strobe being an addressing time; and

varying the addressing time and relative times of addressing each pixel within the frame time to provide a required greyscale intensity interval between different greyscale levels.

2. The method of claim 1 wherein the blanking waveform is replaced by a strobe pulse in combination with two data waveforms.

3. The method of claim 1 wherein the pixels are complete pixels.

4. The method of claim 1 wherein the pixels are formed by combinations of two or more subpixels of the same or different size.

5. The method of claim 4 wherein the relative intensities per unit area between adjacent subpixels is different.

6. The method of claim 1 wherein the addressing sequence of electrodes 1 to M is given by:

(1;

r₂ +r₃ + . . . +r_x +1;

r₃ + . . . +r_x +1;

. . . ;

r_x +1) for electrodes R.y+(1 to R) (y=0, 1, 2, 3, . . . (M/R)-1);

(2;

r₂ +r₃ + . . . +r_x +2;

r₃ + . . . +r_x +2;

. . . ;

r_x

     2) for electrodes 1+ R.y+(1 to R)! (y=0, 1, 2, 3, . . . , (M/R)-1);

(3;

r₂ +r₃ + . . . +r_x +3;

r₃ + . . . +r_x 3;

. . . ;

r_x +3) for electrodes 2+R.y+(1 to R) (y=0, 1, 2, 3, . . . , (M/R)-1);

(R;

r₂ +r₃ + . . . +r_x +R;

r₃ + . . . +r_x +R;

. . . ;

r_x +R) for electrodes R y+(1 to R) (y=0, 1, 2, 3, . . . (M/R)-1)

where r₁ ;

r₂ ;

r₃ ;

. . . ;

r_x (x is number of bits of greyscale), R equal the summation of r_i (for i=1 to x).

7. A multiplex addressed liquid crystal display comprising:

a liquid crystal cell including a layer of ferroelectric smectic liquid crystal material contained between two walls, an m set of electrodes on one wall and an n set of electrodes on the other wall arranged to form collectively an m,n matrix of addressable pixels;

waveform generators for generating m and n waveforms comprising voltage pulses of various dc amplitude and sign in successive time slots (ts) and applying the waveforms to the m and n sets of electrodes through driver circuits;

means for controlling the application of m and n waveforms so that a desired display pattern is obtained;

means for addressing each pixel a first time and a second or more times in a given frame time, the addressing being by application of a blanking waveform followed or preceded by a strobe waveform in combination with one of two data waveforms, the time between application of blanking and strobe being an addressing time; and

varying the addressing time and relative times of addressing each pixel within the frame time to provide a required greyscale intensity interval between different greyscale levels.

8. A multiplex addressed liquid crystal display comprising:

a liquid crystal cell including a layer of ferroelectric smectic liquid crystal material contained between two walls, an m set of electrodes on one wall and an n set of electrodes on the other wall arranged to form collectively an m,n matrix of addressable pixels;

waveform generators for generating m and n waveforms comprising voltage pulses of various dc amplitude and sign in successive time slots (ts), applying the waveforms to the m and n sets of electrodes through driver circuits, controlling the application of m and n waveforms so that a desired display pattern is obtained and addressing each pixel a first time and a second or more times in a given frame time, said addressing by application of a blanking waveform followed or preceded by a strobe waveform in combination with one of two data waveforms, the time between application of blanking and strobe waveforms comprising an addressing time, and varying the addressing time and relative times of addressing each pixel within the frame time to provide a required greyscale intensity interval between different greyscale levels.