Color display device

1. A display layer having a first, viewing surface and a second surface on the opposed side of the display layer from the first surface, the display layer further comprising an electrophoretic medium sandwiched between two electrodes and comprising a solvent or solvent mixture and high positively charged particles, high negatively charged particles, low positively charged particles, and low negatively charged particles all dispersed in the solvent or solvent mixture, the four types of particles having respectively optical characteristics differing from one another, such that:

(a) application between the two electrodes of a high potential difference having a polarity driving the high positively charged particles towards the viewing surface will cause the display layer to display the optical characteristic of the high positively charged particles at the viewing surface;

(b) application between the two electrodes of a high potential difference having a polarity driving the high negatively charged particles towards the viewing surface will cause the display layer to display the optical characteristic of the high negatively charged particles at the viewing surface;

(c) once the optical characteristic of the high positively charged particles is displayed at the viewing surface, application between the two electrodes of a low potential difference having a polarity driving the low negatively charged particles towards the viewing surface will cause the display layer to display the optical characteristic of the low negatively charged particles at the viewing surface; and

(d) once the optical characteristic of the high negatively charged particles is displayed at the viewing surface, application between the two electrodes of a low potential difference having a polarity driving the low positively charged particles towards the viewing surface will cause the display layer to display the optical characteristic of the low positively charged particles at the viewing surface.

2. A display layer according to claim 1 , wherein the charge on the low charged particles is less than 50% of the charge on the high charged particles.

3. A display layer according to claim 1 , wherein the charge on the low charged particles is less than 75% of the charge on the high charged particles.

4. A display layer according to claim 1 , wherein the low potential difference is from 5% to 50% of the high potential difference.

5. A display layer according to claim 1 , wherein the electrophoretic medium further comprises substantially uncharged neutral buoyancy particles.

6. An electrophoretic medium having a first, viewing surface and a second surface on the opposed side of the electrophoretic medium from the first surface, the electrophoretic medium being sandwiched between two electrodes and comprising a solvent or solvent mixture and high positively charged particles, high negatively charged particles, low positively charged particles, and low negatively charged particles all dispersed in the solvent or solvent mixture, the four types of particles having respectively optical characteristics differing from one another, such that:

(a) application between the two electrodes of a high potential difference having a polarity driving the high positively charged particles towards the viewing surface will cause the display layer to display the optical characteristic of the high positively charged particles at the viewing surface;

(b) application between the two electrodes of a high potential difference having a polarity driving the high negatively charged particles towards the viewing surface will cause the display layer to display the optical characteristic of the high negatively charged particles at the viewing surface;

(c) once the optical characteristic of the high positively charged particles is displayed at the viewing surface, application between the two electrodes of a low potential difference having a polarity driving the low negatively charged particles towards the viewing surface will cause the display layer to display the optical characteristic of the low negatively charged particles at the viewing surface; and

(d) once the optical characteristic of the high negatively charged particles is displayed at the viewing surface, application between the two electrodes of a low potential difference having a polarity driving the low positively charged particles towards the viewing surface will cause the display layer to display the optical characteristic of the low positively charged particles at the viewing surface.

7. An electrophoretic medium according to claim 6 , wherein the charge on the low charged particles is less than 50% of the charge on the high charged particles.

8. An electrophoretic medium according to claim 6 , wherein the charge on the low charged particles is less than 75% of the charge on the high charged particles.

9. An electrophoretic medium according to claim 6 , wherein the low potential difference is from 5% to 50% of the high potential difference.

10. An electrophoretic medium according to claim 6 further comprising substantially uncharged neutral buoyancy particles.

11. A method of driving a display comprising an electrophoretic medium and having a first, viewing surface and a second surface on the opposed side of the electrophoretic medium from the first surface, the electrophoretic medium comprising a solvent or solvent mixture and high positively charged particles, high negatively charged particles, low positively charged particles, and low negatively charged particles all dispersed in the solvent or solvent mixture, the four types of particles having respectively optical characteristics differing from one another,

which method comprises, in any order:

(a) applying across the electrophoretic medium a high electric field having a polarity driving the high positively charged particles towards the viewing surface to cause the display layer to display the optical characteristic of the high positively charged particles at the viewing surface;

(b) applying across the electrophoretic medium a high electric field having a polarity driving the high negatively charged particles towards the viewing surface to cause the display layer to display the optical characteristic of the high negatively charged particles at the viewing surface;

(c) once the optical characteristic of the high positively charged particles is displayed at the viewing surface, applying across the electrophoretic medium a low electric field having a polarity driving the low negatively charged particles towards the viewing surface to cause the display layer to display the optical characteristic of the low negatively charged particles at the viewing surface; and

(d) once the optical characteristic of the high negatively charged particles is displayed at the viewing surface, applying across the electrophoretic medium a low electric field having a polarity driving the low positively charged particles towards the viewing surface to cause the display layer to display the optical characteristic of the low positively charged particles at the viewing surface.

12. A driving method according to claim 11 , wherein the low electric field is 5% to 50% of the high electric field.

13. A method of driving an electrophoretic medium comprising a solvent or solvent mixture and high positively charged particles, high negatively charged particles, low positively charged particles, and low negatively charged particles all dispersed in the solvent or solvent mixture, the four types of particles having respectively optical characteristics differing from one another,

which method comprises, in any order:

(a) applying across the electrophoretic medium a high electric field of one polarity to cause the electrophoretic medium to display the optical characteristic of the high positively charged particles;

(b) applying across the electrophoretic medium a high electric field of the opposite polarity to cause the electrophoretic medium to display the optical characteristic of the high negatively charged particles;

(c) once the optical characteristic of the high positively charged particles is displayed, applying across the electrophoretic medium a low electric field of said opposite polarity to cause the electrophoretic medium to display the optical characteristic of the low negatively charged particles; and

(d) once the optical characteristic of the high negatively charged particles is displayed at the viewing surface, applying across the electrophoretic medium a low electric field of said one polarity to cause the electrophoretic medium to display the optical characteristic of the low positively charged particles.

14. A driving method according to claim 13 wherein at least one of steps (a) to (d) comprises application of a shaking waveform prior to application of the electric field.

15. A display device comprising two sub-pixels, each sub-pixel comprising an electrophoretic medium according to claim 6 and comprising black particles and three types of non-black particles, the two sub-pixels combined comprising particles of at least four different non-black colors.

16. A display device according to claim 15 wherein the two sub-pixel combined comprise particles of at least six different non-black colors.