US7746544 was granted on 29 June 2010 and looks to be a great reference patent describing novel electrophoretic displays and materials useful in making such displays. The invention particularly relates to electro-osmotic displays and in particular to those where the encapsulated liquids used in the display contain a dye. The original application contained 9 claims but in the granted patent only three amended claims were allowed. These are shown below:
1. An encapsulated electrophoretic display, comprising a capsule containing at least two immiscible fluids, each fluid having a different refractive index such that the fluids create a first optical effect, wherein at least one of the fluids within the capsule moves to create a second optical effect in response to an electric field, and wherein at least one of the fluids contains a dye.
2. The display of claim 1, wherein a planar index mismatch results from the motion of the fluids in the capsule.
3. The display of claim 1, wherein a non-planar index mismatch results from the motion of the fluids in the capsule.
The E-ink website has some good technical overviews of how EPDs work and the reader can view these by clicking here. Their website does not yet explain the basics of a colour version but the patent describes how a multi-colour display can be made through the use of cyan magenta and yellow particles in the dispersion recreating a full colour display in the way a printed page is produced. Here is the extract from the patent:
As an example of a multi-color, encapsulated electrophoretic display, there may be magenta particles with an average zeta potential of 100 mV, cyan particles with an average zeta potential of 60 mV, and yellow particles with an average zeta potential of 20 mV. To address this display to the magenta state, all the particles are pulled to the back of the cell by applying an electric field in one direction. Subsequently, the field is reversed for just long enough for the magenta particles to move to the top face of the display cell. The cyan and yellow particles will also move in this reversed field, but they will not move as quickly as the magenta particles, and thus will be obscured by the magenta particles.
To address the display to the cyan state, all the particles are pulled to the back of the cell by applying an electric field in one direction. Then the field is reversed for just long enough for the magenta and cyan particles to move to the top face of the display cell. The field is then reversed again and the magenta particles, moving faster than the cyan particles, leave the cyan particles exposed at the top of the display.
Finally, to achieve a yellow display, all the particles are pulled to the front of the display. The field is then reversed and the yellow particles, lagging behind the magenta and cyan particles are exposed at the front of the display.