Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Patent
1996-05-09
1998-11-17
Hjerpe, Richard A.
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
345 87, 345 94, 345 99, 345101, G09G 336
Patent
active
058382925
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method of addressing an optical cell which comprises a layer of material sandwiched between a pair of electrodes, the material having an optical property which is switchable from a first stable state to a second stable state by applying a voltage of one polarity and a given duration between the electrodes and from, the second stable state to the first stable state by applying a voltage of the other polarity and the given duration between the electrodes, the magnitude of the voltage required between the electrodes to switch the optical property from either stable state to the other stable state being subject to different thresholds for different parts of the total area of the layer, which thresholds vary with temperature, in which method a first voltage is applied between the electrodes, the first voltage having the one polarity and a magnitude and duration which are appropriate to ensure that the optical property attains the second stable state over the total area of the layer, after which a second voltage is applied between the electrodes, the second voltage having the other polarity and a magnitude and duration which are appropriate to ensure that, at a given temperature, the optical property is switched by the second voltage from the second stable state to the first stable state over only a portion of the total area of the layer.
A method of the above general kind is disclosed, for example, in EP-A-0240010.
Cells containing material which is electrically addressable to change its optical property, for example between a light-transmissive state and a non light-transmissive state, are commonly proposed for use in displays or printer applications. An array of such cells may be formed, for example, by means of a pair of transparent substrates sandwiching a layer of ferroelectric liquid crystal material between them, and each carrying a set of transparent electrodes oriented so as to cross each other to define a matrix of pixels. In such a case each pixel can be addressed by applying an electrical signal to the corresponding member of each set of electrodes.
Pixels with varying switching thresholds over their areas may be formed, for example, with one or both sets of electrodes having varying thicknesses across their width so that an applied voltage between overlapping electrodes produces differing electric fields across the width of the pixel, which may be sufficient to cause switching of the material in some areas but not in others. As an alternative they may be formed, for example, using an alignment control layer which has different alignment control powers at different regions over the area of each pixel. Both of these possibilities are disclosed in the aforementioned EP-A-0240010. The variation is preferably continuous; if the variation has a stepped form the steps are preferably small and many in number.
A problem with prior art methods of addressing such matrices is that the switching threshold of the material may vary with temperature, so that for a given applied voltage, as the temperature varies so does the amount of the pixel which switches, and thus the brightness level or grey level obtained.
It is an object of the present invention to alleviate this problem of the known prior art.
According to the present invention, a method as defined in the first paragraph is characterised in that, after the application of the second voltage, a third voltage is applied between the electrodes, the third voltage having the one polarity and a magnitude and duration which are appropriate to ensure that, at the given temperature, the optical property is switched back by the third voltage to the second stable state over only a portion of that portion of the total area of the layer over which the optical property has been switched to the first stable state by the second voltage.
The switchings by the second and third voltages may have substantially the same temperature dependance, so that the amount of variation of the brightness level with temperature can be reduced.
If the second volta
Central Research Laboratories Ltd.
Chang Kent
Hjerpe Richard A.
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