Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1994-08-29
2001-01-23
Nguyen, Chanh (Department: 2775)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S904000
Reexamination Certificate
active
06177915
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a display system and more particularly to a display system including a Liquid Crystal Display (LCD) panel comprising an array of individually addressable pixel cells.
BACKGROUND ART
LCD screens for such display systems include passive LCD screens and Thin Film Transistor (TFT) LCD screens.
A passive LCD panel includes two orthogonal arrays of parallel conductive tracks in the form of rows and columns. A layer of liquid crystal material is placed between the two arrays thereby forming a capacitor at each intersection of the orthogonal arrays. The capacitor of an intersection is charged by placing a voltage across the corresponding conductive tracks. When the capacitor is charged, a light path is produced through the liquid crystal material at the intersection thereby generating a pixel cell.
In a TFT LCD, the liquid crystal material is placed between a planar electrode and an array of separate pixel electrodes. Each pixel electrode is coupled to the drain of a transistor switch. The transistor switch is located at the intersection of two orthogonal conductive tracks (row and column tracks). The source of the transistor is coupled to the column track and the gate is coupled to the row track. The transistor switch turns on when a voltage is applied on the row track. In response to the transistor turning on, the capacitor formed between the pixel electrode and the planar electrode charges up to a data voltage applied to the column track. When the transistor is subsequently turned off, the charge stored in the capacitor remains. A light path is produced through the liquid crystal material at the pixel electrode thereby generating the pixel cell.
Passive and TFT LCD screens can exhibit a brightness non-uniformity when the displayed image is generated by a grey scale video signal. The non-uniformity error takes the objectionable visual appearance of spurious brightness variations distributed across the LCD panel. These variations limit the quantity of grey scale brightness levels that can unambiguously be generated.
The brightness non-uniformity can arise from variations in thickness of the liquid crystal layer. This effect is particularly significant where the liquid crystal layer is made thin (typically 4 um) to reduce the transient response period of the LCD panel. In reduced layer thickness LCD screens, any slight variation in the layer thickness causes a corresponding variation in brightness. In colour LCDs, further thickness variations can be introduced by colour filter layers. These further variations add to the effect.
The brightness non-uniformity can also arise from variations in molecular orientation of any liquid crystal alignment layers applied to inner surfaces of the LCD panel.
Furthermore, the brightness non-uniformity can arise from variations in electrical characteristics of the row tracks, the column tracks or the thin film transistors (of a TFT LCD), or any combination thereof.
SUMMARY OF THE INVENTION
An aim of the present invention is therefore to provide a display system having an LCD display panel which does not exhibit spurious brightness variations.
According to the present invention there is now provided a display system for displaying a visual image in response to a video signal, comprising: a liquid crystal display panel divided into a plurality of addressable, variable brightness sections; address means for generating a section address corresponding to a section in response to a timing signal; driver means for varying the brightness of the section in response to a brightness signal derived from the video signal; characterised in that the display system further comprises: a memory for storing a predetermined correction signal corresponding to the section; and control means coupled to the driver means for varying the brightness signal to reduce brightness non-uniformities in the displayed image in response to the video signal and the correction signal in combination.
This has an advantage in that any spurious brightness variations in the image displayed on the LCD panel can now be removed by generating appropriate correction data during the manufacture of the display system and storing this correction data in the memory for retrieval during the operation of the display system.
In a particularly preferred embodiment of the present invention to be described later, each section comprises a plurality of addressable, variable brightness pixel cells. However, it will be appreciated that in other embodiments of the present invention, each section may comprise a single variable brightness pixel cell.
In one preferred embodiment to be described later, the control means comprises a summing circuit for adding the correction signal to the video signal to produce a summed signal for determining the brightness signal.
In another preferred embodiment to be described later the control means includes a voltage control circuit for varying the amplitude of the brightness signal in response to the correction signal.
In the preferred embodiments of the present invention to be described later, the memory comprises a Programmable Read Only Memory wherein each correction signal is stored in the memory in the form of a two bit binary number. In this example of the present invention, the memory is operable for storing a plurality of correction signals in the form of a look up table wherein each correction signal corresponds to a different section of the LCD panel.
In a preferred example of a display system of the present invention, there is provided a section decoder for generating a memory address for addressing the correction signal stored in the the memory in response to the section address. However, it will be appreciated that in other examples of the present invention, the memory address may be generated by a computer system operating under the instruction of a computer program.
These and other embodiments of the present invention have the advantage that the electrical circuitry associated with the display system of the present invention can be included in an inexpensive and simple integrated circuit package. A display system of the present invention can therefore be produced without significantly affecting manufacturing costs.
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Beeteson John S.
Pietrzak Christopher C.
International Business Machines - Corporation
Nguyen Chanh
Scully Scott Murphy & Presser
Underweiser Marian
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