Liquid crystal cells – elements and systems – Particular structure – Interconnection of plural cells in series
Reexamination Certificate
1996-03-27
2001-02-06
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Interconnection of plural cells in series
C349S075000, C349S078000, C349S086000, C349S117000
Reexamination Certificate
active
06184951
ABSTRACT:
The present invention relates to a display. Such a display may, for instance, be used as a high resolution reflective colour display for “personal digital assistant” (PDA) applications and for high light efficiency projection displays.
A known type of display uses polymer dispersed liquid crystals (PDLCs) which are switchable between a clear and a light scattering state. No polariser is necessary for such displays so that, by placing a 100% absorber beneath them, a good approximation to black print-on-paper contrast and brightness can be produced. The electro-optic response curve of known PDLC materials requires the use of active matrix addressing in order to provide an “XY panel”.
Although it is possible to adapt such displays for providing a colour display, the resulting display has disadvantages. For instance, by providing cyan, magenta, and yellow colour absorbers under the PDLC, a maximum absorption of only 33% is obtained. This results in the maximum black level which can be displayed being a relatively bright grey. Using red, green, and blue absorbers under the PDLC gives a 66% maximum absorption resulting in an unsatisfactorily low contrast ratio in reflected light, particularly when back-scatter from the PDLC is included.
A further disadvantage of PDLC displays is that the fully clear state is oily realised for on-axis viewing, whereas it is desirable for high contrast to be provided over a substantial viewing angle. This is caused by the anisotropic liquid crystal being dispersed in an isotropic polymer. Although this problem can be largely overcome by dispersing the liquid crystal in a liquid crystal polymer having similar anisotropy, the anisotropies of the materials have to be matched in planes both perpendicular and parallel to the display cell, for instance by initially poling the structure by a high electric field. This adds to the cost and complexity of manufacturing such displays.
EP-A-0509727 discloses a liquid crystal display of the reflection type in which one layer is pixellated and controls light attenuation and another layer comprises three electrically controllable colour, unpixellated colour filters disposed optically in series. Colour images are obtained by time division multiplexing such that each filter can be activated in turn.
U.S. Pat. No. 4,842,379 disclosed image recording apparatus utilising a liquid crystal shutter array. The shutter has a pixellated layer for controlling light attenuation and an electrically controllable, unpixellated birefringent layer for controlling colour. The shutter works in time multiplexed mode to enable different colours to be passed in turn.
The above-described multiplexing systems have the disadvantage of needing a liquid crystal device capable of being addressed and responding at multiple video-frame rate. This is not required in the present invention
According to the invention, there is provided a display comprising a first layer of pixels, each of which has an independently controllable light attenuation, and a second layer of pixels, each of which provides an independently controllable colour.
Preferred embodiments of the invention are defined in the other appended claims.
The present invention makes it possible to provide a display having a two layer pixel structure in which the first layer of display pixels controls the light intensity of each image pixel (i.e. the pixel perceived by the viewer) and the second layer of display pixels controls the chrominance of each image pixel. Such a display is capable of providing a good range of colours while improving the resolution by a factor of three compared with known RGB displays in which three display pixels are required for colouring each image pixel. Also, the effective brightness of the display can be increased because each display pixel of the second layer can be made to display the required colour. Such displays may be used as transmissive or reflective displays and an appropriate technologies may be used to embody the two layers so as to provide pixellated intensity control and pixellated chrominance control.
For chrominance control technologies which do not provide the full required colour palette, two or more display pixels of the second layer may be allocated to each image pixel so as to extend the perceived colour palette by visual integration of the colours produced by the two display pixels. Although this reduces the resolution, nevertheless the resolution is still greater than for known RGB displays. Only those parts of the display which are required to display such colours need to be controlled in this way, the remaining parts of the display having a single display pixel in the second layer corresponding to each image pixel.
Alternatively, where lower resolution is required, the display pixels may be made larger. This provides enhanced brightness and contrast compared with known displays and relaxes manufacturing requirements so as to improve manufacturing yield and/or reduce manufacturing cost.
In order to obtain a desired colour in HLS (hue, lightness, saturation) colour space, different components may be provided in different ways in reflective and transmissive embodiments of the display. In transmissive embodiments, hue is defined by the second layer, lightness is defined by the first layer, and saturation is defined by controlling an adjacent pixel to be white or grey. In reflective embodiments, hue is defined by the second layer, lightness is defined by controlling an adjacent pixel to be black or grey, and saturation is defined by the first layer.
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Scheffer et al, “Liquid Crystals—Applications and Uses,” vol. 1, pp. 269-270, 1990, 10.3.6.4 Substractive Color STN Displays.
Harrold Jonathan
Tillin Martin David
Chowdhury Tarifur R.
Renner , Otto, Boisselle & Sklar, LLP
Sharp Kabushiki Kaisha
Sikes William L.
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