Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-03-12
2001-03-13
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S201000, C349S033000
Reexamination Certificate
active
06201589
ABSTRACT:
The present invention relates to a spatial light modulator and to a display.
GB 9611993.8 discloses a diffractive spatial light modulator and display. A high resolution electrode structure is used to switch a ferroelectric liquid crystal (FLC) into fine pitch regions suitable for the diffraction of light, for instance for use in a high brightness projection display. Each picture element (pixel) is provided with an interdigitated electrode structure such that alternate strips of the FLC may be switched into the same optical state or into different optical states. When all of the strips are switched to the same state, the pixel does not diffract light, which therefore passes through the pixel into the zeroth order of diffraction. An optical system for gathering light from the pixels is generally arranged not to gather light in this mode so that the pixel appears dark.
When alternate strips of the FLC are switched to different optical states, the pixel acts as a diffraction grating. For instance, the interdigitated strips of FLC may apply different phase delays, for instance differing by 180 degrees, to light passing therethrough. The pixel acts as a diffraction grating with light being diffracted into the non-zeroth diffraction orders where it is collected by the associated optical system so that the pixel appears bright.
According to a first aspect of the invention, there is provided a spatial light modulator comprising an addressing circuit and a plurality of picture elements, each of which has a plurality of first elongate electrodes and a plurality of second elongate electrodes interdigitated with the first electrodes, the first electrodes being connected to the addressing circuit, characterised in that the second electrodes are electrically floating.
The term “electrically floating” as used herein means that there is no electrical connection for establishing a specific voltage on the second electrodes. Thus, the voltage on the second electrodes is determined by the effects of voltages on adjacent conductors and stray capacitance, inductance and resistance resulting from device geometry and materials. This term is well known in the field of electronics and is used in its conventional sense.
The use of floating second electrodes substantially reduces the number of connections which are required. Design and manufacture are therefore simplified and cost is reduced compared with arrangements in which the second electrodes have to be connected to the addressing circuit.
Preferably the modulator comprises a layer of electro-optic material. This material may be a liquid crystal, such as a ferroelectric liquid crystal.
The first and second electrodes may be disposed adjacent a first surface of the layer. Each of the picture elements may have a third electrode disposed adjacent a second surface of the layer.
The picture elements may be arranged as rows and columns and the first and second electrodes may extend in the direction of the columns. At least some of the picture elements in each column may share common first and second electrodes. By providing extended second electrodes which are shared with several rows of picture elements, the voltage of the second electrodes tends to be more stabilised towards a desired voltage. The first electrodes may be connected to a data signal generator of the addressing circuit.
The third electrodes may extend in the direction of the rows. At least some of the picture elements in each row may share a common third electrode. The third electrodes may be connected to a strobe signal generator of the addressing circuit Such arrangements permit the use of conventional passive matrix addressing techniques.
Preferably each of the picture elements is switchable between a diffractive state and a non-diffractive state. The connection arrangements for diffractive spatial light modulators are thus substantially simplified. Further, the first and second interdigitated electrodes can form a parasitic grating which diffracts light, albeit with poor efficiency, irrespective of whether a pixel is in the diffractive or non-diffractive state. Such a parasitic grating has an effective pitch which is different from that of a grating formed by a pixel in the diffractive state. Thus, when in the non-diffractive state, any diffraction produced by the parasitic grating formed by the electrodes directs light at a different angle from the diffraction produced when the pixel is in the diffractive state. The contrast ratio of the modulation of light diffracted into the first order is therefore improved.
According to a second aspect of the invention, there is provided a display characterised by a modulator in accordance with the first aspect of the invention, a light source for illuminating the modulator, and an optical system for gathering light from the modulator.
The spatial light modulator is particularly suitable for use in displays such as high brightness projection displays. Where the picture elements are switchable between a diffractive state and a non-diffractive state, the optical system is preferably arranged to gather light from the picture elements of the modulator when in the diffractive state. This is preferable to the inverse arrangement from the point of view of contrast ratio i.e. the ratio of light produced by each pixel in the bright and dark states. The optical system is preferably a projection optical system.
The invention will be further described, by way of example, with reference to the accompanying drawings, in which:
FIG. 1
is an exploded view of a pixel of a spatial light modulator;
FIG. 2
is a cross-sectional view of the pixel shown in
FIG. 1
;
FIG. 3
is a schematic plan view of a known electrode arrangement for the pixel of
FIGS. 1 and 2
;
FIG. 4
illustrates an alternative electrode arrangement;
FIG. 5
illustrates an electrode arrangement which in combination with the structure shown in
FIGS. 1 and 2
constitutes an embodiment of the invention;
FIG. 6
shows a cross-sectional view of a pixel using the electrode arrangement of
FIG. 5 and a
circuit diagram of the equivalent circuit thereof;
FIG. 7
is a schematic diagram of a projection display;
FIGS. 8
,
9
and
10
are diagrammatic sectional views illustrating diffraction gratings formed by the electrode arrangements of
FIG. 4
, FIG.
3
and
FIG. 5
, respectively;
FIG. 11
is a schematic plan view of an SLM using the electrode arrangement of
FIG. 3
;
FIG. 12
is a schematic plan view of an SLM using the electrode arrangement of FIG.
5
and constituting an embodiment of the invention;
FIG. 13
is a circuit diagram of an equivalent circuit of a floating electrode of the SLM of
FIG. 12
; and
FIG. 14
illustrates diffractive and non-diffractive states of a pixel of the SLM of FIG.
12
.
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patent: 5132819 (1992-07-01), Noriyama et al.
patent: 5182665 (1993-01-01), O'Callaghan et al.
patent: 5237435 (1993-08-01), Kurematsu et al.
patent: 5333004 (1994-07-01), Mourey et al.
patent: 5448385 (1995-09-01), Deffontaines et al.
patent: 5552916 (1996-09-01), O'Callaghan et al.
patent: 2313920 (1997-12-01), None
Search Report for Application No. GB 9705437.3; Dated Jun. 4, 1997.
J.A.M.M. van Haaren et al.; Liquid Crystals, vol. 16, No. 5, pp. 735-748; 1994; “Switching on Stray Electric Fields in Ferroelectric Liquid Crystal Cells”.
D.G. McDonnell et al.; SID 93 Digest; pp. 654-657; 1993; “An Ultra-High-Resolution Ferroelectric Liquid-Crystal Video Display”.
European Search Report Dated Sep. 22, 1999 corresponding to European Patent Application No. 98301936.5.
XP-002114606—“Diffraction Characteristics of Ferroelectric Liquid Crystal Grating”; by Seiji Fukushima and Takashi Kurokawa Jpn. J. Appl. Phys. vol. 33 (1994) pp. 5747-5754, Part 1, No. 10, Oct. 1994.
Bonnett Paul
Mayhew Nicholas
Robinson Michael Geraint
Tombling Craig
Towler Michael John
Parker Kenneth
Renner , Otto, Boisselle & Sklar, LLP
Sharp Kabushiki Kaisha
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