Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-06-10
2001-06-05
Wu, Xiao (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S087000, C345S097000
Reexamination Certificate
active
06243063
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a diffractive spatial light modulator and to a display incorporating such a modulator. Such a display may be of the projection type and may be used to provide large screen TV viewing and business presentations.
DESCRIPTION OF THE RELATED ART
A spatial light modulator using ferroelectric liquid crystal technology is disclosed in a paper entitled “Diffractive Ferroelectric Liquid Crystal Shutters for Unpolarised Light” by M. J. O'Callaghan and M. A. Handschy, Optics Letters, Volume 16 No.10, May 1991, pages 770 to 772. The spatial light modulator disclosed in this paper is switchable between a first state in which it transmits incident light and a second state in which it acts as a phase diffraction grating.
Another spatial light modulator is disclosed in a paper entitled “Improved Transmission in a Two-Level, Phase Only, Spatial Light Modulator” by M. A. A. Neal and E. G. S. Page, Electron. Lett. 30 (5) pages 465-466 1994. This paper discloses a spatial light modulator which is switchable between a transmissive mode and a diffractive mode in which alternative strips of the modulator rotate unpolarised light by plus and minus 45 degrees and an associated half wave retarder further rotates all the polarisation components of the light so as to provide phase-only modulation.
FIGS. 1
a
and
1
b
of the accompanying drawings show a reflection-mode diffractive spatial light modulator (SLM) of the type disclosed in GB 9611993.8. The SLM comprises a rectangular array of rectangular or substantially rectangular picture elements (pixels), only one of which is shown in
FIGS. 1
a
and
1
b.
The SLM comprises upper and lower glass substrates
1
and
2
. The upper substrate
1
is coated with a transparent conducting layer of indium tin oxide (ITO) which is etched to form elongate interdigitated electrodes
3
. The electrodes
3
are covered with an alignment layer
4
for a ferroelectric liquid crystal material. The alignment layer
4
may, for example, be formed by obliquely evaporating silicon oxide at 84 degrees to the normal to the substrate
1
so as to induce the C1 state in ferroelectric liquid crystal material, for instance of the type known as SCE8 available from Merck, and may have a thickness of approximately 10 nanometers. However, other types of alignment layer may be used and the C2 state may be used.
A combined mirror and electrode
5
is formed on the glass substrate
2
by depositing silver to a thickness of approximately 100 nanometers. A static quarter waveplate
6
is formed on the silver mirror and electrode
5
. For example, this may be provided by spinning on a mixture of a reactive mesogen diacrylate such as that known as RM258 in a suitable solvent such as chlorobenzine with a photoinitiator. This is cured for approximately ten minutes under ultraviolet light in an atmosphere of nitrogen. The thickness of the plate
6
is controlled, for instance by varying the mix ratios of the materials and the spin speed, so that it acts as a quarter waveplate for a predetermined bandwidth in the visible spectrum, for instance centred about 633 nanometers. The thickness d is given by the expression
d
=
λ
4
·
Δ
n
where &lgr; is the wavelength of the centre of the band and &Dgr;n is the difference between the ordinary and extraordinary refractive indices of the material of the quarter waveplate
6
. The quarter waveplate
6
therefore typically has a thickness of the order of 800 nanometers.
A further alignment layer
7
is formed on the quarter waveplate
6
, for instance as described hereinbefore for the alignment layer
4
. The substrates
1
and
2
are then spaced apart, for instance by spacer balls of two micrometer diameter, and stuck together so as to form a cell which is filled with the ferroelectric liquid crystal material to form a layer
8
. The spacing provides a layer of ferroelectric liquid crystal material which provides a half wave of retardation so that the liquid crystal layer acts as a half wave retarder whose optic axis is switchable as described hereinafter. In particular, the ferroelectric liquid crystal layer has a thickness d given by
d
=
λ
2
Δ
n
FLC
where &Dgr;n
FLC
is the difference between the ordinary and the extraordinary refractive indices of the ferroelectric liquid crystal material.
In order to optimise the brightness of the display, the reflectivity of each interface should preferably be reduced, for instance by applying antireflection coatings to the substrate
1
and by optically burying the electrodes
3
.
The electrodes
3
and
5
may be arranged to provide for suitable addressing of the pixels of the SLM. For instance, in a passive matrix addressing arrangement, the electrodes
3
may extend throughout the length of the SLM and may be connected to the outputs of a data signal generator for supplying a row of pixel data at a time to the pixels. The electrode
5
may be extended transversely to form a row electrode connected to the output of a strobe signal generator for strobing the data to the SLM a row at a time in a repeating sequence.
For each pixel, the electrode
5
acts as a common electrode which is connectable to a reference voltage line, for instance supplying zero volts, for strobing data to be displayed at the pixel. Alternate ones of the elongate electrodes
3
are connected together to form first and second sets of parallel interdigitated electrodes which are connected to receive suitable data signals. Each pixel is switchable between a reflective state and a diffractive state as described hereinafter.
FIG. 2
of the accompanying drawings illustrates diagrammatically the operation of adjacent strips of the pixel shown in
FIGS. 1
a
and
1
b
when the pixel is in the diffractive mode. The optical path through each pixel is folded by reflection at the mirror
5
but, for the sake of clarity, the path is shown unfolded in FIG.
2
. The SLM acts on unpolarised light, which may be split into components of orthogonal polarisations for the sake of describing operation of the SLM. One of the component polarisations is shown at
10
in FIG.
2
and is at an angle −&phgr; with respect to a predetermined direction
11
.
Voltages which are symmetrical with respect to the reference voltage on the electrode
5
are applied to the first and second sets of alternating interdigitated electrodes
3
a
and
3
b.
Thus, ferroelectric liquid crystal material strips
8
a
and
8
b
disposed between the electrodes
3
a
and
3
b
and the electrode
5
have optic axes aligned at angles of −&thgr; and +&thgr;, respectively, with respect to the direction
11
, where &thgr; is preferably approximately equal to 22.5 degrees.
Each strip
8
a
of ferroelectric liquid crystal material acts as a half wave retarder so that the polarisation of the light component leaving the strip
8
a
is at an angle of &phgr;−2&thgr; with respect to the direction
11
. The light component then passes through the static quarter waveplate
6
, is reflected by the mirror
5
, and again passes through the static quarter waveplate
6
, so that the combination of the quarter waveplate
6
and the mirror
5
acts as a half wave retarder whose optic axis is parallel to the direction
11
. The polarisation direction of light leaving the quarter waveplate
6
and travelling towards the ferroelectric liquid crystal material is “reflected” about the optic axis of the quarter waveplate
13
and thus forms an angle 2&thgr;−&phgr; with respect to the direction
11
. The light component then again passes through the strip
8
a
of ferroelectric liquid crystal material so that the output polarisation as shown at
14
is at an angle of &phgr;−4&thgr; with respect to the direction
11
. Thus, for each input component of arbitrary polarisation direction −&phgr;, the optical path through the SLM via each of the strips
8
a
of ferroelectric liquid crystal material is such that the polarisation direction is
Koden Mitsuhiro
Mayhew Nicholas
Robinson Michael Geraint
Tombling Craig
Renner Otto Boisselle & Sklar
Sharp Kabushiki Kaisha
Wu Xiao
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