U.S. patent application number 09/745913 was filed with the patent office on 2001-06-28 for display device comprising a scanning window.
Invention is credited to Broer, Dirk Jan, De Koning, Hendrik, De Vries, Gosse Charles, Johnson, Mark Thomas.
Application Number | 20010005244 09/745913 |
Document ID | / |
Family ID | 8241100 |
Filed Date | 2001-06-28 |
United States Patent
Application |
20010005244 |
Kind Code |
A1 |
Broer, Dirk Jan ; et
al. |
June 28, 2001 |
Display device comprising a scanning window
Abstract
A display device (1) is provided with a means (15) for
selectively transmitting and blocking light from specific areas of
the image. The means for blocking comprises an LCD cells or a
number of LCD cells, which cells comprises an oriented polymer
network, an LC material with negative .DELTA..epsilon. and a
pleochroic dye.
Inventors: |
Broer, Dirk Jan; (Eindhoven,
NL) ; De Koning, Hendrik; (Eindhoven, NL) ; De
Vries, Gosse Charles; (Eindhoven, NL) ; Johnson, Mark
Thomas; (Eindhoven, NL) |
Correspondence
Address: |
Corporate Patent Counsel
U.S. Philips Corporation
580 White Plains Road
Tarrytown
NY
10591
US
|
Family ID: |
8241100 |
Appl. No.: |
09/745913 |
Filed: |
December 22, 2000 |
Current U.S.
Class: |
349/88 ;
349/143 |
Current CPC
Class: |
G02F 1/133365 20130101;
H01J 29/896 20130101; G02F 1/1334 20130101; H01J 2229/8926
20130101; G02F 1/13345 20210101; G02F 1/13712 20210101; G02F
1/13725 20130101 |
Class at
Publication: |
349/88 ;
349/143 |
International
Class: |
G02F 001/1333; G02F
001/1343 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 24, 1999 |
EP |
99204544.3 |
Claims
1. Display device comprising a display window and means for
generating light at areas of the display window and means for
selectively time-sequentially activating the said areas, said
display device comprising positioned in front of the display
window, or forming part of the display window, a means for
transmitting light from activated areas, and blocking at least
partly light from non-activated areas, characterized in that the
means for transmitting light from activated areas, and blocking at
least partly light from non-activated areas comprise an LCD cell or
LCD cells, the LCD cell(s) comprising an oriented polymer network,
an LC material with negative .DELTA..epsilon. and a pleochroic dye,
the passive relaxation time being less than 2 millisecond and
electrodes.
2. Display device as claimed in claim 1, characterized in that the
content of the oriented polymer matrix lies between 5 and 15%.
3. Display device as claimed in claim 1, characterized in that the
thickness of the LCD material is between 5 and 25 micrometer.
4. Device as claimed in claim 1 characterized in that the
electrodes are formed in a non-straight form.
5. Device as claimed in claim 1, characterized in that the means
comprise an LCD cell comprising opposing surfaces, one of said
surface comprising a common electrode, the opposing surface
comprising a set of separate electrodes and the device comprising
means to provide a common electrode voltage to the common electrode
and selectively providing switching voltages to the set of separate
electrodes.
Description
[0001] The invention relates to a display device comprising a
display window and means for generating light at areas of the
display window and means for selectively time-sequentially
activating the said areas, said display device comprising
positioned in front of the display window, or forming part of the
display window, a means for selectively transmitting light from
activated areas, and blocking at least partly light from
non-activated areas.
[0002] Many display devices of many different types are known, for
instance CRT's (Cathode ray tubes), PDP (Plasma Display Devices),
LCD (Liquid Crystal Displays), and many other types.
[0003] In general a display device comprises a display window. The
image is displayed on the display window. The display window
comprises means to selectively generate light at areas of the
display window. In a CRT for instance the image is built
line-by-line.
[0004] A major problem for display devices is formed by reflection
of ambient light on the display window or at components of the
display device such as phosphor elements (in e.g. CRT's and PDP's).
The viewer sees, apart from the image generated by the device, also
reflections of other light sources, such as lamps and/or the sun
shining on the display window. The reflections of such external
(i.e. outside the display device) light sources reduces the
contrast of the image displayed, and can even make it invisible
especially when bright sunlight shines on the display window. Many
solutions have been proposed ranging from reducing the light
intensity in the room, reducing the reflection coefficient of the
surfaces of the display window (anti-reflection coatings) and using
dark glass for the window (the latter reduces the reflection on the
inside of the display window).
[0005] It is known from European patent application no. 0 000 422
to use a means for transmitting light from activated areas, and
blocking at least partly light from non-activated areas. Areas that
are activated (i.e. emit light) are then seen, whereas non-active
areas are black. Such a means can strongly increase the contrast.
The means shuts out light from non-activated areas and transmits
light from activated areas. The intensity of the image itself is
not, or only to a small amount reduced, whereas the intensity of
the reflected light can be strongly reduced.
[0006] Although this idea has been known for some time, and seems
to be a simple and elegant solution to the problem, as yet it has
not been possible to implement this solution in practice.
[0007] The gain in contrast is in practice not enough to justify
the costs. It is an object of the invention to provide a display
device of the type described in the opening paragraph having an
increased performance.
[0008] To this end the means for transmitting light from activated
areas, and blocking at least partly light from non-activated areas
comprise an LCD cell or LCD cells, the LCD cell(s) comprising an
oriented polymer network, an LC material with negative
.DELTA..epsilon. and a pleochroic dye, dispersed in the oriented
matrix, the passive relaxation time being less than 2
milliseconds.
[0009] The invention is based on the insight that the LC cells have
to `open and close` fast, preferably within 2 milliseconds to be
truly effective. An LCD cell is switched from one state
(transmissive or non-transmissive) to another (non-transmissive or
transmissive) in two different fashions, actively by applying
voltages, and passively when the voltage over the cell is removed.
By using an oriented polymer matrix and an LC material with
negative .DELTA..epsilon. a fast passive relaxation (measured as
the half-way point between an open and shut orientation) when the
voltage is removed of smaller than 2 milliseconds is possible.
[0010] The cell comprises also a pleochroic dye. An LCD cell with
an oriented polymer network and an LC material with negative
.DELTA..epsilon. has in the non-transmissive state a high to very
high scattering coefficient which would increase the reflection of
outside light sources, rather than decrease it, leading to a
decrease in contrast. However, the pleochroic dye absorbs light
inside the LCD cell, reducing the scattered light intensity.
[0011] The presence of the pleochroic dye also decreases the
effective passive relaxation time.
[0012] Preferably the cell thickness lies between 6 and 25
micrometer.
[0013] Preferably the content of the oriented polymer matrix lies
between 5 and 15%. The more oriented polymer matrix the LCD cell
comprises, the faster the passive relaxation is, but the more
scattering occurs, also in the transmissive state. Below 5% it
becomes difficult to obtain fast passive relaxation, above 15%
scattering becomes a problem. The device may comprise a number of
LCD cells, or a single LCD cell with many electrodes.
[0014] These and further aspects of the invention will be explained
in greater detail by way of example and with reference to the
accompanying drawings, in which
[0015] FIG. 1 shows schematically a display device in accordance
with the invention.
[0016] FIG. 2A shows schematically a means for transmitting light
from activated areas, and blocking at least partly light from
non-activated areas.
[0017] FIGS. 2B and 2C show schematically an LCD cell for use in
the means shown in FIG. 2A.
[0018] FIG. 3A illustrates the relative luminance of an image
behind the LCD cell as a function of voltage over the cell.
[0019] FIG. 3B illustrates the switching times both for active and
for passive switching.
[0020] FIG. 4 schematically illustrates the relation between
switching times and the frame time.
[0021] FIGS. 5A and 5B show in a graphical form data on switching
voltages, luminance and switching times as a function of weight
percentage oriented polymer matrix material.
[0022] FIG. 6 illustrates some examples of polymerizable liquid
crystals usable for the oriented polymer network.
[0023] FIG. 7 shows a particular example of a polymerizable liquid
crystal.
[0024] FIG. 8 shows an example of a photoinitiator.
[0025] FIG. 9 shows several examples of pleochroic dyes.
[0026] FIG. 10 illustrates some forms for electrodes.
[0027] FIG. 11 illustrates the voltage versus transmission
curve.
[0028] The Figures are not drawn to scale. In general, like
reference numerals refer to like parts.
[0029] A color display device 1 (FIG. 1) includes an evacuated
envelope 2 comprising a display window 3, a cone portion 4 and a
neck 5. In said neck 5 there is provided an electron gun 6 for
generating three electron beams 7, 8 and 9. A display screen 10 is
present on the inside of the display window. Said display screen 10
comprises a phosphor pattern of phosphor elements luminescing in
red, green and blue. On their way to the display screen the
electron beams 7, 8 and 9 are deflected across the display screen
10 by means of a deflection unit 11 and pass through a shadow mask
12 which is arranged in front of the display window 3 and which
comprises a thin plate having apertures 13. The shadow mask is
suspended in the display window by means of suspension means 14.
The three electron beams converge and pass through the apertures of
the shadow mask at a small angle with respect to each other and,
consequently, each electron beam impinges on phosphor elements of
only one colour. In FIG. 1 the axis (z-axis) of the envelope is
also indicated. In front of the display window 3 a means 15 for
selectively passing and blocking light is provided.
[0030] FIG. 2A shows in more detail a means 15. The means comprises
a number of LCD cells 20 and means 21 to control the transmission
characteristics of the LCD cell. Each cell is opened, i.e.
transmissive to light emanating from the display window when the
area behind the cell is activated, i.e. emits light. It is remarked
that the example in FIG. 2A comprises a number of cells, it is also
possible, and indeed advantageous to use a single cell having a
large number of opposing electrode (for instance many pairs of
opposing electrodes). Application of proper voltages will the
switch the areas between the pairs of electrodes between a
transmissive and a blocking state. Having a single cell is easier
to make and reduce the possibility of difference in
transmission/blocking properties over the display device.
[0031] FIG. 2B illustrates schematically an LCD cell for use in the
invention. The LCD cell comprises an LCD material 23 which
comprises in a oriented polymer network an LCD material with
negative .DELTA..epsilon. and a pleochroic dye, transparent
electrodes (for instance made from ITO) 26, a barrier layer 25, a
polyimide layer 24, a e.g. glass substrate 27 and optionally an
anti-reflection layer 28.
[0032] FIG. 2B shows one half of the set-up, FIG. 2C the cell in
its entirety.
[0033] FIG. 3A shows the relative luminance (L in percentage on the
vertical axis) as a function of the voltage applied on the
electrodes (V in Volts on the horizontal axis. When the cell is in
the transmissive state (L=100%) application of a voltage higher
than approximately 80 Volts will close the cell, which cell in this
example has a thickness of 18 micrometer, and comprises 7% (by
weight) of an oriented polymer network. By application of a voltage
of high enough amplitude the LCD cell can therefor be closed. The
time needed for `closing` the cell is herein called the `active
switching time` .DELTA.t.sub.2. For a proper functioning of the
cell the cell, however, also has to be opened at some time. This is
done by removing the voltages over the electrodes. The LCD material
will then passively, i.e. not driven by outside voltages, convert
back to the `transmissive state`. The oriented polymer network is
oriented in such manner that the LCD material experiences an
internal force driving the LCD material to the `transmissive
state`. The time needed to open a cell is herein called the passive
switching time .DELTA.t.sub.1.
[0034] FIG. 3B illustrates as a function of time the closing of a
cell (falling slopes) and opening of a cell (rising slopes). These
slopes are drawn for may different voltages e.g. for 60 Volts
(slopes A), for 80 Volts (slopes B), for 100 Volts (slopes C) and
for 160 Volts (slopes D). The switching times (i.e. the time needed
to arrive at a point half-way between two states) .DELTA.t.sub.1
and .DELTA.t.sub.2 for to open respectively close a cell are
indicated in the figure. It can be seen that for a voltage higher
than 100 Volt (slopes C) opening and closing time are both less
than 1 millisecond.
[0035] FIG. 4 illustrates schematically the intensity of the
emitted light (unbroken line) and the transmission of the
corresponding LCD cell (broken line) as a function of time, the
times .DELTA.t.sub.1 and .DELTA.t.sub.2 needed to switch the LCD
cell and the transmission T.sub.o in the `closed` state of the
cell.
[0036] Schematically the frame-time to is indicated. Important
parameters for the effectiveness of the LCD cell are the ratio
between the frame time and .DELTA.t.sub.1 (to `open` the LCD cell)
and .DELTA.t.sub.2 (to `close` the LCD cell), the transmission
T.sub.0 in the `closed` state and furthermore (not illustrated in
FIG. 3) the reflection of the LCD cell itself. Both .DELTA.t.sub.1
and .DELTA.t.sub.2 should be substantially smaller than the frame
time t.sub.0, T.sub.o should be small, and the LCD cell should not
reflect much light. The frame time is typically 20 (50 Hz) to 10
(100 Hz) milliseconds. Especially for the shorter frame times (100
Hz) the above set of seems to rule out the use of LCD cells. A
number of different types of LCD cells are known. There are types
that work with interference filters and LCD materials that twist
the polarisation. However, the light coming from the display device
is non-polarised. Thus, using a polarisation filters cuts the
intensity in half, not taking into account that the LCD cell itself
also absorbs light, further reducing the intensity. There are LCD
cells that work on a guest-host system. Such LCD cells, however,
typically have values for .DELTA.t.sub.1 and .DELTA.t.sub.2 of the
order of several milliseconds, which does not enable the LCD cells
to be opened and closed fast enough to be effective. There are LCD
cells that have a `clear` transmissive state and a `closed` state
that highly scatters light. The last thing that seems to be
appropriate is a `closed` state that highly scatters light. This
means that incident light is very efficiently scattered, which
reduces rather than increases the contrast.
[0037] None of the known LCD systems thus is able to meet the
requirements.
[0038] The inventors have, however, realised that using an LCD cell
having an oriented polymer matrix, an LC material with negative
.DELTA..epsilon. and a pleochroic dye it is possible to obtain
values for .DELTA.t.sub.1 (to `open` the LCD cell) and
.DELTA.t.sub.2 (to `close` the LCD cell) of less than 2, preferably
less than 1 millisecond. Less than 1 milliseconds is in particular
suited for devices which operate at more than 50 Hz.
[0039] Using an LCD cell having an oriented polymer matrix, and an
LC material with negative .DELTA..epsilon. enables fast switching
times to be obtained. Such a system, however, in its `closed` state
scatters light very efficiently. Thus such a system seems to be
inappropriate. LCD cells based on `guest-host` systems in which
pleochroic dyes are used are too slow. Surprisingly, however, the
combination of the two enables fast switching times to be obtained
(as fast or even faster than using an oriented polymer matrix and
an LC material with negative .DELTA..epsilon.), high ratio of
transmission and low reflectivity. FIG. 5A shows as a function of
weight percentage of oriented polymer network the relative
luminance (i.e. the percentage light that is transmitted in the
closed state) in line 51 (in percentage), the passive switching
time (in 0.1 milliseconds) in line 53, and the needed switching
voltage (in 10 Volts) in line 52. It is clear that as the
percentage oriented polymer increases a number of parameters are
affected. The switching voltage increases, the passive switching
time decreases and the relative luminance increases. Thus for for
instance 10% by weight of polymer the switching voltage is 140
Volts, the passive switching time 0,25 milliseconds and the
relative luminance 4%. The decrease of passive switching time is a
positive effect, but the increase in relative luminance is not.
Thus preferably the content of the oriented polymer matrix lies
between 5 and 15%. Lower percentages give relatively large passive
switching times whereas higher percentages give a relatively high
luminance even in the closed state. FIG. 5A shows data for an LCD
cell with a thickness of 18 micrometer.
[0040] FIG. 5B show the same data for an LCD cell with a thickness
of 6 micrometer. The same general trends are seen for relative
luminance (line 54), for switching voltage (line 55) and for
switching time (line 56) be it that the relative luminance are
roughly twice as large, the switching voltages are roughly half of
the values given in FIG. 5A. The switching times are
comparable.
[0041] The oriented network is formed be e.g. chain-addition
reaction under influence of UV light of materials such as shown
schematically in FIG. 6. The liquid crystal molecules comprise a
stiff central core A, flexible spacers B and polymerizable end
groups C. Some examples of the stiff central core and the flexible
spacer groups are indicated in FIG. 6. It will be clear that many
variations are possible e.g. by addition of alkyl groups to one or
more of the rings of the central core part or extending the central
core part with an extra ring, or by longer flexible spacer B (i.e.
for instance x larger than 12 or a mixture of the groups
--(CH.sub.2)-- and --O-- (CH.sub.2)--, also the flexible group does
not have to be linear but may have side chains. The flexible group
at either side of the central core may be different, although it
preferably is the same. The polymerizable end groups may e.g. be
chosen from meth(acrylates), vinyl ethers, epoxides and
thiol-enes.
[0042] FIG. 7 shows a particular example of a polymerizable liquid
crystal. It is this material that was used for the measurement
shown in the graphs (figs. SA and 5B).
[0043] In order to polymerise the liquid crystals such as e.g.
shown in FIGS. 6 and 7 a photoinitiator may be used e.g. one as
shown in FIG. 8.
[0044] Some examples of pleochroic dyes are shown in FIG. 9.
[0045] The LC material in the cell may e.g. have the following
characteristics:
[0046] .epsilon.=4.5
[0047] .epsilon.=10.2
[0048] .DELTA..epsilon.=-5.7
[0049] n.sub.o=1.486
[0050] n.sub.e=1.650
[0051] .DELTA..epsilon. is negative. Such LC material may be
obtained by mixing several LC components and can be bought from LC
material manufacturers.
[0052] An means for transmitting/blocking selectively the light is
made e.g. by providing a substrate e.g. a glass substrate with
layers such as shown in FIGS. 2B and 2C by conventional method, the
electrodes being formed in a pattern such as shown in FIG. 2A.
Filling the cell (i.e. the space between two substrates) with a
mixture of a polymerizable liquid crystal material such as e.g.
shown in FIGS. 6 or 7, preferably a photoinitiator (such as e.g.
shown in FIG. 8) and an LC material and a pleochroic dye (such as
e.g. shown in FIG. 9). After filling voltages are put over the
electrodes to orient the polymerizable liquid crystal materials. UV
light is then shone on the cell or cell to polymerise the
polymerisable liquid crystal, forming an oriented polymer network.
Within the oriented polymer network the LC material and the
pleochroic dye is dispersed.
[0053] It will be clear that within the frame work of the invention
many variations are possible.
[0054] FIG. 10 for instance shows the form and shape of electrode
on the substrates. They can for instance be straight (electrodes
101) but especially if the size and shape of electrodes become very
small may be formed in a zig-zag manner to reduce moire effects.
Moire effects occur when a beat frequency occurs between the lines
written in the image and the sequence of cells opened and closed.
Preferably the means comprises a large number of cells (or
switchable areas within a single cells defined by opposing
electrodes) which are switched open and close sequentially i.e.
first the highest, then the one below that one, than the next etc.
until the bottom one has been opened and closed. Of course the
opening and closing must run in step with the lines of the image
being written. For devices using phosphor preferably each cell is
left open for some time after the corresponding line in the image
has been activated. Phosphor emits a substantial amount of their
light in the so-called after-glow, which occurs after the actual
activation of the phosphor. By keeping the cell or cell areas open
for some time this light is also transmitted this light is also
transmitted. More than one cell or cell areas may be open at any
given time. This reduces the change of motion artifacts being
visible in the image. The cell or each cell may have opposing pairs
of electrodes or preferably the cell comprises on one surface a
common electrode and on the opposing surface separate and
separately driven electrodes which define switchable subparts of
the cell. In a preferred embodiment the means for
transmitting/blocking light comprises opposing surface in between
the LCD material is provided, one of said surface comprising a
common electrode, the opposing surface comprising a set of separate
electrodes and the device comprising means to provide a common
electrode voltage to the common electrode and selectively providing
switching voltages to the set of separate electrodes
[0055] FIG. 11 illustrates the voltage (horizontal scale) versus
transmission (vertical scale) curve. Below a threshold value
V.sub.th the cell is transmissive and above a maximum value
V.sub.max the cell blocks light. The driving voltage range .DELTA.V
is indicated in the figure. The driving voltage does not have to
extend beyond V.sub.max and the threshold voltage V.sub.th. By
using a common electrode the complexity of the means and of driving
the means is reduced, which reduces costs and reduces the risk of
malfunctioning. Different driving schemes may be emploed. The
common electrode may be set at a voltage between V.sub.max and
V.sub.th and to provide drive signal with maximum magnitude
(V.sub.max-V.sub.th) to switch the cells. To prevent charge
build-up the polarity of the common electrode voltage may be
switched from time to time. It can also be chosen to set the common
electrode at -V.sub.th, and drive with signals levels 0V and
(V.sub.max-V.sub.the)
[0056] to set the common electrode at -(V.sub.max+V.sub.th)/2, and
drive with signals levels +(V.sub.max-V.sub.th)/2 and -
(V.sub.max-V.sub.th)/2.
[0057] For an LC cell of 18 micrometer (see FIG. 5A) typical
voltages are V.sub.th=30 Volt, V.sub.max=60-80 Volt, the common
electrode may thus be set between -30 and -80 Volt and a votage
swing of 30-50 Volt.
[0058] It is preferred to drive more than one cell (area) at a
time. This improves image perception (less jerky image) and enables
to optimise the colour performance especially if slower phosphors
are used.
[0059] In short the invention may be described by:
[0060] A display device (1) is provided with a means (15) for
selectively transmitting and blocking light from specific areas of
the image. The means for blocking comprises an LCD cells or a
number of LCD cells, which cells comprises an oriented polymer
network, an LC material with negative .DELTA..epsilon. and a
pleochroic dye.
* * * * *