Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ILLUMINATION OF A LIQUID CRYSTAL DISPLAY
TECHNICAL FIELD OF THE INVE'I~TION
The present invention relates generally to liquid crystal display panels and,
more particularly, to a method and apparatus for utilizing thin liquid crystal
displays in a
high contrast frame sequential color display.
BACKGROUND OF THE INVENTION
Light valves having an electro-optically active element comprising a liquid
crystal composite have been used in displays (directly driven, passive matrix,
and active
matrix addressed), windows, and privacy panels. In a liquid crystal composite,
plural
volumes or droplets of a liquid crystal material are dispersecE, encapsulated,
embedded, or
otherwise contained within a matrix material such as a polymer. Exemplary
disclosures
include Fergason, US 4,435,047; West et al., US 4,685,771; Pearlman, US
4,992,201; and
Dainippon Ink, EP 0,313,053, incorporated herein by reference.
The liquid crystal composite is disposed betr~reen electrodes, at least one of
the electrodes typically being patterned to form a matrix. Tlae electrodes are
supported by
substrates. When voltage is applied to a pair of electrodes, an electric field
is created and
the liquid crystal located between the electrodes will become transmissive. In
this optical
state incident light is transmitted through the composite. When the voltage to
the pair of
electrodes is switched off, the electric field no longer exists and the liquid
crystal
composite between the electrodes changes its optical state to one in which
incident light is
substantially scattered andJor absorbed. In this state the mal:er~ial will
typically be opaque
with a frosty appearance if scattering is predominant or dark gray if
absorption is
predominant. By individually controlling the voltage applied to each pair of
electrodes in
an electrode matrix, a graphical image may be generated. The electrode matrix
can be
transparent or reflective and is typically a matrix of thin film transistors
(TFT), MOS
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- transistors, MIM diodes, or crossed patterned electrodes. The graphical
image can be
viewed directly, projected onto a viewing screen, or viewed as a virtual image
in the eye:
By combining red, green, and blue images, either via sequential illumination,
for
example, using field-sequential color with red, green, and blue light or via
dedicated red,
green, and blue pixels, a colored image may be formed.
The prior art approaches to color displays typically use three separate
imaging elements for imparting an image to each of the ligr~t components
(e.g., red, green;
and blue). For example, see Sonehara, US 5,098,183 and K:urematsu et al., US
5,170,194.
Since the imaging elements are generally comprised of liquid crystal
composites, the use
of three different imaging elements is very expensive. This is especially true
for high
resolution displays due to the large number of pixels per imaging element plus
the means
for addressing the individual pixels. Furthermore, many of The prior art color
displays
utilize twisted nematic type liquid crystals in the display elements that may
require a
polarizer, thus leading to a loss of brightness. Therefore a color display
that only requires
a single imaging element that can operate in the absence of a polarizer is
desirable.
Other prior art which may be relevant to the present invention includes US
5,398,081, WO 90/05429 and WO 98/00747.
Although a variety of single imaging element color displays have been
disclosed in the prior art, a simple frame sequential color display that does
not require
relatively complex and expensive filtering means is desired.
SUMMARY OF THE INVENTIfON
The present invention provides a rapid response liquid crystal imaging
element with high brightness and contrast levels. The invention uses a liquid
crystal
composite that is less than 4 micrometers thick, and preferably less than 2.5
micrometers
thick. The display is illuminated by at least one Iight source that is
positioned at an angle
of less than 30 degrees off of the display normal, and preferably less than 20
degrees off
of the normal. The viewing system is positioned on the sanne side of the
display as the
light source and is located approximately normal to the display.
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In one aspect of the invention, the voltage applied across the electrodes of
an individual pixel are varied between a first voltage necess<try to place the
selected pixel
into the on state and a second voltage necessary to place the selected pixel
into the off
state. The second voltage may either be a zero voltage or a non-zero minimum
voltage.
In the preferred embodiment, a non-zero minimum voltage is used in order to
avoid the
contrast reversal that occurs in thin liquid crystal composites at small
illumination angles.
In one embodiment, at least three light sources of different colors (e.g.,
primary colors, complementary colors, etc. ) illuminate a liquid crystal
imaging element.
The sources are all located within an angle of approximately 30 degrees, and
preferably
less than 20 degrees from the imaging element normal. The viewing system is
located on
the same side of the imaging element as the light sources and may be comprised
of a
virtual image viewing system, a direct viewing system, or a projection viewing
system. A
processor is coupled to both the light sources and the imaging element,
thereby allowing
the individual color light sources to be synchronized with the output of the
imaging
element. Preferably the controlled switching rate of the light sources and the
imaging
element are sufficient to avoid frame flicker and color break-up.
A further understanding of the nature and advantages of the present
invention may be realized by reference to the remaining portions of the
specification and
the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a cross-sectional view of a liquid crystal light valve in the on
state
according to the prior art, while Fig. 2 is a cross-sectional view of the
liquid crystal light
valve of Fig. 1 in the off state.
Fig. 3 is a cross-sectional view of a liquid crystal display panel according
to the prior art, while Fig. 4 is a top view of the liquid crystal display
panel of Fig. 3.
Fig. S is an illustration of a liquid crystal display operating in the normal
mode.
Fig. 6 is an illustration of a liquid crystal display operating in the reverse
mode.
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Fig. 7 schematically illustrates a color frame sequential projector.
Fig. 8 is an illustration of an alternative frame sequential color display
according to the prior art.
Fig. 9 is an illustration of the relationship of a liquid crystal display to
an
illumination source and a viewer according to the present invention.
Fig. 10 is an illustration of a testing system used in conjunction with the
invention.
Fig. 11 its a graph comparing the brightness of a thick liquid crystal
composite to a thin liquid crystal composite for three different illumination
angles.
Fig. 12 is a graph illustrating the illumination angle dependence of the
light scattering characteristics of a thin liquid crystal composite.
Fig. 13 is an illustration of a liquid crystal diisplay suitable for use with
the
present invention, while Fig. 14 is an illustration of the Iiqu.id crystal
display shown in
Fig. 13 with a specular reflection controlling reflector
Fig. 15 is an illustration of the invention utilizing multiple light sources.
Fig. 16 is an illustration of the invention for use in a frame sequential
color
display while Fig. 17 illustrates a timing sequence suitable for use with the
embodiment
of the invention shown in Fig. 16.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
Before describing the present invention in detail, several different configu-
rations of prior art liquid crystal displays will be described. Fig. 1 is a
cross-sectional
view of a typical liquid crystal display 100 according to the prior art. A
medium 101
(e.g., a polymer) containing a plurality of liquid crystal volumes or droplets
103 is
sandwiched between a pair of electrodes 105 made of a transparent conductive
material
such as indium tin oxide. Droplets 103 may be individually encapsulated in one
or more
encapsulation layers as taught by Fergason, US 4,435,047; :lZeamey et al., US
5,405,551;
and Havens et al., US 5,585,947, the disclosures of which are incorporated
herein. While
the display is preferably made of encapsulated liquid crystal material, other
types of liquid
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crystal displays, for example smectic A, cholesteric, or dynamic scattering
nematic
displays, may also be employed. Electrodes 105 are coupl<:d to a voltage
source 107.
When voltage source 107 is in an on state, a voltage is applied across
electrodes 105 creating an electric field. Due to the positive dielectric
anisotropy of
liquid crystal droplets 103, the material comprising the droplets aligns
parallel to the
electric field as shown. In this state light incident along a path 109 will
pass through
droplets 103. Depending upon the thickness of the composite, the voltage
applied to
electrodes 105, and the transparency of electrodes 105, medium 101, and
aligned droplets
103, transmission rates of 70% or greater may be achieved.
When voltage source 107 is in an off state as illustrated in Fig. 2, the
electric field between electrodes 105 is effectively zero. As a result, liquid
crystal
droplets 103 no longer are uniformly aligned. Due to the random orientation of
droplets
103, light incident along path 109 is randomly scattered, both in a forward
direction and a
backward direction as illustrated by scatter paths 201. The scattering of the
incident light
causes display 100 to appear opaque or frosty.
Fig. 3 is a cross-sectional view of a liquid crystal display panel 300 that
may be used to display graphical information. As in liquid crystal display
100, panel 300
includes both medium 101 and liquid crystal volumes 103. In at least one
embodiment,
the liquid crystal composite comprising medium 101 and liquid crystal volumes
103 is a .
polymer dispersed liquid crystal (i.e., a PDLC composite).
In marked contrast to display 100, panel 300 includes a plurality of bottom
electrodes 301 and a common top electrode 303 to form a I>lurality of
electrode pairs. The
electrode pairs divide panel 300 into an array of separately controllable
display elements
or pixels. Pane1300 also includes a top support member 305 preferably made of
a
conductive transparent material such as indium tin oxide (i.e., ITO) coated
polyethylene
terephthalate or ITO coated glass. Depending upon the desired application, the
display
can be designed to be either reflective or transparent. If a reflective
display is desired, the
reflective coating may either be applied to a surface of a bottom support
member 307 or
to a surface of pixel electrodes 301. Preferably electrodes :301 are
reflective electrodes
made of aluminum or silver. While the panel co~guration illustrated in Fig. 3
is
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common, it is understood that other configurations are well. known by those of
skill in the
art and that this configuration is intended only to be illustrative, not
limiting.
Electrically coupled to each electrode 301 is. a switching element 309 that
is used to control the application of a voltage across common electrode 303
and electrodes
301. Typically switching elements 309 are thin film transistors when display
300 is a
transparent mode display and MOS transistors (as shown in Fig. 3) when display
300 is a
reflective mode display. Switching elements 309 act as switches for each
electrode "pair"
thus allowing any combination of pixels to be activated. In general, panel 300
is designed
so that the voltage that causes switching elements 309 to operate is the
threshold voltage
of liquid crystal volumes 103. Although in the illustrated embodiment
switching ele-
ments 309 are MOS transistors, other switching elements such as thin film
transistors,
MIMs, diodes, or varistors may be used as an alternative. 'Che application of
voltage
across electrodes 301 and 303, and therefore the activation of individual
pixels, is
controlled by a processor. In some configurations, such as the MOS transistor
configuration illustrated in Fig. 3, capacitive elements 311 are added to the
transistor
circuit in order to store charge.
Fig. 4 is a top view of panel 300. In the illustrated embodiment, panel 300
is comprised of a 20 by 20 array of square pixels 401. Panel 300 may be
comprised,
however, of greater or lesser numbers of pixels. Furthermore, the pixel shape
is not
limited to squares nor is the pixel shape limited to four sided
configurations. Lastly, all of
the pixels within the panel need not be of a uniform shape or size.
Liquid crystal display panels may be utilized in a variety of different
configurations to create direct view, projection, and virtual images. Examples
of direct
view include computer monitor screens and instrument panel readouts. Examples
of
projection systems include front and rear systems projecting to a large screen
or to a
screen in a microdisplay. A virtual microdisplay typically consists ofone or
more light
sources, a liquid crystal composite, electrode elements, andl imaging optics
that form a
virtual image in the eye of the user. Additionally, liquid crystal displays
may be designed
to function in either a transmissive or a reflective mode.
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Basically there are two configurations in which a high contrast image can
be formed; normal mode and reverse mode. In a normal mode configuration, the
image is
formed from the reflected, or transmitted, non-scattered light while the
scattered light is
blocked. Fig. S is an illustration of a reflective liquid crystal display
operating in the
normal mode. In this mode both the Light source SO1 and the viewer 503 are on
the same
side of the panel. As the illustrated panel is a reflective display, either
bottom support
member 505 is reflective or pixels 507 and 509 are reflective. The
construction of
reflective member SOS is well known in the art, see, for example, Rowland, US
3,935,359; Kuney, Jr., US 4,957,335; Nelson et al., US 4,938,563; Belisle et
al., US
4,725,494; Appledorn et al., US 4,775,219; Tung et al., US 4,712,219; Malek,
US
4,712,867; Benson, US 4,703,999; Sick et al., US 4,464,014; Nelson et al., US
4,895,428;
Hedblom, US 4,988,541; Schultz, US 3,922,065; and Linden, US 3,918,795; the
disclosures of which are incorporated herein by reference.
As shown in Fig. 5, the pixels defined by electrodes 507 are in an on state,
thereby causing the liquid crystal volumes in the pixels defined by these
electrodes to
become transparent. Due to the transparency of these pixels, light from source
501 (e.g.,
ambient Light, directed light, etc.) will pass through the pixels and be
reflected by sub-
strate reflector 505 or, in an alternate configuration, by the reflective
electrode. The
reflected specular Light forms a bright image at Location 503, typically after
first passing
ZO through imaging optics 511 and an aperture stop 513. The liquid crystal
volumes in those
pixels defined by non-activated electrodes 509 are scattered in multiple
directions 515,
only a fraction of which will pass through optics 51 l and aperture stop 513
to reach
viewing location 503.
In an alternate configuration of a normal mode display (not shown), neither
the pixel electrodes nor the bottom substrate are reflective, and the image is
formed by the
light transmitted through those pixels in the on state, i.e., pixels 507.
Fig. 6 is an illustration of a display panel operating in the reverse mode.
As noted above, a reverse mode panel may be used either in a reflective
configuration as
illustrated, or in a transparent configuration. This panel is basically the
same as that
illustrated in Fig. 5. However in this configuration it is the scattered light
515 that is
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collected by imaging optics 51 I to form an image at locatiion 503. The
specular light,
either reflected as shown by an exemplary light ray 601 or passing through the
display in
the case of a transparent configuration, is blocked with stop 513. A dark
image is formed
by those pixels 507 in an on state.
Besides the prior art display panel configurations illustrated above, there
are numerous other configurations that are well known by those of skill in the
art.
Furthermore, liquid crystal display panels may also be used to produce color
images. For
example, either pleochroic or isotropic dyes may be included within the liquid
crystal
material, thereby achieving a colored visual effect. Alternatively, colored
filters or
colored source light may be used in conjunction with the liquid crystal
displays to provide
a colored image. By sequentially combining multiple colored images, for
example, red,
green, and blue images, an image of good color purity may be produced.
One type of frame sequential display is schematically illustrated in Fig. 7.
This system is described in detail in Jones, US 5,398,081, the complete
disclosure of
which is incorporated herein. The light from a white light source 701 is
directed at a
color modulator 703, for example, a dichroic cube color separator. The
dichroic cube has
three color selective reflective surfaces positioned behind three sets of
light valves. The
three color selective surfaces may be the three primary colors (e.g., red,
green, and blue)
or some other combination such as three complementary colors (e.g., cyan,
yellow, and
magenta). The light valves are controlled by a computer that switches the
valves between
a substantially transparent state and a substantially non-transparent state.
Thus three
distinct colors may be obtained from the incident white light of source 701.
The color modulated light from modulator ',~03 is directed at an imaging
element 705, also computer-controlled. Element 705 imparts an image onto the
incident
light that corresponds to the particular color of light reaching it.
Projection system 707
sequentially projects the colored images onto a screen, thus creating a
colored image.
Since the computer controls both the switching of color modulator 703 and the
image
presented on imaging element 705, the color output and thE; image may be
synchronized.
Assuming the switching speed is at a high enough rate, the alternating images
are not
resolvable as distinct colors. Thus the viewer only perceives a composite
colored image.
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Fig. 8 is an illustration of an alternative frame sequential color display
according to the prior art. This system is described in detail in Williams et
al., WO
90/05429, the complete disclosure of which is incorporated herein. The light
from a
white light source 801 is incident on a color filtering means 803. Color
filtering means
803 includes a plurality of liquid crystal light valves 805 aligned with a
plurality of color
filters 807. As in the prior approach, in order to achieve an accurate color
composite
three color filters are required (e.g., red, green, and blue). Therefore in
order to transmit
only one color through filtering means 803, for example blue, only light
valves 805
corresponding to blue filters 807 are switched to an on or tr~msparent state
while all
remaining light valves 805 are switched to an off or scattering state. As a
result of this
switching, in this example only blue light would pass through filtering means
803.
The color modulated light exiting filtering means 803 is spread by a light
spreading means 809, such as a lens or diffusion plate. Element 809 insures
that the light
from filtering means 803 is relatively uniform, eliminating the effects of
discrete color
filters 807. The color modulated light then passes through a. liquid crystal
array 811,
array 811 forming the desired image through individual control of the array
pixels. As in
the prior approach, a processor 8i3 synchronizes the image to the image color.
Regardless of the type of frame sequential color display utilized, rapid
switching speeds are required in order to avoid frame flicker and to insure
that the viewer
sees a composite color image rather than a series of discrete single color
images. As a
result, typically switching speeds on the order of at least 90 Hz and
preferably at least 180
Hz are necessary (i.e., 30 to 60 sequences of three colors per second). These
rates
translate to liquid crystal response times of less than 10 milliseconds, and
preferably on
the order of 3 to 5 nuiliseconds.
There are other parameters that are important in selecting liquid crystal
materials besides switching speeds. For example, a high contrast ratio is
desirable since it
leads to improved image quality. Image contrast is defined as the ratio of
light from the
pixels in the on state to that from the pixels in the off state. Another
quality that is
desirable is a low operating voltage, i.e., the voltage required to place a
pixel in an on
state. As the operating voltage increases, the overall power consumption of
the device
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similarly increases. In addition, high voltage transistors are expensive
primarily because
common transistor technology is low voltage. Besides requiring more power, a
high
operating voltage generates more heat that must be dissipated. The higher
temperatures
may also lead to local temperature fluctuations that may result in thermally
induced
mechanical stresses. Typically a low field E9o is preferred for direct view
and projection
applications. A low operating field allows the use of a lov~rer applied
voltage or, for a
given applied voltage, allows for a thicker layer of liquid crystal composite
to be used
thereby yielding a lower transmissivity when the pixel is in the off state
(i.e., Toff), thus
creating a higher contrast image.
The present invention will now be described in more detail. The present
invention utilizes thin liquid crystals, typically with a thickness of less
than 4
micrometers, and preferably with a thickness of less than 2:5 micrometers. An
advantage
of thin liquid crystals is that improved switching speeds m;ay be obtained,
thereby
allowing a colored virtual image to be created using field sequential color
schemes.
Furthermore, with appropriate illumination and viewing angles, improved image
brightness and contrast may also be achieved.
Fig. 9 is an illustration of the relationship oiE a liquid crystal display 901
to
the illumination source and the viewer according to the present invention. The
liquid
crystal composite in display 901 has a thickness of less than 4 micrometers,
and
preferably less than 2.5 micrometers. It is illuminated by at least one light
source 903 that
is positioned at an angle, 8, of less than 30 degrees off of the normal from
display 901,
and preferably less than 20 degrees off of the normal. A viewing system 905 is
located
approximately at the normal to display 901. This configuration provides
superior image
quality in an extremely rapid response time display.
Fig. 10 is an illustration of a testing system 'used to show the
improvements offered by the present invention. A sample 1001 was illuminated
by an
argon laser 1003 operating at a wavelength of 514 nanomel:ers. Three different
illumination angles (i.e., 8) were used during the test; 15, 30, and 45
degrees. A detector
1005 was placed normal to sample 1001 and at a distance o~f 5.25 inches from
the sample.
The detector was 0.25 by 0.25 inches in size.
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Fig. 11 is a graph of the brightness of sample 1001 as a function of the
applied voltage. Two different samples were tested; a 1.83 micrometer thick
liquid
crystal and a 4.9 micrometer thick liquid crystal. At 45 degrees illumination,
the thick
sample (test run # 11 O 1 ) produced approximately twice the brightness of the
thin sample
(test run #I I03). At 30 degrees illumination, the thick sample (test run
#1105) and the
thin sample (test run # 1107) produced approximately the same brightness.
However, at
degrees the thin sample (test run # 1109) produced significantly higher
scatter levels;
and therefore brightness, than the thick sample (test run #1 I 11). This is an
unexpected
result since thick liquid crystal composites were thought to produce more
scatter, and thus
10 more brightness, than thin composites due to the number of available
scattering sites.
Fig. 12 is a graph illustrating the illumination angle dependence of the
light scattering characteristics of a I.83 micrometer thick liquid crystal
composite using
the testing apparatus shown in Fig. 10. The illumination angle, 9, was varied
from 10 to
50 degrees in 2 degree increments. The viewing angle remained constant at the
sample
15 normal. As in Fig. 11, the brightness is shown as a function of the voltage
applied to the
composite.
Regardless of the applied voltage, as the illwnination angle decreases, the
- sample scatter, and therefore brightness; increases: For angles greater than
40 degrees,
there is very little difference in the scatter level as the applied voltage is
varied, resulting
in the creation of low contrast images. For angles greater than 20 degrees,
the brightness
decreases as the voltage increases from 0 to 10 volts. For small illumination
angles, i.e.,
less than 20 degrees, the brightness reaches a maximum value at a non-zero
voltage in the
range of 2 to 3 volts. As a consequence of this unexpected result, the optimwn
contrast is
achieved between the operating voltage and some non-zero voltage. For example,
at a 10
degree illumination angle, the optimum contrast is achieved by setting the
minimum
voltage to 3 volts.
Fig. 13 is an illustration of one embodiment ~of the invention utilizing a
reverse mode configuration. In this embodiment sample 901 is comprised of a
thin liquid
crystal composite 1301, upper electrode 1303, lower pixel electrodes 1305, and
upper and
lower support members 1307 and 1309, respectively. Preferably upper electrode
1303 is
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- made of an optically transparent conductive coating (e.g., ITO) and lower
electrodes 1305
are made of optically reflective and electrically conductive materials (e.g.,
aluminum,
silver, etc.). Alternatively, lower electrodes 1305 may be transparent and
lower support
member 1309 reflective. As previously described, the electrode pairs
controlling the state
of each pixel are controlled by a processor 1311. The lower voltage extreme
may be
either 0 volts or, alternatively, a non-zero voltage. A non-zE:ro voltage is
used in order to
avoid contrast reversal from occurring, as noted above. If a non-zero voltage
is used, it is
typically in the range of 2 to 3 volts.
Source 903 is positioned at an angle of less tlhan 30 degrees off of the
normal from display 901, and preferably less than 20 degreea off of the
normal. As Fig.
13 illustrates a reverse mode configuration, the image is formed by collecting
scattered
rays 1313. ~ Assuming a reflective display as illustrated, the :light rays
passing through
those pixels that are in an on state and reflected back as a specular
reflection are
preferably eliminated with the use of an aperture stop. If both electrodes
1305 and
substrate 1309 are transparent, light rays passing through activated pixels
are either
eliminated with a light trap (e.g., absorbing coating) or reflected away from
the display
and the viewing system.
_ In this embodiment a virtual image is created. for viewing at a location
1315. The virtual image is formed by imaging optics 1307 which collects a
portion of
scattered rays 1313. In one configuration, imaging optics 1:307 are similar to
those used
in a microscope.
In an alternate embodiment, imaging optics 1307 are comprised of a
projection optical system. The projection optical system can be used to
project an image
onto a screen.
Given the relatively small angle between source 903 and the display
normal, it is important to prevent specular reflections from entering the
viewing system.
In an alternate embodiment of display 901 illustrated in Fig. 14, lower member
1309
includes a tilting reflective: member 1401. Reflective member 1401' is used to
direct the
specularly reflected light rays 1403 of source 903 sufficiently away from
viewer 1315 so
that they are not collected by optics I 307. Reflective member 1401 may either
reflect the
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- light rays fiuther away from viewer 1315 as illustrated, or reflect the
light rays back at
source 903. A reflecting member is disclosed by Kamath et al., US 5,132,823,
the
disclosure of which is incorporated herein. In one embodiment, reflective
member 1401
is comprised of a plurality of individual tilted reflective surfaces, each of
said individual
S reflective surfaces corresponding to a display pixel. The individual tilted
surfaces may be
simple planar surfaces or more complex surfaces such as cones, the latter
configuration
suitable for use with multiple light sources.
In an alternate embodiment of the invention, multiple sources 903 are used
to increase the display brightness and uniformity. Fig. 15 is a top view of
display 901.
i 0 As disclosed above, viewing system 905 is positioned norcr~al to the
display. In this
embodiment multiple sources 903 are positioned around viewing system 905 and
within
an angle of approximately 30 degrees, and preferably less than 20 degrees from
the
display normal. Although four sources 903 are shown in Fi.g. 15, both fewer
and greater
numbers of sources may be used with the invention. Additional increases in
contrast
15 levels may be achieved by positioning sources 903 to minimize their
alignment with the
pixel edges as shown in Fig. 15, and as disclosed in co-pending U.S. Patent
Application
Serial No. 09/090,749, filed 4 3une 1998, entitled High Contrast Micro Display
With Off
Axis Illumination.
Fig. 16 is an alternate embodiment of the invention providing a frame
20 sequential color display system. This embodiment is subsW ntially the same
as that
previously illustrated except that at least three different color sources 903
are used to
sequentially illuminate sample 901. For example, sources 903 may be comprised
of a red
source 1601, a blue source 1602, and a green source 1603. If improved
brightness and
uniformity are desired as illustrated in Fig. 15, each color light may be
provided by
25 multiple sources (e.g., sources 1601', 1602' and 1603').
In this embodiment the processor that controls the state of the individual
pixels of display 901 also controls the activation of the individual color
light sources
1601-1603. As disclosed above, the individual colors are synchronized with the
images
displayed on liquid crystal panel 901. Assuming a fast enough switching speed,
only a
30 composite colored image is perceived by the viewer.
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Fig. 17 illustrates a timing sequence suitable; for use with the embodiment
of the invention shown in Fig. 16. Three colors, red, green, and blue, are
shown although
other combinations may be used with the invention as discussed above. As
previously
disclosed, in order to prevent frame flicker and color break-up, a frame rate
of at least 180
Hz is desired. This requires a combination of the liquid crystal response time
and the
active matrix array setup time to be shorter than 5.6 milliseconds. Therefore
if it takes 2
milliseconds to refresh the entire display, the liquid crystal response time
must be shorter
than 3.6 milliseconds as illustrated in Fig. 17. Light sources 903, for
example LEDs, are
preferably not turned on during the array setup time nor du.~~ing the liquid
crystal response
time as illustrated. If the sources are tuned on during either of these times,
there may be
a color shift in the middle of the display.
Typically the active matrix is addressed from the top to the bottom of the
display using a line at a time scheme. Alternatively, the array is divided
into multiple
zones with dedicated light sources per zone, thereby reducing the array setup
time by the
number of zones. Thus by using two zones, the liquid crystal response time
requirement
is reduced from 3.6 milliseconds to 4.6 milliseconds. This .embodiment also
allows the
pulse width of the light sources to be extended, thus increasing the display
brightness.
As will be understood by those familiar with the art, the present invention
may be embodied in other specific forms without departing from the spirit or
essential
characteristics thereof. For example; the invention may be used with a variety
of different
liquid crystal materials, display panel designs, pixel sizes, pixel shapes,
and electrode
configurations. Accordingly, the disclosures and descriptions herein are
intended to be
illustrative, but not limiting, of the scope of the invention which is set
forth in the
following claims.
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