High contrast micro display with off-axis illumination

A liquid crystal display of improved contrast is provided. Image contrast is determined by the ratio of light from the display pixels that are in an on state to the light from the display pixels that are in an off state. The disclosed system improves on the image contrast by selecting the illumination direction and the panel orientation in such a way as to minimize the amount of light entering the viewing field that is due to scattered and/or diffracted light from the fine features of the display panel.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates generally to liquid crystal display panels 
and, more particularly, to a method and apparatus for increasing the 
contrast and over-all image quality of a liquid crystal display panel. 
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 dispersed, encapsulated, embedded, or otherwise contained 
within a matrix material such as a polymer. Exemplary disclosures include 
Fergason, U.S. Pat. Nos. 4,435,047; West et al., 4,685,771; Pearlman, 
4,992,201; and Dainippon Ink, EP 0,313,053, the disclosures of which are 
incorporated herein for all purposes. 
The liquid crystal composite is disposed between electrodes, at least one 
of the electrodes typically being patterned to form a matrix. The 
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 
and/or absorbed. In this state the material 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 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 on 
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. 
Regardless of the exact nature of the liquid crystal composite or of the 
intended application (e.g., reflective versus transmissive, direct viewing 
versus projection versus virtual image viewing, normal mode versus reverse 
mode, etc.), a high contrast display is desirable since it provides 
improved image definition and quality. A number of different displays have 
been designed in response to this need. For example, in U.S. Pat. No. 
4,732,456 a display with enhanced contrast is disclosed in which a lens or 
reflector means is used to direct light transmitted by the display onto a 
light absorbing target. 
U.S. Pat. No. 5,076,668 discloses a system for achieving a high brightness 
display with excellent contrast qualities. The disclosed system utilizes a 
gain reflector disposed behind a display medium comprised of a liquid 
crystal material. The gain reflector is preferably an offset gain 
reflector, thus angularly offsetting the specular reflection from the 
reflected gain. 
Although there are various methods of achieving a high contrast liquid 
crystal display, typically additional elements are required, thus adding 
to the overall complexity and cost of the display panel. Therefore an 
improved high contrast liquid crystal display that is simple to 
manufacture is desired. 
SUMMARY OF THE INVENTION 
The present invention provides a reverse mode liquid crystal display of 
improved contrast. Image contrast is determined by the ratio of light from 
the display pixels that are in the on, ie., bright state to the light from 
the display pixels that are in the off, i.e., dark state. The present 
invention improves on the image contrast by selecting the illumination 
direction and the panel orientation in such a way as to minimize the 
amount of light entering the viewing field that is due to scattered and/or 
diffracted light from the fine features of the display panel. 
A liquid crystal display panel is made up of a plurality of pixels of 
substantially uniform size, shape, and alignment. Due to the alignment of 
the pixel edges of the plurality of panel pixels, multiple pixel edge 
planes are formed For example, if the panel pixels are square, two 
orthogonal pixel edge planes will be formed. Similarly, if delta shaped 
panel pixels are used, three pixel edge planes will be formed, the planes 
being separated by 60 degrees. 
In one aspect of the invention, the illumination is directed at a liquid 
crystal panel such that it is not aligned with a pixel edge plane, thereby 
reducing scattered and/or diffracted light from pixel edges and increasing 
the overall display contrast. Preferably the illumination is directed at 
the panel in a direction that is substantially equidistant from two 
adjacent pixel edge planes, thereby optimizing the image contrast 
Furthermore in at least one embodiment, the location of the illumination 
source is not normal to any portion of the display panel. 
In another aspect of the invention, a liquid crystal display panel is 
illuminated by multiple sources. Preferably none of the illumination from 
the sources is aligned with a pixel edge plane. This aspect of the 
invention provides for improved brightness and illumination uniformity 
while achieving high contrast levels. 
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; 
FIG. 2 is a cross-sectional view of the liquid crystal light valve of FIG. 
1 in the off state; 
FIG. 3 is a sectional view of a liquid crystal display panel according to 
the prior art; 
FIG. 4 is a top view of the liquid crystal display panel of FIG. 3; 
FIG. 5 is an illustration of a reflective liquid crystal display operating 
in the normal mode; 
FIG. 6 is an illustration of a transmissive liquid crystal display 
operating in the normal mode; 
FIG. 7 is an illustration of a reflective liquid crystal display operating 
in the reverse mode; 
FIG. 8 is an illustration of a transmissive liquid crystal display 
operating in the reverse mode; 
FIG. 9 is an illustration of a testing system used to verify the benefits 
of the present invention on a test panel; 
FIG. 10 is a graph of the reflectance of a test panel as a function of the 
applied voltage, the test panel oriented such that the plane of the 
illumination source is aligned with the pixel edges; 
FIG. 11 is a graph of the reflectance of a test panel as a function of the 
applied voltage, the test panel oriented such that the plane of the 
illumination source is not aligned with edges; 
FIG. 12 is an illustration of the orientation of the test panel of FIG. 11; 
FIG. 13 is an illustration of a liquid crystal display panel in which the 
pixels are approximately rectangular in shape; 
FIG. 14 is an illustration of a liquid crystal display panel in which the 
pixels have a delta shape; and 
FIG. 15 is an illustration of a liquid crystal display panel illuminated by 
multiple sources in accordance with the present invention. 
DESCRIPTION OF THE SPECIFIC EMBODIMENTS 
Before describing the present invention in detail, several different 
configurations of liquid crystal displays according to the prior art 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, U.S. Pat. Nos. 4,435,047; Reamey et al., 5,405,551; and Havens 
et al., 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 crystal displays, for example smectic A, 
cholesteric, or dynamic scattering nematic displays, may also be employed. 
Electrodes 105 are coupled 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 
(ie., 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 plurality of 
electrode pairs. The electrode pairs divide panel 300 into an array of 
separately controllable display elements or pixels. Panel 300 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 configuration 
illustrated in FIG. 3 is 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 maximum voltage that appears for each pixel 
electrode is the threshold voltage of liquid crystal volumes 103. Although 
in the illustrated embodiment switching elements 309 are MOS transistors, 
other switching elements such as thin film transistors, MIMs, diodes, or 
varistors may be used as an alternative. The 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 of one or more light sources, a 
liquid crystal composite, electrode elements, and imaging optics that form 
a virtual image on the eye of the user. Additionally, liquid crystal 
displays may be designed to function in either a transmissive or a 
reflective mode. 
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. 5 is an illustration of a 
reflective liquid crystal display operating in the normal mode. In this 
mode both the light source 501 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 505 is well known in the art, see, for 
example, Rowland, U.S. Patent Nos. 3,935,359; Kuney, Jr., 4,957,335; 
Nelson et al., 4,938,563; Belisle et al., 4,725,494; Appledom et al., 
4,775,219; Tung et al., 4,712,219; Malek, 4,712,867; Benson, 4,703,999; 
Sick et al., 4,464,014; Nelson et al., 4,895,428; Hedblom, 4,988,541; 
Schultz, 3,922,065; and Linder, 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 substrate 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 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 511 and aperture stop 513 to reach 
viewing location 503. 
In an alternate configuration of a normal mode display illustrated in FIG. 
6, neither pixel electrodes 507 and 509 nor bottom substrate 505 are 
reflective. In this configuration the image is formed by the light 
transmitted through those pixels in the on state, i.e., pixels 507, as 
illustrated by exemplary light ray 601. As in the reflective 
configuration, only a fraction of scattered rays 515 will pass through 
aperture 513 and optics 511 to be imaged at location 503. 
FIGS. 7 and 8 illustrate a display panel operating in the reverse mode. As 
noted above, a reverse mode panel may be used either in a reflective 
configuration (e.g., FIG. 7) or in a transparent configuration (e.g., FIG. 
8). This panel is basically the same as that illustrated in FIGS. 5 and 6, 
respectively. However in this configuration it is the scattered light 515 
that is collected by imaging optics 511 to form an image at location 503. 
The specular light, either reflected as shown by an exemplary light ray 
701 or passing through the display in the case of a transparent 
configuration as shown by an exemplary light ray 801, 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. See, for example, Jones, U.S. Pat. No. 5,398,081, 
the disclosure of which is incorporated herein. 
The present invention will now be described in more detail. It has been 
discovered that the contrast ratio of an image formed by a liquid crystal 
display is reduced by the scattering and/or diffraction effects of a 
display panel, particularly those effects associated with the fine 
periodic structure (i.e., pixel edges) of the panel matrix. A reduction in 
the contrast ratio results in a reduction in the overall image quality. 
Image contrast is determined by the ratio of light from the pixels in the 
on state to that from the pixels in the off state. 
In the method of the present invention, the scattering and/or diffraction 
effects of the display panel edges are minimized, thereby increasing the 
contrast ratio. Specifically, the illumination direction and the panel 
orientation are selected such that a minimum amount of 
scattered/diffracted light enters the viewing field. Regardless of the 
liquid crystal display configuration utilized, preferably the displayed 
image is of high contrast, thus improving the image quality. 
FIG. 9 is an illustration of a testing system used to verify the benefits 
of the present invention on a test panel 901. Test panel 901 is comprised 
of an approximately 2 micrometer thick liquid crystal display with square 
pixels of approximately 28 micrometers per side in a configuration such as 
that illustrated in FIG. 4. A near collimated halogen light source 903 
illuminates test panel 901 at an angle of 45 degrees from the panel 
normal. An integrating sphere 905 normal to test panel 901 measures the 
reflected light from panel 901, integrating sphere 905 having an f-number 
of 3.5. 
FIGS. 10 and 11 are graphs of the reflectance of test panel 901 as a 
function of the applied voltage. In FIG. 10 the pixel edges of display 901 
are aligned along the x and y-axes of the system while in FIG. 11 the 
display has been rotated 45 degrees around the panel normal as illustrated 
in FIG. 12. Defining the contrast ratio in terms of the reflectance at 0 
volts versus the reflectance at 5 volts, the contrast ratio for the 
rotated panel (i.e., FIG. 11) is approximately 50 percent improved over 
the non-rotated panel. Even more striking is the contrast ratio defined by 
the reflectance at 0 volts versus the reflectance at 10 volts. Using this 
contrast ratio, an improvement of approximately 100 percent is achieved by 
rotating the panel 45 degrees. Similar advantages to rotating the panel 
are obtained if the panel is being used in the transmissive mode. 
Therefore in one aspect of the invention, the illumination is directed at 
the liquid crystal panel, either in the reflective or transmissive mode, 
such that it is not aligned with a pixel edge. To further illustrate this 
aspect of the invention, FIG. 13 shows a display panel 1301 comprised of a 
plurality of pixels 1303. Pixels 1303 may be either square or rectangular 
shaped with pixel edges aligned along the x and y-axes. Assuming a z-axis 
perpendicular to the plane of the figure and given the 0, 90, 180, and 270 
degree directions as indicated, a light source that is in the y-z (i.e., 
0.degree. or 180.degree.) or x-z (i.e., 90.degree. or 270.degree.) planes 
will provide the worst contrast ratios. The optimum contrast ratio is 
achieved with the light source oriented at 45, 135, 225, and 315 degrees, 
i.e., positions 1305-1308, respectively. As a light source is re-oriented, 
becoming closer and closer to a pixel edge, the contrast ratio becomes 
increasingly worse. 
FIG. 14 illustrates the application of the present invention to a liquid 
crystal display panel 1401 in which the pixels 1403 have delta shape. In 
this configuration the edges of the pixels are aligned along the x-axis, 
+30 degrees off of the y-axis, and -30 degrees off of the y-axis. 
Therefore the poorest contrast is achieved when the light source is 
aligned with any of these three pixel edges. Optimum performance is 
achieved when the light source is oriented along the y-axis (i.e., at a 
position 1405 or 1406) or oriented .+-.60 degrees off of the y-axis (i.e., 
at a position 1407-1410). 
In an alternate embodiment of the invention illustrated in FIG. 15, a 
liquid crystal panel such as that shown in FIG. 13 is illuminated by 
multiple sources 1501. The multiple source configuration may be used with 
a panel in either the transmissive or the reflective mode and with any 
pixel shape (see, for example, FIGS. 13-14). Although four sources are 
shown, it is understood that both fewer and greater numbers of sources may 
be used depending upon the desired panel brightness. In addition to 
increasing brightness levels, multiple sources also increase illumination 
uniformity across the display panel. In accordance with the invention, 
each source is directed at panel 1301 in such a manner as to minimize 
scatter/diffraction effects from pixel edges. Thus in the embodiment 
shown, sources 1501 are located at 45, 135, 225, and 315 degrees. 
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. 
Furthermore, the invention is not restricted to either transmissive or 
reflective systems nor is the invention limited to a specific number of 
illumination sources. 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.