U.S. patent application number 09/369465 was filed with the patent office on 2001-11-29 for design features optimized for tiled flat-panel displays.
Invention is credited to GREENE, RAYMOND G., MIWA, KOHICHI, NOGUCHI, MICHIKAZU, SERAPHIM, DONALD P., SKINNER, DEAN W., SUZUKI, SHUNJI, YOST, BORIS.
Application Number | 20010046007 09/369465 |
Document ID | / |
Family ID | 23455583 |
Filed Date | 2001-11-29 |
United States Patent
Application |
20010046007 |
Kind Code |
A1 |
GREENE, RAYMOND G. ; et
al. |
November 29, 2001 |
DESIGN FEATURES OPTIMIZED FOR TILED FLAT-PANEL DISPLAYS
Abstract
The present invention features designs of pixels and designs of
control features for seals on AMLCD tiles optimized for tiling
AMLCD flat panel displays (FPDs) which have visually imperceptible
seams. The FPD structure has an image view plane which is
continuous and remote from the pixel apertures or image source
plane on the inside of the tiles. The image is formed on the view
plane by a distributed ultra low magnification flies-eye optical
system (a screen) that is integrated with the tiles, effectively
excluding and obscuring an image of the seams. The innovations
described herein minimize the defects on the perimeter pixels by
effectively damming the waviness of the front of the seal near the
perimeter pixels on the tiles. Dark space required for the seal
between the interior tile edges and active regions of the pixels is
decreased, as is the space allocated for wiring thereby increasing
the feasible aperture ratios near the mosaic edges and all
apertures. The tile designs make effective use of the area of an
entire manufacturing panel.
Inventors: |
GREENE, RAYMOND G.; (OVID,
NY) ; SERAPHIM, DONALD P.; (VESTAL, NY) ;
SKINNER, DEAN W.; (VESTAL, NY) ; YOST, BORIS;
(ITHACA, NY) ; MIWA, KOHICHI; (YOKOHAMA, JP)
; NOGUCHI, MICHIKAZU; (KANAGAWA-KEN, JP) ; SUZUKI,
SHUNJI; (YOKOHAMA, JP) |
Correspondence
Address: |
MARK LEVY
SALZMAN & LEVY
19 CHENANGO ST STE 606
BINGHAMTON
NY
13901
|
Family ID: |
23455583 |
Appl. No.: |
09/369465 |
Filed: |
August 6, 1999 |
Current U.S.
Class: |
349/73 |
Current CPC
Class: |
G02F 1/13336 20130101;
G02F 1/133512 20130101 |
Class at
Publication: |
349/73 |
International
Class: |
G02F 001/133 |
Claims
What is claimed is:
1. An assembly of four tiles for use in a flat-panel display having
visually imperceptible seams, comprising: a) four display tiles,
each comprising a 400.times.300 sub-array of pixels defining
essentially identical viewing areas, the pixels of said sub-array
of pixels comprising substantially uniform pixel pitch, each of
said pixels of said sub-arrays having an active, central area
surrounded by an inactive, dark area having a predetermined width;
and b) seam regions disposed between adjoining edges of said four
display tiles for maintaining said substantially uniform pixel
pitch across said seam regions, said seam regions comprising thin,
perimeter seals at adjoining edges of each of said four display
tiles, said thin perimeter seals having a width no greater than
said predetermined width of said inactive, dark area.
2. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 1, wherein
each of said pixels in said sub-arrays of pixels comprises a pixel
cell gap.
3. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 2, wherein
said pixel cell gaps in pixels proximate said seam regions are
substantially equal to said pixel cell gaps in pixels disposed in
an interior region of said sub-arrays of pixels.
4. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 3, wherein
said thin, perimeter seals in said dark regions, of said pixels
proximate said adjoining edges of said four display tiles comprise
dam structures to control the spread of a sealing material forming
said thin, perimeter seals.
5. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 4, wherein
said dam structures comprise stripes spaced a predetermined
distance from said pixels proximate said seam regions.
6. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 5, wherein
said sealing material comprises epoxy dispensed according to a
predetermined process and said predetermined distance between said
stripes and said pixels being the width of a contaminating leading
edge of said epoxy, said width being a measurable characteristic
associated with said epoxy and said predetermined process.
7. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 3, wherein
said thin perimeter seals in said dark region of said pixels,
proximate said adjoining edges of said four-tile display, comprise
color filter dispense pads for controlling the central location and
profile of seal material as it is dispensed.
8. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 7, wherein
the width and location of said color filter dispense pads is
determined by at least one parameter representative of the width,
volume and front edge location relative to the pixels of said seal
and at least one characteristic of said seal material.
9. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 8, wherein
said pixels comprise sub-pixels arranged in a predetermined
pattern.
10. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 9, wherein
each of said sub-pixels comprises a red, a blue and green
sub-pixel.
11. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 10, wherein
said predetermined pattern comprises a rectangle.
12. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams count as recited in claim 8,
further comprising wiring selectively disposed in said inactive,
dark areas of said pixels.
13. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 8, further
comprising a fiducial structure used to locate components of said
four display tiles precisely with respect to one another and with
respect to said front and said rear masking means.
14. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 8, wherein
said four display tiles are AMLCD tiles comprising a liquid crystal
layer and, further, wherein said liquid crystal layer comprises
spacer means.
15. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 14, wherein
said spacer means comprises spacing spheres distributed in said
liquid crystal layer.
16. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 15, wherein
said liquid crystal layer of each of said four display tiles has an
identifiable rubbing direction where by the alignment of said
liquid crystal layer is maintained in a uniform direction.
17. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 16, wherein
said thin, perimeter seals are formed by dispensing in a
predetermined pattern and location relative to said pixels and said
color filter dams.
18. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 17 wherein
said predetermined pattern comprises at least two unique,
identifiable, predetermined patterns.
19. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 18, wherein
said at least two unique, identifiable, predetermined patterns are
identified on an exterior portion of said four display tiles.
20. An assembly of four tiles for use in a flat-panel display
having visually imperceptible seams, comprising: a) four display
tiles, each comprising a 400.times.300 sub-array of pixels defining
essentially identical viewing areas, the pixels of said sub-array
of pixels comprising a substantially uniform pixel pitch, each
having an active, central area surrounded by an inactive, dark area
having a predetermined width; b) seam regions disposed between
adjoining edges of said four display tiles for maintaining said
substantially uniform pixel pitch across said seam region, said
seam regions comprising thin, perimeter seals at adjoining edges of
each of said four display tiles, said thin perimeter seals being
formed from a flowable sealing material dispensed according to a
predetermined process and having a finished width no greater than
said predetermined width of said inactive, dark areas adjacent said
seams, said thin, perimeter seals further comprising dispense pads
for controlling the central location and profile of said flowable
sealing material as it is dispensed; and c) dam structures to
control the spread of said flowable sealing material, said dam
structures comprising stripes spaced a predetermined distance from
said pixels, said predetermined distance comprising the width of a
contaminating leading edge of said flowable sealing material, said
width being a measurable characteristic of said flowable sealing
material and said predetermined process.
21. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 20, wherein
said dam structures each comprise a first, continuous dam structure
extending essentially to at least one corner of each of said four
display tiles and a second dam structure proximate at least one of
said corners of said display tiles and said dispense pads to
control the position and profile of said thin perimeter seals
proximate said corners.
22. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 21, wherein
each of said pixels in said sub-arrays of pixels comprises a pixel
cell gap.
23. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 22, further
comprising a cell gap control structure for maintaining said pixel
cell gaps in pixels proximate said seams substantially equal to
said pixel cell gaps in pixels disposed at an interior region of
said sub-arrays of pixels.
24. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 23, wherein
said cell gap control structure comprises at least one from the
group of: dams, dispense pads, external spacers, stripes external
to said pixel area.
25. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 23, wherein
said cell gap control structure comprises external spacers matched
to spacers located in the liquid crystal area of said display
tiles.
26. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 25, wherein
each of said four display tiles has a physical structure different
from one another, each of said physical structures having a unique
identification.
27. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 26, said
physical structure of each of said tiles comprising a different
location for a liquid crystal fill port for each of said uniquely
identified display tiles.
28. An assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 20, further
comprising: d) interconnection means operatively connected to each
of said pixels for providing externally-generated, electrical drive
signals thereto; and e) electrostatic discharge protection means
operatively connected to said interconnection means for dissipating
electrical charges to prevent damage to said display tiles.
29. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 28, wherein
said electrostatic discharge protection means comprises diodes.
30. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 29, wherein
said diodes are located inside said narrow perimeter seals.
31. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 30, wherein
said diodes are located under said narrow perimeter seals.
32. The assembly of four tiles for use in a flat-panel display
having visually imperceptible seams as recited in claim 31, further
comprising a redundant transistor for controlling drive signals to
at least one of said subpixels.
Description
RELATED PATENT APPLICATION
[0001] The present patent application is related to U.S. Pat. No.
5,661,531 granted Aug. 26, 1997 for TILED FLAT PANEL DISPLAYS, and
co-pending U.S. patent application Ser. No. 09/221,096, filed Dec.
28, 1998, both assigned to the common assignee, and hereby
incorporated by reference.
FIELD OF THE INVENTION
[0002] The invention pertains to flat-panel electronic displays
and, more particularly, to large, flat-panel electronic displays
that are composed of a plurality of joined, smaller building blocks
(tiles) having seams therebetween. The tiles may be viewed as
though they were a single, monolithic display (i.e., as a display
having visually imperceptible seams).
BACKGROUND OF THE INVENTION
[0003] Images on electronic displays are derived from an array of
small picture elements known as pixels. In color displays, these
pixels comprise three color elements that produce the primary
colors: red, blue and green (R, B and G), for example. Usually
arranged in rectangular arrays, these pixels can be characterized
by a pixel pitch, P, a quantity that measures the spacing of pixels
in one direction. A typical cathode-ray tube (CRT) display used for
computer applications has a pixel pitch of 0.3 mm and a pixel array
width:height ratio of 4:3. Typical, standardized arrays in computer
displays are comprised of 640.times.480 (VGA) or 800.times.600
pixels (SVGA).
[0004] Large displays can be constructed of a plurality of adjacent
tiles, with each having a single pixel or an array thereof. Such
assembled tiled displays contain visually disturbing seams,
resulting from the gaps between adjacent pixels on the same and/or
adjacent tiles. Such seams may incorporate interconnect adhesives,
seals, mechanical alignment means and other components resulting in
visible optical discontinuities in displayed images. Some of these
structures are described in the aforementioned U.S. Pat. No.
5,661,531. As a consequence, the image portrayed on seamed displays
appears segmented and disjointed. Therefore, it is desirable to
fabricate tiled, flat-panel displays which do not have noticeable
or perceptible seams under the intended viewing conditions.
[0005] The pixel pitch in electronic displays must be set so that a
continuous image is produced when the display is viewed at
distances greater than the minimum viewing distance. For example,
with a pixel pitch of P=0.3 mm, the minimum viewing distance is on
the order of 1 m. Even though the minimum viewing distance
increases in proportion to the pixel pitch, it still limits the
pixel pitch for most computer and consumer displays. Since space
for the tiling functions must be provided in areas smaller in size
than the pixel pitch, it is difficult to develop structures and
methods for constructing tiled displays.
[0006] Flat-panel displays (FPD) provide the best choice for
constructing "seamless", tiled screens. Flat-panel displays include
backlighted and self-lighted displays. Liquid crystal displays
(LCDs) are the most common backlighted displays.
[0007] Flat-panel displays depend on the microfabrication of key
components that carry the pixel patterns.
[0008] Unfortunately, microfabrication techniques are not viable
for very large displays currently greater than 20 inches diagonal,
due to the fact that manufacturing yield declines rapidly with
increasing area of the display. Therefore, the inventors have
determined that tiles with arrays of pixels can be microfabricated
and then assembled together to form a larger electronic
display.
[0009] The present invention provides unique designs and methods
for achieving such large, seamless, tiled panels for color or
gray-scale displays. This invention particularly focuses on
displays of the transparent, lightvalve type. In such displays,
light from a uniform, backlight source is transmitted through the
display assembly and directly viewed from the front side of the
display. The lightvalves control the amount of primary light rays
transmitted through each of the color elements in the pixels. At a
sufficient viewing distance, the viewer's eyes merge the primary
light from the pixels to form a continuous image. Because of a
number of secondary processes, low-level light emanates from the
spaces between the pixels. These phenomena include reflection and
light guiding, all of which must be kept to a minimum in order to
achieve sufficient brightness and contrast. The spaces between
pixels on the same tile, and the spaces between pixels on adjacent
tiles have different structures. Consequently, the presence of
seams between the pixels at the edge of the tiles affects both
primary and secondary light rays, thus increasing the difficulties
for constructing seamless, tiled displays.
[0010] The inventors have identified three design principles in
making large-scale, seamless, flat panels that may be viewed as
though they were single monolithic displays: (a) the intra-tile
pixel pitch on the view plane for the tiles must be matched to the
intertile pixel pitch; (b) the primary light paths through the
lightvalves must not be affected by the presence of the seam or any
other structures or components used in the tile assembly; (c) the
inter-pixel regions must be designed so that intra-tile and
inter-tile pixel regions, which have different physical structures,
present approximately the same visual appearance to the viewer
under transmitted and reflected light. This has largely been
accomplished by applying the technology described in U.S. Pat. No.
5,661,531 to fabricated tiled AMLCD functional models. However,
design improvements can still be made to increase manufacturing
yields and to maximize optical performance of the tiled displays
and their component parts, particularly the tiles.
SUMMARY OF THE INVENTION
[0011] The present invention describes a tiled, flat-panel display
having visually imperceptible seams between tiles disposed in an
interior portion thereof, so that the display is perceived by a
human observer as a single, monolithic display, when viewed at a
distance equal or greater than the intended minimum viewing
distance. This invention applies primarily to lightvalve-type,
flat-panel displays with a backlight.
[0012] The panel comprises an image source plane having
spaced-apart pixels with active areas which control the
primary-color, light-transmitting elements (e.g., red, blue and
green). It should be understood that the primary colors need not be
red, blue and green but may be other colors, and not necessarily
limited to three. Included in the image source plane may be a color
filter (CF) layer. Alternatively, the CF may be included with
screen and polarizer outside of the tiles continuous across the
mosaic. Surrounding the active area of each pixel is an inactive
(dark) area. This dark area can be used for a variety of purposes
without affecting the light output and/or visual appearance of the
display. For example, electrical circuitry, such as transistors,
are situated in the dark spaces. Most importantly, thin, perimeter
seals at the edges of the AMLCD tiles may utilize that portion on
the dark areas of the pixels adjacent to the edge. Wiring may also
be placed in the pixel dark areas, as required.
[0013] Each of the pixels is disposed along the image source plane
at a given pitch greater than approximately 0.2 mm and preferably
0.98 mm. A plurality of adjacently-disposed tiles is located in the
image source plane. The invention includes a number of methods for
the design, construction and assembly of tiled displays with
invisible seams which are significant compliments to the technique
disclosed in U.S. Pat. No. 5,661,531. These can be grouped into the
following distinct categories: (1) alteration of the
characteristics of the image source plane, (2) preferred
positioning of the masks, polarizer and image view plane (screen)
to enhance hiding of the seams between tiles, (3) enhancement of
the brightness of the display assembly by optimizing the backlight
collimation angles, and (4) improvements in color matching between
tiles.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] A complete understanding of the present invention may be
obtained by reference to the accompanying drawings, when considered
in conjunction with the subsequent detailed description, in
which:
[0015] FIG. 1 shows a schematic, plan view of a typical, tiled,
prototype array of pixels in a color, electronic display, in
accordance with this invention;
[0016] FIG. 2 illustrates a schematic, cross-sectional view of a
lightvalve used in a flat-panel display with a backlight (not
shown);
[0017] FIG. 3 depicts a graph of a typical light transmission
voltage curve for a lightvalve in an active matrix liquid-crystal
display;
[0018] FIG. 4 shows a schematic diagram of a color pixel with three
lightvalves, three column lines and one row line for the selection
of each color valve, including devices for activating the
lightvalves;
[0019] FIG. 5 illustrates the floor plan of a color pixel with
three lightvalves, matching color filters with dark space
surrounding the sub-pixels;
[0020] FIG. 6 is a schematic, cross-sectional diagram of pixels
with three lightvalves for an active matrix liquid-crystal tiled
color display near a seam with cover and back plate, polarizers,
masks, and screen;
[0021] FIG. 7 is a graph of intensity versus light distribution for
the light source to be used with a tiled display;
[0022] FIG. 8 shows a schematic diagram of the limiting angles of
light rays passing through pixels and seam areas in the FPD
prototype of the current invention;
[0023] FIG. 9 depicts the location of CF dams, CF dispense pad, in
reference to pixels and tile edges and corners near seams, showing
also the approximate seal location and the outer edge of a
tile;
[0024] FIG. 10 is an illustration of the seal flow after squeeze of
the CF substrate to the TFT substrate with spacer balls determining
the cell gap therebetween;
[0025] FIG. 11a is a schematic, composite view of a single color
filter substrate showing four possible LC fill port locations and
four different rubbing directions allowing configuration as one of
four different part numbers in a tile array, depending upon the
chosen seal-dispense pattern;
[0026] FIG. 11b is a detailed, schematic view of a portion of the
composite color filter shown in FIG. 11a, showing a corner seal
configuration for an "A" color filter configuration;
[0027] FIG. 11c is a detailed, schematic view of a narrow-seal (non
fill port) corner of the composite color filter shown in FIG.
11a;
[0028] FIG. 11d is a detailed, schematic view of a narrow-seal (no
fill port) corner of the composite color filter of FIG. 11a,
showing narrow dam structures attached to dispense pads; and
[0029] FIG. 12 is a schematic view of a four-tile FPD constructed
from four unique, single part number color filters as shown in FIG.
11a, each directionally rubbed in an appropriate direction as
required by its position in the four-tile FPD assembly.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] Generally speaking, the present invention features a tiled,
flat-panel, color display that is visually seamless under the
intended viewing conditions. The seams become effectively invisible
when they do not produce image segmentation, and their brightness,
color and texture appear equal to the spaces between the
lightvalves residing on the same tile. A number of techniques are
described that affect the design, construction and assembly of the
tiled display, making the display appear seamless.
[0031] FIG. 1 shows a schematic, plan view of a typical, tiled
display having arrays of pixels 11 arranged into tiles with seams 8
therebetween. Each of the arrays of pixels 11 comprises primary
color elements R, B and G (red, blue and green) in the preferred
embodiments. The number and selection of the primary colors is not
limited to this set, however.
[0032] Referring to FIG. 2, a cross-sectional view of a typical
lightvalve 12 used in flat-panel displays is illustrated. In a
flat-panel, liquid-crystal display (LCD), light is generated in a
separate backlight assembly, not shown, and projected (arrow 5)
through the lightvalve 12 towards the viewer, not shown. The
lightvalve 12 is formed by two polarizer sheets placed on opposite
sides of an optically-active, liquid-crystal layer 6. Light passing
from the backlight through the lower polarizer sheet 21 becomes
linearly polarized. When an electric field is applied to the
liquid-crystal layer 6, it turns the plane of polarization of the
transmitted light 5 by an amount that monotonically increases with
the magnitude of the applied electric field. The top polarizer
layer 21 lets pass only the polarization component of the light
that is parallel to its polarization plane. By varying the
magnitude of the applied voltage, the lightvalve 12 thus modulates
the intensity of the transmitted light in a continuous fashion,
from fully off to fully on. A typical, light-transmission,
applied-voltage curve for LCD materials used in active matrix
liquid-crystal displays (AMLCDs) is depicted in FIG. 3.
[0033] Referring to FIG. 4, single lightvalves 12 are shown
covering a pixel area 16 for color display applications. In a
conventional AMLCD, the lightvalves 12 comprises a thin film
transistor (TFT) and a storage capacitor, in addition to the
liquid-crystal cell, the transparent electrodes and the polarizers.
The TFT is used as the active non-linear device, in combination
with row 17 and column 15 lines to achieve matrix addressing of all
pixels in the display.
[0034] In electronic color displays, separately controlled
lightvalves 12 are placed in the pixel 16, as shown in FIG. 5. One
color element is assigned to each of the primary colors. A color
filter layer 18 is placed on top of the lightvalves 12 into the
pixel 16. Light having the desired wavelength spectrum
corresponding to only one of the color filter regions 18 passes
through the lightvalve 12 and the aligned color filter layer
18.
[0035] Assume that the dimensions of each of the apertures is
W.times.H, as shown in FIG. 5. The W dimension is somewhat smaller
than the pitch P, divided by 3 for color displays (FIG. 5). H is
also somewhat smaller than P. A further fraction of this light
passes through the second matching aperture defined in the color
filter layer 18 (FIG. 5), with the given dimensions of W'.times.H',
where W'>W, and H'>H, in order to allow for misalignment
during assembly.
[0036] A tiled lightvalve assembly as described above is shown in
FIG. 6 in cross-section. It consists of tile components 19, bottom
plate 20, with thin film TFT structures 24, and a second tile
component, top plate 21, containing the color filter described
above and dark spaces 30. These are enclosed by the glass cover
plate 22 and glass back plate 26, and masks 23. The lightvalve
apertures 18 form the actual image source plane of the display in
this preferred assembly, while the screen 25 which forms the view
plane is external to the glass cover plate 22 and polarizer sheet
21. Other positions for screens, masks and polarizers presented in
the sequence to the light are also effective for tiled displays.
For example, the screen may be on either side of the mask on the
bottom surface of the cover plate with the polarizer positioned
between these components and the tiles. Also, the polarizer and the
back plate may be on either side of the mask.
[0037] The color filter layer 18 in this preferred embodiment is
inserted on the topside of the LCD fill material into close
proximity of the image source plane in all conventional LCDs, in
order to avoid parallax. Typically, the thickness of the LCD layer
is less than 10 .mu.m. However, if collimated or partially
collimated light is used, the color filter layer 18 may be located
alternatively further from the image source plane, for example, on
the cover plate below the screen. The typical glass sheet thickness
used in LCDs is between 0.7 and 1.1 mm. The tile component glass
sheets 20 and 21 carry the transparent electrodes in their
thin-film layers and usually comprise indium-tin-oxide material
(ITO). The lower glass plate 20 usually carries the X-Y
interconnect for matrix addressing, in addition to the non-linear
TFT control devices and the storage capacitors for the lightvalves,
for image stabilization. The upper glass sheet 21 carries another
transparent electrode and the patterned color filter layer 18. The
backlight acts as a diffuse source, with light rays emanating into
the full half-space above the source. A fraction of this light
passes through the aperture of a lightvalve in a specific pixel
defined in the thin film layer 24.
[0038] The spacing d between these two thin film apertures is
determined by the optical design of the display, with the optical
path length through the liquid-crystal layer being the primary
factor. The spacing d is always much smaller than W or H, FIG. 5,
and is typically about 5 .mu.m for an AMLCD. As a consequence of
the very small aspect ratio d:W, a very wide, angular range of
light rays can pass through the display stack 154 (FIG. 6). For a
conventional AMLCD with pixel width P=300 .mu.m for the three
subpixels and pixel pitch in the range of 400 .mu.m, and d=5 .mu.m,
the limiting rays form angles of greater than 750 with the surface
normal of the display. Therefore, light normally spreads in the top
glass plate over a wide lateral distance, overlapping several other
pixels. Only an angle of 15.20 normal to the surface is required
for a 1.1 mm-thick top plate 21 for the light to reach the adjacent
pixel with the above, sample parameters in conventional, non-tiled,
AMLCD displays.
[0039] In addition to these two apertures, reflection and
refraction processes take place at each optical interface where the
refractive index changes or a reflective material is encountered.
For a glass-to-air interface, with refractive indices of 1.5 and
1.0, for example, the angle for total internal reflection is 56.30.
Therefore, the limiting primary rays escaping from the display
stack towards the viewer are not limited by the aspect ratio of the
aperture, but by total internal reflection. Nevertheless, the
permissible angles for the limiting rays are much larger than the
angle required to overlap adjacent pixels.
[0040] A great number of secondary light rays traverse in the
transparent glass stack, in addition to the primary rays
originating from the backlight and passing through the lightvalves.
When diffuse light emanating from the backlight is passed through
the glass stack, it undergoes optical refraction and reflection
processes, including lateral reflective and refractive waveguiding.
These processes redistribute the secondary rays in the glass stack,
so that some light is transmitted through all points of the display
outside the primary rays controlled by the lightvalve apertures.
Secondary rays, in combination with ambient light entering the
display from the top surface, form background light that influences
the contrast of the display. In order to maximize the contrast, the
intensity of secondary rays must be minimized. Contrast ratios as
large as 100:1 have been demonstrated in state-of-the-art
AMLCDs.
[0041] The range of perception thresholds for image segmentation
and discrimination of brightness and color differences are
determined by a human observer, as described in detail
hereinbelow.
[0042] Monolithic displays are laterally uniform, and secondary
light does not pose any special optical problems, apart from the
edge pixels that can be extended and covered over. In tiled
displays, however, the situation is completely different. The
structure abruptly changes at the seam of each tile. Therefore,
both primary and secondary rays are affected by the presence of a
seam, and any seam is generally visible unless it is significantly
modified. The visibility of the seam can be rigorously demonstrated
using the following model. Assume that the brightness of two
adjacent tiles is the same, but undergoes an offset at the seam, as
shown in FIG. 6. By performing a Fourier analysis of the resulting
light intensity profile, and relating this to the resolving power
of the human eye, the following equation for the threshold width
.theta. of the seam, under high illumination conditions (500 nit or
cd/m.sup.2), is:
.theta.=3.5(.DELTA.1/1) arc sec (1)
[0043] where .DELTA.1/1 is the relative intensity modulation at the
seam. (See Alphonse, G. A. and Lubin, J., "Psychophysical
Requirements for Tiled Large Screen Displays", SPIE Vol. 1664, High
Resolution Displays and Projection Systems, 1992.) Equation 1 has
been confirmed by psychophysical testing, showing that both bright
and dark seams are equally visible. For a relative intensity
modulation of 1 or 100%, at a viewing distance of 50 cm, Equation 1
shows that the maximum width for an invisible seam is 8.5 .mu.m for
this intensity modulation. Since tiling functions cannot easily be
accomplished in 8.5 .mu.m seam widths today, tiled displays cannot
be constructed without special designs that drastically reduce the
intensity modulation at the seam.
[0044] The techniques presented in U.S. Pat. No. 5,661,531 for
designing, constructing and assembling tiled displays with
invisible seams was grouped into the following six distinct
categories, described hereinbelow in detail:
[0045] (1) alteration of the image plane,
[0046] (2) generation of an image view plane apart from the image
source plane,
[0047] (3) collimation, or partial collimation, of light to prevent
primary light rays from reaching the seams,
[0048] (4) suppression of secondary rays emanating from the gaps
between the lightvalves in the pixels,
[0049] (5) enhancement of the range of view angles presented to the
observer by the tiled display, and
[0050] (6) enhancement of the brightness of the tiled display
assembly.
[0051] This invention deals with optimization of the same six
categories and, in addition, deals with color richness of the
individual sub-pixels in a large tiled display employing monolithic
masks. The monolithic masks, preferably placed on cover and back
plates, cover the seams and all dark spaces between pixels,
smoothing out the appearance of optical differences between the
seam areas and the dark spaces. The masks counteract the
uncertainty of the locations of the edges of the tiles and their
positioning accuracy, since these areas are well hidden from the
collimated light. In this preferred embodiment, the blue color is
now intentionally placed at the center of the three sub-pixels so
that the mask never shades any of the blue area. A shift in
position of the tile either covering more sub-pixel or opening more
sub-pixel to the light coming through the masks changes only the
intensity level of red or green colors and are easily counteracted
in intensity modulation by the software and electronics. This is
the current preferred embodiment but many other arrangements would
suffice.
[0052] Furthermore, this preferred tiling design clusters the
sub-pixels together as close as possible for the entire pixel as
shown in FIG. 4. This is accomplished by moving the wiring for all
columns 15 and row lines 17 to the dark space area rather than
between sub-pixels. These are unique design modifications focused
on improving the tiles for application to tiled FPDs and have the
effect of increasing the aperture ratio. The image source plane of
a tiled display is preferably designed and the masking external to
the tiles is arranged so that the image appears as a uniform array
of pixels with a constant pixel pitch, both in transmitted and
reflected light, irrespective of the presence of the seams. For
close distance viewing this arrangement may appear grainy, but for
appropriate distances for larger displays, the images are better
than most large display technologies.
[0053] First, therefore, all physical space required by tiling must
fit into the space provided by the uniform pixel pitch determined
by the monolithic masks within the tiles. For LCDs, the seam must
accommodate two liquid-crystal seals, substantial tolerance
deficiencies in the location of tiles one to another and possibly
some space for metal interconnect for the matrix addressing of each
pixel. This requirement limits the achievable minimum pixel pitch
in tiled displays. Second, the space between lightvalves on
adjacent tiles must be made to appear the same optically as the
pixel spaces on the same tile. This can be accomplished by placing
light shields and/or selected color filter patterns into the image
source plane between adjacent lightvalves and by minimizing the
tile to tile spacing so that these light shields almost fill the
space between tiles. The non-transparent, thin film materials used
for making the TFT device interconnect or CF light shields can be
used for light shielding on the tile. The separate light shields
noted above are preferably placed to block direct light rays from
passing through the gap, and are aligned to the thin film masking
within the tiles, during the process of assembling the tiles to
cover and back plates. Finally, the front side optical reflectivity
of all light shield layers placed into the spaces between
lightvalves should be as uniform as possible. Furthermore, the
light shields plus the absorption effects of the CF decrease the
impact of the intense back light used in tiled displays on the
light activated leakage of the TFTs.
[0054] Generation of Image View Plane
[0055] The image source plane in a flat-panel LCD is formed by the
lightvalve apertures in the thin film layer underneath the
optically-active liquid-crystal layer. For practical purposes, the
color filter can be considered to reside in the image source plane
as well, since the thickness of the liquid-crystal layer is on the
order of only 5 .mu.m. Even with the state-of-the-art,
high-resolution pixel pitch of 0.2 mm, this gives a height-to-width
aspect ratio of 0.075 for the color elements, which produces a
negligible parallax error for normal viewing conditions. However,
if mask layers or aperture plates are used on the top surface of
the thinnest available, upper glass sheet with a thickness of 0.7
mm, the height-to-width aspect ratio with the same pixel pitch
increases to 16.5. This results in an unacceptably large parallax
error, unless the image source plane is viewed close to the
direction of the surface normal. In order to avoid this parallax
problem, the image source plane must be projected into a separate
image view plane, which must be generated from the image source
plane using a number of well-known optical techniques. This allows
the CF if desired to be proximate the image view plane in
alternative assembly embodiments.
[0056] First, as noted above, the seams are hidden from direct view
by placing a monolithic face mask on the common coverplate over all
seams and dark spaces between pixels.
[0057] Preferably the cross section design will minimize the
distance between the mask and the tiles. This may be achieved by
placing the polarizers on the outside of the cover plate and back
plate surfaces and by placing the masks on the inside surfaces as
close to the tiles as the composite adhesive system 40, 41 allows,
as shown in FIG. 6. The composite adhesive thickness is preferably
minimized between cover plate and tiles, and between back plate and
tiles. Furthermore, it is preferred that the tiles are made with
0.7 mm or thinner glass to minimize the seam area and to improve
the limiting optical angles. It is desirable to cover the gaps
between the lightvalves on the same tiles, as well, with the same
face mask, in order to match the light reflection characteristics
with those of the seam gaps, and in order to control secondary
rays, as described hereinbelow.
[0058] Second, optical elements can be used to perform the actual
forward projection of the image. A number of optical techniques,
including but not limited to arrays of refractive microlenses,
holographic lenses, diffusive screens, lenticular screens and
Fresnel screens can be used to perform the projection. These
optical techniques can be designed to meet or exceed the typical
view angle requirements of state-of-the-art AMLCDs. Since the image
quality of the tiled display depends on this projection, care must
be taken to maintain a uniform focus and contrast over the entire
area of the display.
[0059] Collimation of Primary Rays
[0060] The primary rays should preferably be limited, so that they
do not pass through any structures used for tiling, when passing
through lightvalves adjacent or close to the seams.
[0061] The placing of a monolithic black mask behind and in front
of this seam "hides" it from view, thereby rendering the display
seamless in a forward direction, within defined angles. A seamless
display at large angles is created, however, when light is
collimated to the extent of minimizing the primary and secondary
light in the seam area.
[0062] FIG. 6 shows a cross-section of a particular functioning
embodiment of such a display. The display is illuminated by a
collimated light source, not shown. Light enters the display
through a polarizer, then a rear mask, the LCD tile panels, a front
mask, a front polarizer, and finally a diffusion screen located at
the image view plane.
[0063] FIG. 7 shows a measured example of a practical collimated
light source. Maximum brightness occurs at normal incidence, with
increasing attenuation at increasing "off-normal" angles. A
preferred allowable clipping level is established, which then
defines the collimation angle for a particular angular distribution
of light coming from the back light behind the masks.
[0064] FIG. 8 shows a cross-section (not to scale) of physical
dimensions in the display. Several angles can be derived from these
dimensions. These angles have visual significance for seamlessness,
shadowing, resolution, cross talk and light transmission efficiency
to the viewer. If the collimated light entering the display exceeds
limiting angles, these parameters are affected.
[0065] Perfect seamlessness is accomplished by complete blockage of
the light entering and exiting the display by the front and rear
masks near the seam. This technique requires an illumination source
with a clipping level of zero at a collimation angle defined by
"A". Larger angle A, which improves the case of achieving a
seamless appearance, increases with increasing mask line width,
increases with decreasing tile thickness, and also increases with
decreasing adhesive thickness between the cover plate or back plate
and the tiles. The light passing through angles, greater than A is
also substantially blocked by the combination of the two
polarizers, the collimating efficiency of the optics behind the
mask and the light blocking efficiency of the structure in the seam
area.
[0066] In a practical sense the clipping level shown in FIG. 7 need
not be zero; a lower limit of light, not detectable through the
seam, is determined by a percentage of the light permitted through
a pixel when it is in the black state. This state is determined by
the contrast of the polarizers blocking the light entering and
exiting the seam and the depolarized secondary light rays caused by
internal reflections for rays that have passed through the front
polarizer. For this reason, the adhesive material between the glass
plates is chosen to be substantially equal in index of refraction
to that of glass. However, secondary rays may also result from
internal reflections from the color filter dark areas, side walls
of the tile enclosures and from non-collimated, secondary light
entering from the front face of the display. Furthermore, the glass
sidewalls in the seam area may be damaged to depths of several
light wave lengths, i.e., in the range of a micron, also causing
diffracted rays.
[0067] The rear mask casts a shadow on the pixel if the collimated
light exceeds the angle defined by "B". Larger collimation angles,
and more efficient lighting, result from using thinner tiles and
thinner adhesive layers. Light exceeding angles B directly affect
the color balance of the light exiting the display. If a
collimation angle greater than "B" is chosen, the sub-pixel sizes,
spectral content of the illumination source and spectral effects of
optical components must be compensated for in order to produce a
good "white" state. In addition, placement error tolerances of the
rear mask to the pixel cause color shifts and imbalance between
adjacent LCD panels, terminating at the seam, thereby adding to the
visual detection of the seam.
[0068] Perfect resolution occurs when all of the light entering a
rear mask aperture illuminates only one pixel. This is defined at a
collimation angle of "C". If the collimation angle exceeds "C", the
image produced at a pixel is projected into the adjacent pixel's
aperture in the front mask, thereby affecting contrast.
[0069] One type of cross talk is defined by resolution. Another
type can be defined as the limit where light from an adjacent rear
mask aperture cannot exit the opposing adjacent front mask
aperture. This is defined as angle "D". In reality, the collimation
angle defined by adequate resolution or cross talk (angles C and D)
can be practically larger since light must travel through a greater
LC distance, which has less optical transmissibility.
[0070] A practical collimation angle for a tiled seamless display
lies between angles C and D. The diffusion screen location relative
to the other geometry can also affect seamlessness favorably by
defocusing the image of the seam. In the ideal case the projected
pixel images should not overlap but should fill the projected image
plane. An overlap produces a light colored seam, while an underlap
will create a dark seam. In the current design, the important
angles are controlled by the masks and by a method of collimation
which can be varied conveniently to achieve practical cut off
angles as described in a copending patent application. In addition
to the collimation of primary rays, the optical elements help to
suppress secondary rays and enhance image contrast and focus.
[0071] The light transmission efficiency is determined by the
product of the efficiency of transmission through each optical
element. Referring to FIG. 8, one important contribution is
dependent on the aperture ratio which is approximately
p.sup.2/S.sup.2, where the pixel is approximately square and where
S is the pixel pitch. Therefore the seam width (S-p) is a major
contribution to lighting efficiency.
[0072] Now referring to our current functional SVGA tiled display
prototype in the area of the seam, refer to FIG. 9. The allocation
of dimensions is determined by the control of the seal front (inner
edge). The seal front is controlled by a dam structure designed
into the color filter. The dams 92 are actual vertical walls of CF
with spaces between. The walls are of the order of 1 to 3 .mu.m
high or higher. The elements of the design are as follows: a) the
buffer zone 91 between the pixels and the seal front adjacent to
the pixels 90, b) the nominal width 93 of the seal from the front
to the glass edges, c) the tolerance for location of the finished
glass edges (cut line) 100, and d) the assembly tolerances (not
shown) for tile placement relative to fiducial location accuracy of
tiles with cover plates and back plates, and machine assembly
location repeatability. The buffer zone (approximately 50 .mu.m)
between pixels and visible seal is experimentally determined for
the seal material components; a non-visible contaminant which
impacts the twisting behavior of the LC approximately 50 microns in
front of the seal. The seal front location is determined by the
accuracy in position of the dispense tool syringe and the control
of the volume of material dispensed as well as the accuracy of the
spacer ball diameter and the lamination pressure in determining the
cell gap on lamination of the CF to the TFT substrate. Dispense
pads of CF are located precisely in reference to the pixels and the
final desired objective seal width. The dispense pad 94 is wetted
preferentially by the seal, assisting in locating accurately the
deposited seal material (prior to lamination). Thus, the choice of
s and p is a careful design tradeoff determined by in-depth
knowledge of the technology and process parameters. For example, in
the current design, FIG. 9, the objective seal width is
approximately 800 .mu.m, of which an approximate objective 400
.mu.m flows toward the pixels, and an approximate 400 .mu.m flows
away from the pixels. The approximate objective seal front
resulting is about 100 .mu.m from the pixels.
[0073] The largest portion of s-p is related to the control of the
seal material and process. The choice of these nominal dimensions
determines angles B, C and D, discussed above. It is desired, as
noted above, to decrease s-p, optimizing these angles for minimum
shadowing, maximum resolution, and minimum cross talk while
maximizing the aperture ratio for light transmission
efficiency.
[0074] Two problems with seal polymeric material are that it
extrudes into a wavy front during lamination of the CF plate to the
TFT plate, and it generally contains an active or bonding diluting
liquid which readily wets the CF structure and structures on the
TFT substrate. If this front extrudes into the pixel aperture area,
it prevents the LC from twisting and creates a defect in the
desired pixel array. As shown in FIG. 9, this wavy liquid front is
controlled by CF dams 92 configurations spaced a precise distance
from the pixels. The presently preferably used configuration is a
double dam structure (FIG. 9) which is preferably spaced in
coordination with the choice of seal volume and ultimate laminated
seal width to be in the middle of the wavy liquid front. The seal
front waviness without the dams is typically approximately 100
.mu.m for 800 .mu.m width seals and is thereby decreased to less
than 50 .mu.m when the dams are present, allowing the seal to be
placed substantially closer to the pixels as compared to seals that
are not dammed. As a result, with dams, less space is used for the
seals, allowing s-p to be small and the aperture ratio to be
increased, as compared to structures without dams. The current dam
design structure is one example of many which can be applied to
improve the tiled display optical efficiency.
[0075] Control of the seal material at corners is described in a
separately filed patent. It is also important to control the cell
gap to be uniform near the tile edges. A detailed description of
methods for maintaining cell gap uniformity, especially near
corners, is included in our co-pending application, Ser. No.
09/221,096.
[0076] Suppression of Secondary Rays
[0077] Secondary rays can originate either from the backside or
frontside of the display. Backside secondary rays emanate from the
backlight and undergo a number of refractive and reflective
processes. Ambient light provides the source for frontside
secondary rays. Secondary rays have complex and essentially
unpredictable paths in the display stack. In addition to the
uncertainty of their behavior, additional optical phenomena occur
in the structures that are tiled, such as reflection and refraction
at the edges of the glass plates forming the display tiles;
blockage of light rays in the seal materials; line-of-sight
transmission of light rays through the gap between the tiles; and
waveguiding of light through the gap between the tiles. In order to
minimize the intensity modulation at the seams, the inter-pixel
spaces in the interior of the tiles and at the edges of the tiles
should preferably be made similar, from the optical point of
view.
[0078] Secondary ray effects can be managed using the following
techniques: (a) inserting light shields in the lightvalve layers
(thin film or color filter levels) to block all rays outside the
primary-ray envelopes in the image source plane; (b) inserting
light shields into the gap between each adjacent tile surrounding
each tile; (c) inserting further light shields into the regions on
the tiles that are used for interconnect functions at the edges
thereof; (d) inserting further non-transparent regions into the
outer, light-shield layers used for light collimation, so as to
block direct rays from passing through the display stack regions
between lightvalves on the tiles or in the seams; (e) preparing the
edges of tiles to well-defined optical characteristics to influence
edge-scattering of light, for example, by making them fully
transmissive, fully reflective or diffusive; (f) filling the gaps
between back plates and the tiles 40 and the gaps between the cover
plates and the tiles 41 with an index-matching,
optically-transparent compound; (g) inserting a face plate pattern
on the bottom surface of the cover plate, with opaque patterns
above all regions not overlapping lightvalves in the image view
plane, whether on the tiles or atop the seams therebetween; and (h)
inserting light shields into the areas used for interconnection on
the backplate or on tile carriers described in the aforementioned
related patent application, Ser. No. 08/571,208.
[0079] Techniques in (a) block direct light rays from passing
through the regions between the lightvalves in the image source
layer. The technique (b) is preferably used in order to block
line-of-sight rays from passing up through the gap between the two
vertical faces of the tile plates, and to match the gap light
transmission with that of the spaces between the lightvalves on the
tiles. Technique (c) is also needed to match the optical
transmission characteristics of the interconnect areas to the gaps
between lightvalves in the interior of the tiles. The addition of
matching light shields in (d) is effective both for the partial
collimation of primary rays and the blockage of stray light rays.
The need for technique (e) depends upon the optical quality of the
edges of the tile glass plates. Scribing and cleaving, the usual
way of cutting the tiles from larger sheets of glass, produces a
near optical-quality surface that has a residual surface topology
of more than several micrometers. Glass surfaces cut with a
rotating diamond wheel may be topologically smooth, but often have
a "milky" visual appearance, because of a fine surface roughness
that depends on parameters of the grinding process including the
grit size of the wheel. In either case, additional optical
preparation of the edge of the glass can be performed, if required,
using well-known techniques. The technique in (f) facilitates the
lateral transport of optical energy associated with the secondary
rays across the gap between the tiles above the image source plane,
in a fashion similar to that atop pixel gaps on top of the tiles.
Finally, technique (g) is required to match the front surface
reflectivities of seam regions with those between the lightvalves
on the tiles, primarily for improved appearance in ambient
light.
[0080] View Angle Enhancement of Tiled Display
[0081] While collimation or partial collimation helps to focus
primary light rays into channels passing through the lightvalves,
it limits the front side viewing angles to a rather small, solid
angle from the surface normal. In contrast, single-user electronic
displays often are required to sustain a viewing angle distribution
of .+-.300 and multi-user displays of up to .div.700 from the
surface normal. Therefore, the view angle distribution limited by
collimation may be enhanced, depending on the intended application.
This can be accomplished by inserting an array of lenses, or, in
the current preferred design, by inserting a dispersive screen into
the view plane. The lens array may consist of refractive
microlenses or holographic microlenses, and it can be made using
microfabrication techniques. The lens array or screen may reside on
a separate transparent plate or, alternatively, it can be
integrated into one of the existing glass sheets used in the tiles
or the cover plate.
[0082] Brightness Enhancement of Tiled Display
[0083] The second problem arising from collimation or partial
collimation of the primary rays is that collimation tends to limit
the amount of light collected by each lightvalve and consequently
reduces the brightness of the display. For example, if aperture
plates are used for collimation, the total light flux is reduced in
proportion to the aperture ratio of the light shield facing the
backlight source. Since reduced-brightness displays require low
ambient light viewing conditions, the brightness may have to be
enhanced. This can be done in several different ways. The intensity
of the backlight source itself can be increased by boosting the
electrical energy input or by using a greater number of light
sources and/or reflective light concentrators. Alternatively, the
efficiency for collecting the backlight into the collimated light
channels can be increased by using microlens or holographic lens
arrays, or other optical devices. These optical elements may also
be placed between the backlight source and the image plane of the
display.
[0084] This invention covers all techniques discussed above, and
all of their combinations, for designing, constructing and
assembling seamless, tiled, flat-panel displays. Which of these
techniques or combinations thereof are used for a given, tiled
display depends on the aperture ratio, the fraction of the pixel
pitch allocated for tiling functions, the assembly techniques, the
specifications of the display and the viewing conditions. In order
to clarify such combinations, this specific, preferred embodiment
employs concepts that allow the placement of structures both in
front of and behind the view plane, in order to make the seams
appear invisible, under normal viewing conditions intended for the
tiled display. This embodiment is useful for tiled displays having
larger viewing angles and a medium-to-large
view-plane-to-image-plane distance and pixel pitch ratio.
[0085] The specific, preferred embodiment of the seamless, tiled
display of this invention is illustrated with scaled
cross-sectional view in FIG. 6. The seamless display 154 comprises
an image source plane 24 composed of a lightvalve aperture layer 18
and a color filter layer in close proximity. The tiles are formed
by the top and bottom glass layers 20 and 21, respectively. The
inter-tile space 160 is covered by an inserted light shield layer
23. The intra-tile pixel gaps are covered by an opaque, thin-film,
lightshield layer 30.
[0086] The space between the tile glass sheets forming two adjacent
tiles and the spaces 40 between back plate and tiles and cover
plate and tiles 41 are filled with a transparent material having an
optical refractive index closely matched to that of the glass
tiles.
[0087] A light blocking monolithic mask layer 23 covers all inter-
or intra-tile lightvalve gaps between adjacent pixels. This gives
the seam regions the same appearance as the lightvalve gaps on the
tiles, in reflected light. A screen microlens array 25 is placed on
top of the glass cover plate, or it is integrated therein. The
screen microlens array generates the image view plane and enhances
the view angle distribution. Lightshield layers 23 are also used
for further collimating the light emanating from the collimated
backlight assembly.
[0088] The amount of light collimation can be controlled by shaping
and sizing the apertures in the light shield layers so that the
divergence of the rays passing through the image plane produces the
desired view plane characteristics. The spacing of the light
shields from the image plane also affects the light ray
distributions; they are chosen so that the desired degree of
collimation is achieved. A commercially available microlens array
for focusing light rays from the diffuse backlight assembly into
the partially collimating light apertures of the display stack has
been attached to, or integrated into, the lower surface of the
bottom glass plate facing the light source, in order to boost the
brightness of the display.
[0089] Having described the principal design factors in a vertical
plane and the effect of the horizontal plane dimensions in
determining critical angles for optics that are significant in
creating a monolithic seamless appearance with good human factors
including view angle and contrast, it is now equally important to
show the design configurations in the horizontal plane which allow
practical aperture ratios, pixel densities, and sealing
configurations and are efficient for production of tiles in a
typical AMLCD manufacturing line.
[0090] Consider an example of a design (FIG. 11a) which uses the
full panel size in a generation 2 AMLCD manufacturing line, which
typically employs glass panel sizes in the range of 20 inches
diagonal. A tile containing 400.times.300 pixels with a pitch of
0.98 mm can be manufactured on this sized panel. Four such panels
tiled in a 2.times.2 configuration (FIG. 12) produce an
approximately 40 inch diagonal FPD with active area resulting in an
SVGA standard (800.times.600 pixels). For comparison, a slightly
smaller pixel pitch of 0.85 mm in a slightly larger tile, still
fitting within a generation 2 manufacturing line panel, could be
used to make an XGA FPD with 1024.times.780 pixels. Both of these
designs make very efficient use of the area of a generation 2
panel. The XGA tile will require more tightly held tolerances,
seals decreased in width by about 25 .mu.m and tile edge location
tolerances reduced by about 15 .mu.m to maintain aperture ratios
closely equal to those for the SVGA FPD. Alternatively, a small
decrease in aperture ratios will ease the tolerances for the XGA
design.
[0091] It should be understood that the inventive apparatus and/or
methods are not limited to the pixel densities disclosed
hereinabove, but may be applied to panels of any range of pixel
densities. In addition, the disclosed pixel densities all fall in a
4.times.3 aspect. The invention may also be applied to tiles of
other aspect ratios such as the 16.times.9 aspect ratio defined for
high-definition TV (HDTV). Furthermore, tiles may also be produced
in larger sizes incorporating greater numbers of pixels on larger
substrates.
[0092] For example, it is anticipated that improvements in epoxies,
dispensing techniques, tile size, seam fabrication techniques, and
epoxy flow control structures will allow pixel counts in the range
of 1600.times.1200 for tiled flat-panel displays.
[0093] Referring again to FIG. 11a, there is shown a color filter
design external to the active area which is common for the four
different tile part numbers to be used in a FPD with a 2.times.2
array of tiles. The rubbing direction for the polyimide 70, which
orients the liquid crystal, and the locations of the fill ports are
unique to each of the four part numbers (i.e., "A", "B', "C", "D")
to be tiled. The location for the seal 93, the LC fill port 95, the
dams 92, and the dispense pads 94 are shown in plan view in FIG.
11b specifically for part number A in the wide seal area. This is a
magnified view of the wide seal corner of FIG. 11a with the seal 93
shown as it is deposited to define part number A. The fill port 95
is a gap left in the seal 93 perimeter near the corner. The seal is
drawn between the CF dispense pads 94 and the CF areas 80 used, in
cooperation with spacer means 110, to control cell gap.
[0094] Referring now to FIG. 11c, there is shown a detailed view of
the narrow seal corner opposite the wide seal corner (FIG. 11a) of
the composite color filter shown in FIG. 11a. This view of the
narrow seal corner for part number "A" also shows the seal location
after the CF substrate and the TFT substrate for part number A are
squeezed together. The CF dispense pads are eliminated in the
narrow seal corner and in all other corners. The reason for this
design is to balance the increase in width due to the extra seal
per unit length deposited as the syringe changes direction in
rounding a corner. The volume of CF eliminated is the
width.times.length.times.height of the CF and this is designed to
match the extra volume deposited at a corner.
[0095] A second factor in determining corner shape is due to the
momentum of the dispense platform causing an overshoot. For these
narrow seal designs the dispense speed is decreased to the minimum
allowed by the dispense machines. This allows the achievement of a
smaller radius at the corner.
[0096] Still another factor in determining corner shape of the seal
is the adhesive strength of the seal as the syringe effectively
pulls the adhesive around a corner. A preferable design for
decreasing this effect is shown in FIG. 11d. In this design, narrow
dams linked to the edges of dispense pads have the effect of
centering the position of the seal at the corners after deposition
and prior to squeeze.
[0097] Referring now to FIG. 12, the cut line 100 is the final
determination of the tile type A, B, C or D for the CF substrate.
These concepts of a common CF part number are used in the current
prototype 2.times.2 tile array FPD and are applicable to 1.times.2
and 2.times.N arrays with some design modifications.
[0098] In the case of using a single part number for the CF, there
are two different wiring patterns for the TFT substrate, one of
which is shown in FIG. 4. This preferred alternative uses only two
different TFT part numbers (A=C) and (B=D). Alternatively, each
tile CF and TFT substrate may be uniquely designed. The rubbing
directions for the TFT substrates are orthogonal to those for the
CF.
[0099] The CF designs outside of the seal, with the CF profile
height equal to the CF height in the areas containing the pixels
(FIG. 10), assists in maintaining the cell gap uniform in the area
of the seam during lamination. The subject matter of FIG. 10 is
more completely described in published Japanese Patent Application
No. JP10-311454 (1998). The cell gap may be varied by using a
choice of different sized spacer spheres 110 of glass or polymer in
the seal material, as compared to those in the active area. This
cell gap is maintained by the mechanical strength of the seal
material even after the dummy CF is cut away. Without a uniform
cell gap across the seam, gray scale color changes may be visible
in the seam area. The small differences in cell gap and TV curve
response may be corrected near the seam as described in copending
patent application Ser. No. 09/221,096, filed Dec. 28, 1998.
[0100] There are several design configurations of the CF that are
instrumental in controlling the waviness of the seal front and the
position of the seal. Shown in FIGS. 9 and 10 is the CF pad 94,
used to receive the epoxy seal material as it is dispensed. This CF
is readily wetted by the epoxy and thereby establishes the initial
mean location of the dispensed material more precisely than does
the location of the syringe dispensing the seal. This pad is
designed in width (270 microns) to match the width of wet seal
material seeking its equilibrium position, due to surface tension,
so that no excess material overflows the pad. The pad thereby
defines a highly accurate location for the dispensed seal material.
The pad is also designed to be a precise distance from the pixels,
depending on the seal width desired. For example if the seal front
is desired to be a nominal distance of x microns from the pixels
and the half width of the seal is y microns the center of the CF
dispense pad is placed at x+y=approximately 500 microns from the
pixels.
[0101] As the seal is squeezed out, the front becomes wavy,
typically in the range of approximately .+-.50 microns amplitude
from the seal front mean position, for seals in the range of 800
microns in width. The waviness increases by about 10 microns or
more for each additional 100 microns of width. Random neck downs in
the seal increase with seals that are narrower than 800 microns.
Therefore it is preferred to use seal widths in the 800 to 840
micron range to maintain a compromise between waviness and
neckdowns. In addition to the waviness, a defective area appears,
due to unknown material (probably the reactive solvent which
combines molecularly with the epoxy) that contaminates the
polyimide surface a measured distance approximately 50 microns in
front of the visible seal material for the commonly used seal
materials. The affect on the response of the liquid crystal is
obvious only when the pixels that are contaminated are switched or
viewed carefully with polarizers and analyzer rotations. CF dams
placed assiduously decrease the seal waviness and to some degree
also appear to decrease the defective area noted above.
[0102] A dam design that works effectively is shown in FIGS. 9 and
10. In this case, as discussed above, the seal front is chosen to
be 80 microns from the pixels (50 microns to allow for the defect
area and 30 microns to allow for the waviness). The mean seal front
is designed to be disposed between two dams, providing the
smoothing of the front by wetting action along the dams and
blocking of the liquid front by the dams. The defect area is
therefore maintained at a safe distance from the pixels. In this
design, lack of control of volume of seal equivalent to a width of
approximately .+-.40 .mu.m is still acceptable for maintaining
clean pixels for the objective seal width of 800 to 840 .mu.m.
[0103] These CF designs are key to minimizing the dark space needed
between the pixels at the tile edges and to controlling the seal
front to prevent contamination of the pixels. Once these design
parameters are chosen, the total space required at the edge can be
calculated based on assembly location accuracy (about +25 microns,
currently), glass edge location accuracy, and seal width required
for strength and for preventing leakage. In the current design
example, the outer edge of the seal is chosen to be nominally at
approximately 200 microns from the pixels. This is the intended cut
or scribe and break line to meet the tolerances required for final
assembly. Thus, the allocation for location tolerances and the
outer glass edge distance from the pixels for the two neighboring
tile edges sums to approximately 420 to 450 microns. Then the
design for the tiled display evolves depending on the choice of
density standard. In the example discussed herein, a tiled panel
with SVGA density requires tiles to contain an array of
400.times.300 pixels and fit within the area of a generation 2
glass panel. A certain amount of space outside of the pixels at the
edges of the panel is required for attachment of electronics,
jigging and fixtures for sealing, scribing, breaking, etc. A
convenient compromise size active area is 11.58".times.15.55" which
will contain the 400.times.300 pixels with pitch of 0.98 mm. The
dark space for all of the pixels is chosen as noted above. This is
equal to the dark space of 420 microns between pixels on
neighboring tiles. Since a monolithic mask covers all four tiles
and electronics are used to balance color and intensity across the
seam, there is less requirement for precision as compared to a
design where the tiles are butted against each other. There is a
desire, however, to minimize the dark space and maintain an
aperture ratio for the highest practical light transfer efficiency.
With this design it is also possible to decrease the seam space and
improve seamlessness as the cutting and assembly tolerances are
improved.
[0104] In the sequence of processing the TFT substrate and the CF
substrate components of the tile making them ready for the assembly
operation a thin film of polyimide is deposited on each substrate.
As shown earlier there is a particular rubbing direction for each
substrate defining the part number A, B, C, or D. When the tiles
are later assembled into an FPD, the rubbing directions line up so
that they are all in one direction for the TFT and in the
orthogonal direction to that for the CF. A unique problem arises
from these rubs at the tile level in that half of the tiles are
rubbed from the narrow seal side while the second half are rubbed
from the wide seal side. Rubbing is one of the most severe
generators of electrostatic discharge. In non-tiled displays, the
rubbing entry point may be on the wide seal side which contains
protective diodes which substantially prevent damage to the
internal electronics, particularly the TFTs. Unless such
preventative measures are taken for the tiles on the narrow seal
sides for the TFTS, there is a risk that the ESD will create
damage. Therefore, a preferred design for tiling encompasses
protective diodes on the narrow seal sides as well as on the wide
seal sides. A second line of defense is to add redundant TFTs for
all sub pixels neighboring the narrow seal sides or preferably for
all sub-pixels.
[0105] Since other combinations, modifications and changes varied
to fit particular operating requirements and environments will be
apparent to those skilled in the art, the invention is not
considered limited to the chosen preferred embodiments for purposes
of this disclosure, but covers all changes and modifications which
do not constitute departures from the true spirit and scope of this
invention.
[0106] Having thus described the invention, what is desired to be
protected by Letters Patent is presented in the subsequently
appended claims.
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