Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-12-19
2003-07-29
Dudek, James (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06600528
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to direct view displays, and more particularly to direct view liquid crystal displays employing integrated prism sheets.
2. Description of the Related Art
Backlight systems in liquid crystal displays (LCDs) tend to be inefficient, with typically only about 5-10% of the light incident on the back being transmitted through the polarizers (40%), open aperture (60%), and the color filters (35%). The operation of a conventional backlight can be understood with reference to FIG.
1
.
Referring to
FIG. 1
, light from a triband cold cathode fluorescent light (CCFL)
10
is directed into the Acrylic light guide
12
by a reflector
15
. The light only escapes from the light guide
12
if the angle of incidence at the acrylic/air interface is less than the critical angle. The light guide
12
is in the form of a wedge so that the incident angle of the light at the acrylic/air interface is gradually decreased as it propagates down the light guide by reflecting between the top and bottom surfaces
14
until it is less than the critical angle and escapes. The intensity of the light which escapes is controlled by a pattern of dots
17
printed on the bottom of the light guide
12
where the density of the dots
17
is adjusted to result in a uniform illumination of the display. Any light which exits the bottom of the guide
12
is redirected upward by a white diffuse scattering sheet
16
.
The dots
17
cause diffuse scattering which results in more of the light escaping the light guide
12
in the area of the dots
17
. The light typically comes out of the light guide
12
with a peak brightness at about a 70 degree angle from the normal to the display. A ridge collimating film sheet or sheets
20
(see Suzuki et. al. U.S. Pat. No. 5,600,462, for example) are used to redirect the white light so that the peak brightness is normal to the display. Additionally, the light has an appropriate angular distribution (a half brightness at 25 degrees or more off the display normal in both the vertical and horizontal directions) to provide an adequate viewing angle. With 90 degree twisted nematic (TN) mode liquid crystal
28
, typically used for portable active matrix liquid crystal displays (AMLCDs), the viewing angle is usually limited by the off-normal contrast ratio and color inversion due to the liquid crystal.
For desktop monitors, a broader distribution of light is desirable: a half brightness at about 40 degrees or more off normal in both the vertical and horizontal directions. Note that it is also desirable to have a wider horizontal than vertical viewing angle. After the ridge sheet(s)
20
, the light passes through a back polarizer
24
, a thin film transistor (TFT) plate
26
, a liquid crystal (LC) layer
28
, a color filter plate
22
(in color filter displays), and a front polarizer
30
which reduces the intensity of the transmitted light to only 5-10% of that incident, as described above.
One way of improving the backlight efficiency is to use the combination of a totally internally reflecting light guide, a diffraction grating and lenticular lens to separate the Red, Green, and Blue (RGB) light by angles and then focus the individual colors through the appropriate sub-pixels, see, e.g. the commonly assigned application, to Y. Taira entitled “COLOR FLAT PANEL DISPLAY,” PCT Application number JP00/00912, filed Feb. 12, 1999, designating the United States and incorporated herein by reference. This can improve the efficiency by removing the color filters (about 3× improvement) and by focusing the light into the open aperture (about 1.3× improvement). The general operation of the color filterless (CF-less) backlight can be understood with reference to FIG.
2
.
Referring to
FIG. 2
, a light source
112
(CCFL) and reflector
113
direct light into an acrylic light guide
116
which has no printed dot pattern on it so that light can only escape when it's angle of incidence is less than the critical angle. A low index coating
117
(with the refractive index, n, equal to 1.29, for example) along the bottom surface of the light guide
116
results in the light only exiting on a bottom surface
114
of the light guide
116
with a fairly narrow distribution of angles. The CCFL
112
has triband phosphors, so the light produced is mainly in three distinct Red, Green, and Blue bands. A reflective diffraction grating
115
is attached to the low index coating
117
and serves to decompose the white light into the three individual colors and redirect them upward at slightly different angles for each color. Note that a transmissive grating sheet could be used in an alternative configuration in which case no low index coating is required and a mirror sheet is placed below the light guide. A lenticular lens sheet
120
on the bottom of a back polarizer
122
of the AMLCD then focuses the angularly separated RGB light through the appropriate sub-pixels
123
. With this approach, the peak intensity for the Red and Blue light is directed at an angle to the display normal. This peak off-normal Red and Blue light will lead to lateral color shifts when viewing the display. A diffuser
121
is included to improve viewing angle.
The lateral color shift problem will now illustratively be described with reference to FIG.
3
. Referring to
FIG. 3
, the optimum focal length of a lenticular sheet
214
is fixed by the sub-pixel pitch of the display and the angular separation between the Red, Green, and Blue light. The Red
202
, Green
204
, and Blue
206
light are focused through the appropriate sub-pixel apertures by lenticular lens
214
. The Red, Green and Blue light is focused through apertures
208
in black matrix
211
, but the blue and red light are laterally shifted away from the normal (or Green light). An observer at point “O” would see more blue in the displayed image while an observer at point “P” would observe more red in the displayed image. Polarizers
212
and glass substrates
210
are provided as is known in the art. Note that in some cases, the peak red and blue intensities are not equally separated from the green intensity peak, so it may be desirable to tilt the green light slightly off normal so that the deviation of red and blue from the normal is minimized. In this case, the green light will also have a slight lateral color shift.
A color filterless backlight system has been described by van Raalte in U.S. Pat. No. 4,798,448, entitled HIGH EFFICIENCY ILLUMINATION SYSTEM FOR DISPLAY DEVICES, which used lenticular lens and a transmissive diffraction grating but did not describe any means of correcting for the lateral color shifts when viewing the display.
When microlenses are used for projection displays, as has been described by H. Hamada in “Optical systems for high-luminance LC rear projection”, SID '96 Digest, pp. 911-914, a projection lens is used in front of the AMLCD which images the black matrix plane onto the screen and hence no correction of the lateral color shift is needed. For direct view displays, in particular liquid crystal displays, a key attribute is the thickness of the display which should be as thin as possible. The use of dichroic mirrors for the angular separation of the colors, as described by Hamada, requires too much depth to be used and since the displays are viewed directly a projection lens cannot be used to fix the lateral color shift problem. An additional problem with this arrangement in projection displays is that a large projection lens is needed to collect the divergent light. A preferred method of angular color separation may include the combination of a totally internal reflecting light guide and a diffraction grating, as has been described by Taira, cited above since this is very compact and requires no additional space.
The use of combined microlenses and microprisms has been described by Nishihara in U.S. Pat. No. 5,764,319 entitled TRANSMISSIVE DISPLAY DEVICE WITH MICROLENSES AND MICROPRISMS ADJACENT COUNTER ELECTRODE, where the microlens f
Colgan Evan G.
Singh Rama N.
Taira Yoichi
Wisnieff Robert L.
Yamada Fumiaki
Dudek James
International Business Machines - Corporation
Trepp Robert M.
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