Colored liquid crystal display having a reflector which reflects different wavelengths of light for different incident angles

A colored reflective liquid crystal display employs a liquid crystal light valve switchable between a light scattering and a transmissive state. Positioned on the non-viewing side of the light valve is a reflector which reflects light of selected wavelengths and transmits the other wavelengths.

TECHNICAL FIELD OF THE INVENTION 
This invention relates to a colored reflective liquid crystal display. 
BACKGROUND OF THE INVENTION 
The commonly used twisted nematic (TN) liquid crystal display shows dark 
characters on a gray or silvery background. For some applications, a 
display showing bright colored characters or other items of information is 
desirable. This has been achieved by the inclusion of dyes or filters, but 
the color intensity, brightness, and/or contrast are not always 
satisfactory. It has been proposed to combine holographic reflectors with 
TN or super-twisted nematic (STN) displays to produce a backlit, colored 
appearance. Chen et al., SID 95 Digest, 176-179 (1995). 
Colored displays can also be made using encapsulated liquid crystal 
materials, in which plural volumes of a liquid crystal material are 
dispersed or encapsulated in a matrix material. In one approach, a 
dichroic dye is included in the liquid crystal material. See, for example, 
Fergason, U.S. Pat. No. 4,435,047 (1984) and Drzaic et al., SID 92 Digest, 
571-573 (1992). In another approach, a non-pleochroic dye is included in 
the liquid crystal material or the matrix material, and the color effect 
is enhanced by a scattering effect combined with total internal reflection 
of the scattered light. A tuned dielectric interference layer may be 
positioned behind the liquid crystal material. Fergason, U.S. Pat. No. 
4,596,445 (1986). 
SUMMARY OF THE INVENTION 
This invention provides a liquid crystal display having improved color 
intensity, brightness, and/or contrast compared to prior art devices. 
Accordingly, there is provided a colored reflective liquid crystal 
display, comprising 
a light valve having a viewing and a non-viewing side and switchable 
between a substantially light scattering state and a substantially light 
transmissive state, at least some of the light scattering being in a 
forwardly direction, said light valve comprising a liquid crystal 
composite wherein plural volumes of liquid crystal material are dispersed 
in an encapsulating material; and 
a reflector disposed on the non-viewing side of said light valve, said 
reflector being a holographic or polymeric cholesteric reflector wherein 
the wavelength of light reflected by said reflector is dependent on the 
incidence angle of the light. 
Optionally, a dark absorber may be placed behind the reflector, optically 
coupled thereto.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference is made to FIGS. 1a and 1b for explaining the operation of the 
instant invention. Display 10 comprises light valve 11, reflector 12, and 
optional dark absorber 13. Light valve 11 has a viewing side 14 and a 
non-viewing side 15. Light valve 11 comprises a liquid crystal composite 
17 in which droplets or volumes 19 of nematic liquid crystal material 20 
having a positive dielectric anisotropy are dispersed in an encapsulating 
material 18. Composite 17 is sandwiched between first and second 
electrodes 16a and 16b, respectively, made from a transparent conductor 
such as indium tin oxide ("ITO"). Electrodes 16 and 16b may be supported 
by respective supporting materials (not shown), such as polyester or 
glass. 
The application or not of a voltage across electrodes 16a and 16b from 
power source 21 is controlled by switch 22, shown here in the open 
position ("off-state"). As a result, no voltage is impressed across 
composite 17 and the electric field experienced by liquid crystal material 
20 is effectively zero. Due to surface interactions, the liquid crystal 
molecules preferentially lie with their long axes parallel to the curved 
interface with encapsulating material 18, resulting in a generally 
curvilinear alignment within each droplet. The curvilinear axes in 
different droplets 19 are randomly oriented. Liquid crystal material 20 
has an extraordinary index of refraction n.sub.e which is different from 
the index of refraction n.sub.p of encapsulating material 18 and an 
ordinary index of refraction no which is the substantially the same as 
n.sub.p. (Herein, two indices or refraction as said to be substantially 
the same, or matched, if they differ by less than 0.05, preferably less 
than 0.02.) (Some recent work suggests that the random alignment of the 
liquid crystal material between adjacent domains may contribute 
substantially to the scattering effect in the off-state. See Drzaic, Mol. 
Cryst. Liq. Cryst. 261, 383-392 (1995) and Drzaic et al., Appl. Phys. 
Lett. 62(12), 1332-1334 (1993). Incident light rays 23 traveling through 
composite 17 have a high statistical probability of encountering at least 
one interface between encapsulating material 18 and liquid crystal 
material 20 in which the liquid crystal index of refraction with which it 
operatively interacts is n.sub.e. Since n.sub.e is different from n.sub.p, 
there is refraction, or scattering of light rays 23, both forwardly and 
backwardly. As a result of multiple scattering interactions with composite 
17, the substantial collimation of incident light rays 23 is destroyed, 
and exiting light rays 23a are incident on reflector 12 from an 
essentially random assortment of angles. 
Reflector 12 reflects some of light rays 23a, the wavelength of the 
reflected light reflected being dependent on the incidence angle. That is, 
reflector 12 is an angularly dependent color reflector. However, because, 
as noted above, the incidence angle is essentially random, reflected light 
25 comprises essentially all wavelengths, so that reflector 12 has the 
same generally white appearance of composite 17 and is hardly noticeable 
to a viewer 24 positioned on viewing side 14. In effect, under these 
conditions, reflector 12 has the appearance of a diffuse reflector. This 
is a significant advantage compared to using a fluorescent reflector, 
where the masking effect of composite 17 in the off-state would not be as 
efficient. 
FIG. 1b shows light valve 11 in the on-state, with switch 22 closed. An 
electric field is applied between electrodes 16a and 16b and across 
composite 17, with a directionality indicated by arrow 25. (An alternating 
electric field may be applied, in which case the field direction changes 
periodically by 180.degree..) Liquid crystal material 20, being positively 
dielectrically anisotropic, aligns parallel to the electric field 
direction. (The required voltage is dependent inter alia on the thickness 
of the composite and typically is between 3 and 50 volts.) Further, this 
alignment with the field occurs in each droplet 19, so that there is order 
among the directors from droplet to droplet. When the liquid crystal 
molecules are aligned in this manner, the liquid crystal index of 
refraction with which incident light rays 23 operatively interact is 
n.sub.o. Because n.sub.o is substantially the same as n.sub.p, there is no 
scattering at the liquid crystal-encapsulating material interface. As a 
result, rays 23 are transmitted through composite 17, i.e. composite 17 
becomes substantially transparent. As incident rays 23 are at least 
partially collimated and the collimation has not been destroyed by their 
passage through composite 17, exiting light rays 23b are also 
substantially collimated, i.e., are incident upon reflector 12 at 
substantially the same angle. Consequently, the bandwidth of reflected 
light 25a is relatively narrow--that is, reflected light 25a is intensely 
colored. The exact wavelength (or more accurately, waveband) of reflected 
light 25a will depend on the incidence angle. A very high degree of 
collimation is not needed for the operation of this display. Outdoor 
sunlight will be sufficiently collimated, as will normal indoor lighting, 
including ordinary ceiling and fluorescent fixtures. The color of light 
reflected by reflector 12 can be predetermined to a certain extent by 
taking into consideration the likely incidence angles of light under 
typical conditions in which the display is used. Incident light rays 25b 
having wavelengths outside of the reflection band are passed through 
reflector 12 to be absorbed by absorber 13. Preferably, absorber 13 is 
optically coupled to reflector 12, meaning that the two are in direct 
contact, without any intervening material (including air, as in an air 
gap). Absorber 13 can be made of any suitable black or dark material, for 
example a sheet of black paper or plastic, or a painted surface. Thus, by 
applying or not an electric field, display 10 can be made to appear white, 
black, or intensely colored, with bright, saturated colors, without the 
need for a backlight and relying only on ambient lighting. 
Composite 17 can be made by any number of methods known in the prior art. 
Exemplary disclosures include Fergason, U.S. Pat. No. 4,435,047 (1984); 
West et al., U.S. Pat. No. 4,685,771 (1987); Pearlman, U.S. Pat. No. 
4,992,201 (1991); and Dainippon Ink, EP 0,313,053 (1989); the disclosures 
of which are incorporated herein by reference. Composite 17 optionally may 
include a pleochroic dye, but a construction in which dye is essentially 
absent is also contemplated. Suitable encapsulating materials include 
polyurethane, poly(vinyl alcohol), epoxies, poly(vinyl pyrrolidone), 
poly(ethylene glycol), poly(acrylic acid) and its copolymers, poly(hydroxy 
acrylate), cellulose derivatives, silicones, acrylates, polyesters, 
styrene-acrylic acid-acrylate terpolymers, and mixtures thereof. Composite 
17 may be made by an emulsion techniques (see, e.g., Fergason '047, 
supra), latex techniques (see, e.g., Pearlman '201, supra) or phase 
separation techniques (see, e.g., West '771, supra). 
For sake of clarity, in FIGS. 1a and 1b the discussion of the invention was 
exemplified with a composite 17 in which the off-state is light scattering 
(also referred to as the "normal mode"). Those skilled in the art will 
appreciate that "reverse mode" composites (i.e., which are transparent in 
the off-state and scattering in the on-state) are known and are equally 
usable in the instant invention. It is not important whether it is the 
on-state or the off-state which is scattering--what is important is that 
one of the states be scattering and that the composite be switchable 
between that state and a transparent one. For an illustration on the 
preparation of a reverse mode composite, see Ma et al., U.S. Pat. No. 
5,056,898 (1991), the disclosure of which is incorporated herein by 
reference. 
Also suitable for use in this invention are composites in which the liquid 
crystal material is a cholesteric material (also known in the art as a 
chiral nematic material). The techniques used to make the cholesteric 
composites are substantially analogous to those employed for making 
nematic composites. As in the instance of the nematic composites, 
cholesteric composites can operate in either the normal or the reverse 
mode. The switching of a cholesteric based display involves the 
reorientation and realignment of the cholesteric domains. Exemplary 
disclosures of cholesteric composites include West et al., Appl. Phys. 
Lett. 63(11), 1471-1473 (1993); Yang et al., J. Appl. Phys. 76(2), 
1331-1333 (1994); and Doane et al., Japan Display 1992, 73 (1992); the 
disclosures of which are incorporated herein by reference. 
Generally, the reflector contains periodic refractive index variations in 
the material forming it. The wavelength of incident light reflected is 
also a function of this periodicity, given approximately by the equation: 
EQU .lambda.=2d sin .theta. 
where .lambda. is the wavelength of reflected light, d is the periodicity 
of the refractive index variations in the reflector, and .theta. is the 
angle of incidence. Thus, the reflective characteristics of the reflector 
can be determine at the manufacturing stage, by establishing a particular 
periodicity in the refractive index variations therein. 
A reflector suitable for use in this invention is a holographic reflector. 
Preferably, the holographic reflector is a volume phase hologram, also 
known as a refractive index modulated hologram, in which holographic 
fringes are formed by the photo-polymerization or -crosslinking of a 
photocurable material after exposure to a light interference pattern 
generated by two interfering laser beams. Examples of photocurable 
materials which can be made for making volume phase holograms include 
dichromated gelatin and acrylates. A preferred material is DMP-128, from 
Polaroid Corporation. Disclosures of suitable holographic reflectors 
include Fielding et al., U.S. Pat. No. 4,535,041 (1985); Fielding et al., 
U.S. Pat. No. 4,588,664 (1986); Ingwall et al., U.S. Pat. No. 4,970,129 
(1990); and Ingwall et al., Opt. Eng., 28(6), 586-591 (1989), the 
disclosures of which are incorporated herein by reference. To increase the 
viewing cone, the incidence angle dependence of the reflector can be 
reduced by deliberately introducing a slight randomization of the 
reflection planes of the holographic reflector. In any event, for hand 
held devices, where the viewing angle can be readily manipulated, the 
breadth of the viewing cone is not a major problem. 
Another reflector suitable for use in this invention is a polymeric 
cholesteric reflector, made for example by curing a system containing a 
reactive chiral monomer to produce a cholesteric liquid crystalline 
structure whose pitch corresponds to a wavelength of visible light, which 
selectively reflects light of wavelengths centered around the pitch. Those 
skilled in the art will understand that cholesteric structures are chiral 
and that the circular polarization of light reflected thereby will 
correspond to the handedness of the chirality. Examples of suitable 
cholesteric reflectors are disclosed in Consortium Electrochem. Ind., EP 
0,631,157 A1 (1994); Philips, EP 0,578,302 A1 (1994); Derwent abstract no. 
87-010570/02 (abstract of Matsushita, JP 61- 267702 (1986)); Derwent 
abstract no. 86-242007/37 (abstract of Matsushita, JP 61-170704 (1986)); 
and Derwent abstract no. 86-085477/13 (abstract of Matsushita, JP 
61-032801 (1986)); the disclosures of which are incorporated by reference. 
Those skilled in the art will appreciate that the present invention is not 
restricted to displays capable of only a single color or a limited set of 
images. By combining plural pixels, alternately having a red, a green, and 
a blue reflector, an RGB full-color display can be prepared, capable of 
displaying an indefinite variety of images, ranging from text to graphics. 
The alternating pattern of red, green, and blue reflectors can be made by 
individually positioning discrete reflectors. However, for making high 
resolution displays where many pixels are needed, a more efficient way may 
be to sequentially expose a photopolymerizable material such as DMP-128 
from Polaroid Corp. with successively different wavelengths of laser light 
or by varying the relative angle of the interfering laser beams. A 
photomask can be used at each exposure to expose the desired pixel areas 
to the laser light while shielding the areas which should not be exposed. 
At the next exposure, the photomask can be shifted over by one pixel, to 
permit exposure of the adjoining pixels to a different wavelength of light 
and the production of a different colored holographic reflector. 
Alternatively, the photomask can be dispensed with, by using highly 
focused laser beams to write at each pixel location. 
For driving such a display, a semiconductor active matrix is preferred, for 
example a thin-film transistor array. For some applications, it is 
aesthetically desirable to have a display with a dark front. This effect 
can be accomplished by including a pleochroic dye in a normal mode nematic 
liquid crystal composite. In the off-state, the pleochroic dye 
substantially absorbs incident light, creating a dark or colored 
appearance. (To produce the color white, red, green, and blue pixels can 
be turned on simultaneously.) 
The foregoing detailed description of the invention includes passages which 
are chiefly or exclusively concerned with particular parts or aspects of 
the invention. It is to be understood that this is for clarity and 
convenience, that a particular feature may be relevant in more than just 
passage in which it is disclosed, and that the disclosure herein includes 
all the appropriate combinations of information found in the different 
passages. Similarly, although the various figures and descriptions thereof 
relate to specific embodiments of the invention, it is to be understood 
that where a specific feature is disclosed in the context of a particular 
figure, such feature can also be used, to the extent appropriate, in the 
context of another figure, in combination with another feature, or in the 
invention in general.