Liquid crystal display panel having a phase grating formed of liquid crystal molecules

A liquid crystal display panel includes a first substrate and a second substrate, at least one of which possesses light transmissivity, each having an electrode layer formed thereon, the electrode layers confronting each other, and a liquid crystal layer sandwiched between the first substrate and the second substrate, divided into plural regions, and including liquid crystal molecules. The liquid crystal molecules are oriented in the same direction in each of first regions, and in a different direction in second regions each located between two adjacent first regions. The second regions are spaced at a regular periodicity. Without an electric field applied to the liquid crystal layer, the different orientations of the liquid crystal molecules create differences in the refractive indices at which the light is transmitted so as to serve as a phase grating the diffracts incident light. When an electric field is applied, the difference in the refractive indices decreases, and the incident light is not diffracted but propagates as is through the panel. The liquid crystal display panel may be incorporated into a projection display apparatus having a light source and projection lens.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a liquid crystal display panel for forming 
an optical image by changing the diffraction of light, its method of 
manufacture, and a projection display apparatus for magnifying the optical 
image displayed on the liquid crystal display panel and projecting the 
image onto a screen. 
2. Description of the Prior Art 
A large screen display is recently attracting attention, such for use as a 
home theater and for use in making a presentation. Many projection 
apparatus using light valve have been proposed. Lately, liquid crystal 
projection apparatus for displaying an image on a wide screen using a 
small liquid crystal display panel and a projection lens or the like, have 
been developed. 
The liquid crystal display panel is designed to create a display image 
mainly by varying its optical characteristic electrically, and is 
classified based on many kinds of operating principles. A twisted nematic 
(TN) liquid crystal display panel used in existing liquid crystal 
projection apparatus makes use of an electric field to change the 
polarization of the liquid crystal. The TN liquid crystal display panel, 
however, requires a polarizer for modulating the light, and hence the 
efficiency is poor. 
Known methods of controlling the light without using a polarizer include 
methods based on using the scattering or diffraction of light. Liquid 
crystal display panels for forming an optical image by varying the state 
in which light is scattered include phase change (PC), dynamic scattering 
mode (DSM), and polymer dispersion liquid crystal panels. In particular, 
the polymer dispersion liquid crystal panel has been under intensive 
consideration recently. Such a panel is disclosed, for example, in U.S. 
Pat. No. 4,435,047. However, in a liquid crystal projection apparatus 
using this polymer dispersion liquid crystal panel there is a compromise 
between the image brightness and contrast (Tomita, Proc. of SID, p. 579, 
1992), and such an apparatus capable of forming an image having both 
satisfactory brightness and contrast has not yet been realized. 
On the other hand, a liquid crystal display panel for forming an optical 
image by changing the state under which light is diffracted is disclosed 
in U.S. Pat. No. 4,729,640. The basic structure and operation of this type 
of liquid crystal display panel are shown in FIGS. 13(a) and 13(b). 
Transparent electrodes 135 and 136 are formed on glass substrates 131 and 
132, and an irregular sectional layer 134 is formed on a surface of at 
least one of the glass substrates confronting a liquid crystal layer 133. 
The irregular sectional layer 134 has a periodic configuration, and the 
refractive index of the irregular sectional layer 134 is nearly equal to 
the ordinary refractive index no of a liquid crystal 137. FIG. 13(a) shows 
a case in which an electric field is not applied to the liquid crystal 
layer 133, and the liquid crystal 137 is oriented in a homogeneous state 
with its molecular long axis parallel to the longitudinal direction of the 
stripes of the irregular sectional layer 134. A ray of light 138 entering 
this liquid crystal panel is transmitted with a refractive index of 
n.sub.o in the convex portion of the irregular sectional layer 134. 
However, a polarized component 138a of the light is transmitted in the 
concave portion of irregular sectional layer, i.e. in the liquid crystal, 
with an extraordinary refractive index of n.sub.c. Thus the irregular 
sectional layer acts as a phase grating, and the ray of light 138 is 
modulated. On the other hand, FIG. 13(b) shows a case in which a 
sufficiently large electric field is applied to the liquid crystal layer 
133 so that the liquid crystal layer 137, having a positive dielectric 
anisotropy, is oriented in a direction normal to the glass substrates 131, 
132. Accordingly, the light is transmitted in the concave portion, i.e. in 
the liquid crystal 137, with an ordinary refractive index n.sub.o which is 
the same as the refractive index of the convex portion. Accordingly, the 
incident ray of light 138 is not diffracted but propagates straight 
through the panel. 
An example of a projection display apparatus using the diffraction type of 
liquid crystal display panel of FIG. 13 is shown in FIG. 14. The light 
emitted from a lamp 141 is converted by a concave mirror 142, passes 
through a polarizer 145, and enters a liquid crystal display panel 143. 
Natural light emitted from the lamp 141 is half-absorbed by the polarizer 
145, and the polarized light enters the liquid crystal display panel 143. 
The light entering the liquid crystal panel 143, if not modulated, is 
completely led into a projection lens 144. A matrix pixel electrode and a 
grating are provided on one side of the liquid crystal layer 133 of the 
liquid crystal panel 143. An optical image can be formed on the liquid 
crystal panel 143 by changing the state of diffraction of the light with 
video signals. The light transmitted from a pixel to which a sufficient 
voltage is applied completely enters the projection lens 144 and reaches a 
screen 148, and a bright spot is displayed at a corresponding position on 
the screen 148. Diffracted light is emitted from the pixels across which 
no voltage is applied. The diffracted light is transmitted from the 
projection lens 144 and does not reach the screen 148, and a dark spot is 
displayed at a corresponding position on the screen 148. 
Problems of the conventional diffraction type of liquid crystal panel are 
discussed below. While the liquid crystal display panel is in the 
"diffraction state" the polarized component 138a of the light 138, 
oscillating in a direction perpendicular to the sheet of FIG. 13, is 
transmitted through the liquid crystal with the extraordinary refractive 
index n.sub.e, and the ray of light is diffracted and modulated. However, 
a polarized component 138b of the light 138, oscillating in a direction 
parallel to the sheet of paper, passes through the liquid crystal with an 
ordinary refractive index n.sub.o. Hence, the ray of light is not 
modulated. That is, only 50% of the incident ray of light is diffracted 
and modulated, while the remaining 50% of the ray of light directly passes 
through the panel. Therefore, in the projection type display apparatus 
shown in FIG. 14, the polarized 145 is used to transmit only polarized 
light capable of being diffracted and modulated by the liquid crystal 
panel 143. Hence, the efficiency of light utilization is 50%. 
To solve this problem, a diffraction type of liquid crystal display panel 
has been proposed in U.S. Pat. No. 4,251,137. In this panel, gratings are 
formed on upper and lower substrates, respectively, and are oriented 
orthogonally. U.S. Pat. No. 4,856,869 similarly discloses an apparatus in 
which two diffraction liquid crystal display panels are arranged so that 
the gratings of the respective panels are orthogonal. 
However, each grating in these panels must have an approximate height of 
several microns, and a period of about several microns to about 20 
microns. It is extremely difficult to form the gratings uniformly over the 
entire display area. 
Further, if the grating is formed of a substance having a dielectric 
constant different from that of the liquid crystal, when a sufficient 
electric field is applied between upper and lower electrodes, the 
direction of electric lines of force is inclined toward that element 
having the lower dielectric constant. Accordingly, the liquid crystal 
aligns in the direction of the electric line of force, whereupon the 
refractive index of the liquid crystal does not match the grating. 
Moreover, if the refractive indices of the liquid crystal and the grating 
are matched for a certain wavelength of light, light of another wavelength 
may be transmitted through the liquid crystal and grating with different 
refractive indices. In this case, the light may be diffracted. 
Similarly, if the refractive indices of the liquid crystal and grating are 
matched at a certain temperature, light may be transmitted with different 
refractive indices due to temperature changes, thereby causing 
diffraction. 
SUMMARY OF THE INVENTION 
It is hence a primary object of the invention to provide a liquid crystal 
panel which utilizes with high efficiency and forms an optical image of 
high contrast, and to provide a projection type display apparatus using 
such a liquid crystal panel. 
To achieve the object, the liquid crystal display panel comprises a first 
substrate and a second substrate, at least one of which possesses light 
transmissivity, and each which have electrode layer formed thereon, the 
electrode layers confronting each other, and a liquid crystal layer 
sandwiched between the first substrate and the second substrate, divided 
into plural regions, and including liquid crystal molecules, wherein the 
liquid crystal molecules are oriented in the same direction in each of 
first regions and in a different direction in each of second regions each 
defined between two adjacent first regions, the regions being formed with 
a regular periodicity. 
In particular, the liquid crystal molecules in the second regions are 
preferably oriented in a direction extending at 90.degree. to the 
direction in which the molecules are oriented in the first regions. This 
is to provide the greatest difference in refractive indices throughout the 
phase, while allowing the thickness of the liquid crystal layer to be 
minimal. 
The projection display apparatus of the invention comprises the liquid 
crystal display panel, light generating means for generating light, a 
first optical element for leading the light to the liquid crystal display 
panel, and a second optical element part for projecting the light 
modulated by the liquid crystal display panel. 
In the liquid crystal display panel for forming an optical image, since the 
phase grating having a non-uniform refractive index is formed of liquid 
crystal molecules, light can be modulated without concern for polarization 
of the incident light. Therefore, the liquid crystal display panel 
utilizes light with high efficiency. Further, the present invention is 
free from effects of refractive index changes due to wavelength of 
incident light, incident angle or temperature. Accordingly, a liquid 
crystal display panel capable of displaying a quality image having 
excellent contrast can be realized. It is not necessary to form a separate 
grating, and hence the liquid crystal display panel can be manufactured 
easily and inexpensively. By using this liquid crystal panel, a projection 
display apparatus capable of displaying a bright image with a high degree 
of contrast image can be realized.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The liquid crystal display panel of the invention controls the diffraction 
state with an electric field, and acts as a grating capable of modulating 
light. The principle behind the operation of a diffraction type of liquid 
crystal display panel of the invention is shown in FIGS. 12(a) and 12(b). 
A liquid crystal layer 123 is sandwiched between two transparent substrates 
121, 122. Transparent electrodes 124, 125 are formed on the sides of the 
liquid crystal layer 123 confronting the two transparent substrates 121, 
122. Natural light 126 enters from an input side of the liquid crystal 
display panel. 
FIG. 12(a) shows the state in which an electric field is not applied to the 
liquid crystal panel. In this state, the direction of orientation of 
liquid crystal molecules 127 of the liquid crystal layer 123 in the panel 
is homogeneous, and the liquid crystals in the portions A of the layer are 
oriented with their molecular long axes perpendicular to the sheet of 
paper. The portions A and the portions B, in which the directions of 
orientation of the liquid crystal molecules differ, are striped as viewed 
in a direction perpendicular to the substrate, and are disposed 
alternately at equal intervals. The liquid crystal panel in this state 
acts as a phase grating since the portions A and B have different 
refractive indices which periodically repeat. Supposing the difference in 
the refractive indices of mutually adjacent portions A and B to be an, the 
wavelength of natural light to be .lambda., and the thickness of the 
liquid crystal layer 123 to be t, the diffraction efficiency .eta. of 
transmitted diffracted light of the 0-th order, that is, the 
transmissivity of straight light is approximately expressed as 
EQU .eta.o=0.5.times.(1+cos.delta.) 
EQU (where .delta.=4.pi..DELTA.n d/.lambda.) (1) 
The natural light 126 entering the liquid crystal display panel is divided 
into polarized light 128 oscillating in a direction perpendicular to the 
sheet of paper, and polarized light 129 oscillating in a direction 
parallel to the sheet of paper. The polarized light 128 oscillating in a 
direction perpendicular to the sheet of paper passes through the portion A 
of the liquid crystal layer with an extraordinary refractive index 
n.sub.o, and passes through portion B of the liquid crystal layer with an 
ordinary refractive index n.sub.o. The polarized light 129 oscillating in 
a direction parallel to the sheet of paper passes the portion A with an 
ordinary refractive index no, and passes through the portion B with an 
extraordinary refractive index n.sub.e. Both the polarized light 128 and 
129 experience a refractive index difference an .DELTA.n=n.sub.e -n.sub.o, 
and therefore the natural light 126 is transmitted by diffracting at the 
efficiency expressed in formula (1) wherein the refractive index 
difference .DELTA.n=n.sub.e -n.sub.o. 
On the other hand, when an electric field is applied to the liquid crystal 
panel, as shown in FIG. 12(b), the direction of orientation of the liquid 
crystal molecules 127 changes to a homeotropic state. When an electric 
field is applied to the liquid crystal having a positive dielectric 
anisotropy, the long axes of the liquid crystal molecules align in the 
direction of the electric field, that is, in a direction perpendicular to 
the substrate. As a result, the liquid crystal state the same throughout 
the portions A and B of the liquid crystal layer 123, and refractive 
indices of the portions A, B of the ordinary refractive index n.sub.o are 
always experienced by both the polarized light 128 oscillating in a 
direction perpendicular to the sheet of paper and the polarized light 129 
oscillating in a direction parallel to the sheet of paper. Hence the 
refractive index difference an .DELTA.n=n.sub.o -n.sub.o =0, and the 
diffraction efficiency of light of the 0-th order is 1 from equation (1). 
That is, diffraction does not occur, and the incident light is emitted as 
it is. 
When the panel is in a diffraction state, the angle of diffracted light is 
expressed as follows. 
EQU sin.theta.=m.lambda./(n p) (2) 
where m is the diffraction order, .lambda. is the wavelength of light, n is 
the refractive index of the transparent substrate 122, and P is period of 
the grating. 
So far, only a crystal layer in which the direction of orientation of the 
liquid crystal molecules of the portion B of the layer extends at an angle 
of 90.degree. to the direction of orientation of liquid crystal molecules 
of the portion A has been described. Supposing that this angle is .theta., 
the ray of light 128 oscillating in a direction perpendicular to the sheet 
of paper passes through the portion A of the liquid crystal layer with an 
extraordinary refractive index n.sub.e, and the ray of light 129 
oscillating in a direction parallel to the sheet of paper passes through 
the portion B of the liquid crystal layer with an ordinary refractive 
index n.sub.o, as has already been mentioned above. In the portion B of 
the liquid crystal layer, however, the refractive index n, experienced by 
the ray of light 128 oscillating in a direction perpendicular to the sheet 
of paper is 
##EQU1## 
and the refractive index n.sub.p experienced by the polarized light 129 
oscillating in a direction parallel to the sheet of paper is 
##EQU2## 
When the difference between refractive indices n, and n.sub.e and the 
difference between the refractive indices n.sub.p and n.sub.o are placed 
in the formula (1) an efficiency, by which each light is diffracted, is 
obtained. When an electric field is applied, the efficiency can be 
determined in exactly the same manner, and hence an explanation thereof is 
omitted. 
Preferred embodiments of the invention will now be described below while 
referring to the drawings. 
A first embodiment of a liquid crystal display panel according to the 
invention is shown in FIGS. 1(a) and 1(b). FIG. 1(a) is a sectional view 
of the liquid crystal display panel, and FIG. 1(b) is a plan view thereof 
schematically showing the direction of orientation of liquid crystal 
molecules in the plane of the substrate of the liquid crystal display 
panel. In the liquid crystal display panel of the invention, a liquid 
crystal layer 13 is held between two transparent substrate 11, 12. At the 
liquid crystal layer sides of the substrates 11, 12, a counter electrode 
16 and a pixel electrode 17 are formed as transparent electrodes. The 
pixel electrode is formed in a matrix, and a thin film transistor (TFT) 18 
is disposed near each pixel electrode 17 as a switching element. Each TFT 
18 is connected to a source signal wire (not shown) and a gate signal wire 
(not shown), and respectively connected to a signal feed circuit and a 
scanning circuit, and a signal voltage is applied to each pixel. A black 
matrix 19 shields each signal wire and TFT 18. 
The liquid crystal layer 13 consists of portions A and B in which the 
direction of orientation of liquid crystal molecules differ, and they are 
alternately disposed with a regular periodicity. That is, in the portion 
A, the molecular long axes of the liquid crystal molecules 14 are oriented 
in a direction perpendicular to the sheet of paper in a homogeneous state. 
In the portion B, on the other hand, although in homogeneous state, the 
molecular long axes are oriented at a certain angle .theta. to the 
direction perpendicular to the sheet of paper. More specifically, 
observing the direction of orientation of liquid crystal molecules 14, 15 
of the liquid crystal layer 13 through the transparent substrate in FIG. 
1(b), the liquid crystal molecules 15 in portion B are oriented at a 
certain angle .theta. relative to the liquid crystal molecules 14 of 
portion A, as shown in FIG. 12(b). Furthermore, portions A and B are 
configured as stripes alternately disposed at equal intervals. With such a 
structure, the liquid crystal display panel of the invention functions as 
s so-called phase grating, in which portion having different refractive 
indices provide the periodicity of the grating. 
The operation of the liquid crystal display panel of the first embodiment 
is described below. The incident light 20, of which is natural light, 
includes light oscillating in a direction perpendicular to the sheet of 
paper (polarized component 20a) and light oscillating in a direction 
parallel to the sheet of paper (polarized component 20b). The light 20a 
oscillating in a direction perpendicular to the sheet of paper passes 
through the portion A of the liquid crystal layer 13 with a refractive 
index n.sub.e, and passes through the portion B with a refractive index 
n.sub.s expressed in formula (3). The light 20b oscillating in a direction 
parallel to the sheet of paper passes through the portion A of the liquid 
crystal layer 13 with a refractive index no, and passes through the 
portion B with a refractive index n. expressed in formula (4). As a 
result, depending on each refractive index difference, the incident light 
can be modulated at a diffraction efficiency determined in formula (1). 
The diffracted light is divided from the primary diffracted light as 
diffracted light of high order at an angle determined in formula (2). On 
the other hand, when an electric field is applied between electrodes 16 
and 17, the liquid crystal molecules 14 in portion A and liquid crystal 
molecules 15 in portion B are aligned in a direction perpendicular to the 
substrate and hence, the liquid crystal layer 13 exhibits a uniform 
monodomain, so that the incident light is not modulated. That is, when a 
sufficient electric field is applied to the liquid crystal layer 13, the 
difference in refractive indices between the liquid crystal molecules 14 
in portion A and liquid crystal molecules 15 in portion B is nullified, 
and the incident light is allowed to propagate straight through the panel, 
and when an electric field is not applied, the incident light is 
diffracted by an amount corresponding to the difference in refractive 
indices between the liquid crystal molecules 14 in portion A and liquid 
crystal molecules 15 in portion B. Therefore, the light diffraction state 
of the liquid crystal layer of each pixel can be controlled by the applied 
voltage. Thus, the liquid crystal display panel of the invention can form 
an optical image by changing the diffraction state in dependence on a 
video signal. Moreover, it is possible to modulate natural light 
completely. 
A second embodiment of a liquid crystal display panel according to the 
invention is shown in FIGS. 2(a) and 2(b). FIG. 2(a) is a sectional view 
of the liquid crystal display panel, and FIG. 2(b) is a plan view thereof 
schematically showing the direction of orientation of liquid crystal in 
the plane of the substrate of the liquid crystal display panel. To avoid 
duplicating the preceding description of the first embodiment, only 
differences between the first and second embodiments will be described 
below. The same applies to the following embodiments. In this liquid 
crystal display panel of the invention, the direction of orientation of 
the liquid crystal molecules 15 in portion B of the liquid crystal layer 
13 is parallel to the sheet of paper. This is identical to the first 
embodiment when the angle .theta. in the first embodiment is 90.degree.. 
At this time, the light 20a oscillating in a direction perpendicular to 
the sheet of paper is transmitted through portion A of the liquid crystal 
layer with an extraordinary refractive index n.sub.e, and through the 
portion B of the liquid crystal layer with an ordinary refractive index 
n.sub.o. On the other hand, the light 20b oscillating in a direction 
parallel to the sheet of paper is transmitted through the portion A with 
an ordinary refractive index n.sub.o, and through the portion B with an 
extraordinary refractive index of n.sub.e. Both the light 20a and 20b 
experience a refractive index difference .DELTA.n=n.sub.e -n.sub.o and 
therefore, the light 20 is transmitted by diffracting at an efficiency 
expressed in formula (1) wherein the refractive index difference 
.DELTA.n=n.sub.e -n.sub.o. That is, the light is diffracted at the same 
efficiency regardless of its direction of polarization In this case, i.e. 
when .theta.=90.degree., the largest refractive index difference is 
obtained, and hence the thickness of the liquid crystal layer can be 
minimized to attain a desired diffraction efficiency. Further, the driving 
voltage of the liquid crystal can be lowered. For example, the ordinary 
refractive index n.sub.o of nematic liquid crystal is about 1.4 to 1.5, 
and the extraordinary refractive index n.sub.c thereof is in a range of 
1.5 to 1.8 when the nematic liquid crystal is cyanobiphenyl, the 
difference between n.sub.e and n.sub.o is 0.2, which is the greatest 
difference that can be attained. 
A liquid crystal display panel was made using this liquid crystal. The 
thickness of the liquid crystal layer was about 1.5 .mu.m. Without 
applying an electric field, the incident light was diffracted, and the 
straight exit light was set to 0, that is, the diffraction efficiency of 
0-th order light was 0. The period of the regions having liquid crystal 
molecules oriented in directions differing by 90.degree. was about 100 
.mu.m, and the diffraction angle of the primary diffracted light was about 
0.2.degree. to the normal of the panel. When an electric field was applied 
to the liquid crystal panel, the diffraction was eliminated, and a 
transmissivity of about 80% was obtained. 
A third embodiment of a liquid crystal display panel according to the 
invention is shown in FIG. 3, which shows a section of the liquid crystal 
display panel. In this panel, a liquid crystal layer 13 is held between 
two transparent substrates 11, 12. At the liquid crystal layer sides of 
the substrate 11, 12, a counter electrode 16 and a pixel electrode 17 are 
formed as transparent electrodes. Alignment layers 31, 32 for controlling 
the direction of orientation of liquid crystal are formed on the 
electrodes 16, 17. However, the orientation of the molecules in portion C 
and the orientation of the molecules in portion D differ by an angle 
.theta., and these portions are alternately disposed with regular 
periodicity. 
An example of a method of manufacturing the liquid crystal display panels 
will be described with reference to FIGS. 4(a)-4(g). First, a transparent 
electrode layer 42 is formed on the surface of a transparent glass 
substrate 41 as shown in FIGS. 4(a) and 4(b). At this time, signal wires 
and switching elements may also be formed. An alignment layer 43 is formed 
on the electrode layer surface of the substrate shown in FIG. 4(b). An 
ordinary polyimide resin is used as the material of the alignment layer. A 
polyimide solution dissolved in solvent, or a polyamic acid solution which 
is a precursor of polyimide is applied on the electrode 42 in an effective 
display region of the substrate 41 by a printing method or the like, and 
then baked to form a polyimide resin thin film 43 as shown in FIG. 4(c). A 
resist film 44 is formed thereon, openings in the form of stripes 1 .mu.m 
to 10 .mu.m wide are formed at a pitch of 2 .mu.m to 20 .mu.m by 
developing the resist. As shown in FIG. 4(d), the surface of this 
substrate is rubbed with a roller 45 wound with cloth or the like in one 
direction to give a rubbing treatment. The resist film 44 is peeled off, 
and a new resist film 46 is formed. The portion that was rubbed is now 
shielded, and the previously shielded portion is now exposed. As shown in 
FIG. 4(e), the substrate is rubbed again in a different direction by the 
roller 45. Then, the resist film 46 is peeled off, whereby the substrate 
to be used in the invention is obtained as shown in FIG. 4(f). Although 
the resist film was described as used for a mask, it is also possible to 
mask the film 43 directly with other materials such as metal. 
Two of the substrates shown in FIG. 4(f) are used as upper and lower 
substrates, and the two substrates positioned so that the directions of 
orientation of their alignment layers (film 43) coincide. A liquid crystal 
layer 47 is sandwiched between the mutually opposing upper and lower 
substrates, thereby providing a liquid crystal display panel as shown in 
FIG. 4(g). The panel operates in the same manner as the first embodiment. 
A fourth embodiment of the liquid crystal panel according to the invention 
will be described with reference to FIG. 5. FIG. 5 shows a section of the 
liquid crystal display panel, in which a liquid crystal layer 13 is held 
between two transparent substrates 11, 12. At the liquid crystal layer 
sides of the substrates 11, 12, a counter electrode 16 and a pixel 
electrode 17 are formed as transparent electrodes. Alignment layers 51, 52 
for controlling the direction of orientation of liquid crystal are formed 
on the electrodes 16, 17, respectively. The direction of orientation of 
the liquid crystal molecules in portions C of the liquid crystal layer 13 
and the direction of orientation of the liquid crystal molecules in 
portions D differ by 90.degree.. The portions C and D are alternately 
disposed at a specific periodicity. This is achieved by carrying out the 
steps shown in FIG. 4(e) and FIG. 4(d). The panel operates in the same 
manner as the second embodiment. 
A first embodiment of a projection display apparatus according to the 
invention is shown in FIG. 6. Numeral 61 denotes a liquid crystal display 
panel of the invention 62 a light source, 65 a projection lens, and 66 a 
screen. 
The liquid crystal display panel 61 includes a container formed of two 
glass substrates 11, 12 and a seal member, and a liquid crystal layer 13 
injected into the container. In this instance, a liquid crystal display 
panel of the second embodiment of the invention is used. The molecules of 
the liquid crystal layer 13 align in the direction of the electric field 
so that the panel assumes a homeotropic state in which all of the light is 
transmitted with the same refractive index so as to propagate straight 
through the panel. When an electric field is not applied, the incident 
light is diffracted due to differences in the refractive index of the 
crystal layer 13 created by the different directions of orientation of the 
molecules of the liquid crystal layer 13. By impressing a voltage across 
the liquid crystal layer of each pixel, the state of diffraction of light 
passing therethrough can be controlled. Thus, an optical image can be 
formed on the liquid crystal panel 61 based on a video signal. 
The light source 62 is formed of a lamp 63 and a concave mirror 64. The 
light leaving the lamp 63 is converged by the concave mirror 64, so that 
light of a relatively high directivity is emitted. The exit light from the 
light source 62 passes through a field lens 67 and the liquid crystal 
panel 61 sequentially, and enters the projection lens 65. The size of the 
pupil of the projection lens 65 is set so that about 90% of the quantity 
of light transmitted from the pixels at the center of the liquid crystal 
display panel 61 are admitted into the pupil. The field lens 67 is used 
for refracting inwardly the light passing toward the peripheral part of 
the display region of the liquid crystal panel 61 so that such light will 
be admitted into the pupil of the projection lens 65, whereby the 
peripheral area of the projected image will not be dark. The focus of the 
projected image is adjusted by moving the projection lens 65 along the 
optical axis 68. 
An optical image is formed on the liquid crystal display panel 61 depending 
on the state of diffraction created by the video signal. The projection 
lens 65 receives the portion light within a certain solid angle of the 
light emitted from each pixel. When the state of diffraction of exit light 
from each pixel varies, the quantity of light within the solid angle is 
changed, and therefore, the optical image formed on the liquid crystal 
display panel 61 changes, i.e. changes in the state of diffraction are 
converted into changes in the illumination of the screen 66. The optical 
image formed on the liquid crystal display panel 61 is magnified and 
projected onto the screen 66 by the projection lens 65. 
When the projection display apparatus of the invention uses the second 
embodiment of the liquid crystal display panel, it is possible to modulate 
even natural light, regardless of the state of polarization of the light 
entering the liquid crystal display panel 61. Accordingly, the light is 
transmitted with a high degree of efficiency. Further, regardless of the 
wavelength of the incident light, the angle of incidence of the light or 
the temperature, the order of diffraction is 0 when no voltage is applied. 
That is, the quantity of light transmitted during a "black display" is 0, 
which contributes to enhancing the contrast of the display image. 
As for the liquid crystal display panel 61, the width of the strips of the 
crystal layer (portions A and portions B in FIGS. 2(a) and 2(b)) is 
preferred to be 0.5 .mu.m to 20 .mu.m. That is, the period of the grating 
derived from formula (2) is preferred to be 1 .mu.m to 40 .mu.m. When the 
stripes are wide, the diffraction angle is narrow, and when the stripes 
are narrow, the diffraction angle is wide. In particular, in the 
projection display apparatus in which the liquid crystal display panel of 
the invention is used as light valve, the light cannot be separated if the 
diffraction angle is too narrow and the light cannot be picked up by the 
projection lens if the diffraction angle is too wide. 
The second embodiment of a projection display apparatus of the invention is 
shown in FIG. 7. Numeral 61 denotes a liquid crystal display panel, and 62 
a light source. The liquid crystal display panel 61 is the same as that 
shown in FIGS. 2(a) and 2(b). 
Moreover, a schlieren lens 71, and an input mask 72 and an output mask 73, 
serving as schlieren stops, are intended to provide a white display when 
the liquid crystal display panel 61 is in a diffracting state, and a black 
display when in a non-diffracting state. In the optical system of the 
projection display apparatus of the first embodiment, a black display is 
produced when the liquid crystal display panel 61 is in a diffracting 
state, and a white display in a non-diffracting state, which is opposite 
to this embodiment. In the projection display apparatus of the second 
embodiment, the display is black when an electric field is applied to the 
liquid crystal display panel 61, so that a black display of high quality 
is provided stably. The schlieren lens 71 is situated between the input 
mask 72 and the output mask 73, so as to form a schlieren optical system 
in which the image at the input mask 72 is focused on the output mask 73. 
The liquid crystal display panel 61 of the invention is disposed in the 
schlieren optical system. The light propagating straight through the 
liquid crystal display panel 61 is shielded by the output mask 73, and 
only the diffracted light is transmitted through the openings of the 
output mask 73 onto the screen 66 via the projection lens 65. A fly-eye 
lens 74 and a field lens array 75 are disposed between the light source 62 
and input mask 72. The field lens array 75 is disposed near the input mask 
72. The fly-eye lens 74 is used to form the light source image in the 
openings of the input mask 72, which therefore acts as a miniature light 
source array. These optical elements enhance the efficiency of the light 
source. The fly-eye lens 74 and field lens array 75 may, however, be 
omitted. 
The third embodiment of a liquid crystal projection display apparatus 
according to the invention is shown in FIG. 8. Numerals 61a, 61b, 61c 
denote liquid crystal panels, 62 a light source, 65 a projection lens 81, 
82, 83, 84 dichroic mirrors, and 85 to 86 planar mirrors. 
The liquid crystal display panels 61a, 61b, 61c are each identical to the 
liquid crystal display panels of the invention shown in FIGS. 2(a) and 
2(b). 
The light source 62 is formed of a lamp 63, a concave mirror 64, and a 
filter 87. The lamp 63 is a metal halide lamp, and emits light composed of 
the three primary colors of red (R), green (G), and blue (B). The concave 
mirror 64 is made of glass, and a multilayer film (formed thereon by a 
conventional vapor deposition technique) which reflects visible light and 
allows infrared light to pass therethrough. The filter 87 comprises a 
glass substrate, and a multilayer film (also formed by vapor deposition) 
which allows visible light to pass therethrough and wherein infrared light 
and ultraviolet light is reflected at the glass substrate. The visible 
light in the radiation from the lamp 63 is reflected at the reflection 
panel of the concave mirror 64, so as to propagate close to parallel. The 
infrared light and ultraviolet light emitted from the concave mirror 64 
are removed from the system by the filter 87. 
The transmitted light from the light source 62 enters a color separation 
optical system comprising the dichroic mirrors 81, 82 and the planar 
mirror 85, and is separated into light of the three primary colors. Each 
light of a primary color passes through a positive lens 88a, 88b, 88c 
functioning as a field lens, and enters a liquid crystal display panel 
61a, 61b, 61c. The light emitted from the liquid crystal display panels 
61a, 61b, 61c is combined by a color synthesis optical system comprising 
dichroic mirrors 83, 84 and the planar mirror 86, and enters the 
projection lens 65. 
Since the three liquid crystal display panels 61a, 61b, 61c are used for 
red, green and blue components of the light, respectively, a projection 
image of excellent brightness and resolution is obtained. At least one of 
the three liquid crystal display panels 61a, 61b, 61c has a liquid crystal 
layer of a thickness, refractive index or period that is different from 
that of the other panels. As is clear from formula (1) and formula (2), 
the diffraction efficiency and diffraction angle depend on the wavelength 
of light. Therefore, the liquid crystal layer thickness at which the 
diffraction efficiency is maximized or the liquid crystal layer thickness 
at which the diffraction efficiency is nullified should vary slightly 
among the panels 61a, 61b, 61c. For example, to set the diffraction 
efficiency of light of the 0-th order to 0 at a refractive index 
difference an of 0.2, the liquid crystal layer thicknesses of the panels 
are 1.5 .mu.m, 1.3 .mu.m, and 1.1 .mu.m, respectively. Similarly, the 
diffraction angle should differ slightly among the panels. For example, 
when the diffraction angle of first order light is 2.degree., the pitches 
of the regions of the liquid crystal layers of the panels should be 12 
.mu.m, 10 .mu.m, and 8 .mu.m, respectively. 
The dichroic mirror used in the color separation and color synthesis 
optical system of the invention may be a mere absorption type of color 
filter. Alternatively, the color synthesis optical system may be omitted 
in which case three projection lenses are overlaid on the screen. 
An example of a projection display apparatus using three such projection 
lenses will now be explained. FIG. 9 is a schematic diagram of a fourth 
embodiment of a projection display apparatus according to the invention. 
In FIG. 9, numeral 91 denotes a collective optical system having a concave 
mirror and a metal halide lamp of 250 W as a light source. The concave 
mirror is designed to reflect only visible light. An ultraviolet ray 
cut-off filter is disposed at the exit end of the collective optical 
system 91. Numeral 92 denotes an infrared ray cut-off mirror which allows 
infrared rays to pass therethrough and reflects only visible light. 
However, needless to say, the infrared ray cut-off mirror 92 may be 
incorporated by the collective optical system 91. Meanwhile, reference 
numeral 93a denotes a blue reflection dichroic mirror (BDM), 93b a green 
reflection dichroic mirror (GDM), and 93c a red reflection dichroic mirror 
(RDM). The present invention is not limited to the illustrated sequence of 
dichroic mirrors. In fact, the final RDM 93c may be replaced by a full 
reflection mirror. 
Numerals 97a, 97b, and 97c denote liquid crystal display panels. Among the 
liquid crystal display panels, the thickness d of the liquid crystal layer 
of the liquid crystal display panel 97c for modulating the light R is 
about 0.2 .mu.m to 1.0 .mu.m greater than the thickness of the liquid 
crystal layers of the other liquid crystal display panels. This is because 
the degree of diffraction depends on the wavelength of the light to be 
modulated. Moreover, if necessary, the thickness of the liquid crystal 
layer of the liquid crystal panel 97a for modulating blue light is 0.2 
.mu.m to 1.0 .mu.m less than that for green light. Numerals 95a, 95b, 95c 
denote input masks, 98a, 98b, 98c output masks, and 96a, 96b, 96c 
schlieren lenses. Numerals 94a, 94b and 94c denote fly-eye lenses, and 
99a, 99b and 99c projection lenses. 
The fly-eye lenses 94a, 94b, 94c and input masks 95a, 95b, 95c may be 
integrated and disposed between the collective optical system 91 and 
dichroic mirror 93a. The schlieren lenses 96a, 96b, 96c may be disposed 
between the liquid crystal display panels 97a, 97b, 97c and output masks 
98a, 98b, 98c. 
In the projection display apparatus of the invention, the light enters from 
the confronting substrate side of the liquid crystal display panel, but 
this is not limitative, and the same effects are obtained if entering from 
the array substrate side. Thus, the projection display apparatus of the 
invention does not depend on the input direction of light. 
In the projection display apparatus of the third and fourth embodiments, 
liquid crystal panels for modulating the R, G, and B lights are provided. 
However, the present invention is not so limited. For example, a mosaic 
color filter may be attached to one liquid crystal display panel, and 
light may be transmitted from the pixels of the panel as in the 
single-panel projection display apparatus of the first and second 
embodiments. 
In the embodiments of the projection display apparatus of FIGS. 6 to 9, the 
liquid crystal display panel may be replaced with any of the liquid 
crystal display panels of the embodiments of FIGS. 1, 3, 5, and depending 
upon the particular application of and restrictions imposed upon the 
projection display apparatus. 
The fifth embodiment of a projection display apparatus according to the 
invention is shown in FIGS. 10 and 11. Numeral 62 denotes a light source, 
101 a liquid crystal display panel, and 104 a projection lens. The light 
source 62 is the same as that shown in FIG. 6. 
The liquid crystal display panel 101 is a reflection type of liquid crystal 
display panel. The liquid crystal display panel 101 includes an enclosed 
space formed by a first glass substrate 112, a second glass substrate 111, 
and a seal member, and a liquid crystal layer 113 injected into the space. 
As shown in FIG. 22, a TFT 114 is formed in a matrix on the first glass 
substrate, and a pixel electrode 116 of aluminum is formed thereon through 
an intervening insulation layer 115. Each pixel electrode 116 is connected 
to a drain electrode 117 of each TFT 114. A common electrode 118 
comprising a transparent conductive film is formed on the second glass 
substrate 111. In the liquid crystal layer 113, as shown in FIG. 2, the 
direction of orientation of liquid crystal of adjacent regions differs by 
90.degree., which regions are defined with a specific period. 
A projection lens member 104 comprises a first lens group 105 closer to the 
liquid crystal panel 101, and a second lens group 106 closer to the 
screen, and a planar mirror 107 is disposed between the first lens group 
105 and the second lens group 106. The scattered light emitted from the 
pixel located in the center of the liquid crystal panel 101 passes through 
the first lens group 105, and about half of that enters the planar mirror 
107, while the remainder thereof does not enter the planar mirror 107 but 
enters the second lens group 106. The normal reflection plane of the 
planar mirror 107 is inclined 45.degree. to the optical axis 103 of the 
projection lens 104. The light from the light source 62 is reflected by 
the plane mirror 107, passes through the first lens group 105, and enters 
the liquid crystal display panel 101. The light reflected from the liquid 
crystal display panel 101 passes sequentially through the first lens group 
105 and second lens group 106 and reaches the screen. The light from the 
center of the diaphragm of the projection lens 104 and propagating toward 
the liquid crystal display panel 101 is designed to enter the liquid 
crystal layer 113 almost vertically, that is, to be telocentric. 
In the projection display apparatus using the diffraction type of liquid 
crystal panel, to improve the contrast of the projected image, the 
diffraction efficiency of 0-th order light must be as small as possible, 
preferably close to 0. From formula (1), if the depth d of the grating is 
constant, the refractive index difference an of the liquid crystal layer 
should be as large as possible. In existing liquid crystal layers, 
however, the maximum extraordinary refractive index is 1.7 to 1.8, and 
hence .DELTA.n=n.sub.e -n.sub.o is about 0.2. At this refractive index 
difference, therefore, to obtain a diffraction efficiency for light of the 
0-th order at 0, the thickness of the liquid crystal layer must be 2 
.mu.m. In a reflection type of liquid crystal display, the light passes 
through the liquid crystal layer twice. Therefore, the thickness of the 
liquid crystal layer may be about half that of the transmission type of 
liquid crystal display panel. Accordingly, the contrast of the projected 
image is more enhanced in a reflection type of liquid display panel than 
in the transmission type. That is, by reducing the thickness of the liquid 
crystal layer, a projected image of low voltage drive and excellent 
contrast is obtained.