Reflective display apparatus using compensator cell and ferroelectric liquid crystal at 90.degree. to enhance contrast ratio

A reflective display apparatus enhancing a contrast ratio does not use a quarter wavelength plate and adjusts a switching angle of each molecular axis of a compensator and a ferroelectric liquid crystal (FLC) display device, to thereby enhance a contrast ratio. The reflective display apparatus is used for manufacturing an FLC projection TV using a polarizing beam splitter (PBS).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a reflective display apparatus using a 
polarizing beam splitter (PBS), and more particularly, to a reflective 
display apparatus enhancing a contrast ratio by controlling a switching 
angle with respect to each molecular axis of a compensator and 
ferroelectric liquid crystal display device. 
2. Description of the Related Art 
A contrast ratio is a scale representing how definitely an image is viewed. 
An image is viewed better as a difference in luminance is greater. The 
contrast ratio is a value which is obtained by dividing a luminance value 
of the white state by that of the black state at the center of a panel. 
The luminance of the black state has a value smaller than that of the 
white state. Thus, it can be seen that the contrast ratio is influenced 
more by the luminance of the black state. Finally, the contrast ratio 
becomes higher as the luminance value of the black state is smaller. 
FIG. 1 is a schematic diagram showing a configuration of a conventional 
reflective display apparatus using a PBS. 
In FIG. 1, a P polarized beam of a beam incident to a polarizing beam 
splitter (PBS) 11 transmits through the PBS 11 and an S polarized beam 
reflects from the PBS 11. The beam having transmitted through the PBS 11 
proceeds to a quarter wavelength plate 12. The beam having proceeded to 
the quarter wavelength plate 12 is converted into a circular polarized 
beam in the quarter wavelength plate 12 and then proceeds to a compensator 
13. The beam having proceeded to the compensator 13 is converted into a 
linear polarized beam in the compensator 13 and then proceeds to a 
ferroelectric liquid crystal (FLC) display device 14. The beam incident to 
the FLC display device 14 is converted into a circular polarized beam in 
the FLC display device 14 and then reflected from the FLC display device 
14. The reflected beam is converted into a linear polarized beam in the 
FLC display device 14 and proceeds to the compensator 13. The beam having 
proceeded to the compensator 13 is converted into a circular polarized 
beam in the compensator 13 and then proceeds to the quarter wavelength 
plate 12. The beam incident to the quarter wavelength plate 12 is 
converted into a linear polarized beam in the quarter wavelength plate 12 
and then proceeds to the PBS 11. In the beam proceeding from the quarter 
wavelength plate 12 to the PBS 11, a P polarized beam transmits through 
the PBS 11 and an S polarized beam reflects from the PBS 11 to proceed to 
a projection lens. 
In FIG. 1, the compensator 13 is made of the same material as that of the 
FLC display device 14. The compensator 13 converts the polarized beam of 
the incident beam. The molecular axis of each pixel of the compensator 13 
is aligned in the disorder state when a driving voltage is not applied 
thereto. The molecular axis of each pixel of the compensator 13 is aligned 
in any one direction of 0.degree. direction and +45.degree. direction 
based on a vertical axis according to an applied driving voltage. The 
compensator 13 looks as if the whole compensator 13 is made of a single 
pixel when a molecular axis of each pixel is aligned in the same 
direction. An angle where a molecular axis is aligned to be 0.degree. or 
+45.degree. is called a switching angle (referring to FIG. 2A). 
In FIG. 1, the FLC display device 14 is a reflective liquid crystal display 
device and converts a polarized beam of the incident beam. The molecular 
axis of each pixel of the FLC display device 14 is aligned in the disorder 
state when a driving voltage is not applied thereto. The molecular axis of 
each pixel of the FLC display device 14 is aligned in any one direction of 
0.degree. direction and -45.degree. direction based on a horizontal axis 
according to an applied driving voltage. An angle where a molecular axis 
is aligned to be 0.degree. or -45.degree. is called a switching angle 
(referring to FIG. 2B). 
The compensator 13 and the FLC display device 14 of FIG. 1 are integrally 
formed. 
FIG. 3 illustrates a table showing a switching angle with respect to a 
molecular axis of the compensator 13, a switching angle with respect to a 
molecular axis of the FLC display device 14, and transmissivity of a beam 
with respect to a projection lens, in the cases that the beam proceeding 
from the quarter wavelength plate 12 to the PBS 11 reflects from the PBS 
11 and then proceeds to the projection lens (the white state), and 
transmits through the PBS 11 to then not proceed to the projection lens 
(the black state). 
In FIG. 3, in the white state, each molecular axis of the compensator 13 
and the FLC display device 14 is aligned to have a switching angle of 
0.degree. and 0.degree. or +45.degree. and -45.degree.. Here, the 
transmissivity of the beam with respect to the projection lens is 
substantially 100%. Meanwhile, in the black state, each molecular axis of 
the compensator 13 and the FLC display device 14 is aligned to have a 
switching angle of 0.degree. and -45.degree. or +45.degree. and 0.degree.. 
Here, the transmissivity of the beam with respect to the projection lens 
has a small value. 
However, since the beam transmitted from the quarter wavelength plate 12 
transmits through the PBS 11 but does not proceed to the projection lens 
in the black state, the transmissivity of the beam with respect to the 
projection lens should be substantially 0%. However, as shown in FIG. 3, 
it can be seen that the transmissivity is not 0% in the black state. 
Also, in order to maintain the transmissivity to be 0% in the black state, 
an angle of 90.degree. should be formed between the molecular axis of the 
compensator 13 and that of the FLC display device 14. However, as can be 
seen from FIG. 3, an angle of 135.degree. or 45.degree. is formed between 
the molecular axis of the compensator 13 and that of the FLC display 
device 14. Thus, the reflective display apparatus has a problem where a 
contrast ratio is lowered due to the luminance of a small amount of the 
beam transmitted through the projection lens in the black state. 
SUMMARY OF THE INVENTION 
To solve the above problems, it is an object of the present invention to 
provide a reflective display apparatus enhancing a contrast ratio by 
adjusting a switching angle with respect to each molecular axis of a 
compensator and an FLC display device to thereby make transmissivity of a 
beam with respect to a projection lens in the black state substantially 
0%. 
To accomplish the above object of the present invention, there is provided 
a reflective display apparatus comprising: a polarizing beam splitter 
(PBS) transmitting a P polarized beam of an incident beam and reflecting 
an S polarized beam thereof; a compensator for converting the incident 
beam from the PBS into a polarized beam and then transmitting the 
polarized beam, and converting a beam returning after the polarized beam 
has been transmitted into a polarized beam and proceeding to the PBS, in 
which a molecular axis of each pixel is aligned in any one direction among 
.+-..alpha..degree. directions based on the vertical axis when a driving 
voltage is applied; and a reflective liquid crystal display device for 
converting the incident beam after transmitting through the compensator 
into a polarized beam and then reflecting the polarized beam, and 
converting the reflected beam into a polarized beam and proceeding to the 
compensator, where a molecular axis of each pixel is aligned in any one 
direction among .+-..beta..degree. directions based on the horizontal axis 
when a driving voltage is applied, wherein an angle between the molecular 
axis of the compensator and that of the reflective liquid crystal display 
device is formed as 90.degree. in the case that the beam transmitting 
through the compensator and proceeding to the PBS transmits through the 
PBS, thereby enhancing a contrast ratio.

DETAILED DESCRIPTION OF THE INVENTION 
A preferred embodiment of the present invention will be described with 
reference to the accompanying drawings. 
In FIG. 4 showing a configuration of a reflective display apparatus using a 
PBS according to the present invention, a quarter wavelength plate in the 
reflective display apparatus shown in FIG. 1 is excluded. 
In FIG. 4, a P polarized beam of a beam incident to a polarizing beam 
splitter (PBS) 21 transmits through the PBS 21 and an S polarized beam 
reflects from the PBS 21. The beam having transmitted through the PBS 21 
proceeds to a compensator 23. The beam having proceeded to the compensator 
23 is converted into a circular polarized beam in the compensator 23 and 
then proceeds to a FLC display device 24. The beam incident to the FLC 
display device 24 is converted into a linear polarized beam in the FLC 
display device 24 and then reflected from the FLC display device 24. The 
reflected beam is converted into a circular polarized beam in the FLC 
display device 24 and proceeds to the compensator 23. The beam having 
proceeded to the compensator 23 is converted into a linear polarized beam 
in the compensator 23 and then proceeds to the PBS 21. In the beam 
proceeding from the compensator 23 to the PBS 21, a P polarized beam 
transmits through the PBS 21 and an S polarized beam reflects from the PBS 
21 to proceed to a projection lens. 
Here, the compensator 23 is made of the same material as that of the FLC 
display device 24. The compensator 23 converts the polarized beam of the 
incident beam. The molecular axis of each pixel of the compensator 23 is 
aligned in the disorder state when a driving voltage is not applied 
thereto. The molecular axis of each pixel of the compensator 23 is aligned 
in any one direction of -22.5.degree. direction and +22.5.degree. 
direction based on a vertical axis according to an applied driving 
voltage. An angle where a molecular axis is aligned to be -22.5.degree. or 
+22.5.degree. is called a switching angle (referring to FIG. 5A). The 
present invention illustrates only the case where a switching angle with 
respect to the molecular axis of the compensator 23 is .+-.22.5.degree., 
in which an absolute value of the switching angle satisfies an inequality 
0&lt;absolute value of switching angle .ltoreq.22.5. 
In FIG. 4, the FLC display device 24 is a reflective liquid crystal device 
and converts a polarized beam of the incident beam. The molecular axis of 
each pixel of the FLC display device 24 is aligned in the disorder state 
when a driving voltage is not applied thereto. The molecular axis of each 
pixel of the FLC display device 24 is aligned in any one direction of 
+22.5.degree. direction and -22.5.degree. direction based on a horizontal 
axis according to an applied driving voltage. An angle where a molecular 
axis is aligned to be +22.5.degree. or -22.5.degree. is called a switching 
angle (referring to FIG. 5B). The present invention illustrates only the 
case where a switching angle with respect to the molecular axis of the FLC 
display device 24 is .+-.22.5.degree., in which an absolute value of the 
switching angle satisfies an inequality 0&lt;absolute value of switching 
angle .ltoreq.22.5. 
Here, the switching angle absolute value of the compensator 23 and that of 
the FLC display device 24 are established as the same value. That is, if 
the switching angle absolute value of the compensator 23 is five, that of 
the FLC display device 24 is established as five. 
The compensator 23 and the FLC display device 24 of FIG. 4 are integrally 
formed. 
FIG. 6 illustrates a table showing a switching angle with respect to a 
molecular axis of the compensator 23, a switching angle with respect to a 
molecular axis of the FLC display device 24, and transmissivity of a beam 
with respect to a projection lens, in the cases where the beam proceeding 
from the compensator 23 to the PBS 21 reflects from the PBS 21 and then 
proceeds to the projection lens (the white state), and transmits through 
the PBS 21 to then not proceed to the projection lens (the black state). 
In FIG. 6, in the white state, each molecular axis of the compensator 23 
and the FLC display device 24 is aligned to have a switching angle of 
-22.5.degree. and -22.5.degree. or +22.5.degree. and +22.5.degree.. Here, 
the transmissivity of the beam with respect to the projection lens is 
substantially 100%. Meanwhile, in the black state, each molecular axis of 
the compensator 23 and the FLC display device 24 is aligned to have a 
switching angle of -22.5.degree. and +22.5.degree. or +22.5.degree. and 
-22.5.degree. . It can be seen that an angle of 90.degree. is formed 
between the molecular axis of the compensator 23 and that of the FLC 
display device 24. Thus, it can be seen that the transmissivity of the 
beam with respect to the projection lens is substantially 0% in the black 
state. 
As described above, the reflective display apparatus according to the 
present invention adjusts a switching angle with respect to each molecular 
axis of the compensator and the FLC display device 24, except for the 
quarter wavelength plate, to maintain the transmissivity of the beam with 
respect to the projection lens to be substantially 0% in the black state. 
Thus, the reflective display apparatus according to the present invention 
does not substantially influence a contrast ratio with the luminance in 
the black state. Therefore, the reflective display apparatus according to 
the present invention has a higher contrast ratio to thereby enable a 
clearer image to be displayed. 
The reflective display apparatus according to the present invention is used 
for manufacturing an FLC projection TV using a PBS.