Liquid crystal cells – elements and systems – Liquid crystal system – Projector including liquid crystal cell
Reexamination Certificate
2002-08-14
2004-08-17
Nguyen, Dung (Department: 2871)
Liquid crystal cells, elements and systems
Liquid crystal system
Projector including liquid crystal cell
C349S096000, C349S119000, C353S038000
Reexamination Certificate
active
06778228
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a projection-type display apparatus (also called a liquid crystal projector) employing liquid crystal light valves composed of liquid crystal panels.
2. Description of Related Art
The liquid crystal projector
1100
of
FIG. 9
is an example of a generally-used projection-type projector employing transmissive liquid crystal panels as light valves. In
FIG. 9
, light from a lamp unit
1102
, serving as a light source, is reflected by a mirror
11061
and enters a light guide
1104
where the light is separated into three primary light beams, a red R light beam, a green G light beam, and a blue B light beam, by two dichroic mirrors
11081
and
11082
. The blue B light beam, separated from the light by the dichroic mirror
11082
, is reflected by a mirror
11062
, and enters a liquid crystal light valve
1110
B. The green G light beam, reflected by the dichroic mirror
11081
, enters a liquid crystal light valve
1110
G The red R light beam, transmitted through the dichroic mirror
11081
, is reflected by two mirrors
11063
, and enters a liquid crystal light valve
1110
R.
The three light valves
1110
R,
1110
G, and
1110
B are used for modulating the light beams incident thereto in accordance with their respective color image information in order to form an image. The light beams, modulated by the liquid crystal light valves
1110
R,
1110
G, and
1110
B enter a dichroic prism
1112
from three directions. The dichroic prism
1112
consists of four right-angled prisms whose apices are brought into alignment and bonded together, with two types of wavelength selection reflecting films formed so as to form an X-shape along the bonded surfaces. Accordingly, the red R light beam is reflected by one of the two types of wavelength selection reflecting films towards a projection lens
1114
. The blue B light beam is reflected by the other of the two types of wavelength selection reflecting films towards the projection lens
1114
. The green G light beam transmits through the two types of wavelength selection reflecting films and arrives at the projection lens
1114
. In other words, the images formed by the three liquid crystal light valves
1110
R,
1110
G, and
1110
B, are synthesized by the dichroic prism
1112
in order to project the resultant image onto a projection surface of, for example, a screen through the projection lens
1114
.
FIG. 10
is a schematic view of a liquid crystal light valve. Conventionally, as shown in
FIG. 10
, each of the liquid crystal light valves
1110
R,
1110
G, and
1110
B consists of a liquid crystal panel
804
, a light-incident side polarizer
803
, and a light-emitting side polarizer
805
, with the polarizer
803
being spaced from the light-incident surface of the liquid crystal panel
804
and the polarizer
805
being provided on the light-emitting surface of the liquid crystal panel
804
.
In
FIG. 10
, reference numeral
803
denotes the light-incident side polarizer, which transmits, for example, a p-polarization axis component light beam
801
of the incident light (symbol
in
FIG. 10
represents the p-polarization axis) and absorbs an s-polarization axis component light beam
802
(symbol ⊙ in
FIG. 10
represents the s-polarization axis). The p-polarized light beams
801
, transmitted through the polarizer
803
, enters the liquid crystal panel
804
. The liquid crystal panel
804
is a twisted nematic (TN) type liquid crystal panel, which causes the p-polarized light beam
801
, incident upon a pixel to which a voltage is not applied, to leave it as an s-polarized light beam
809
by rotating the polarization axis through an angle of about 90 degrees. On the other hand, the p-polarized light beam
801
, incident upon a pixel to which a voltage is applied, leaves the liquid crystal panel
804
as a p-polarized light beam
808
. Reference numeral
805
denotes the light-emitting side polarizer. When the polarization axis of the polarizer
805
is set to that allowing transmission of s-polarized light beams, the light beam
809
, which has left the liquid crystal panel
804
as an s-polarized light beam, transmits through the polarizer
805
unchanged. On the other hand, when the light beam
808
, which has left the liquid crystal panel
804
as a p-polarized light beam, is absorbed by the polarizer
805
. In the liquid crystal panel
804
, the proportion of liquid crystal twisting can be controlled by controlling the voltage applied to the liquid crystal for each pixel based on each color image information, making it possible to control the amount of rotation of the polarization axis of the light beam
801
, transmitted through the light-incident side polarizer
803
, which enters the liquid crystal panel
804
. Thus, it is possible to control the quantity of light passing through the light-emitting side polarizer
805
for every pixel in order to form an image.
The path taken by the s-polarized light beam
802
is shown at the right side in FIG.
10
. The s-polarized light beam
802
is absorbed by the light-incident side polarizer
803
and is converted into heat. The light beam
808
, absorbed by the light-emitting side polarizer
805
, is also converted into heat.
Conventional polarizers were of the type absorbing a non-transmissive polarization axis (hereinafter referred to as an absorptive-type polarizer), so that of the randomly polarized light beams illuminating a polarizer, approximately half of it is absorbed by the polarizer, and converted into heat, deteriorating the polarization characteristics of the polarizer. In addition, when the heat produced at the polarizer is transmitted to the light crystal panel
804
, liquid crystal characteristics change, or a large quantity of leakage current flows at a thin film transistor (TFT) disposed at each pixel of the liquid crystal panel, resulting in display variations.
Therefore, in conventional projection-type display apparatuses, it is necessary, for example, to use a highly heat-resistant liquid crystal, since heat is generated as a result of absorption of about half of the incident light by the light-incident side polarizer of a liquid crystal light valve. In other words, it is necessary to use a liquid crystal with a high N-I point, since physical properties, such as the refractive index, the anisotropy of dielectric constant, or the elastic constant, of the liquid crystal change with temperature, and the change becomes greater the closer the transition point (the N-I point) is to the isotropic phase. At present, a high N-I point material is mixed with ten types of materials or so, so that within, for example, the threshold voltage or response speed range providing satisfactory performance, the liquid crystal material obtained has a high N-I point at a temperature equal to or greater than 100 degrees. Thus, liquid crystals become expensive, which causes liquid crystal panels to become expensive.
In addition, in order to make the image projected onto a screen brighter, there has been a trend, in recent years, to increase the luminance of a light source lamp, resulting in the problem of increased heat generation from the liquid crystal light valves. A cooling fan for cooling the light-incident side polarizer and the liquid crystal panels is provided in order to cool the liquid crystal light valves. The designing of such a cooling fan is sophisticated since, for example, the rotational speed or the size of the cooling fan must be made large.
Further, since about half of the light from the light source lamp is absorbed by the light-incident side polarizer and converted into heat, light is used with very low efficiency, preventing a bright display from being obtained.
SUMMARY OF THE INVENTION
Accordingly, in order to overcome the above-described problems of conventional devices, it is an object of the present invention to provide a projection-type display apparatus which reduces the amount of heat generated by a liquid crystal light valve, serving as a polarizing means, and maintains excellent
Itoh Yoshitaka
Murakami Osamu
Yajima Akitaka
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