JP2000180844A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a liquid crystal display device which can display images with high contrast and high brightness. SOLUTION: The utilization factor of the light from a light source 10 is improved by providing a polarized light selecting and separating plate 50 between the light source 10 and a first polarizing plate 30 arranged on the incidence plane side of a liquid crystal cell 20 and by transforming natural light from the light source 10 into partially polarized light having an axis of polarization parallel with the transmission axis of the first polarizing plate 30. Also by improving the degree of partial polarization (a) of the light (partially polarized light) transmitted by the polarized light selecting and separating plate 50, the degree of polarizing (P) of the first polarizing plate 30 on which the partially polarized light falls is reduced, thereby improving the transmissivity of the first polarizing plate 30.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device which improves light use efficiency to realize high contrast and high brightness.

[0002]

2. Description of the Related Art Various types of liquid crystal display devices have been proposed by utilizing the function of a liquid crystal having a light modulation function.
These liquid crystal display devices are provided with a polarizing plate on a light incident surface side and a light emitting surface side of a liquid crystal cell for selectively transmitting a linearly polarized light component having a predetermined polarization axis. This polarizing plate has a role of visualizing a change in the orientation of liquid crystal due to an electric field in a liquid crystal display device. In the case of a polarizing plate used for a liquid crystal display device, the transmittance of non-polarized light alone is generally 50%.
% Or less. Further, in the case of a high-contrast type polarizing plate having a degree of polarization of 99.9% or more used in an active matrix type liquid crystal display device, the transmittance of non-polarized light alone is in the range of about 40 to 44%. .

[0003] However, such a polarizing plate has the following problems.

That is, in order to obtain practical linearly polarized light, the polarizing plate transmits only a linearly polarized light component having a polarization axis parallel to the transmission axis of the polarizing plate itself, and absorbs other polarized light components. The light utilization efficiency is at least １ ／
There is a problem that it is restricted as follows.

For this reason, in recent years, in order to improve the utilization efficiency of light from the light source, a linearly polarized light component having a polarization axis parallel to the transmission axis of the polarizing plate is taken out and the light source side is absorbed without absorbing other polarized light components. There has been proposed a liquid crystal display device using a polarization selective separation plate that reflects light back to and reuses it.

This polarization selective separation plate uses a circularly polarized light selective reflection effect of cholesteric liquid crystal, and converts the transmitted circularly polarized light into linearly polarized light having a predetermined polarization axis via a １ ／ λ phase difference plate. And a method in which retardation films having different refractive indices are laminated have been put to practical use.

Generally, the transmission axis of the polarization selective separation plate is arranged so as to be parallel to the transmission axis of the polarization plate disposed on the incident surface side of the liquid crystal cell so that the light use efficiency is maximized.

[0008]

The light transmitted through the polarization selective separation plate as described above is partially polarized light. That is,
If the degree of polarization of the light transmitted through the polarization selective separation plate is 99.9% or more, it can be regarded as perfect linearly polarized light, and a polarizing plate to be disposed on the incident surface side of the liquid crystal cell is practically unnecessary. Become. However, in reality, the degree of polarization of light transmitted through the polarization selective separation plate is about 70 to 90%, and therefore, it is necessary to dispose the polarization plate on the incident surface side.

However, when such a partially polarized light is used in combination with the incident-surface-side polarizing plate, the contrast ratio of the liquid crystal display device is improved in principle. However, the contrast ratio of about 300 is sufficient even in the current liquid crystal display device, and the pigment dispersion type color filter used in the currently mainstream color liquid crystal display device is 300 to 300.
According to the conventional technology, the incident light incident on the incident-side polarizing plate is a partially polarized light because, for example, it has a depolarization characteristic of about 500 and a remarkable improvement in contrast ratio cannot be expected any more. There is a problem that features are not fully utilized.

In general, it is known that the degree of polarization and the transmittance of a polarizing plate have a trade-off relationship. Therefore, the degree of polarization and transmittance of the polarizing plate are determined according to the design of the liquid crystal display device. The characteristics required of the polarizing plate are that the degree of polarization and the transmittance can be both achieved at a higher level.

In a polarizing plate applied to an active matrix type liquid crystal display device, when the light source is non-polarized light,
The degree of polarization is required to be about 99.9%. At this time, the transmittance of the most commonly used iodine-based high-contrast polarizing plate for non-polarized light is about 40 to 44%. That is, if the degree of polarization of the polarizing plate can be reduced, it can contribute to the improvement of the transmittance, and the light use efficiency can be improved. However, when the current light source is non-polarized light, if the degree of polarization of the polarizing plate is reduced, the contrast ratio of the liquid crystal display device is remarkably reduced, which is not practical.

SUMMARY OF THE INVENTION An object of the present invention is to provide a liquid crystal display capable of displaying a high-contrast and high-luminance image in view of the above-mentioned problems.

[0013]

According to the first aspect of the present invention, a liquid crystal is interposed between a light source and a pair of substrates, and has a light modulation function. A liquid crystal cell, a first polarizer disposed as a polarizer on an incident side of the liquid crystal cell on which a light beam from the light source is incident, and having a transmittance Y1 and a degree of polarization P1 with respect to an unpolarized beam; A second polarizing plate having a transmittance Y2 and a degree of polarization P2 with respect to an unpolarized beam, disposed between the light source and the first polarizing plate; A polarization selective separation plate having a transmission axis for transmitting only a predetermined polarization component of the light beam and reflecting another polarization component to the light source side; The separation plate
The transmission axis is arranged so as to be parallel to the transmission axis of the first polarizing plate, the degree of polarization a of light incident on the first polarizing plate is set to a> 1/2, and the first polarizing plate is set. And the degree of polarization P2 of the second polarizer is P1 <P
A liquid crystal display device characterized by satisfying the relationship 2 is provided.

According to the second aspect of the present invention, a liquid crystal cell having a light source, a liquid crystal sandwiched between a pair of substrates and having a light modulation function, and the liquid crystal cell to which a light beam from the light source is incident A first polarizer having a transmittance Y1 and a degree of polarization P1 with respect to a non-polarized beam for the unpolarized beam; and a first polarizer arranged as an analyzer on the exit side of the liquid crystal cell;
A second polarizing plate having a transmittance Y2 and a degree of polarization P2 with respect to an unpolarized beam; and a predetermined polarizing component of the light beam from the light source, which is disposed between the light source and the first polarizing plate. And a polarization selective separation plate having a transmission axis for transmitting light and reflecting another polarization component to the light source side, wherein the polarization selective separation plate has the transmission axis of the It is disposed so as to be parallel to the transmission axis of the first polarizing plate, the degree of polarization a of light incident on the first polarizing plate is set to a> 1/2, the transmittance Y1 of the first polarizing plate, and The transmittance Y2 of the second polarizing plate is Y1
> Y2 is provided.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the liquid crystal display device of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view schematically showing the structure of the liquid crystal display device of the present invention.

As shown in FIG. 1, the liquid crystal display device 1 includes a light source unit 10, a liquid crystal cell 20 having a liquid crystal having a light modulation function, and a liquid crystal cell 20 into which light from the liquid crystal cell 20 is incident.
Polarizing plate 30 disposed as a polarizer on the incident surface side of
A second polarizer 40 disposed as an analyzer on the exit surface side of the liquid crystal cell 20, and a second polarizer 40 disposed between the light source unit 10 and the first polarizer 30. A polarization selective separation plate 50 having a transmission axis that transmits only the polarized light component and reflecting other polarized light components toward the light source unit.

The light source unit 10 includes a thin light guide plate 11 as a diffusion unit made of an acrylic resin, a cold cathode tube 12 as a light source disposed on a short end face of the light guide plate 11, and a cold cathode tube 12. A reflection plate 13 surrounding the periphery and reflecting light from the cold cathode tube 12 toward the end face of the light guide plate 11; and a reflection plate 13 disposed on the back side of the light guide plate 11 to selectively transmit light propagating in the light guide plate 11. A reflection sheet 14 on which a milky white dot pattern or the like is printed so as to emit light from the main surface.

The liquid crystal cell 20 has an array substrate, a counter substrate disposed opposite to the array substrate, and a liquid crystal composition 60 disposed between the array substrate and the counter substrate.

The array substrate is made of glass of 0.7 mm
Although not shown, 1024 × 3 signal lines and 768 scanning lines arranged in a matrix pattern on a thick transparent insulating substrate 21, and switch elements arranged near intersections of the signal lines and the scanning lines A TFT 22; a pixel electrode 23 composed of ITO connected to the TFT 22;
Film 2 for orienting liquid crystal molecules contained in liquid crystal molecules in a predetermined direction
4 is provided.

The opposing substrate is a transparent insulating substrate 25 made of glass and having a thickness of 0.7 mm.
It comprises a counter electrode 26 made of TO and an alignment film 27 for aligning liquid crystal molecules contained in the liquid crystal composition 60 in a predetermined direction.

The liquid crystal composition 60 sandwiched between the array substrate and the counter substrate is, for example, a TN type liquid crystal composition.

The first polarizing plate 30 is arranged so that its transmission axis is oriented in a predetermined direction. Also, the second polarizing plate 40
Are arranged such that their transmission axes are orthogonal to the transmission axis of the first polarizing plate 30.

The polarization selective separation plate 50 has, for example, a cholesteric liquid crystal layer, and uses the circular polarization selective reflection effect of this liquid crystal layer to convert the transmitted circularly polarized light to a predetermined polarization through a λλ phase difference plate. Convert to linearly polarized light with axis. In addition, as the polarization selective separation plate 50, one having a structure in which at least two types of retardation films having different refractive indexes are stacked may be applied.

That is, the polarization selective separation plate 50 transmits only the polarized light component having a predetermined polarization axis out of the light from the light source unit 10 and reflects the other polarized light components to the light source unit 10 side. The polarization selective separation plate 50 transmits, for example, the P-polarized light component of the light from the light source unit 10, reflects the light toward the light source unit 10 without absorbing the S-polarized light component, and reuses the light. The light transmitted through the polarization selective separation plate 50 is not completely linearly polarized light, but is partially polarized light having a degree of polarization of 70 to 90%.

The polarization selective separation plate 50 is disposed such that the polarization axis of the transmitted light is parallel to the transmission axis of the first polarization plate 30.

By the way, the transmittance and the degree of polarization of the first polarizing plate 30 and the second polarizing plate 40 applied to the above-described liquid crystal display device 1 are set so as to satisfy the following conditions.

That is, the transmittance of the polarizing plate is the transmittance T when ideal linearly polarized light parallel to the transmission axis of the polarizing plate is incident.
_trans. and transmittance T _abs. when an ideal linearly polarized light parallel to the absorption axis perpendicular to the transmission axis is incident _.
= 1 and T _abs. = 0 are ideal.

Here, the transmittance for white light is represented by a transmittance Y multiplied by a visibility correction value, and the transmittance when two polarizing plates are combined is Y when the transmission axes are parallel to each other.
_{When the} transmission axes are perpendicular to each other and represented by _para , they are represented by Y _cross .

Now, considering the case where two polarizing plates have the same characteristics with general natural light, the following equation is given. Y = (T _trans. + T _abs. ) / 2 (1) Y _para = (T _trans. ² + T _abs. ² ) / 2 (2) Y _cross = T _trans. · T _abs. (3) Polarization of polarizing plate From the equation P = (T _trans. −T _abs. ) / (T _trans. + T _abs. ) (4), P ² = (Y _para −Y _cross ) / (Y _para + Y _cross ) (5) expressed.

Next, consider a case where the light incident on the polarizing plate is partially polarized light and the characteristics of the polarizing plate are different.

Assuming that the luminance of the light source is L and the real number representing the degree of polarization of the incident light incident on the polarizer is a (where 0 <a <1), the first polarizer disposed on the incident surface side of the liquid crystal cell The intensity of a polarized light component having a polarization axis parallel to the transmission axis of the first polarizing plate of the incident light incident on the first polarizing plate is represented by a · L, and displaces the polarization axis orthogonal to the transmission axis of the first polarizing plate. The intensity of the polarized light component is represented by (1-a) · L. In the case of non-polarized light, a = 1/2.

As for the transmittance of the first polarizing plate disposed on the incident surface side of the liquid crystal cell, the transmittance in the direction parallel to the transmission axis of the first polarizing plate is represented by T1 _trans. Is defined as T1 _abs., And the transmittance of the second polarizing plate disposed on the exit surface side of the liquid crystal cell in the direction parallel to the transmission axis of the second polarizing plate is defined as T2 _trans. If the transmittance in the direction parallel to the absorption axis of the second polarizing plate is T2 _abs.
When the transmission axes of the first polarizing plate and the second polarizing plate are parallel to each other, the luminance Y _{para of} the light transmitted through the second polarizing plate is (1),
From (2) and (3), they are expressed as follows. Y _para = {a · T1 _trans. · T2 _trans. + (1-a) · T1 _abs. · T2 _abs. ｝ · L (6) Further, the transmission axes of the first polarizing plate and the second polarizing plate are orthogonal to each other. In this case, the luminance Y _cross of the light transmitted through the second polarizing plate is
Similarly, it is expressed as follows. Y _cross = {a · T1 _trans. · T2 _abs. + (1-a) · T1 _abs. · T2 _trans. ｝ · L (7) At this time, the degree of polarization P ′ is obtained from (5) as follows: Is represented by _{^{P '2 = (T2 trans -T2}} abs..) / · (T2 trans + T2 abs..) {A · T1 trans -.. (1-a) · T1 abs}. / {A · T1 trans + (1 −a) · T1 _abs. ｝ (8) Now, consider the structure of a conventional liquid crystal display device. That is, the light source is non-polarized light, that is, a = 1/2, and the first polarizer and the second polarizer have the same characteristics, that is, T1 _trans. = T2 _trans. T1 _abs. = T2 _abs. when, _{^{(8), P 2 = {(T1 trans}} . -T1 abs.) / (T1 trans. + T1 abs.)} 2 (9) , and the (4) is guided.

Here, in order to make the contrast ratio of the liquid crystal display device the same as in the prior art, the condition of P = P '(10) is necessary, so that (8), (9) and (10) The following relationship is obtained. (T2 _trans. -T2 _abs. ) / (T2 _trans. + T2 _abs. ) · ｛A · T1 _trans .-(1-a) · T1 _abs. ｝ / ｛A · T1 _trans. + (1-a) · T1 _abs. ｝ = {(T _trans.− T _abs. ) / (T _trans. + T _abs. )} ² (11) When (11) is arranged, a / (1−a) = (T2 _trans./T2 ) _abs. ) / (T1 _{trans./T1 abs.} ) (12)

That is, if a> 1/2 (that is, the light source is polarized light and its polarization axis coincides with the transmission axis of the first polarizer), then (T2 _trans. / T2 _abs. )> ( T1 _trans. / T1 _abs. ) (13) holds.

The degree of single polarization P of the polarizing plate is represented by P = (T _trans.− T _abs. ) / (T _trans. + T _abs. ) (4), but usually T _trans. ≫T _abs. (4) can be approximated as P ≒ (1-2 · T _abs. / T _trans. ) (15)

Therefore, from the relations (13) and (15), P1 <P2 (17) is derived.

Incidentally, the polarizing plate applied to the liquid crystal display device of this embodiment is an iodine-based high-contrast type polarizing plate and has a degree of polarization-transmittance characteristic as shown in FIG. There is a trade-off between the transmittance of the polarizing plate and the degree of polarization. That is, as shown in FIG.
The polarizing plate has such a property that when the transmittance is improved, the degree of polarization is reduced, and when the degree of polarization is improved, the transmittance is reduced. Therefore, the degree of polarization and transmittance of the polarizing plate are determined according to the design of the liquid crystal display device.

When the light incident on the first polarizing plate is natural light,
Since a = 1/2, the required polarization degree P of the polarizing plate
Is 99.9%. At this time, as shown in FIG.
The transmittance of the polarizing plate is 43.7%.

On the other hand, when the light incident on the first polarizing plate is a partially polarized light, that is, as shown in FIG. 1, a polarization selective separation plate 50 is disposed between the light source unit 10 and the first polarizing plate 30. When natural light from the light source unit 10 is converted to linearly polarized light having a polarization axis parallel to the transmission axis of the first polarizing plate 30 to some extent,
The degree a of polarization of the light incident on the first polarizing plate 30 is about 0.9.

In this case, the condition (10) and the expression (12)
From (15), P = 99.1%, and the degree of polarization can be reduced by about one digit. At this time, as shown in FIG. 2, the transmittance of the polarizing plate can be improved to 45.3%.

On the basis of the above-described examination results, a liquid crystal display device was actually manufactured and evaluated. Here, the brightness and contrast ratio of the display image of the liquid crystal display device were evaluated based on the combination of the presence or absence of the polarization selective separation plate and the two types of first polarization plates.

FIG. 3 is a diagram showing three comparative examples, a configuration of the embodiment of the present invention, and respective evaluation results.

That is, in Comparative Example 1, since the polarization selective separation plate was not used, the degree of polarization of the incident light on the first polarization plate was a
Is 0.5, and the degree of polarization P of the first and second polarizers is
Were both 99.9%. The luminance of the display image obtained at this time is 200, and the contrast ratio is 300
Met.

In Comparative Example 2, since the polarization selective separation plate was not used, the degree of polarization a of the incident light on the first polarizing plate was 0.5, and the degree of polarization P of the first polarizing plate was 99.1%. And the degree of polarization P of the second polarizing plate was set to 99.9%. Although the brightness of the display image obtained at this time is slightly improved as compared with Comparative Example 1, since the incident light is non-polarized (a = 0.5),
The contrast ratio becomes 30, which is about one digit lower. Therefore, the configuration as in Comparative Example 2 is not practical.

In Comparative Example 3, since the polarization selective separation plate was used, the degree of polarization a of the light incident on the first polarization plate was 0.9, and the polarization degree P of the first and second polarization plates was 0.9. 99.9%
And In the configuration of Comparative Example 3, since the polarization selective separation plate is used, the light use efficiency is improved. For this reason,
The brightness of the display image obtained at this time was improved by about 35% as compared with Comparative Example 1. Further, the contrast ratio was 300, which is equivalent to that of Comparative Example 1. The contrast ratio is comparative example 1 despite the incident light being deflected.
It is considered that the reason why it is not improved is that the depolarization characteristic of the pigment dispersion type color filter used in this embodiment is generally only about 300, so that a further improvement in contrast cannot be expected.

On the other hand, in the embodiment of the present invention, since the polarization selective separating plate is used, the degree of polarization a of the incident light on the first polarizing plate is 0.9, and the degree of polarization P of the first polarizing plate is 0.9. 9
9.1%, and the degree of polarization P of the second polarizing plate was 99.9%. In the configuration of this embodiment, the use efficiency of light is improved because the polarization selective separation plate is used. Therefore, the brightness of the display image obtained at this time is improved by about 40% as compared with Comparative Example 1. The contrast ratio is 30
0, which makes it possible to maintain the same value as in Comparative Example 1. This contrast ratio falls within a range of 300 or more, which is sufficiently practical.

As described above, according to the liquid crystal display device of the present invention, the polarization selective separation plate is provided between the light source unit and the first polarizing plate disposed on the incident surface side of the liquid crystal cell. Is converted into partial polarized light having a polarization axis parallel to the transmission axis of the first polarizing plate, thereby making it possible to improve the use efficiency of light from the light source unit. Further, by improving the degree of polarization a of the light (partially polarized light) transmitted through the polarization selective separation plate, it becomes possible to reduce the degree of polarization P of the first polarizing plate on which the partial polarized light is incident. 1
It is possible to improve the transmittance of the polarizing plate.

Accordingly, the light from the light source is converted into partial polarization when passing through the polarization selective separation plate, and the first polarization plate,
The brightness of a display image formed by the light transmitted through the liquid crystal cell and the second polarizing plate can be improved, and at the same time, a sufficient contrast ratio can be secured.

[0050]

As described above, according to the present invention, it is possible to provide a liquid crystal display device capable of displaying a high-contrast and high-luminance image.

[Brief description of the drawings]

FIG. 1 is a sectional view schematically showing a structure of a liquid crystal display device according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a polarization degree-transmittance characteristic of a polarizing plate applied to the liquid crystal display device illustrated in FIG.

FIG. 3 is a diagram illustrating a configuration of a comparative example and an embodiment of the present invention, and respective evaluation results.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Liquid crystal display device 10 ... Light source part 20 ... Liquid crystal cell 30 ... First polarizing plate 40 ... Second polarizing plate 50 ... Polarization selective separation plate 60 ... Liquid crystal composition

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference) 2H049 BA02 BA03 BA05 BA07 BA27 BA43 BB03 BC22 2H091 FA08X FA08Z FA10Z FA11Z FA14Z FA23Z FA31Z FA41Z FA42Z FB02 FD06 FD07 FD08 FD10 GA13 HA07 LA03 LA17 5G435 AA02 BB03 CB03 FF05 FF08 GG12 GG24

Claims (6)

[Claims] 1. A light source, a liquid crystal cell having a liquid crystal sandwiched between a pair of substrates and having a light modulation function, and a polarizer disposed on a light incident side of the liquid crystal cell on which light from the light source is incident. A first polarizing plate having a transmittance Y1 and a degree of polarization P1 with respect to a second polarizing plate disposed as an analyzer on the emission side of the liquid crystal cell and having a transmittance Y2 and a degree of polarization P2 with respect to non-polarized light; A polarization selector disposed between the light source and the first polarizing plate, having a transmission axis that transmits only a predetermined polarization component of the light from the light source, and reflecting another polarization component toward the light source; A light-transmitting liquid crystal display device comprising: a light-transmitting liquid crystal display device comprising: a polarizing plate; and a polarization-selective separating plate, wherein the polarization-selective separating plate is arranged such that the transmission axis is parallel to a transmission axis of the first polarizing plate. The degree of polarization a of light incident on Rutotomoni, the polarization P1 of the first polarizer, and polarization P2 of the second polarizer liquid crystal display device which satisfies the relationship of P1 <P2. 2. A light source, a liquid crystal cell having a liquid crystal sandwiched between a pair of substrates and having a light modulation function, and a light polarizer disposed on an incident side of the liquid crystal cell on which light from the light source is incident. A first polarizing plate having a transmittance Y1 and a degree of polarization P1 with respect to a second polarizing plate disposed as an analyzer on the emission side of the liquid crystal cell and having a transmittance Y2 and a degree of polarization P2 with respect to non-polarized light; A polarization selector disposed between the light source and the first polarizing plate, having a transmission axis that transmits only a predetermined polarization component of the light from the light source, and reflecting another polarization component toward the light source; A light-transmitting liquid crystal display device comprising: a light-transmitting liquid crystal display device comprising: a light-transmitting liquid crystal display device; The degree of polarization a of light incident on Rutotomoni, the transmittance of the first polarizing plate Y1, and a liquid crystal display device in which the transmittance Y2 of the second polarizer and satisfies the relationship of Y1> Y2. 3. The degree of polarization a by the polarization selective separation plate.
Is 0.7 to 0.9.
Or the liquid crystal display device according to 2. 4. The liquid crystal display device according to claim 1, wherein the polarization selective separation plate has a cholesteric liquid crystal layer. 5. The liquid crystal display device according to claim 1, wherein the polarization selective separation plate is a multilayer laminated film having at least two kinds of refractive indexes. 6. The contrast ratio C of the liquid crystal display device is as follows:
2. A contrast ratio C 'of a liquid crystal display device in which the second polarizer is disposed on the incident side and the output side of the liquid crystal cell and which does not use the polarization selective separation plate. Or the liquid crystal display device according to 2.