JP5789564B2 - Method for improving visibility of liquid crystal display device, and liquid crystal display device using the same - Google Patents

Description

  The present invention relates to a method for improving the visibility of a liquid crystal display device capable of ensuring good visibility regardless of the viewing angle when the screen is observed through a polarizing plate such as sunglasses, and a liquid crystal display device using the same. .

  In recent years, the use of liquid crystal display devices (LCD) has been expanded, and LCDs are also used for various display objects used outdoors. For example, the use is expanding to instrument panels such as cars, ships, airplanes, in-car navigation systems, digital cameras, mobile devices such as mobile phones and personal computers, or digital signage used in buildings, supermarkets, and the like.

  The LCD performs display by transmitting / shielding external light or light generated by a light source such as a front light and a backlight on a liquid crystal panel having a liquid crystal cell sandwiched between two polarizing plates. As the backlight light source, a fluorescent tube such as a cold cathode tube or a hot cathode tube is generally used. The spectral distribution of a fluorescent lamp such as a cold cathode tube or a hot cathode tube shows an emission spectrum having a plurality of peaks, and these discontinuous emission spectra are combined to obtain a white light source. On the other hand, use of a light-emitting diode with low power consumption is being studied from the viewpoint of energy saving. In particular, white light emitting diodes (white LEDs) have a continuous and broad emission spectrum as compared with fluorescent tubes, and are excellent in luminous efficiency.

  By the way, in an environment such as outdoors where the sunlight is strong, in order to eliminate the glare, the LCD may be visually recognized in a state of wearing sunglasses having polarization characteristics. In this case, since the observer visually recognizes the light having linearly polarized light emitted from the LCD through the polarizing plate, the absorption axis of the polarizing plate built in the LCD and the absorption axis of the polarizing plate such as sunglasses are formed. Depending on the angle, the screen disappears.

  In order to solve the above problem, for example, Patent Document 1 proposes a method for depolarizing a polarizing plate by converting a linearly polarized light into a circularly polarized light by obliquely laminating a retardation (1/4 wavelength) plate on the LCD surface. Yes.

  Patent Document 2 proposes to solve the above problem by using a high birefringence material such as calcite or artificial quartz as a depolarizing element. This is because when a high birefringence material such as calcite or artificial quartz is inserted between orthogonal polarizing plates, light transmitted through the high birefringence material having a high retardation (more than 100000 nm as retardation) has various wavelengths. Therefore, it is used that the light interferes with each other and emits white light.

JP 2005-352068 A Japanese Patent Laid-Open No. 10-10522

  However, even a phase difference (quarter wavelength) plate only achieves a quarter wavelength only for light in a specific wavelength region, and it is uniformly four minutes over a wide visible light region. A material that achieves one wavelength is not obtained. Therefore, the method of Patent Document 1 cannot obtain a sufficient visibility improvement effect.

  Furthermore, in the method of Patent Document 2, a high birefringence material made of a special material such as calcite or artificial quartz must be used, and the configuration of the liquid crystal display device becomes complicated, and the degree of freedom such as an increase in screen size and weight reduction is required. It was difficult to adopt a high configuration, and practicality was poor.

  The present invention has been made to solve such a problem, and its purpose is to ensure a highly good visibility regardless of the observation angle when the screen is observed through a polarizing plate such as sunglasses. The object is to provide a liquid crystal display device.

  As a result of intensive studies to achieve the above object, the present inventors have found that the above problem can be solved by using a specific backlight light source and a polymer film having a specific retardation in combination. It was completed.

That is, the present invention relates to the following (i) to ( ix ).
(I) In a liquid crystal display device having at least a backlight light source, a liquid crystal cell, and a polarizing plate disposed on the viewing side of the liquid crystal cell, a light source having a continuous emission spectrum is used as the backlight light source, and the polarizing plate on the viewing side, a polymer film having retardation of 3000～30000Nm (excluding oriented polyethylene terephthalate film) improve visibility method of a liquid crystal display device which comprises using by distributing the.
(Ii) The method for improving the visibility of the liquid crystal display device, wherein the polymer film is an oriented polyester film .
(Iii) The method for improving the visibility of the liquid crystal display device, wherein the polymer film has a thickness of 25 to 500 μm.
( Iv ) A liquid crystal display device using the visibility improving method.
( V ) A polymer film used for the visibility improving method.
(Vi) In a liquid crystal display device having at least a backlight light source, a liquid crystal cell, and a polarizing plate disposed on the viewing side of the liquid crystal cell, a light source having a continuous emission spectrum is used as the backlight light source, and the polarizing plate On the viewing side, an oriented polyethylene terephthalate film having a retardation of 3000 to 30000 nm is used so that the angle formed by the absorption axis of the polarizing plate and the slow axis of the oriented polyethylene terephthalate film is about 45 degrees. A method for improving the visibility of a liquid crystal display device.
(Vii) The method for improving the visibility of the liquid crystal display device, wherein the polymer film has a thickness of 25 to 500 μm.
(Viii) A liquid crystal display device using the visibility improving method.
(Ix) A polymer film used for the visibility improving method.

  In the method of the present invention, a white light-emitting diode light source having a continuous and broad emission spectrum efficiently eliminates linearly polarized light and obtains a spectrum that approximates the light source. Therefore, when observing a liquid crystal display screen through a polarizing plate such as sunglasses. However, good visibility can be ensured regardless of the observation angle.

It is the interference color of a polycarbonate film observed using a white LED light source. It is the interference color chart of a polycarbonate film calculated using the emission spectrum of a white LED light source. The emission spectrum of the white LED light source and the spectrum of the light transmitted through the crossed Nicols at Re = 8000 nm are calculated. It is the image which observed the screen of the liquid crystal display device obtained by the method of this invention through sunglasses. The emission spectrum of the cold cathode tube and the spectrum of the light transmitted through the crossed Nicols at Re = 8000 nm are calculated. It is the interference color chart of a polycarbonate film calculated using the emission spectrum of a cold cathode tube.

  In general, the liquid crystal panel includes a rear module, a liquid crystal cell, and a front module in order from the side facing the backlight light source toward the image display side (viewing side). The rear module and the front module are generally composed of a transparent substrate, a transparent conductive film formed on the liquid crystal cell side surface, and a polarizing plate disposed on the opposite side. Here, the polarizing plate is arranged on the side facing the backlight light source in the rear module, and is arranged on the side (viewing side) displaying the image in the front module.

  The liquid crystal display device (LCD) of the present invention includes at least a backlight light source, a liquid crystal cell, and a polarizing plate disposed on the viewing side of the liquid crystal cell. As described above, since the liquid crystal cell is generally disposed between two polarizing plates on the backlight source side and the viewing side, a polarizing plate is disposed on the opposite side of the viewing side of the liquid crystal cell. It doesn't matter. Moreover, you may have suitably other structures other than these, for example, a color filter, a lens film, a diffusion sheet, an antireflection film etc. suitably.

  In the present invention, it is necessary to use a white light emitting diode (white LED) as a backlight light source of a liquid crystal display (LCD). The white LED is an element that emits white by combining a phosphor type with a phosphor type, that is, a light emitting diode that emits blue light or ultraviolet light using a compound semiconductor. In particular, white light-emitting diodes, which are composed of light-emitting elements that combine blue light-emitting diodes using compound semiconductors with yttrium, aluminum, and garnet-based yellow phosphors, have a continuous and broad emission spectrum and have high luminous efficiency. Therefore, it is suitable as the backlight light source of the present invention. In addition, since the white LED with low power consumption can be widely used by the method of the present invention, it is possible to achieve an energy saving effect.

  In addition, a method of using LEDs that emit red, green, and blue colors in combination as a white light source has been put into practical use. However, since the emission spectrum is narrow and discontinuous in this method, the expected effect of the present invention is achieved. This is not preferable because it is expected to be difficult to obtain.

  In addition, fluorescent tubes such as cold cathode tubes and hot cathode tubes that have been widely used as backlight light sources in the past cannot be used in the present invention. Because the emission spectrum of these light sources has only a discontinuous emission spectrum having a peak at a specific wavelength, in order to produce white light by interference color from such an emission spectrum, it has a retardation exceeding 100,000 nm. This is because a special inorganic material has to be used, which greatly restricts the device design of the liquid crystal display device.

In the present invention, a polymer film having a specific range of retardation is arranged on the viewing side of the polarizing plate. The inventor has focused on the envelope shape of the interference color spectrum by the transmitted light that has passed through the birefringent body, and obtained the idea of the present invention. That is, it found that the visibility by the envelope shape of the interference color spectrum by transmitted light transmitted through the emission spectrum and the birefringence of the light source is similar phase is remarkably improved, and have reached the present invention . Specifically, the effect that the visibility is improved by the configuration of the present invention is based on the following technical idea.

  When a polymer film having birefringence is disposed between two orthogonal polarizing plates, the linearly polarized light emitted from the polarizing plate is disturbed when passing through the polymer film, and light is transmitted. The transmitted light shows an interference color peculiar to retardation which is a product of birefringence and thickness of the polymer film. In the present invention, a white LED having a continuous emission spectrum is used as a light source. For this reason, it becomes possible to approximate the envelope shape of the spectrum of the transmitted light showing the interference color to the emission spectrum of the light source by controlling to a specific retardation range that can also be achieved by the polymer film. This has led to the improvement of visibility. (See Figure 3)

  In order to achieve the above effect, the polymer film used in the present invention must have a retardation of 3000 to 30000 nm. When the retardation is less than 3000 nm, when the screen is observed through a polarizing plate such as sunglasses, a strong interference color is exhibited. Therefore, the envelope shape is different from the emission spectrum of the light source, and good visibility cannot be ensured. A preferable lower limit of retardation is 4500 nm, a more preferable lower limit is 6000 nm, a still more preferable lower limit is 8000 nm, and a still more preferable lower limit is 10000 nm.

  On the other hand, the upper limit of retardation is 30000 nm. Even if a polymer film having a retardation higher than that is used, the effect of improving the visibility is not substantially obtained, and the thickness of the film is considerably increased, so that the handleability as an industrial material is lowered. It is not preferable.

  In addition, the retardation of this invention can also be calculated | required by measuring the refractive index and thickness of a biaxial direction, and can also be calculated | required using commercially available automatic birefringence measuring apparatuses, such as KOBRA-21ADH (Oji Scientific Instruments). it can.

  Since the present invention uses a white LED having a broad emission spectrum as a light source, setting the retardation of the polymer film in the above range allows the spectrum of the transmitted light to have an envelope shape with only a relatively simple configuration. It is possible to approximate to That is, since the conventional technique uses a light source having a discontinuous emission spectrum, the visibility cannot be improved unless a birefringent body having an extremely high retardation (over 100,000 nm) is used. Using the property of a white LED light source having a spectrum, a unique effect of improving the visibility with a relatively simple configuration as described above is achieved.

  In the polymer film used in the present invention, the angle formed by the absorption axis of the polarizing plate and the slow axis of the polymer film is approximately 45 degrees on the viewing side of the polarizing plate disposed on the viewing side of the liquid crystal cell. It is used by arranging. As a method of arranging the polymer film on the viewing side of the polarizing plate, the polymer film may be laminated directly on the outermost layer of the polarizing plate, or may be arranged via another transparent member. Further, a polymer film may be installed and bonded to the outermost surface on the viewing side of the liquid crystal display device. When the polymer film is arranged directly or via another transparent member, it is also a preferred embodiment to use a polymer film provided with an adhesive layer.

  When the polymer film is arranged, it is desirable that the angle formed by the absorption axis of the polarizing plate and the slow axis of the polymer film is about 45 degrees. Thereby, high transmitted light can be obtained regardless of the angle of the polarizing plate such as sunglasses. The angle does not need to be strictly 45 degrees, and may be adjusted as necessary as long as the effect of the present invention is not impaired. A preferable range of the angle is 30 to 60 degrees, and more preferably 40 to 50 degrees.

  The material of the polymer film used in the present invention is not particularly limited and is arbitrary. Examples thereof include polyesters such as polyethylene terephthalate and polyethylene naphthalate, polycarbonate, polystyrene, polyether ether ketone, polyphenylene sulfide, and cycloolefin polymer. Among these, polycarbonate and polyester are exemplified as particularly preferable materials. These resins are excellent in transparency and excellent in thermal and mechanical properties, and the retardation can be easily controlled by stretching. In particular, polyesters typified by polyethylene terephthalate are the most suitable materials because they have a large intrinsic birefringence and relatively large retardation can be obtained relatively easily even when the film thickness is thin.

  Next, since the polymer film in the present invention has a specific birefringence, it is desirable to use an oriented film. However, the production method is not particularly limited as long as the film characteristics defined in the present invention are satisfied. Absent.

  In the case of a polycarbonate film, an unoriented sheet obtained by melting polycarbonate and extruding it into a sheet is stretched in one direction (or two directions if necessary) at a temperature equal to or higher than the glass transition temperature, and has a specific retardation. A polycarbonate film can be obtained. As the non-oriented polycarbonate sheet, a commercially available product or a solution prepared by solution film formation can be suitably used.

  Further, in the case of a polyester film, there is a method in which a polyester is melted, and a non-oriented polyester extruded and formed into a sheet is subjected to a heat treatment after transverse stretching with a tenter at a temperature equal to or higher than the glass transition temperature.

  Specifically, the transverse stretching temperature is preferably 80 to 130 ° C, particularly preferably 90 to 120 ° C. The transverse draw ratio is preferably 2.5 to 6.0 times, and particularly preferably 3.0 to 5.5 times. When the draw ratio is too high, the transparency of the resulting film tends to be lowered. On the other hand, if the draw ratio is too low, the draw tension is also small, so the birefringence of the resulting film is small, and the retardation is small, such being undesirable. In the subsequent heat treatment, the treatment temperature is preferably 100 to 250 ° C, particularly preferably 180 to 245 ° C.

  In order to control the retardation of the polymer film within a specific range, the stretching ratio, the stretching temperature, and the thickness of the film can be appropriately set. For example, the higher the stretching ratio, the lower the stretching temperature, and the thicker the film, the higher the retardation. Conversely, the lower the stretching ratio, the higher the stretching temperature, and the thinner the film, the lower the retardation.

  The polymer film according to the present invention has a film surface for the purpose of improving adhesiveness, water resistance, chemical resistance, and the like with layers formed on the film such as an adhesive layer, a release layer, and an antistatic layer. Surface treatment, that is, corona discharge treatment (in air, nitrogen, carbon dioxide, etc.) or easy adhesion treatment may be performed by a known method. For the easy adhesion treatment, various known methods can be used, and a method of applying various known easy adhesives to the film after uniaxial or biaxial stretching is preferably employed.

  Although the thickness of the polymer film used for this invention is arbitrary, it is preferable that it is the range of 25-500 micrometers. Even with a film thickness of less than 25 μm, it is possible in principle to obtain a retardation of 3000 nm or more. However, in that case, the anisotropy of the mechanical properties of the film becomes remarkable, and it becomes easy to cause tearing, tearing, etc., and the practicality as an industrial material is remarkably lowered. A particularly preferable lower limit of the thickness is 35 μm. On the other hand, a film exceeding 500 μm is not very preferable because it is very rigid and the flexibility specific to the polymer film is lowered, and the practicality as an industrial material is also lowered. A particularly preferred upper limit of the film thickness is 350 μm.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and is implemented with appropriate modifications within a range that can be adapted to the gist of the present invention. These are all included in the technical scope of the present invention. In addition, the evaluation method of the physical property in the following examples is as follows.

<Retardation>
This is a parameter defined by the product (ΔN × d) of the biaxial refractive index anisotropy (ΔN = | Nx−Ny |) and the film thickness d (nm) on the film. It is a scale showing isotropic and anisotropy. The biaxial refractive index anisotropy (ΔN) is determined by the following method. Using two polarizing plates, the orientation axis direction of the film was determined, and a 4 cm × 2 cm rectangle was cut out so that the orientation axis directions were perpendicular to each other, and used as a measurement sample. For this sample, the biaxial refractive index (Nx, Ny) perpendicular to each other and the refractive index (Nz) in the thickness direction were determined by an Abbe refractometer (NAR-4T, manufactured by Atago Co., Ltd.). The absolute value of the difference (| Nx−Ny |) was defined as the anisotropy (ΔN) of the refractive index. The thickness d (nm) of the film was measured using an electric micrometer (manufactured by Fine Reef, Millitron 1245D), and the unit was converted to nm. Retardation (Re) was determined from the product (ΔN × d) of refractive index anisotropy (ΔN) and film thickness d (nm). Of the orientation axes, an axis showing a higher refractive index is defined as a slow axis.

Experimental example 1
Here, an example in which a polycarbonate film is used as the polymer film is shown.
Polycarbonate (manufactured by Aldrich) was dissolved in methylene chloride 4 times by weight to prepare a polymer solution. The polymer solution was developed into a smooth glass plate with a knife coater and allowed to stand at room temperature to dry the solvent. Thereafter, the polycarbonate sheet was peeled off from the glass plate and dried in a vacuum dryer at 90 ° C. for 24 hours. A sample was cut out in a dumbbell shape from the obtained polycarbonate sheet. The cut sheet sample was heated to about 160 ° C. using Tensilon (manufactured by Orientec) and stretched in the uniaxial direction. The oriented polycarbonate film which has a desired retardation was obtained by adjusting the draw ratio and the draw speed in that case.

  The interference color of the obtained oriented polycarbonate film was observed. As the light source, a white light emitting diode (Nichia, NSPW500CS) composed of a light emitting element in which a blue light emitting diode and a yttrium / aluminum / garnet yellow phosphor were combined was used.

  FIG. 1 shows the relationship between the retardation of the oriented polycarbonate film and the interference color actually measured in this manner.

Next, an interference color chart of the oriented polycarbonate film was created by simulation.
When a birefringent material is arranged in a diagonal position between orthogonal Nicols and a white light source is used as a backlight and light passing through the orthogonal Nicols is defined as an interference color, the light transmittance is expressed by the following equation (1). expressed.
I / I ₀ = 1/2 · sin ² (π · Re / λ) (1)
Here, I ₀ is the intensity of light incident on the crossed Nicols, I is the intensity of the light transmitted through the crossed Nicols, and Re is the retardation of the birefringent body. Thus, since the transmittance (I / I ₀ ) varies depending on the retardation and the wavelength of light, an interference color peculiar to the retardation value is observed.

However, the polymer material has a large wavelength dispersion of the refractive index (particularly, the short wavelength region of visible light), and the wavelength dispersion varies depending on the polymer material. If the birefringence has wavelength dependency, the birefringence changes for each wavelength. Therefore, when the above formula (1) is applied to the oriented polycarbonate, the following formula (2) is applied in this experimental example in consideration of the wavelength dispersion characteristic of the polycarbonate.
I / I ₀ = 1/2 · sin ² (π · f (λ) · Re / λ) (2)
Here, f (λ) is a function representing the wavelength dispersion of birefringence.

  Based on the above formula (2), a program for calculating the interference color in consideration of the birefringence wavelength dispersion of the oriented polycarbonate film was created, and an interference color chart representing the relationship between retardation and interference color was created. FIG. 2 shows an interference color chart of the oriented polycarbonate film calculated using the emission spectrum of the white light emitting diode shown in FIG. 1 and 2, it is found that the colors are the same in the actual measurement and the simulation, and the variation of the interference color is remarkably reduced when the retardation (Re) ≧ 3000 nm, and the interference color is almost constant at about Re ≧ 8000 nm. .

FIG. 3 shows a spectrum of light transmitted through crossed Nicols at Re = 8000 nm. In the figure, P (λ) is the emission spectrum of the light source (white LED), and T (λ) is the spectrum of the transmitted light. Since the envelope of the transmitted light spectrum has a shape which becomes similar to the shape of the emission spectrum of the light source, the interference color of the oriented polycarbonate film became constant, be a light emitting spectrum of effectively source It became clear. It was also confirmed that the intensity of transmitted light was ¼ of the intensity of the light source.

  FIG. 4 shows a white LED (Nichia Corporation, NSPW500CS) composed of a light-emitting element obtained by combining a blue light-emitting diode and an yttrium / aluminum / garnet-based yellow phosphor with the oriented polycarbonate film (Re = 9087 nm) obtained by the above method. Is placed on a liquid crystal display device (with a liquid crystal cell sandwiched between two polarizing plates) with a backlight source, and the screen is viewed through polarized sunglasses. Regardless of the angle formed by the polarizing plate of the liquid crystal display device and the polarized sunglasses, the display color can be seen without change, so that the created film can be used as a visibility improving device for a liquid crystal display device using a white LED as a backlight light source. confirmed.

  FIG. 5 shows the emission spectrum of a general cold cathode tube and the spectrum of light transmitted through crossed Nicols at Re = 8000 nm. In the figure, P (λ) is the emission spectrum of the light source, and T (λ) is the spectrum of the transmitted light. It was suggested that the transmitted light spectrum does not preserve the shape of the emission spectrum of the light source and exhibits a transmitted light color different from that of the light source.

  FIG. 6 shows an interference color chart of the oriented polycarbonate film calculated using the emission spectrum of the cold cathode tube shown in FIG. From the comparison between FIG. 5 and FIG. 2, it is understood that the fluctuation of the interference color with respect to retardation varies greatly depending on the spectrum of the light source, and the visibility improving effect of the present invention cannot be obtained when a cold cathode tube is used as the backlight light source. did it.

Experimental example 2
Here, an example in which an oriented polyethylene terephthalate (PET) film is used as the polymer film is shown.
1000 parts of dimethyl terephthalate, 700 parts of ethylene glycol, and 0.16 part of manganese acetate tetrahydrate were charged into a transesterification can, subjected to a transesterification reaction at 120 to 210 ° C., and produced methanol was distilled off. When the transesterification reaction was completed, 0.13 part of antimony triacid and 0.017 part of normal phosphoric acid were added, and the pressure in the system was gradually reduced to 133 Pa in 75 minutes. At the same time, the temperature was gradually raised to 280 ° C. A polycondensation reaction was carried out for 70 minutes under these conditions, and the molten polymer was extruded into water from a discharge nozzle to obtain a PET resin having an intrinsic viscosity of 0.62 dl / g.

  A PET resin having an intrinsic viscosity of 0.62 dl / g was extruded through a film-forming die onto a water-cooled rotating quench drum to prepare an unstretched film. This unstretched film was stretched 4.0 times at 100 ° C. in the width direction, heat-set at 150 ° C., and further subjected to a 3% relaxation treatment in the width direction while cooling from 130 ° C. to 100 ° C. A 38 μm oriented PET film (PET film-1) was obtained.

  Moreover, the orientation PET film (PET film-2) with a thickness of 200 micrometers was obtained by changing the thickness of an unstretched film using the method similar to PET film-1.

  An unstretched film prepared using the same method as PET film-1 is heated to 105 ° C. using a heated roll group and an infrared heater, and then longitudinally (running direction) in a roll group having a difference in peripheral speed. The film was stretched 3.4 times to obtain an oriented PET film (PET film-3) having a thickness of 700 μm.

  The properties of the oriented PET film obtained by the above method are shown in Table 1. In addition, a liquid crystal display device (two liquid crystal cells) using these films as a light source (Nichia Chemical, NSPW500CS) with a white LED composed of a blue light emitting diode and a yttrium / aluminum / garnet yellow phosphor as a combination. Placed on a liquid crystal display device having a cold cathode tube as a backlight light source (having a structure in which a liquid crystal cell is sandwiched between two polarizing plates) and passing through sunglasses. Table 2 shows the situation when viewing the screen.

  From the above results, the produced film has a visibility improvement effect when a white LED is used as a backlight light source, but has a visibility improvement effect when a cold cathode tube is used as a backlight light source. I understood that I could not get it.

  The method for improving the visibility of a liquid crystal display device according to the present invention can be applied to a liquid crystal display device used outdoors, such as an instrument panel such as a car, a ship or an airplane, an in-vehicle car navigation system, a digital camera, a mobile device such as a mobile phone or a personal computer, or a building. It can be suitably used for digital signage used in supermarkets.

Claims (7)

In a liquid crystal display device having at least a backlight source, a liquid crystal cell, and a polarizing plate disposed on the viewing side of the liquid crystal cell,
While using a light source having a continuous emission spectrum as a backlight light source,
A method for improving the visibility of a liquid crystal display device, comprising using a polymer film having a retardation of 3000 to 30000 nm (excluding an oriented polyethylene terephthalate film) on the viewing side of the polarizing plate.   The method for improving the visibility of a liquid crystal display device according to claim 1, wherein the polymer film is an oriented polyester film.   The method for improving the visibility of a liquid crystal display device according to claim 1, wherein the polymer film has a thickness of 25 to 500 μm.   A liquid crystal display device using the visibility improving method according to claim 1. In a liquid crystal display device having at least a backlight source, a liquid crystal cell, and a polarizing plate disposed on the viewing side of the liquid crystal cell,
While using a light source having a continuous emission spectrum as a backlight light source,
An oriented polyethylene terephthalate film having a retardation of 3000 to 30000 nm is arranged on the viewing side of the polarizing plate so that the angle formed by the absorption axis of the polarizing plate and the slow axis of the oriented polyethylene terephthalate film is about 45 degrees. And improving the visibility of the liquid crystal display device. 6. The method for improving the visibility of a liquid crystal display device according to claim 5, wherein the oriented polyethylene terephthalate film has a thickness of 25 to 500 [mu] m.   A liquid crystal display device using the visibility improving method according to claim 5.