JP2002105447A - Liquid crystal display - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a liquid crystal display which is capable of giving pictures with such a high grade of an image as having no occurrence of moving-picture shading and color blurring at the time of PWM-dimming a fluorescent lamp and of displaying pictures on a liquid crystal panel. SOLUTION: The liquid crystal display, displays pictures by steps of dimming a fluorescent lamp 51 having a phosphor screen emitting a red, a green and a blue light by pulse-wide modulation lighting through a lighting circuit 61, writing in picture signals 13 on a liquid crystal panel 1 and functioning the fluorescent lamp as a backlight for the liquid crystal panel, is characterized in that the fluorescent lamp has a phosphor screen emitting such a green light as the time after the light-out to reach one-tenth the light amount at the lamp- lighting time is not more than 1 millisecond.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a liquid crystal display device, and more particularly to a phosphor technology and a lighting control technology of a fluorescent lamp functioning as a backlight of a liquid crystal panel.

[0002]

2. Description of the Related Art Liquid crystal displays (hereinafter abbreviated as LCDs), which were first born in the 1960's, were initially limited to use as small still image displays, but in recent years, device technology and electronics technology have been developed. As a result, a display performance close to that of a CRT was achieved. More recently, due to improvements in manufacturing techniques and the like, they have been used as large-sized computer displays and television receivers.

As shown in FIG. 6, a conventional liquid crystal display device comprises a liquid crystal panel 1 and a driver 2 for driving the liquid crystal panel 1.
1 and 22, a video signal processing circuit 4 for processing video signals, a control circuit 3 for processing signals output to the drivers 21 and 22, a fluorescent lamp 50 serving as a backlight, and a light source for lighting the fluorescent lamp 50. It consisted of a lighting circuit 60 and a power supply 11.

With such a configuration, as shown in FIG. 7, writing of a video signal to the liquid crystal panel 1 is performed for one field period (1/60 sec. = 1 in the NTSC broadcasting standard).
(6.6 milliseconds), and the fluorescent lamp 50 is generally continuously lit by the lighting circuit 60.

[0005]

The conventional LCD has a CR
Compared to T, still image display performance (for example, viewing angle, contrast, color reproducibility, etc.) has reached the same level, but in moving image display performance such as television,
Since phenomena such as blurred display contents (hereinafter, referred to as moving image blur) and color bleeding of moving image outline occur, the performance of the CRT is not reached.

[0006] The main cause of such moving image blur is CR
Whereas T is an impulse-type image display device that emits and displays bright light instantaneously, the LCD is a hold-type image display device that emits and displays light of constant brightness during one field period. And the length of the liquid crystal response time, and the length of the response time and afterglow time of the light output in the fluorescent lamp. Therefore, the moving image display performance of LCD
In order to approach the moving image display performance, it is necessary to solve all of the above problems.

[0007] The reason why the moving image is blurred when a moving image is displayed on the hold-type image display device is said to be due to the following movement of the human eye, the time integration effect of vision, and the non-linearity of visual response to light stimulus intensity. ing. (Detail is,
"Image quality of moving image display on hold-type display" by Kurita (IEICE technical report, EID99-10,
p. p. 55-60 (1999-06)) As a method for solving the problem due to the fact that the LCD is a hold type display device, for example, Japanese Patent Application Laid-Open No. Hei 8-500
915 (Japanese Patent Application No. 7-503382). According to this method, as shown in FIG. 8A, sequential field display information of a video signal is written into a liquid crystal panel in a period sufficiently shorter than one field period.
As shown in (b), the fluorescent lamp is turned on during a part of the time interval during which the video signal is not written, and is turned off at a part of the time interval during which the video signal is written, thereby achieving the impulse driving of the CRT. The same effect is simulated in an LCD. (The lighting method of the fluorescent lamp is hereinafter referred to as pulse width modulation (PWM) dimming.) To realize such PWM dimming, the response time of the liquid crystal panel, the response of the fluorescent lamp, and the afterglow The time must be sufficiently shorter than one field period. Currently, the most widely used twisted nematic liquid crystal (TN mode liquid crystal) has a response time equivalent to one field period, and an OCB (Optically Compensated Bend) mode characterized by high-speed driving. In the liquid crystal, a response time of less than a half field (about 8 milliseconds) is achieved.

FIG. 9 shows a conventional general use of a fluorescent lamp by a PWM dimming control signal (a).
The lamp current waveform peak value (b), light output waveform (c) and drive signal waveform (d) of the OCB mode liquid crystal at the time of M dimming (focusing on a certain liquid crystal cell, the highest gradation and the lowest gradation for each field) FIG. 8 is a timing chart showing a case where a key is alternately displayed).

As shown in FIG. 9D, the drive signal waveform of the OCB mode liquid crystal responds at a high speed (3 milliseconds or less). The response and afterglow time at the light output of the lamp is long (approximately 5 milliseconds at one tenth afterglow time (time for light output to drop to 10%)). By focusing on the end point of a certain display field, the response of the liquid crystal cell to the next field ends before the light of the illuminated fluorescent lamp is turned off. Will be mixed. Such a phenomenon is a cause of moving image blur when displaying a moving image.

[0010] The afterglow time of the light amount L by the phosphor used in the fluorescent lamp is different for the red, green, and blue phosphors, as shown in FIG. The afterglow time is the shortest when the light amount L'b of the blue light-emitting phosphor that has been conventionally used is the shortest (about 1/10 of the afterglow time is about 0.3 mm).
1 msec), and then the light amount L'r of the red-emitting phosphor
Is short (less than about 3 milliseconds in 1/10 afterglow time), and the amount of light L'g by the phosphor emitting green light is about 1/2 field period (about 8 milliseconds) in 1/10 afterglow time. Also.

On the other hand, as shown in FIG.
The response time of the driving voltage Vd of the mode liquid crystal is about 3 milliseconds, and the afterglow time of the light amounts L′ b and L′ r by the blue and red light emitting phosphors is equal to the response time of the driving voltage Vd of the liquid crystal panel. Although it is shorter than that, the afterglow time of the light amount L′ g by the green light emitting phosphor is equal to the driving voltage V of the liquid crystal panel.
It is more than twice as long as the response time of d.

Therefore, at the end of a certain display field, even after the response of the liquid crystal to the next field has ended, only afterglow by the green light-emitting phosphor is present. When a black moving image window 15 is displayed on a white background 14 as shown in FIG. 11, for example, as shown in FIG.
Will be accompanied. In particular, green has a high visibility and is easily noticeable, greatly deteriorating the display quality of moving images.

Accordingly, the present invention has been made to solve the above problems, and an object of the present invention is to cause blurring of moving images and color bleeding when a fluorescent lamp is subjected to PWM dimming and an image is displayed on a liquid crystal panel. It is an object of the present invention to provide a liquid crystal display device capable of obtaining an image with high display quality without any problem.

[0014]

In order to achieve the above object, a first liquid crystal display device according to the present invention has a red, green,
A fluorescent lamp having a blue light-emitting phosphor film is pulse-modulated and lit by a lighting circuit to perform dimming, write a video signal to a liquid crystal panel, and display an image by making the fluorescent lamp function as a backlight of the liquid crystal panel. In a liquid crystal display device, the fluorescent lamp has a green light-emitting phosphor film in which a time when a light amount becomes 1/10 of a lighting time after being turned off is 1 millisecond or less. In this case, the phosphor film for emitting green light is preferably made of europium-activated strontium aluminate.

According to the first liquid crystal display device, the fluorescent lamp as a backlight is provided with a time in which the amount of light is reduced to one-tenth of that when the fluorescent lamp is turned on after the light is turned off, that is, a one-tenth afterglow time is 1 millisecond or less. Green light-emitting phosphor film, especially SrAl which is europium-activated strontium aluminate
Using a phosphor film made of ₂ O ₄ : Eu ²⁺ , for example, Y ₂ O ₃ :
A red light-emitting phosphor film made of Eu ³⁺ (1/10 afterglow time is 3 milliseconds or less) is also used, for example, BaMgAl ₁₀ O
₁₇ : Eu ²⁺ (1/10 afterglow time is 0.1 ms or less)
The use of a blue light-emitting phosphor film made of
Since the 1/0 persistence time is shorter than the response time to the off state of the driving voltage of the liquid crystal panel for all the red, blue and green phosphor films, the moving image is displayed on the liquid crystal panel. In this case, the so-called moving image blur, in which the images in the two display fields are mixed and the display content is blurred, and the color bleeding of the contour in the moving direction of the moving image due to only a long afterglow time due to the green light-emitting phosphor film. , The effect on display can be improved, and an image with high display quality can be obtained.

In order to achieve the above object, a second liquid crystal display device according to the present invention provides dimming by illuminating a fluorescent lamp having red, green, and blue light emitting phosphor films by pulse width modulation using a lighting circuit. A liquid crystal display device that displays an image by writing an image signal to a liquid crystal panel and causing the fluorescent lamp to function as a backlight of the liquid crystal panel, wherein a lighting circuit includes a lamp current flowing through the fluorescent lamp during a pulse width modulation lighting period. The current dimming control is characterized in that the peak value of the current attenuates at a predetermined degree from the lighting start time to the lighting end time.

According to the second liquid crystal display device, the afterglow time of the fluorescent lamp can be shortened by the lighting circuit as compared with the related art, so that when a moving image is displayed on the liquid crystal panel, moving image blur or color bleeding occurs. To improve the display impact of
An image with high display quality can be obtained.

In order to achieve the above object, a third liquid crystal display device according to the present invention comprises three red, green, and blue monochromatic fluorescent lamps each having a red, green, and blue light emitting phosphor film formed thereon. A liquid crystal that displays images by using a single lighting circuit to independently illuminate with pulse width modulation, dimming, writing video signals to the liquid crystal panel, and using the red, green, and blue monochromatic fluorescent lamps as the backlight of the liquid crystal panel A display device, wherein the three lighting circuits are:
Current dimming so that the peak value of the lamp current flowing through the fluorescent lamp during the pulse width modulation lighting period is attenuated to a different degree from the lighting start time to the lighting end time and according to the red, green, and blue monochromatic fluorescent lamps. It is characterized by controlling.

According to the third liquid crystal display device, each lighting circuit can reduce the afterglow time of the red, green, and blue monochromatic fluorescent lamps and the difference in the afterglow time between them. When a moving image is displayed on the panel, it is possible to improve the effect of moving image blur and color bleeding on the display, and obtain an image with high display quality.

[0020]

Embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment) FIG. 1 is a block diagram showing the configuration of a liquid crystal display device according to a first embodiment of the present invention.

In FIG. 1, the liquid crystal display device according to the present embodiment includes a fluorescent lamp 51, a lighting circuit 61 for controlling the fluorescent lamp 51 to perform PWM lighting control, and a liquid crystal panel 1.
Drivers 21 and 22 for driving the liquid crystal panel 1, a control circuit 3 for processing signals sent to the drivers 21 and 22, a video signal processing circuit 4 for receiving the video signal 13 and performing signal processing such as pixel conversion, and a power supply 11 It is composed of

The fluorescent lamp 51 is a green light-emitting phosphor.
Europium activated strontium aluminate phosphor,
For example, SrAl_TwoO_Four: Eu²⁺, Red light emitting phosphor
For example, Y_TwoO_Three: Eu³⁺, And blue light emitting phosphor
BaMgAl_TenO₁₇: Eu ²⁺Through the phosphor film mixed with
Provided on the inner wall surface of the light-tight container.

The lighting circuit 61 receives the PWM dimming control signal 12 output from the video signal processing circuit 4,
A PWM oscillation circuit 90 for generating a WM oscillation signal;
DC-DC that receives an oscillation signal and converts the power supply voltage level
It comprises a converter 10, an inverter 8 for generating a sine wave from the voltage received from the DC-DC converter 10, and a transformer 7 for boosting the sine wave.

In such a configuration, when the power supply 11 is turned on and the PWM dimming control signal 12 and the video signal 13 are input, the fluorescent lamp 51 is subjected to PWM dimming by the lighting circuit 61 to be lit, and the liquid crystal panel 1 is turned on. The image is displayed.

Here, as shown in FIG. 2, the afterglow time of the light quantity L'g by the conventional green light emitting phosphor is about 8 milliseconds, which is about 3 milliseconds, which is the response time of the driving voltage of the liquid crystal panel 1. On the other hand, the afterglow time of the light amount Lg by the green light-emitting phosphor in this embodiment is 1 millisecond or less in 1/10 afterglow time, so that a moving image is displayed on the liquid crystal panel 1. In this case, it is possible to prevent the occurrence of blurred moving images and color bleeding of the outline.

Alternatively, even if moving image blurring or outline color bleeding occurs, the time interval is short enough to be invisible and does not affect the deterioration of moving image quality.

(Second Embodiment) FIG. 3 is a block diagram showing the configuration of a liquid crystal display according to a second embodiment of the present invention. Note that the present embodiment differs from the first embodiment in that the fluorescent material of the fluorescent lamp has a conventional configuration and the method of controlling the lamp current by the lighting circuit is changed.

In FIG. 3, the liquid crystal display device according to the present embodiment includes a fluorescent lamp 50 and a lighting circuit 62 for controlling the PWM of the fluorescent lamp 50 and the current attenuation to control the light.
A liquid crystal panel 1, drivers 21 and 22 for driving the liquid crystal panel 1, a control circuit 3 for processing signals sent to the drivers 21 and 22, and a video signal for receiving the video signal 13 and performing signal processing such as pixel conversion. It comprises a processing circuit 4 and a power supply 11.

The fluorescent lamp 50 has a phosphor film on which the same red, green, and blue light emitting phosphors are mixed as in the prior art, on the inner wall surface of the translucent airtight container.

The lighting circuit 62 receives the PWM dimming control signal 12 output from the video signal processing circuit 4 and
A PWM oscillation / current control circuit 91 for generating a WM oscillation / current control signal, a DC-DC converter 10 for receiving the PWM oscillation / current control signal and converting a power supply voltage level;
An inverter 8 generates a sine wave from the voltage received from the C-DC converter 10 and a transformer 7 boosts the sine wave.

In such a configuration, when the power supply 11 is turned on and the PWM dimming control signal 12 and the video signal 13 are input, the fluorescent lamp 50 is illuminated by the PWM dimming by the lighting circuit 62 and the liquid crystal panel 1 is turned on. The image is displayed.

Here, the PWM oscillation / current control circuit 91
As shown in the timing chart of FIG. 4, the peak value of the lamp current (FIG. 4B) is the PWM dimming control signal 1
2 (FIG. 4A) suddenly rises immediately after the logic "H" is activated, and the PWM dimming control signal 12
Performs a current dimming control such that the current attenuates at a predetermined degree during the logic “H”.

By controlling the lighting of the fluorescent lamp 50 in this manner, the peak value of the lamp current is low immediately before the PWM dimming control signal 12 becomes the inactive state of the logic “L”, and the fluorescent lamp 50 is turned off immediately before being turned off. Therefore, in the light emission amount of the fluorescent lamp 50 (FIG. 4C), the afterglow time after the PWM dimming control signal 12 becomes inactive becomes short. As a result, when a moving image is displayed on the liquid crystal panel 1, the afterglow time of the PWM dimming is shortened and improved, so that the moving image blur can be improved and a high-quality image can be obtained.

(Third Embodiment) FIG. 5 is a block diagram showing the structure of a liquid crystal display according to a third embodiment of the present invention.

In FIG. 5, the liquid crystal display device according to the present embodiment has three types of monochromatic fluorescent lamps 5r, 5g, each having a red, green, and blue light emitting phosphor film formed on the inner wall surface of a light-transmitting airtight container. 5b and three types of monochromatic fluorescent lamps 5
Lighting circuit 6 for lighting each of r, 5g and 5b independently
, 6g, and 6b, a backlight, a liquid crystal panel 1, and drivers 21 and 2 for driving the liquid crystal panel 1.
2. a control circuit 3 for processing signals to be sent to the drivers 21 and 22;
It comprises a video signal processing circuit 4 for generating a PWM dimming control signal 12 to be sent to each of the lighting circuits 6r, 6g, 6b, and a power supply 11.

Each of the lighting circuits 6r, 6g, 6b has the same configuration as the lighting circuit 62 in the second embodiment.
The oscillation / current control circuits 9r, 9g, and 9b cause the peak value of the lamp current to rise sharply immediately after the PWM dimming control signal 12 becomes the logic "H" active state.
To perform current dimming control such that the WM dimming control signal 12 attenuates at a predetermined rate during the logic “H”, P
The afterglow time of WM dimming is shortened and improved.

Further, each PWM oscillation / current control circuit 9
r, 9g, and 9b are configured such that the degree of attenuating the peak value of the lamp current differs for each lighting circuit 6r, 6g, and 6b. Thereby, the shift of the afterglow time between the red, green, and blue monochromatic fluorescent lamps 5r, 5g, and 5b can be improved. For example, as shown in FIG. 10A, the afterglow time of the light amount L′ g by the green light-emitting phosphor is longer than that of the blue light-emitting phosphor by 7 ms or more. In the lighting period of the PWM dimming, the afterglow time of the green single-color fluorescent lamp 5g is set by setting the lamp current of the green single-color fluorescent lamp 5g to be larger than the lamp current of the blue single-color fluorescent lamp 5b. And the difference in apparent afterglow time is reduced. As a result, when a moving image is displayed on the liquid crystal panel 1, the red, green, and blue monochromatic fluorescent lamps 5r, 5g, 5b
Since the difference in the afterglow time for each image is small and improved, the color blur of the outline of the moving image is improved.

[0039]

As described above, according to the liquid crystal display device of the present invention, when the fluorescent lamp is subjected to PWM dimming and an image is displayed on the liquid crystal panel, moving image blur and color bleeding do not occur. Thus, an image with high display quality can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a liquid crystal display device according to a first embodiment of the present invention.

FIG. 2 is a graph showing a persistence time characteristic of each fluorescent light emitting phosphor of the fluorescent lamp according to the first embodiment of the present invention with a solid line, and a persistence time characteristic of a conventional fluorescent lamp with a green light emitting phosphor as a broken line.

FIG. 3 is a block diagram showing a configuration of a liquid crystal display device according to a second embodiment of the present invention.

4 is a timing chart showing a PWM dimming control signal (a), a peak value of a lamp current (b), and a light emission amount (c) of a fluorescent lamp of the liquid crystal display device shown in FIG.

FIG. 5 is a block diagram illustrating a configuration of a liquid crystal display device according to a third embodiment of the present invention.

FIG. 6 is a block diagram illustrating a configuration of a conventional liquid crystal display device.

FIG. 7 is a diagram showing a timing of writing a video signal to a liquid crystal panel (a) and a turning on / off state of a fluorescent lamp (b) in a liquid crystal display device according to one conventional example.

FIG. 8 is a diagram showing a timing of writing a video signal to a liquid crystal panel (a) and a turning on / off state (b) of a fluorescent lamp in a liquid crystal display device according to another conventional example.

FIG. 9 shows a PWM dimming control signal of a liquid crystal display device according to another conventional example (a), a peak value of a lamp current (b),
Timing chart showing light output (c) of a fluorescent lamp and drive voltage (d) of an OCB mode liquid crystal panel

FIG. 10 shows the afterglow time characteristics (a) and OCB of each fluorescent light emitting phosphor of a fluorescent lamp in a conventional liquid crystal display device.
FIG. 7 is a diagram showing response time characteristics (b) of the drive voltage of the mode liquid crystal panel.

FIG. 11 is a diagram showing an example in which a moving image (black window) is displayed on a liquid crystal panel in a conventional liquid crystal display device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Liquid crystal panel 21, 22 Driver 3 Control circuit 4 Video signal processing circuit 50, 51 Fluorescent lamp 5r Red monochromatic fluorescent lamp 5g Green monochromatic fluorescent lamp 5b Blue monochromatic fluorescent lamp 60, 61, 62, 6r, 6g, 6b Lighting circuit 7 Transformer Reference Signs List 8 inverter 90 PWM oscillation circuit 91, 9r, 9g, 9b PWM oscillation / current control circuit 10 DC-DC converter 11 power supply 12 PWM dimming control signal 13 video signal 14 white background 15 black window 16 outline color blur

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G09G 3/36 G09G 3/36 5G435 H01J 61/44 H01J 61/44 N (72) Inventor Hirofumi Yamashita Osaka 1006 Kadoma, Kadoma City Matsushita Electric Industrial Co., Ltd. JJ02 JJ04 JJ05 KK43 5G435 AA04 BB03 BB12 BB15 EE26 GG26 GG27

Claims (4)

[Claims] 1. A fluorescent lamp having a phosphor film for emitting red, green, and blue light is subjected to pulse width modulation lighting by a lighting circuit to perform dimming, and a video signal is written to a liquid crystal panel. A liquid crystal display device that displays an image by functioning as a backlight, wherein the fluorescent lamp emits the green light having a time that is 1/10 or less of a time when the light is turned on after being turned off. A liquid crystal display device having a body film. 2. The liquid crystal display device according to claim 1, wherein the green light emitting phosphor film is made of europium-activated strontium aluminate. 3. A fluorescent lamp having a phosphor film for emitting red, green and blue light is subjected to pulse width modulation lighting by a lighting circuit to perform dimming, and a video signal is written to a liquid crystal panel. A liquid crystal display device that displays an image by functioning as a backlight, wherein the lighting circuit has a peak value of a lamp current flowing through the fluorescent lamp during a pulse width modulation lighting period, from a lighting start time to a lighting end time. A current dimming control such that the current dimming is attenuated to a predetermined degree. 4. A monochromatic fluorescent lamp of red, green, and blue having a phosphor film for emitting red, green, and blue light, respectively, is subjected to pulse-width modulation lighting by three lighting circuits independently to perform dimming. A liquid crystal display device that displays an image by writing a video signal to a panel and causing the red, green, and blue monochromatic fluorescent lamps to function as a backlight of the liquid crystal panel, wherein the three lighting circuits include pulse width modulation. Current dimming control so that the peak value of the lamp current flowing through the fluorescent lamp during the lighting period is attenuated to a different degree from the lighting start time to the lighting end time and according to the red, green, and blue monochromatic fluorescent lamps. A liquid crystal display device comprising: