JP2008191248A - Backlight device for liquid crystal display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a back light device for a liquid crystal display capable of suppressing the occurrence of a color irregularity. <P>SOLUTION: The back light device for the liquid crystal display causes each of LEDs 3 to 5 to flicker in a specified light emission period in each one field period, puts off all the LEDs 3 to 5 in the remaining light out period and controls the integration value of the respective lighting times of the LEDs 3 to 5 in the light emission period. Consequently, the time when the other LEDs are put out while the certain LED is lighted can be shortened and the occurrence of the color breakup can be suppressed. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a backlight device for a liquid crystal display, and more particularly to a backlight device for a liquid crystal display that provides transmitted light to a liquid crystal panel.

  In recent years, a liquid crystal display (LCD) has become widespread as a display for image display, in addition to a display using a conventional cathode ray tube (CRT). In the liquid crystal display, a backlight light source is installed on the back side of a transmissive liquid crystal panel, and light emitted from the backlight light source is transmitted through the liquid crystal panel to display an image.

  As one of image display methods, an interlace method is known. The interlace method is a method for displaying one image (one frame) divided into a plurality of field periods. In the NTSC (National Television Standards Committee) system used in Japanese television broadcasting, half the information of one image is updated in one field period.

  Here, unlike a display using a CRT, the LCD employs a hold-type driving method in which the luminance of a displayed image is maintained substantially constant over one field period. In this type of display, the image quality of the moving image deteriorates, such as tailing, blurring, blurring, and blurring, due to the effect of the retinal afterimage. That is, in this type of display, images are always displayed during one field period, so images of different field periods are superimposed on the retina of the observer. For this reason, the observer feels tailing, that is, blurring and rounding of the outline.

  As a means for improving the tailing of the hold type driving display, there is a so-called impulse lighting method. In the impulse-type lighting system, as shown in FIGS. 9A and 9B, a light emission period in which the backlight light source is turned on and a light-out period in which the backlight light source is turned off are provided in each one field period. It is. The vertical synchronization signal is a signal that becomes “L” level in a pulse manner for each field period T. The light source is turned off (OFF) in response to the falling edge of the vertical synchronization signal, and turned on (ON) after a predetermined time. Thereby, the influence of the retinal afterimage is reduced, and the tailing of the display of the hold-type drive system is improved.

  In order to realize such an impulse-type lighting system, it is necessary to blink the backlight light source in a short period as described above. There is an LED (Light Emitting Diode) as a light source that can easily blink in a short period of time. Currently, CCFL (Cold Cathode Flurescent Lamp) is mainly used as a backlight light source, but the response speed of the LED is faster than the response speed of CCFL. Also, by using R (red), G (green), and B (blue) LEDs, very good color reproducibility can be obtained as compared with CCFL. For this reason, a technique for performing impulse driving using R, G, B LEDs as a backlight light source has been studied.

In Patent Document 1, R, G, and B LEDs of a backlight source are turned on simultaneously, R, G, and B are optically mixed to achieve white, and further, these LEDs are driven by an impulse lighting method. This improves the tailing of the display of the hold-type drive system. Further, in order to eliminate a white balance shift caused by variations in luminance of R, G, and B LEDs, the luminance of the LEDs of each color is detected by a luminance sensor, and the luminance of the LEDs of each color is controlled.
JP-A-2005-208486

  Patent Document 1 does not disclose a specific method for controlling the luminance of the LED. However, since the amount of current and the luminance are not proportional to the LED, the luminance of the LED is generally controlled by changing the lighting time.

  FIGS. 10A to 10D are time charts showing a method of controlling the luminance of the LED by controlling the lighting time of the LED. FIG. 10A shows the waveform of the vertical synchronizing signal, and FIGS. 10B to 10D show the operation of the R, G, and B LEDs, respectively. Each LED is driven by an impulse lighting system, and has an extinguishing period and a lighting period within one field period. Here, when the required lighting times of the red LED, the green LED, and the blue LED are expressed by α, β, and γ, by setting the lighting time of each LED so as to satisfy the relationship of α> β> γ, a desired white can be obtained. You can get a balance.

  However, when the time difference Δt between the lighting period of the red LED and the lighting period of another LED becomes large, optical color mixing does not work well, and a so-called color breaking phenomenon in which a specific color is recognized occurs. It will occur. Although the degree of color breakage that can be recognized varies depending on the observer, once the color breakup is recognized, it becomes very unpleasant.

  Therefore, a main object of the present invention is to provide a backlight device for a liquid crystal display capable of suppressing the occurrence of a color break phenomenon.

  A backlight device for a liquid crystal display according to the present invention is a backlight device for a liquid crystal display that provides transmitted light to a liquid crystal panel, and includes a backlight source including a plurality of light sources that emit light of different colors, and each light emission A luminance sensor for detecting the luminance of the light source, and the light emission source is blinked during a predetermined first period of each one field period, and a plurality of light emission sources are turned off during the second period. And a controller for controlling the integrated value of the lighting time of each light source in the first period so that the luminance of each light source emitted matches the predetermined target value for that light source. It is.

  Further, another backlight device for a liquid crystal display according to the present invention is a backlight device for a liquid crystal display that gives transmitted light to a liquid crystal panel, and includes a plurality of light emitting sources that emit light of different colors. And a luminance sensor for detecting the luminance of each light source, and in the first period of each one field period, the light source having the longest required lighting time among a plurality of light sources is kept on and the other light sources The light source is blinked, the plurality of light emission sources are turned off in the second period, and each light emission is set so that the luminance of each light emission source detected by the luminance sensor matches a target value predetermined for the light emission source. And a control unit that controls an integrated value of the lighting time of the source.

  Preferably, the control unit turns on the plurality of light emission sources blinking in the first period at different timings.

Preferably, the plurality of light emitting sources respectively emit red, blue, and green light.
Also preferably, each light source includes a light emitting diode.

  In the backlight device for a liquid crystal display according to the present invention, each of the light emitting sources blinks during a predetermined first period of each one field period, and the plurality of light emitting sources are extinguished during the second period. The integrated value of the lighting time of each light emitting source in the first period is controlled so that the luminance of each light emitting source detected by the above matches the target value predetermined for the light emitting source. Therefore, it is possible to reduce the time during which another light source is turned off while a certain light source is turned on, and the occurrence of the color breakup phenomenon can be suppressed.

  In another backlight device for a liquid crystal display according to the present invention, the light emitting source having the longest required lighting time among the plurality of light emitting sources continues to be lit during the first period of each one field period, and Each light source is blinked, and the plurality of light sources are turned off in the second period, so that the luminance of each light source detected by the luminance sensor matches a target value predetermined for the light source. The integrated value of the lighting time of each light source is controlled. Therefore, it is possible to reduce the time during which another light source is turned off while a certain light source is turned on, and the occurrence of the color breakup phenomenon can be suppressed.

[Embodiment 1]
FIG. 1 is a diagram showing a main part of a backlight device 1 for a liquid crystal display according to Embodiment 1 of the present invention, and shows a state where a liquid crystal panel and optical sheets are removed from the liquid crystal display. The backlight device 1 is a so-called direct-type backlight device that is disposed directly below (back side) of a liquid crystal panel and applies transmitted light to the liquid crystal panel.

  The backlight device 1 includes a plurality of LED light sources 2 arranged in a matrix. The plurality of LED light sources 2 constitutes a backlight light source. Each LED light source 2 includes a red LED 3, a green LED 4, and a blue LED 5. The LEDs 3 to 5 emit R, G, and B light, respectively. A sensor unit 6 is provided at a predetermined position in the gap between the plurality of LED light sources 2. The sensor unit 6 receives the reflected light from the liquid crystal panel and detects the luminance of each of the LEDs 3 to 5.

  FIG. 1 shows an example in which the present invention is applied to a direct type backlight device 1 in which the LED light source 2 is disposed directly below the liquid crystal panel. However, the present invention is not limited to this. The present invention can also be applied to a so-called side edge type backlight device in which the LED light sources 2 are arranged at the upper and lower ends or the left and right ends of the liquid crystal panel, and the light emitted from the LED light sources 2 is spread over the entire screen by a light guide plate or the like.

  FIG. 2 is a block diagram showing a configuration of the backlight device 1. In FIG. 2, the sensor unit 6 includes luminance sensors 7 to 9. Each of the luminance sensors 7 to 9 includes a photo sensor and a color filter formed thereon. The brightness sensors 7 to 9 detect the brightness of the LEDs 3 to 5 and output a signal indicating the detected value to the control unit 10.

  The control unit 10 controls the entire backlight device 1 and includes a CPU or a DSP. Based on the vertical synchronization signal and the output signals of the luminance sensors 7 to 9, the control unit 10 controls each of the LEDs 3 to 5 via the driver unit 11 so that the luminance of each of the LEDs 3 to 5 matches the target luminance. Control lighting time. The driver unit 11 includes drivers 12 to 14 that control the LEDs 3 to 5 in response to output signals of the control unit 10, respectively.

  In addition, although the sensor part 6 is divided | segmented into the three sensors 7-9 for convenience of explanation, the three sensors 7-9 may be accommodated in the same package. Similarly, the brightness of each of the LEDs 3 to 5 may be detected by using only a photosensor without providing a color filter and utilizing a shift in the light emission time of the LEDs 3 to 5.

  FIG. 3 is a block diagram illustrating a configuration of the control unit 1. In FIG. 3, the control unit 1 includes memory units 21 to 23, differential units 24 to 26, gain units 27 to 29, integration units 30 to 32, and PWM (Pulse Width Modulation) value calculation units 33 to 33. 35. The target luminances of the LEDs 3 to 5 are stored in advance in the memory units 21 to 23, respectively. The control unit 10 performs feedback control so that the luminances of the LEDs 3 to 5 detected by the sensors 7 to 9 become the target luminances stored in the memory units 21 to 23, respectively.

  The differential units 24 to 26 calculate an error between the target luminance stored in the memory units 21 to 23 and the luminance detected by the sensors 7 to 9, respectively. The gain units 27 to 29 multiply the outputs of the differential units 24 to 26 by a predetermined gain, respectively. These gain sections 27 to 29 set the control band of the feedback control system. The integrating units 30 to 32 integrate the R, G, and B luminance errors multiplied by the predetermined gains by the gain units 27 to 29, respectively, and control the luminance of the LEDs 3 to 5 based on the integrated value. The brightness of each LED matches the target brightness by feedback control so that the integral value converges to a constant value.

  The PWM value calculation units 33 to 35 convert the integration values that are the outputs of the integration units 30 to 32, respectively, into pulse widths. The LEDs 3 to 5 are driven according to the PWM values calculated by the PWM value calculation units 33 to 35, respectively. Further, the vertical synchronization signal is input to the PWM value calculation units 33 to 35, and the PWM value is updated according to the vertical synchronization signal. That is, the PWM value is updated for each field, and the LEDs are always driven so that the luminance of each color LED matches the target luminance.

  4A to 4D are time charts showing the operation of the backlight device 1. As shown in FIG. 4A, the vertical synchronization signal is pulsed to “L” level for each field period. In the case of the NTSC system, the frequency of the vertical synchronization signal is 60 Hz, and one field period is 16.7 msec. 4B to 4D show the light emission states of the red LED 3, the green LED 4, and the blue LED 5, respectively. In the ON period in the figure, current is supplied to the LED and the LED is turned on. In the OFF period in the figure, the current to the LED is interrupted and the LED is turned off.

  A light emission period and a light extinction period are provided within one field period, and during the light emission period, each of the LEDs 3 to 5 is PWM driven, and each of the LEDs 3 to 5 blinks. For example, if the light emission period in one field period is set to 4 msec, and the LEDs 3 to 5 need to be lit by α, β, and γ (msec), respectively, in order to obtain a desired white balance, the control unit No. 10 controls the lighting pulse width of each of the LEDs 3 to 5 so that the integrated values of the lighting times of the LEDs 3 to 5 become α, β, and γ (msec) in the light emission period (4 msec), respectively. Moreover, the control part 1 makes each LED3-5 light in multiple times in each light emission period. Thereby, each of LED3-5 light-emits uniformly in the light emission period.

  4A to 4D, the light emission period comes in the latter half of each field period. This assumes that the image on the liquid crystal panel is switched in the first half of the field period. It depends. That is, by providing the light emission period in the latter half of the field period, it means that the LEDs 3 to 5 are caused to emit light after the image of the liquid crystal panel is completely switched.

  Here, since the lighting times of the LEDs 3 to 5 are controlled to be α, β, and γ (msec), respectively, the integrated values of the lighting times of the LEDs 3 to 5 in the light emission period are shown in FIGS. As in the conventional example shown in Fig. 1, the difference in the lighting times of the LEDs 3 to 5 per lighting operation is shortened, although it differs for each color. For example, in FIGS. 4B to 4D, since the LEDs 3 to 5 are blinked four times during the light emission period, the deviation of the lighting time per lighting time is Δt in the conventional example. In the first embodiment, Δt / 4. In general, the shorter the difference in the lighting times of the LEDs 3 to 5 per lighting, the more difficult it is to recognize the color breaks, so that the color breaks felt by the observer by using the lighting method of Embodiment 1 are as follows. It is greatly reduced.

  As described above, according to the first embodiment, the tailing inherent to the display of the hold-type drive can be improved by providing the light emission period and the light extinction period in one field period, and the LED 3 in the light emission period can be improved. By causing each of ˜5 to blink a plurality of times, color breakup can be made difficult to be observed.

[Embodiment 2]
FIG. 5 is a block diagram showing a configuration of control unit 40 of the backlight device for a liquid crystal display according to the second embodiment of the present invention, and is a diagram to be compared with FIG. Referring to FIG. 5, this control unit 40 is different from control unit 10 in FIG. 3 in that PWM value calculation units 33 to 35 are replaced with PWM calculation unit 41.

  The PWM value calculation unit 41 receives the vertical synchronization signal and the output signal of the integration units 30 to 32, and the LED 3 based on the output signal of the integration units 30 to 32 every time the vertical synchronization signal becomes a pulse “L” level. The required lighting time for each of ˜5 is calculated. Then, the LED that requires the longest lighting time among the LEDs 3 to 5 is selected, and the required lighting time of the LED is set as the light emission period in one field period. Since the required lighting time of each other LED is shorter than the set light emission period, each other LED is blinked within the set light emission period. That is, a desired white balance is obtained by continuously lighting the LED that needs to be lit for the longest time during the light emission period within one field period and blinking the other LEDs during the light emission period.

  FIG. 6 is a flowchart showing the operation of the PWM value calculation unit 41. In FIG. 6, the PWM value calculation unit 41 waits until the vertical synchronization signal becomes the activation level “L” level (step S <b> 1), and when the vertical synchronization signal becomes the activation level, the outputs of the integration units 30 to 32. A signal is taken in and the required lighting time of each of the LEDs 3 to 5 is calculated (step S2). Next, the LED having the longest required lighting time is selected based on the calculation result (step S3), and the longest lighting time is set as the light emission period of the corresponding field (step S4). Next, the ratio of the required lighting time of each of the other LEDs to the light emission period set in step S4 is calculated (step S5), and the other LEDs are PWM-driven within the light emission period so that the calculated ratio is obtained. The LED that needs to emit light multiple times and needs to be lit for the longest time continues to emit light during the light emission period (step S6). When the light emission period ends, all the LEDs 3 to 5 are turned off, and the process returns to step S1 (step S7). In such a procedure, the PWM value calculation unit 41 calculates the lighting time of each of the LEDs 3 to 5 and drives each of the LEDs 3 to 5.

  7A to 7D are time charts showing the operation of the backlight device. 7A shows the waveform of the vertical synchronizing signal, and FIGS. 7B to 7D show the light emission states of the red LED 3, the green LED 4, and the blue LED 5 in one field period, respectively. Here, the required lighting time of the red LED 3 is the longest, and the required lighting times of the LEDs 3 to 5 are α, β, and γ (msec), respectively. Therefore, α> β, γ. Of course, α is equal to or shorter than one field period T (16.7 msec in the NTSC system).

  7A to 7D, when the vertical synchronization signal is lowered to the “L” level of the activation level, the lighting times of the respective LEDs 3 to 5 necessary for obtaining a desired white balance are shown. Calculated. In this case, since the lighting time α of the red LED 3 is the maximum, the light emission period of the backlight device within the corresponding field period is set to α. When the relative lighting time of each of the other two color LEDs 4 and 5 is calculated, the required lighting time of the green LED 4 is β, so the ratio of the lighting time in the light emission period α is β / α. Similarly, the lighting time ratio of the blue LED 5 is γ / α. Then, since the period until the image of the liquid crystal panel is completely switched is set as the extinguishing period, the lighting of the LEDs 3 to 5 is started after a lapse of a certain period after the vertical synchronization signal becomes the activation level.

  Here, the red LED 3 is driven so as to continue to be lit only during the period α. The green LED 4 may be lit only during the period β / α of the lighting period α. FIG. 7C shows an example in which the green LED 4 is turned on twice in the light emission period. For this reason, the green LED 4 is first turned on for the period β / 2, turned off for the period (α−β) / 2, then turned on again for the period β / 2, and turned off for the period (α−β) / 2. To be driven. As a result, the green LED 4 is lit during the β / α period of the light emission period α. Similarly, the blue LED 5 is first turned on for a period of γ / 2, turned off for a period of (α−γ) / 2, then turned on again for a period of γ / 2, and turned off for a period of (α−γ) / 2. Thus, the light was lit during the period β / α of the light emission period α. In this way, the lighting times of the LEDs 3 to 5 are α, β, and γ, respectively, and a desired white balance is obtained.

  By driving the LEDs 3 to 5 in this way, an impulse type lighting system is possible. In addition, by blinking each of the LEDs 4 and 5 a plurality of times during the light emission period, the difference in the light emission period per time becomes small, so that it is difficult for the observer to perceive color breakup. Since the light emission period in one field period is equal to the light emission time of the LED that needs to emit light for the longest time, it can be set to the shortest light emission period in realizing a desired white balance. In other words, this corresponds to performing impulse-type lighting with the shortest light emission period (the longest extinction period) in one field. Therefore, it is possible to realize a driving method that can most effectively improve the trailing of the hold type display driving.

  In the second embodiment, the number of blinks in the light emission period of the green LED 4 and the blue LED 5 is two. However, the blink is not limited to two, and the more the number of blinks, the more the color breakup phenomenon is recognized. It becomes difficult.

[Embodiment 3]
FIG. 8 is a time chart showing the operation of the backlight device for a liquid crystal display according to the third embodiment of the present invention, and is a diagram compared with FIG. The configuration of the backlight device is the same as that of the second embodiment.

  Here, for convenience of explanation, it is assumed that the required lighting time is longer in the order of the red LED 3, the green LED 4, and the blue LED 5. That is, the necessary lighting times α, β, and γ (msec) of the LEDs 3 to 5 are in a relationship of α> β> γ.

  8A to 8D, as described above, since the necessary lighting time α of the red LED 3 is the longest, α is set as the light emission period in the corresponding field period. The green LED 4 and the blue LED 5 are blinked a plurality of times during this light emission period, but the timing at which the green LED 4 and the blue LED 5 start lighting is shifted.

  Specifically, the red LED 3 and the green LED 4 are turned on at the start of the light emission period (time t1). The green LED 4 is turned on for the period β / 2, turned off for the period (α−β) / 2, and then turned on again for the period β / 2. On the other hand, the blue LED 5 does not start lighting at the time t1, starts lighting at the time t2 after (α−β) / 2 from the time t1, turns on for a period of γ / 2, and (α−γ) / 2. After the light is turned off, the light is turned on again for γ / 2.

  In the third embodiment, the lighting start time of the blue LED 5 is shifted. This is because there is a relation of β> γ. That is, the lighting start time of the LED with the shortest lighting time is shifted. As a result, the light emission times of the LEDs 3 to 5 become α, β, and γ, respectively, and a desired white balance is maintained.

  In addition, by shifting the lighting start time of the LED having the shortest lighting time in this way, the period in which the LEDs 4 and 5 other than the red LED 3 are simultaneously turned off, that is, the period in which only the red LED 3 is lit is shortened. be able to. For this reason, it is possible to make it difficult to recognize the color breakup phenomenon.

  Also in this Embodiment 3, the number of blinks in the light emission period of the green LED 4 and the blue LED 5 is set to two, but the blink is not limited to two. It becomes difficult to do.

  It should be considered that the disclosed embodiments are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is a figure which shows the principal part of the backlight apparatus for liquid crystal displays by Embodiment 1 of this invention. It is a block diagram which shows the structure of the backlight apparatus for liquid crystal displays shown in FIG. It is a block diagram which shows the structure of the control part shown in FIG. It is a time chart figure which shows operation | movement of the backlight apparatus for liquid crystal displays shown in FIG. It is a block diagram which shows the structure of the control part of the backlight apparatus for liquid crystal displays by Embodiment 2 of this invention. 6 is a flowchart illustrating an operation of a PWM value calculation unit illustrated in FIG. 5. FIG. 6 is a time chart showing the operation of the liquid crystal display backlight device described in FIG. 5. It is a time chart which shows operation | movement of the backlight apparatus for liquid crystal displays by Embodiment 3 of this invention. It is a time chart which shows operation | movement of the backlight apparatus for conventional liquid crystal displays. It is a time chart which shows operation | movement of the other conventional backlight apparatus for liquid crystal displays.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Backlight apparatus for liquid crystal displays, 2 LED light source, 3 red LED, 4 green LED, 5 blue LED, 6 sensor part, 7-9 sensor, 10,40 control part, 11 driver part, 12-14 driver, 21-21 23 Memory part, 24-26 Differential part, 27-29 Gain part, 30-32 Integration part, 33-35, 41 PWM value calculation part.

Claims (5)

A backlight device for a liquid crystal display that gives transmitted light to a liquid crystal panel,
A backlight source including a plurality of light emitting sources that emit light of different colors;
A luminance sensor for detecting the luminance of each light source;
Each light source is blinked in a predetermined first period of each one field period, and the plurality of light sources are turned off in a second period. The luminance of each light source detected by the luminance sensor is A backlight device for a liquid crystal display, comprising: a control unit that controls an integrated value of a lighting time of each light emitting source in the first period so as to coincide with a predetermined target value for the light emitting source. A backlight device for a liquid crystal display that gives transmitted light to a liquid crystal panel,
A backlight source including a plurality of light emitting sources that emit light of different colors;
A luminance sensor for detecting the luminance of each light source;
In the first period of each one field period, the light emitting source having the longest required lighting time among the plurality of light emitting sources is kept on and the other light emitting sources are blinked. Control for turning off the light emitting source and controlling the integrated value of the lighting time of each light emitting source so that the luminance of each light emitting source detected by the luminance sensor matches a target value predetermined for the light emitting source. And a backlight device for a liquid crystal display.   3. The backlight device for a liquid crystal display according to claim 2, wherein the control unit lights a plurality of light emitting sources blinking in the first period at different timings.   The backlight device for a liquid crystal display according to any one of claims 1 to 3, wherein the plurality of light emitting sources respectively emit red, blue, and green light.   The backlight device for a liquid crystal display according to any one of claims 1 to 4, wherein each light source includes a light emitting diode.

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