Patent Publication Number: US-7911438-B2

Title: Area lighting device and liquid crystal display device having the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an area lighting device and a liquid crystal display device having the same, particularly to an area lighting device using an LED as a light source and a liquid crystal display device having the same. 
     2. Description of the Related Art 
     A liquid crystal display device has a liquid crystal display panel provided with a plurality of pixels arranged in a matrix, and a backlight unit which illuminates light from behind the liquid crystal display panel. The liquid crystal display device drives liquid crystals to control the transmittance of light from the backlight unit at each of the pixels for display. In recent years, in order to expand the color reproduction range (color gamut) of the liquid crystal display device, it is studied to use an LED for the light source of the backlight unit, becoming practical use. In using a plurality of the LEDs, it is necessary to control the balance of the light quantity of each LED because the properties of LEDs are different from one another. Particularly, when LEDs that emit single color lights of red (R), green (G), and blue (B) are combined for use, it is required to control the balance of the light quantity of each of red, green and blue LEDs. 
     For that scheme, there is a technique in which an optical sensor is disposed on one part of a backlight unit (for example, on the back side of a light guide plate), the optical sensor senses the chromaticity or brightness of light combined with each color of R, G and B, each of the LEDs is feedback controlled based on the sensed chromaticity or brightness, and the balance of the quantity of R, G and B lights is optimized. The optical sensor senses the chromaticity and brightness of the combined lights, and outputs signals depending thereon. The LEDs are driven based on the signals to control the light emission brightness of each of the LEDs, and to control the chromaticity and brightness of light emitted from the backlight unit. 
     Patent Reference 1: JP-A-2004-29141 
     To a viewer who views the display screen of a liquid crystal display device, light enters the viewer&#39;s eyes which has come out of a backlight unit and passed through a liquid crystal display panel. The optical property of the liquid crystal display panel is changed because of changes in the wavelength property of liquid crystals due to temperature and aging (for example, deterioration in color filters and aging of the wavelength property of liquid crystals). Therefore, since changes in the optical property of the liquid crystal display panel are not reflected even though the chromaticity and brightness of light emitted from the backlight unit is controlled as described above, the display chromaticity and display brightness of the liquid crystals display device are sometimes varied. 
       FIG. 9  is a graph illustrating the variation over time in display chromaticity in which almost white is displayed on the display screen of a liquid crystal display device of related art. The horizontal axis of the graph depicts the lighting time (minute) of LEDs of a backlight unit, and the vertical axis depicts the display chromaticity of the liquid crystal display device. A line connecting black circles depicts a chromaticity x, and a line connecting white circles depicts a chromaticity y. As shown in  FIG. 9 , the display chromaticity of the liquid crystal display device is relatively greatly varied right after the LEDs are lighted, and it takes a long time until it becomes stable. As described above, the liquid crystal display device of related art has a problem that display chromaticity and display brightness are varied over time after the LEDs are lighted and display quality is not stabilized. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an area lighting device which can provide stable display quality and a liquid crystal display device having the same. 
     The object can be achieved by an area lighting device including: a light source part in a plane which has a plurality of LEDs; a dummy liquid crystal panel which has a pair of substrates and a liquid crystal layer encapsulated between the pair of the substrates, and to which light from the light source part partially enters; an optical sensor part which senses a chromaticity and/or brightness of light transmitted through the dummy liquid crystal panel; and an LED control part which controls the plurality of the LEDs based on the chromaticity and/or brightness. 
     According to the invention, an area lighting device which can provide stable display quality and a liquid crystal display device having the same can be implemented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view illustrating the configuration of a liquid crystals display device of a first embodiment according to the invention; 
         FIG. 2  is a cross section illustrating the configuration of the liquid crystal display device of the first embodiment according to the invention; 
         FIG. 3  is a block diagram illustrating the schematic configuration of the liquid crystal display device of the first embodiment according to the invention; 
         FIG. 4  is a graph illustrating the variation over time in the display chromaticity of the liquid crystal display device of the first embodiment according to the invention; 
         FIG. 5  is graph illustrating the variations over time in the display brightness of the liquid crystal display device of the first embodiment according to the invention and a liquid crystal display device of related art; 
         FIG. 6  is an exploded perspective view illustrating the configuration of a liquid crystal display device of a second embodiment according to the invention; 
         FIG. 7  is a cross section illustrating the configuration of the liquid crystal display device the second embodiment according to the invention; 
         FIG. 8  is a block diagram illustrating the schematic configuration of the liquid crystal display device of the second embodiment according to the invention; and 
         FIG. 9  is a graph illustrating the variation over time in the display chromaticity of the liquid crystal display device of related art. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     First Embodiment 
     An area lighting device and a liquid crystal display device having the same of a first embodiment according to the invention will be described with reference to  FIGS. 1 to 5 .  FIG. 1  is an exploded perspective view illustrating the configuration of the liquid crystal display device according to the embodiment, and  FIG. 2  is a cross section illustrating the configuration of the liquid crystal display device according to the embodiment. Furthermore,  FIG. 3  is a block diagram illustrating the schematic configuration of the liquid crystal display device according to the embodiment. As shown in  FIGS. 1 to 3 , a liquid crystal display device  1  has a transmissive liquid crystal display panel  2 , and a backlight unit (an area lighting device)  3  which illuminates the liquid crystal display panel  2 . The liquid crystal display panel  2  has a pair of substrates, a liquid crystal layer encapsulated between the both substrates, and a pair of polarizers which sandwich the both substrates and are disposed thereoutside. Between the liquid crystal display panel  2  and the backlight unit  3 , optical sheets such as a polarizing sheet  11  and a diffusing sheet  12  are disposed. The liquid crystal display panel  2  performs display by driving liquid crystals to control the transmittance of light from the backlight unit  3  at each of pixels. 
     The backlight unit  3  has a light source part  30  which has a light guide plate  13  in a plane with a light emitting surface  13   a  in a rectangular plane shape, for example, on the liquid crystal display panel  2  side, and an LED module which is, for example, disposed on two end sides facing each other of the light guide plate  13  and formed of a plurality of LEDs  14  having luminous colors different from one another, for example. The LED  14 , for example, includes a plurality of R-LEDs which emits red light, a plurality of G-LEDs which emits green light, and a plurality of B-LEDs which emits blue light. The R-LEDs, G-LEDs and B-LEDs can be driven separately, and emit light in the light emission brightness proportional to the drive current of each LED, for example. Between the light emitting surface  13   a  and the diffusing sheet  12 , for example, a predetermined thickness of an air space  19  is disposed. On the back side of the light emitting surface  13   a  of the light guide plate  13 , diffusion dots are disposed. Furthermore, a reflective sheet  15  is disposed as it faces the surface on which the diffusion dots of the light guide plate  13  are disposed. The light emitted from each of the LEDs  14  enters the light guide plate  13  in which the light is guided, and mainly comes out of the light emitting surface  13   a  through the diffusion dots and the reflective sheet  15 . The light emitted from the light emitting surface  13   a  passes through the air space  19  and the optical sheets, and enters the liquid crystal display panel  2 . The light incident to the liquid crystal display panel  2  transmits the liquid crystal display panel  2  at a predetermined transmittance for each of the pixels, and enters the eyes of a viewer. The backlight unit  3 , the optical sheets, and the liquid crystal display panel  2  are accommodated and held by a plastic frame  24  and a front cover  25 . 
     Nearly at the center of the reflective sheet  15 , a circular opening  16 , for example, is formed. The light guided in the light guide plate  13  is partially emitted on the back side of the light emitting surface  13   a  through the opening  16 . On the back side of the reflective sheet  15 , a dummy liquid crystal panel  17  is disposed which is actually not used for display. The light emitted on the back side of the light emitting surface  13   a  through the opening  16  enters the dummy liquid crystal panel  17 , as it does not pass through the liquid crystal display panel  2 . The dummy liquid crystal panel  17  has a pair of substrates, a liquid crystal layer encapsulated between the both substrates, and a pair of polarizers disposed outside as sandwiching the both substrates. Liquid crystals used for the dummy liquid crystal panel  17  are the same as the liquid crystals used for the liquid crystal display panel  2 , for example. Desirably, on the both substrates, electrodes, an insulating film, color filters and so on are formed almost the same layer configuration as that of the pair of the substrates of the liquid crystal display panel  2 . Moreover, preferably, an alignment film is formed on the interface between the both substrates and the liquid crystal layer. The dummy liquid crystal panel  17  has almost the same or similar configuration as that of the liquid crystal display panel  2 , except that the panel area is smaller than that of the liquid crystal display panel  2 , and has almost the same or similar optical property (transmission property) as that of the liquid crystal display panel  2 . It is fine that the optical property of the dummy liquid crystal panel  17  is changed almost the same as the optical property of the liquid crystal display panel  2  caused by changes in the wavelength property of liquid crystals due to temperature and aging. Therefore, for example, the dummy liquid crystal panel  17  may not have color filters. When the color filter is not provided, the factor of changes in the color filter is not reflected, but the property of liquid crystals is reflected. Thus, the object of the invention can be approximately achieved. Similarly, the dummy liquid crystal panel can be simplified when the optical property resembles the liquid crystal display panel  2 . Furthermore, for example, when the liquid crystal display panel  2  is in a normally black mode, the dummy liquid crystal panel  17  is also set in the normally black mode. A voltage applying part, not shown, always applies white voltage, for example, to the liquid crystal layer of the dummy liquid crystal panel  17 , and the dummy liquid crystal panel  17  is in the state to transmit light. 
     When the liquid crystal display panel  2  is in a normally white mode, the dummy liquid crystal panel  17  is also set in the normally white mode. Moreover, even though the liquid crystal display panel  2  is in the normally black mode, the dummy liquid crystal panel  17  may be set in the normally white mode. The dummy liquid crystal panel  17  is set in the normally white mode, and then light is made to transmit the dummy liquid crystal panel  17  even though voltage is not applied to the liquid crystal layer. Therefore, the voltage applying part is unnecessary which applies voltage to the liquid crystal layer of the dummy liquid crystal panel  17 , allowing implementation of reductions in dimensions, cost, and power consumption. In the case in which the liquid crystal display panel  2  in the normally black mode, for example, has vertically aligned liquid crystals with negative dielectric constant anisotropy and polarizers in cross nicol arrangement, in order to set the dummy liquid crystal panel  17  to the normally white mode as the configuration resembled to that of the liquid crystal display panel  2  is maintained, the polarizers of the dummy liquid crystal panel  17  may be arranged in parallel nicol. 
     On the back side of the dummy liquid crystal panel  17 , a chromaticity sensor (an optical sensor part)  18  is disposed. The chromaticity sensor  18  has a light receiving surface on which light is received that has entered the dummy liquid crystal panel  17  from the light guide plate  13  and transmitted the dummy liquid crystal panel  17 . The chromaticity sensor  18  senses the chromaticity of the received light, and outputs a predetermined chromaticity signal in accordance with the chromaticity. In addition, instead of the chromaticity sensor  18 , a brightness sensor may be used as the optical sensor part, which senses the brightness of light and outputs a predetermined brightness in accordance with the brightness, or both of the chromaticity sensor  18  and the brightness sensor may be used. 
     The chromaticity signal outputted from the chromaticity sensor  18  is inputted to a sensor controller IC  20  of an LED control part  22 . The sensor controller IC  20  to which the chromaticity signal is inputted compares the chromaticity of light transmitted through the dummy liquid crystal panel  17  with a preset target value of chromaticity, and creates and outputs PWM signals that control the currents to be carried through the R-LED, the G-LED and the B-LED so that the chromaticity becomes close to the target value. The PWM signals outputted from the sensor controller IC  20  are inputted to a constant current power source circuit  21  of an LED drive part. The constant current power source circuit  21  flows a predetermined current through each of the R-LED, the G-LED and the B-LED based on the inputted PWM signals. As described above, the LED control part  22  feedback controls the current amount carried through the LED  14  based on the chromaticity (or brightness) of light transmitted through the dummy liquid crystal panel  17 . 
       FIG. 4  is a graph illustrating the variation over time in the display chromaticity of the liquid crystal display device according to the embodiment. The horizontal axis of the graph depicts the lighting time (minute) of the LED, and the vertical axis depicts the display chromaticity of the liquid crystal display device. Line a connecting black circles depicts the variation over time in chromaticity x, and line b connecting white circles depicts the variation over time in chromaticity y. As already shown in  FIG. 9 , the display chromaticity of the liquid crystal display device of related art is relatively greatly varied right after the LED is lighted, and it takes time until it becomes stable. This is caused by lighting the LED to rise the temperature of the liquid crystal display panel to vary the wavelength property of liquid crystals. On the other hand, in the liquid crystal display device according to the embodiment, as shown in  FIG. 4 , the display chromaticity is relatively stable from the time right after the LED is lighted. 
       FIG. 5  is a graph illustrating the variations over time in the display brightness of the liquid crystal display device according to the embodiment and the liquid crystal display device of related art. The horizontal axis of the graph depicts the lighting time (minute) of the LED, and the vertical axis depicts the display brightness of the liquid crystal display device. Here, the display brightness is normalized as the display brightness after stabilized is one. Line c depicts the variation over time in the display brightness of the liquid crystal display device according to the embodiment, and line d depicts the variation over time in the display brightness of the liquid crystal display device of related art. As shown in  FIG. 5 , the display brightness of the liquid crystal display device of related art is the highest right after the LED is lighted, gradually dropped over the LED lighting time, and stabilized after the LED is lighted for about 150 minutes (line d). On the other hand, the display brightness of the liquid crystal display device according to the embodiment is almost constant regardless of the LED lighting time (line c). 
     The embodiment is provided with the dummy liquid crystal panel  17  which has almost the same optical property as that of the liquid crystal display panel  2  and to which the light from the light source part  30  partially enters, and the optical sensor part which senses the chromaticity of light and/or brightness transmitted through the dummy liquid crystal panel  17 . The optical property of the dummy liquid crystal panel  17  is varied almost similar to the optical property of the liquid crystal display panel  2  caused by changes in the wavelength property of liquid crystals due to temperature and aging. The current amount to be carried through the LED  14  is feedback controlled based on the chromaticity and/or brightness of light transmitted through the dummy liquid crystal panel  17 , the display brightness and display chromaticity of the liquid crystal display device are not varied even though changes occur in the optical property of the liquid crystal display panel  2 , and stable display quality can be obtained. 
     Second Embodiment 
     Next, a liquid crystal display device of a second embodiment according to the invention will be described with reference to  FIGS. 6 to 8 . In the embodiment, in the case in which the aging of the optical property of a liquid crystal display panel  2  can be ignored, changes in the wavelength property of liquid crystals due to temperature are determined beforehand, and the current amount to be carried through LEDs  14  is controlled based on the chromaticity of light and/or brightness from a light source part  30  and the temperature near a liquid crystal display panel  2 .  FIG. 6  is an exploded perspective view illustrating the configuration of a liquid crystal display device according to the embodiment, and  FIG. 7  is a cross section illustrating the configuration of the liquid crystal display device according to the embodiment. Furthermore,  FIG. 8  is a block diagram illustrating the schematic configuration of the liquid crystal display device according to the embodiment. As shown in  FIGS. 6 to 8 , the liquid crystal display device according to the embodiment has a temperature sensor  26  which senses the temperature near the liquid crystal display panel  2 , and outputs a temperature signal. For example, the temperature sensor  26  is disposed on a printed circuit board  27  on which peripheral circuits are mounted. The printed circuit board  27  is connected to the liquid crystal display panel  2  through a flexible substrate  28  on which a driver IC is mounted. Moreover, in the embodiment, a sensor controller IC  20  of an LED control part  22  corrects a target value of chromaticity based on the temperature near the liquid crystal display panel  2  so as to cancel the temperature change in the optical property of the liquid crystal display panel  2 . Furthermore, in the embodiment, it is different from the first embodiment in that the dummy liquid crystal panel  17  is not disposed and the light emitted from the back side of a light guide plate  13  through an opening  16  directly enters a chromaticity sensor (or brightness sensor)  18 . 
     The chromaticity sensor  18  senses the chromaticity of light from the light guide plate  13 , and outputs a chromaticity signal. The chromaticity signal outputted from the chromaticity sensor  18  is inputted to the sensor controller IC  20  of the LED control part  22 . Furthermore, a temperature signal outputted from the temperature sensor  26  is also inputted to the sensor controller IC  20 . The sensor controller IC  20  to which the chromaticity signal and the temperature signal are inputted corrects a preset target value of chromaticity based on the temperature near the liquid crystal display panel  2 . Subsequently, the sensor controller IC  20  compares the chromaticity of light emitted from the light guide plate  13  with the corrected target value, determines the current amount to be carried through the R-LED, the G-LED and the B-LED so that the chromaticity becomes close to the corrected target value, and creates and outputs PWM signals. In addition, the target value may not be corrected based on the temperature. The current amount may be corrected based on the temperature in which a preset target value is used to determine the current amount and then the temperature change in the optical property of the liquid crystal display panel  2  is cancelled. The PWM signals outputted from the sensor controller IC  20  are inputted to a constant current power source circuit  21  of an LED drive part. The constant current power source circuit  21  flows a predetermined current to each of the R-LED, the G-LED and the B-LED based on the inputted PWM signals. 
     In the embodiment, the current amount to be carried through the LED  14  is controlled based on the chromaticity (or brightness) of light emitted from the light guide plate  13  and the temperature near the liquid crystal display panel  2 . The target value of chromaticity is corrected so as to cancel the temperature change in the optical property of the liquid crystal display panel  2 . Accordingly, even though temperature changes occur in the optical property of the liquid crystal display panel  2 , the display brightness and display chromaticity of the liquid crystal display device are hardly varied, and stable display quality can be obtained. 
     The invention can be modified variously, not limited to the embodiment. 
     For example, in the embodiment, a sidelight backlight unit is taken as an example in which an LED is linearly disposed near the edge part of the light guide plate, but the invention is not limited thereto, which can be applied to a direct backlight unit as well.