Display device and display device drive method for controlling luminance of RGBW subpixels and a lighting unit

In a display device, pixels each including first to fourth subpixels that respectively display first to third primary colors and fourth color are arranged on an image display panel. A lighting unit emits light to the panel from the rear thereof. A control unit calculates a required luminance value for each block of the display surface of the panel based on an input image signal, determines a light source lighting amount of the lighting unit based on luminance distribution information on the lighting unit so as to satisfy the required luminance value, generates luminance information on each pixel based on the luminance distribution information and light source lighting amount, generates an output image signal that drives the subpixels based on the luminance information and input image signal, controls the lighting unit by the light source lighting amount, and controls the panel by the output image signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2014-065803, filed on Mar. 27, 2014, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a display device and a display device drive method.

BACKGROUND

In recent years, for example, the screen definition of display devices has become higher and the color reproduction ranges of display devices have become larger. The power consumption of such high performance display devices increases. For example, to solve this problem, there has been known the technique of forming a pixel of four subpixels obtained by adding a fourth subpixel which displays a fourth color to a first subpixel which displays a first primary color, a second subpixel which displays a second primary color, and a third subpixel which displays a third primary color. With this technique the fourth subpixel increases luminance. This makes it possible to decrease the luminance of a backlight. As a result, power consumption is reduced. Furthermore, the technique of controlling the luminance of a backlight according to an input image signal for reducing power consumption further is known (see, for example, Japanese Laid-open Patent Publication No. 2011-248352).

SUMMARY

There are provided a display device and a display device drive method which reduce power consumption. Alternatively, there are provided a display device and a display device drive method which improve image quality.

According to an aspect, there is provided a display device including: an image display panel including a plurality of pixels each including a first subpixel which displays a first primary color, a second subpixel which displays a second primary color, a third subpixel which displays a third primary color, and a fourth subpixel which displays a fourth color; a lighting unit which emits light to the image display panel from a rear of the image display panel; and a control unit which calculates a required luminance value for each of blocks obtained by dividing a display surface of the image display panel on the basis of an input image signal, which determines a light source lighting amount of the lighting unit on the basis of luminance distribution information on the lighting unit stored in advance so as to satisfy the required luminance value, which generates luminance information on each pixel on the basis of the luminance distribution information and the light source lighting amount, which generates an output image signal which drives the first subpixel, the second subpixel, the third subpixel, and the fourth subpixel on the basis of the luminance information and the input image signal, which controls the lighting unit by the light source lighting amount, and which controls the image display panel by the output image signal.

DESCRIPTION OF EMBODIMENTS

Embodiments will now be described with reference to the accompanying drawings.

Disclosed embodiments are simple examples. It is a matter of course that a proper change which suits the spirit of the invention and which will readily occur to those skilled in the art falls within the scope of the present invention. Furthermore, in order to make description clearer, the width, thickness, shape, or the like of each component may schematically be illustrated in the drawings compared with the actual state. However, it is a simple example and the interpretation of the present invention is not restricted.

In addition, in the present invention and the drawings the same components that have already been described in previous drawings are marked with the same numerals and detailed descriptions of them may be omitted according to circumstances.

First Embodiment

A display device according to a first embodiment will be described by the use ofFIG. 1.FIG. 1illustrates an example of the structure of a display device according to a first embodiment. A display device1illustrated inFIG. 1includes a control unit2, an image display panel unit3, and a lighting unit5.

The control unit2receives an input image signal from the outside, controls the luminance of the lighting unit5which lights the image display panel unit3and image display by the image display panel unit3, and displays an image of the input image signal.

The image display panel unit3includes pixels arranged in a matrix of Q columns and P rows, each of which includes a first subpixel which displays a first primary color, a second subpixel which displays a second primary color, a third subpixel which displays a third primary color, and a fourth subpixel which displays a fourth color. For example, the first primary color is red, the second primary color is green, and the third primary color is blue. The fourth color is a color which contributes to an increase in the luminance of a pixel, and is, for example, white or yellow. The operation of each subpixel is controlled by an output image signal.

The lighting unit5is a backlight which emits light from the rear of the image display panel unit3, and emits white light to the display surface of the image display panel unit3. The lighting unit5adjusts a light source lighting amount of a light source. By doing so, division drive control by which luminance is controlled according to areas is performed. For example, a plurality of light sources which operate independently of one another are used and division drive control of luminance is performed by their lighting patterns. Division drive control may be performed by arranging between the light sources and the image display panel unit3a plurality of adjustment units each of which adjusts the amount of the light of a light source that reaches the image display panel unit3. In this case, a light source lighting amount may be kept constant. A case where the lighting unit5includes a plurality of light sources will now be described. However, an adjustment amount by each adjustment unit is determined in the same way.

Processes performed by the control unit2will be described. The control unit2performs required luminance value calculation2a, light source lighting amount determination2b, luminance information generation2c, and output image signal generation2d.

Description will be given in order of process. An input image signal inputted to the control unit2includes an input signal value x1(p,q)for the first primary color, an input signal value x2(p,q)for the second primary color, and an input signal value x3(p,q)for the third primary color. “p” and “q” are integers which satisfy 1≦p≦P and 1≦p≦Q respectively.

In the required luminance value calculation2aa required luminance value is calculated for each of the blocks obtained by dividing the display surface of the image display panel unit3on the basis of an input image signal. As stated above, the input image signal includes an input signal value x1(p,q)for the first primary color, an input signal value x2(p,q)for the second primary color, and an input signal value x3(p,q)for the third primary color. When an image of the input image signal is reproduced on each pixel of the image display panel unit3including the fourth subpixel, an increase in the luminance of the image is realized. Furthermore, the luminance of the lighting unit5can be reduced according to the increase in the luminance of the image. In the required luminance value calculation2athe lowest luminance of the lighting unit5that enables color reproduction is found for all pixels each including the fourth subpixel in each block. By doing so, a required luminance value is calculated.

In the light source lighting amount determination2ba light source lighting amount which satisfies a required luminance value for each block is determined on the basis of luminance distribution information2estored in advance in a storage unit. The lighting unit5includes a plurality of light sources which operate independently of one another. Luminance information on the lighting unit5at the time of lighting each light source in advance at a determined amount of light is stored in the luminance distribution information2e. In the light source lighting amount determination2ba lighting amount of each light source is adjusted so as to satisfy a required luminance value for each block, and a lighting pattern is determined.

In the luminance information generation2cluminance information on the lighting unit5for each pixel is generated on the basis of the luminance distribution information2eand a light source lighting amount. To be concrete, luminance distribution information on the lighting unit5at the time of driving the lighting unit5at a light source lighting amount determined by the use of the luminance distribution information2eis calculated. When the calculated luminance distribution information is not indicated on a pixel-by-pixel basis, the calculated luminance distribution information is converted to pixel-by-pixel information. By doing so, luminance information for each pixel on the lighting unit5is obtained.

In the output image signal generation2dan output image signal is generated for each pixel on the basis of luminance information on the lighting unit5for the pixel and the input image signal. The output image signal includes an output signal value X1(p,q)corresponding to the first subpixel, an output signal value X2(p,q)corresponding to the second subpixel, an output signal value X3(p,q)corresponding to the third subpixel, and an output signal value X4(p,q)corresponding to the fourth subpixel. As stated above, the first subpixel displays the first primary color, the second subpixel displays the second primary color, the third subpixel displays the third primary color, and the fourth subpixel displays the fourth color. Accordingly, the output signal value X1(p,q), the output signal value X2(p,q), the output signal value X3(p,q), and the output signal value X4(p,q)included in the output image signal correspond to the first primary color, the second primary color, the third primary color, and the fourth color respectively.

As stated above, the luminance of the lighting unit5can be reduced according to an increase in the luminance of an image. There is such a correspondence between the luminance of an image and the luminance of the lighting unit5. Accordingly, display is performed more properly by generating an output image signal in which luminance information on the lighting unit5calculated for each pixel is reflected.

With the display device1a light source lighting amount of the lighting unit5is determined so as to satisfy a required luminance value for each block calculated by the use of an input image signal. As a result, the luminance of the lighting unit5can be reduced for a block in which the luminance of an image is low. This leads to a reduction in power consumption. Furthermore, luminance information on the lighting unit5corresponding to the determined light source lighting amount is found for each pixel and an output image signal in which the luminance information on the lighting unit5found for each pixel is reflected is determined. As a result, the luminance of the lighting unit5matches the output image signal on a pixel-by-pixel basis and image quality improves.

Second Embodiment

A display device according to a second embodiment will now be described. First the structure of a display device will be described, and then a display control process performed by the display device will be described.

FIG. 2illustrates an example of the structure of a display device according to a second embodiment.

A display device10illustrated inFIG. 2includes an image output unit11, a signal processing unit20, an image display panel30, an image display panel drive unit40, a surface light source device50, and a light source drive unit60. The display device10is an embodiment of the display device1illustrated inFIG. 1.

The image output unit11outputs an input signal SRGB to the signal processing unit20. The input signal SRGB includes an input signal value x1(p,q)for a first primary color, an input signal value x2(p,q)for a second primary color, and an input signal value x3(p,q)for a third primary color. In the second embodiment it is assumed that the first primary color is red, the second primary color is green, and the third primary color is blue.

The signal processing unit20is connected to the image display panel drive unit40which drives the image display panel30and the light source drive unit60which drives the surface light source device50. The signal processing unit20division-controls the luminance of the surface light source device50for each block. Furthermore, the signal processing unit20calculates luminance information for each pixel on the surface light source device50and generates an output signal SRGBW in which it is reflected. By doing so, the signal processing unit20controls image display. In addition to an output signal value X1(p,q)corresponding to a first subpixel, an output signal value X2(p,q)corresponding to a second subpixel, and an output signal value X3(p,q)corresponding to a third subpixel, the output signal SRGBW includes an output signal value X4(p,q)corresponding to a fourth subpixel which displays a fourth color. In the second embodiment it is assumed that the fourth color is white. The signal processing unit20is an embodiment of the control unit2.

The image display panel30is made up of (P×Q) pixels48arranged in a two-dimensional matrix. The image display panel drive unit40includes a signal output circuit41and a scanning circuit42and drives the image display panel30. The image display panel30and the image display panel drive unit40are an embodiment of the image display panel unit3.

The surface light source device50is arranged on the rear side of the image display panel30and emits light to the image display panel30. By doing so, the surface light source device50lights the image display panel30. The light source drive unit60controls the luminance of the surface light source device50on the basis of a light source control signal SBL outputted from the signal processing unit20. The surface light source device50and the light source drive unit60are an example of the lighting unit5.

The image display panel30and the surface light source device50will now be described by the use ofFIGS. 3 and 4respectively.

The image display panel30will be described first.FIG. 3illustrates an example of the arrangement of pixels on the image display panel in the second embodiment.

With the image display panel30illustrated inFIG. 3, each of the pixels48arranged in a two-dimensional matrix includes a first subpixel49R, a second subpixel49G, a third subpixel49B, and a fourth subpixel49W. In the second embodiment, the first subpixel49R displays red, the second subpixel49G displays green, the third subpixel49B displays blue, and the fourth subpixel49W displays white. However, colors which the first subpixel49R, the second subpixel49G, and the third subpixel49B display are not limited to them. The first subpixel49R, the second subpixel49G, and the third subpixel49B may display other different colors. For example, the first subpixel49R, the second subpixel49G, and the third subpixel49B may display the complementary colors of red, green, and blue respectively. Furthermore, a color which the fourth subpixel49W displays is not limited to white. For example, the fourth subpixel49W may display yellow. However, white is effective in reducing power consumption. It is desirable that if a light source lights the first subpixel49R, the second subpixel49G, the third subpixel49B, and the fourth subpixel49W at the same light source lighting amount, the fourth subpixel49W is brighter than the first subpixel49R, the second subpixel49G, and the third subpixel49B. If there is no need to distinguish among the first subpixel49R, the second subpixel49G, the third subpixel49B, and the fourth subpixel49W, then the term “subpixels49” will be employed in the following description.

More specifically, the image display panel30is a transmission type color liquid crystal display panel. Color filters which transmit red light, green light, and blue light are disposed between the first subpixel49R, the second subpixel49G, and the third subpixel49B, respectively, and an observer of an image. Furthermore, a color filter is not disposed between the fourth subpixel49W and an observer of an image. The fourth subpixel49W may include a transparent resin layer in place of a color filter. If a color filter is not disposed between the fourth subpixel49W and an observer of an image, a great difference in level arises between the fourth subpixel49W and the first subpixel49R, the second subpixel49G, and the third subpixel49B. The formation of a transparent resin layer prevents a great difference in level from arising between the fourth subpixel49W and the first subpixel49R, the second subpixel49G, and the third subpixel49B.

The signal output circuit41and the scanning circuit42included in the image display panel drive unit40are electrically connected to the subpixels49R,49G,49B, and49W of the image display panel30via signal lines DTL and signal lines SCL respectively. The subpixels49are connected not only to the signal lines DTL but also to the signal lines SCL via switching elements (such as TFTs (Thin Film Transistors)). The image display panel drive unit40selects subpixels49by the scanning circuit and outputs image signals in order from the signal output circuit41. By doing so, the image display panel drive unit40controls the operation (light transmittance) of the subpixels49.

Next, the surface light source device50will be described by the use ofFIG. 4.FIG. 4illustrates an example of the structure of the surface light source device in the second embodiment.

The surface light source device50illustrated inFIG. 4includes a light guide plate54and a sidelight light source52in which light sources56A,56B,56C,56D,56E,56F,56G,56H,56I, and56J are arranged opposite an incident surface E that is at least one side of the light guide plate54. The light sources56A,56B,56C,56D,56E,56F,56G,56H,56I, and56J are LEDs (Light-Emitting Diodes) which emit light of the same color (white, for example), and control current values or duty ratios independently of one another. If there is no need to distinguish among the light sources56A,56B,56C,56D,56E,56F,56G,56H,56I, and56J, then the term “light sources56” will be employed in the following description. The light sources56are arranged along the one side of the light guide plate54. It is assumed that the direction in which the light sources56are arranged is a light source arrangement direction LY. Light emitted from the light sources56is inputted from the incident surface E to the light guide plate54in an incident direction LX perpendicular to the light source arrangement direction LY.

The light source drive unit60adjusts the values of current supplied to the light sources56or duty ratios on the basis of a light source control signal SBL outputted from the signal processing unit20. By doing so, the light source drive unit60controls the amount of the light of the light sources56and controls the luminance (intensity of the light) of the surface light source device50.

Lights which are inputted from the light sources56and which are emitted from the light guide plate54to the rear of the image display panel30have different luminance distributions according to the positions at which the light sources56are arranged. The luminance distribution of light on which each light source56acts will be described by the use ofFIGS. 5 and 6.

FIG. 5illustrates an example of the luminance distribution of light on which one light source of the sidelight light source acts.FIG. 5illustrates the distribution of the intensity of light which is inputted from the light source56A and which is emitted from the light guide plate54to the rear of the image display panel30in the case of only the light source56A lighting. As illustrated inFIG. 4, the light source56A is arranged at the end of the sidelight light source52. LX inFIG. 5indicates a direction in which light is inputted from each light source of the sidelight light source52. LY perpendicular to the incident direction LX indicates a light source arrangement direction of the sidelight light source52. LZ perpendicular to the incident direction LX and the light source arrangement direction LY indicates a direction in which the image display panel30is lighted from the rear. When light emitted from the light source56A is inputted from the incident surface E to the light guide plate54, the light guide plate54emits light in the lighting direction LZ.

FIG. 6illustrates an example of the luminance distribution of light on which another light source of the sidelight light source acts.FIG. 6illustrates the distribution of the intensity of light which is inputted from the light source56C and which is emitted from the light guide plate54to the rear of the image display panel30in the case of only the light source56C lighting. As illustrated inFIG. 4, the light source56C is arranged between the light sources56A and56J which are arranged at both ends of the sidelight light source52. When light emitted from the light source56C is inputted from the incident surface E to the light guide plate54, the light guide plate54emits light in the lighting direction LZ.

Both ends of the light guide plate54which appear in the light source arrangement direction LY reflect light. As a result, the luminance distribution ofFIG. 5realized by the light source56A near both ends of the light guide plate54which appear in the light source arrangement direction LY and the luminance distribution ofFIG. 6realized by the light source56C arranged between the light sources56A and56J, which are arranged at both ends of the sidelight light source52, differ. The signal processing unit20considers that luminance distributions realized by the light sources56differ, and controls a lighting amount of each light source56.

The hardware configuration of the display device10will now be described.FIG. 7illustrates an example of the hardware configuration of the display device according to the second embodiment.

The whole of the display device10is controlled by a device control unit100. The device control unit100includes a CPU (Central Processing Unit)101. A RAM (Random Access Memory)102, a ROM (Read Only Memory)103, and a plurality of peripheral units are connected to the CPU101via a bus108.

The RAM102is used as main storage of the device control unit100. The RAM102temporarily stores at least a part of an OS (Operating System) program or an application program executed by the CPU101. In addition, the RAM102stores various pieces of data which the CPU101needs to perform a process.

The ROM103is a read only semiconductor memory and stores an OS program, an application program, and fixed data which is not rewritten. Furthermore, a semiconductor memory, such as a flash memory, may be used as auxiliary storage in place of the ROM103or in addition to the ROM103.

The CPU101controls the whole of the display device10on the basis of an OS program and an application program stored in the ROM103and various pieces of data expanded in the RAM102. When the CPU101performs a process, the CPU101may operate by an OS program or an application program temporarily stored in the RAM102.

The plurality of peripheral units connected to the bus108are a display driver IC (Integrated Circuit)104, an LED driver IC105, an input interface106, and a communication interface107.

The image display panel30is connected to the display driver IC104via the image display panel drive unit40. The display driver IC104outputs an output signal SRGBW to the image display panel drive unit40. The image display panel drive unit40outputs a control signal corresponding to the output signal SRGBW to display an image on the image display panel30.

The surface light source device50is connected to the LED driver IC105. The LED driver IC105drives the light sources56according to a light source control signal SBL and controls the luminance of the surface light source device50. The LED driver IC105realizes at least a part of the function of the light source drive unit60.

An input device used for inputting user's instructions is connected to the input interface106. An input device, such as a keyboard, a mouse used as a pointing device, or a touch panel, is connected. The input interface106transmits to the CPU101a signal transmitted from the input device.

The communication interface107is connected to a network200. The communication interface107transmits data to or receives data from another computer or a communication apparatus via the network200.

By adopting the above hardware configuration, the processing functions in the second embodiment are realized.

The processing operation of the signal processing unit20is realized by the display driver IC104or the CPU101.

If the processing operation of the signal processing unit20is realized by the display driver IC104, then an input signal SRGB is inputted via the CPU101to the display driver IC104. The display driver IC104generates an output signal SRGBW to control the image display panel30. Furthermore, the display driver IC104generates a light source control signal SBL and transmits it to the LED driver IC105via the bus108.

If the processing operation of the signal processing unit20is realized by the CPU101, then an output signal SRGBW is inputted from the CPU101to the display driver IC104. A light source control signal SBL is also generated by the CPU101and is transmitted to the LED driver IC105via the bus108.

The functions of the signal processing unit20will now be described.FIG. 8is a functional block diagram of the signal processing unit in the second embodiment.

The signal processing unit20includes a timing generation unit21, an image processing unit22, an image analysis unit23, a light source data storage unit24, a lighting pattern determination unit25, and a luminance information calculation unit26. An input signal SRGB is inputted from the image output unit11to the signal processing unit20. The input signal SRGB includes color information on an image displayed at the position of each pixel48. The timing generation unit21generates synchronization signal STM for synchronizing the operation timing of the image display panel drive unit40with that of the light source drive unit60every image display frame. The timing generation unit21outputs the generated synchronization signal STM to the image display panel drive unit40and the light source drive unit60.

The image processing unit22generates an output signal SRGBW on the basis of an input signal SRGB and luminance information for each pixel on the surface light source device50inputted from the luminance information calculation unit26.

On the basis of an input signal SRGB, the image analysis unit23calculates a required luminance value of the surface light source device50needed for each of the blocks obtained by dividing a display surface of the image display panel30. Each pixel48includes the fourth subpixel49W, so its luminance can be adjusted. An index for adjusting the luminance of each pixel48is determined according to the input signal SRGB. With division drive control of the surface light source device50, the luminance of each pixel48is adjusted and the luminance of the surface light source device50is reduced according to an increase in the luminance of each pixel48. That is to say, there is a correspondence between the index for adjusting the luminance of each pixel48and an index for adjusting the luminance of the surface light source device50. The image analysis unit23analyzes the input signal SRGB corresponding to each block, calculates a block correspondence index for adjusting the luminance of the surface light source device50for each block, and determines a required luminance value for each block. For example, the image analysis unit23calculates a block correspondence index on the basis of at least one of saturation and a value of the input signal SRGB corresponding to each block.

The light source data storage unit24stores luminance distribution information on the light sources56. As illustrated inFIGS. 5 and 6, the light sources56differ in luminance distribution. Accordingly, the light source data storage unit24stores as luminance distribution information a luminance value on the entire surface of the surface light source device50detected at the time of lighting each light source56at a determined lighting amount. Luminance distribution information will be described by the use ofFIGS. 9 and 10.

FIG. 9is a schematic view for describing luminance distribution information. As illustrated inFIG. 9, luminance distribution information indicates a luminance value of the surface light source device50detected for each of the (m×n) areas (m is any integer which satisfies 1≦p≦P and n is any integer which satisfies 1≦p≦Q) obtained by dividing the display surface of the image display panel30(or an output surface of the surface light source device50). The number of areas obtained by division is set to any number, but it does not exceed the number of pixels. If each area obtained by division corresponds to one pixel, then the luminance value for each pixel is stored as luminance distribution information. If each area obtained by division corresponds to more than one pixel, then a pixel at a determined position in each area is considered as a representative pixel and the luminance value of the surface light source device50for the representative pixel is stored. In the example ofFIG. 9, the luminance value L1is set as a luminance value for a representative pixel in an area inside a luminance (L1) distribution line indicative of the luminance value L1. The light source data storage unit24stores luminance distribution information in which luminance values for (m×n) areas are set for each light source56in a tabular form. In the following description the luminance distribution information in a tabular form for each light source will be referred to as a light-source-specific LUT (LookUp Table). Light-source-specific lookup tables are information specific to the display device10, so they are created in advance and are stored in the light source data storage unit24.

FIG. 10illustrates light-source-specific lookup tables in the second embodiment. A light-source-specific lookup table240is prepared for each of the light sources56A,56B,56C,56D,56E,56F,56G,56H,56I, and56J. Luminance values detected for (m×n) areas at the time of lighting only the light source56A are recorded in a tabular form in a LUTA241a. Similarly, LUTs are prepared in the same way for the light sources56B,56C,56D,56E,56F,56G,56H,56I, and56J.FIG. 10illustrates a LUTI241ifor the light source56I and a LUTJ241jfor the light source56J. If a luminance value for a representative pixel which represents a determined area is used, the size of the light-source-specific lookup table240becomes smaller and the storage capacity of the light source data storage unit24is reduced. When a luminance value for each pixel is needed, it is calculated by interpolation calculation. The light-source-specific lookup table240is information obtained by lighting one light source56at a time. However, a light-source-specific lookup table obtained by simultaneously lighting a combination of the light sources56A and56B, a combination of the light sources56C and56D, or the like may be created and stored. This reduces the amount of work for creating light-source-specific lookup tables and the storage capacity of the light source data storage unit24.

Furthermore, luminance values are set in a corrected state in the light-source-specific lookup tables240so as to accommodate correction of luminance irregularity. By using the light source-specific lookup tables240, correction of luminance irregularity and lighting pattern determination are performed at the same time.

Description will return toFIG. 8.

The lighting pattern determination unit25determines a lighting pattern of the sidelight light source52on the basis of a required luminance value for each block calculated by the image analysis unit23and the light-source-specific lookup tables240stored in the light source data storage unit24. The lighting pattern determination unit25may find a lighting pattern of the sidelight light source52by calculation. Furthermore, the lighting pattern determination unit25may set a tentative lighting pattern of the sidelight light source52, calculate tentative luminance distribution information for the tentative lighting pattern by the use of the light-source-specific lookup tables240, compare the required luminance value with the tentative luminance distribution information to make a correction, and determine a lighting pattern. The lighting pattern determination unit25generates a light source control signal SBL on the basis of the lighting pattern and outputs it to the light source drive unit60.

The luminance information calculation unit26uses a lighting pattern and the light source-specific lookup tables240stored in the light source data storage unit24for calculating luminance information for each pixel on the surface light source device50at the time of lighting the sidelight light source52according to the lighting pattern. First the luminance information calculation unit26uses the light-source-specific lookup tables240for calculating actual luminance distribution information for each light source at the time of actually lighting the sidelight light source52according to the lighting pattern. If pixel-by-pixel information is not obtained from the light-source-specific lookup tables240, then the luminance information calculation unit26performs interpolation calculation for calculating actual luminance distribution information for each light source. The luminance information calculation unit26then combines the actual luminance distribution information for the light sources for finding actual luminance distribution information on the sidelight light source52, and transmits it to the image processing unit22. A Luminance value of the surface light source device50is set for each pixel in the calculated actual luminance distribution information on the sidelight light source52.

A process performed by the image processing unit22which acquires actual luminance distribution information from the luminance information calculation unit26will be described. The image processing unit22obtains a luminance value of the surface light source device50for each pixel from the actual luminance distribution information. As stated above, the luminance of the surface light source device50is calculated by the index for reducing the luminance. In addition, when there is a determined correspondence between the index for reducing the luminance and the index for increasing the luminance of each pixel48, display is performed with proper luminance. The image processing unit22calculates, from the luminance value of the surface light source device50for each pixel, a first pixel correspondence index for reducing the luminance of the surface light source device50. Furthermore, the image processing unit22calculates a second pixel correspondence index for increasing the luminance of each pixel48which corresponds to the first pixel correspondence index, and generates an output signal SRGBW by the use of the second pixel correspondence index.

A case where the expansion coefficient α is used as the index for increasing the luminance of each pixel48or the index for reducing the luminance of the surface light source device50will now be described.

Each pixel48of the display device10includes the fourth subpixel49W which outputs the fourth color (white). This extends the dynamic range of a value in reproduction HSV color space which can be reproduced by the display device10. “H” represents hue, “S” represents saturation, and “V” represents a value.

FIG. 11is a schematic view of reproduction HSV color space which can be reproduced by the display device according to the second embodiment. As illustrated inFIG. 11, the reproduction HSV color space to which the fourth color has been added has a shape obtained by putting an approximately trapezoid solid in which, as the saturation S increases, the maximum value of the value V becomes smaller on cylindrical HSV color space which the first subpixel49R, the second subpixel49G, and the third subpixel49B display. The signal processing unit20stores the maximum value Vmax(S) of a value expressed with the saturation S in the reproduction HSV color space which has been extended by adding the fourth color as a variable. That is to say, the signal processing unit20stores the maximum value Vmax(S) of a value by the coordinates (values) of the saturation S and the hue H for the solid shape of the reproduction HSV color space illustrated inFIG. 11.

An input signal SRGB includes input signal values corresponding to the first, second, and third primary colors, so HSV color space of the input signal SRGB has a cylindrical shape, that is to say, has the same shape as a cylindrical portion of the reproduction HSV color space illustrated inFIG. 11has. Accordingly, an output signal SRGBW is calculated as an expanded image signal obtained by expanding the input signal SRGB to make it fall within the reproduction HSV color space. The input signal SRGB is expanded by the use of the expansion coefficient α determined by comparing the value levels of subpixels of the input signal SRGB in the reproduction HSV color space. By expanding the level of an input image signal by the use of the expansion coefficient α, an output signal value corresponding to the fourth subpixel49W can be made large. This increases the luminance of an entire image. At this time the luminance of the surface light source device50is reduced to 1/α according to an increase in the luminance of the entire image caused by the use of the expansion coefficient α. By doing so, display is performed with exactly the same luminance as with the input signal SRGB.

The expansion of an input signal SRGB will now be described.

An output signal value X1(p, q)corresponding to the first subpixel49R, an output signal value X2(p, q)corresponding to the second subpixel49G, and an output signal value X3(p, q)corresponding to the third subpixel49B for a (p, q)th pixel (or a combination of the first subpixel49R, the second subpixel49G, and the third subpixel49B) are expressed as:
X1(p,q)=α·x1(p,q)−χ−X4(p,q)(1)
X2(p,q)=α·x2(p,q)−χ−X4(p,q)(2)
X3(p,q)=α·x3(p,q)−χ−X4(p,q)(3)
where α is an expansion coefficient and χ is a constant which depends on the display device10. χ will be described later.

In addition, an output signal value X4(p, q)is calculated on the basis of the product of Min(p, q)and the expansion coefficient α, where Min(p, q)is the minimum value of an input signal value x1(p, q)corresponding to the first subpixel49R, an input signal value x2(p, q)corresponding to the second subpixel49G, and an input signal value x3(p, q)corresponding to the third subpixel49B. To be concrete, an output signal value X4(p, q)is found on the basis of
X4(p,q)=Min(p,q)·α/χ  (4)

In expression (4), the product of Min(p, q)and the expansion coefficient α is divided by χ. However, another calculation method may be adopted. Furthermore, the expansion coefficient α is determined every image display frame.

These points will now be described.

On the basis of an input signal SRGB for the (p, q)th pixel including an input signal value x1(p, q)corresponding to the first subpixel49R, an input signal value x2(p, q)corresponding to the second subpixel49G, and an input signal value x3(p, q)corresponding to the third subpixel49B, usually saturation S(p, q)and value V(S)(p, q)in the cylindrical HSV color space are found from
S(p,q)=(Max(p,q)−Min(p,q))/Max(p,q)(5)
V(S)(p,q)=Max(p,q)(6)
where Max(p, q)is the maximum value of the input signal value x1(p, q)for the first subpixel49R, the input signal value x2(p, q)for the second subpixel49G, and the input signal value x3(p, q)for the third subpixel49B, Min(p, q)as stated above, is the minimum value of the input signal value x1(p, q)for the first subpixel49R, the input signal value x2(p, q)for the second subpixel49G, and the input signal value x3(p, q)for the third subpixel49B, the saturation S has a value in the range of 0 to 1, and the value V(S) has a value in the range of 0 to (2n−1), where n is a display gradation bit number.

A color filter is not disposed between the fourth subpixel49W which displays white and an observer of an image. If a light source lights the first subpixel49R which displays the first primary color, the second subpixel49G which displays the second primary color, the third subpixel49B which displays the third primary color, and the fourth subpixel49W which displays the fourth color at the same light source lighting amount, then the fourth subpixel49W is brighter than the first subpixel49R, the second subpixel49G, and the third subpixel49B. It is assumed that when a signal value corresponding to the maximum value of output signal values corresponding to the first subpixels49R is inputted to a first subpixel49R, a signal value corresponding to the maximum value of output signal values corresponding to the second subpixels49G is inputted to a second subpixel49G, and a signal value corresponding to the maximum value of output signal values corresponding to the third subpixels49B is inputted to a third subpixel49B, the luminance of a set of a first subpixel49R, a second subpixel49G, and a third subpixel49B included in each pixel48or the luminance of a set of first subpixels49R, second subpixels49G, and third subpixels49B included in a group of pixels48is BN1-3. Furthermore, it is assumed that when a signal value corresponding to the maximum value of output signal values corresponding to a fourth subpixel49W included in each pixel48or fourth subpixels49W included in a group of pixels48is inputted to a fourth subpixel49W, the luminance of the fourth subpixel49W is BN4. That is to say, white which has the maximum luminance is displayed by a set of a first subpixel49R, a second subpixel49G, and a third subpixel49B and the luminance of white is BN1-3. As a result, the constant χ which depends on the display device10is expressed as
χ=BN4/BN1-3

By the way, if the output signal value X4(p, q)is given by the above expression (4), the maximum value Vmax(S) of a value is expressed, with the saturation S in the reproduction HSV color space as a variable, as:

The maximum value Vmax(S) of a value which is expressed with the saturation S in the reproduction HSV color space that has been extended by adding the fourth color as a variable and which is obtained in this way is stored in, for example, the signal processing unit20as a type of lookup table. Alternatively, the maximum value Vmax(S) of a value expressed with the saturation S in the reproduction HSV color space as a variable is found every time by the signal processing unit20.

The expansion coefficient α is used for expanding the value V(S) in the HSV color space into the reproduction HSV color space and is expressed as
α(S)=Vmax(S)/V(S)  (9)

In expansion calculation, the expansion coefficient α is determined on the basis of, for example, α(S) found for plural pixels48.

Signal processing performed by the signal processing unit20by the use of the expansion coefficient α will now be described. The following signal processing is performed so that the ratio among the luminance of the first primary color displayed by (first subpixel49R+fourth subpixel49W), the luminance of the second primary color displayed by (second subpixel49G+fourth subpixel49W), and the luminance of the third primary color displayed by (third subpixel49B+fourth subpixel49W) will be held, so that a color tone will be held (maintained), and so that a gradation-luminance characteristic (γ characteristic) will be held (maintained). Furthermore, if all input signal values are 0 or small for a pixel48or a group of pixels48, then the expansion coefficient α may be calculated with the pixel48or the group of pixels48excluded.

A process performed by the image analysis unit23will be described. On the basis of an input signal SRGB for plural pixels48included in a block, the image analysis unit23finds the saturation S and the value V(S) of the plural pixels48. To be concrete, the image analysis unit23uses an input signal value x1(p, q), an input signal value x2(p, q), and an input signal value x3(p, q)for a (p, q)th pixel48and finds S(p, q)and V(S)(p, q)from expressions (5) and (6) respectively. The image analysis unit23performs this process on all pixels in the block. As a result, combinations of (S(p, q), V(S)(p, q)) the number of which corresponds to the number of pixels48in the block are obtained. Next, the image analysis unit23finds the expansion coefficient α on the basis of at least one of α(S) values found for the pixels48in the block. For example, the image analysis unit23considers the smallest value of α(S) values found for the pixels48in the block as the expansion coefficient α for the block. The image analysis unit23calculates the expansion coefficient α for the block in this way.

The image analysis unit23repeats this procedure for each block and calculates the expansion coefficient α for each block. Luminance required for a block is calculated by the use of 1/α which is the reciprocal of the expansion coefficient α. 1/α is an example of a block correspondence index.

FIG. 12illustrates an example of a required luminance value for each block in the second embodiment. Information regarding a required luminance value for each of the 27 (=3×9) blocks obtained by dividing an emission surface of the surface light source device50is set in required luminance value information270illustrated inFIG. 12. Information regarding a required luminance value may be, for example, the expansion coefficient α, 1/α, or a luminance value after conversion calculated for each block. As stated above, the required luminance values illustrated inFIG. 12is an example. In addition, the number of blocks obtained by division is not limited to 27 and is arbitrarily selected.

A process performed by the lighting pattern determination unit25will now be described. The lighting pattern determination unit25determines a lighting pattern of the sidelight light source52on the basis of the required luminance value information270acquired from the image analysis unit23and the light-source-specific lookup tables240stored in the light source data storage unit24.

First the lighting pattern determination unit25sets a tentative lighting pattern of the sidelight light source52. The lighting pattern determination unit25then uses the light-source-specific lookup tables240for combining tentative luminance distribution information at the time of lighting the sidelight light source52according to the tentative lighting pattern. For example, the lighting pattern determination unit25uses the light-source-specific lookup table LUTA241aregarding the light source56A for calculating tentative luminance distribution information at the time of lighting the light source56A at a lighting amount of the tentative lighting pattern. Similarly, the lighting pattern determination unit25calculates tentative luminance distribution information at the time of lighting each of the light sources56B,56C,56D,56E,56F,56G,56H,56I, and56J at a lighting amount of the tentative lighting pattern. Thus calculated tentative luminance distribution information for the light sources is combined to obtain tentative luminance distribution information on the sidelight light source52. The tentative luminance distribution information T(i, j)of the sidelight light source52is represented, for example, by

T⁡(i,j)=∑k=0n⁢⁢Tk⁡(i,j)·ak(10)
where Tkis a light-source-specific lookup table regarding each light source and akis a lighting amount set for each light source56. The lighting pattern determination unit25calculates the tentative luminance distribution information on the sidelight light source52in this way by referring to the light-source-specific lookup tables240in place of performing calculations by the use of expression (10), so the amount of calculation is reduced.

Next, the lighting pattern determination unit25compares the obtained tentative luminance distribution information on the sidelight light source52with a required luminance value for each block. If there is a difference between them, then the lighting pattern determination unit25corrects the tentative lighting pattern.

Correction of the tentative lighting pattern will be described.FIG. 13illustrates the relationship between a required luminance value and luminance distribution in the second embodiment.FIG. 13is a sectional view taken in the direction LY. The same applies to a sectional view taken in the direction LX.

As illustrated inFIG. 13, a required luminance value271is determined for each block, so luminance changes like steps in the direction LY. On the other hand, luminance distribution272continuously changes at the time of lighting the sidelight light source52. The tentative lighting pattern is corrected so that the luminance distribution272at the time of lighting the sidelight light source52will not be lower than the required luminance value271in any area.

After the lighting pattern determination unit25corrects the tentative lighting pattern, the lighting pattern determination unit25uses a tentative lighting pattern after the correction for repeating the above procedure. By doing so, the lighting pattern determination unit25determines a lighting pattern which satisfies a required luminance value for each block.

Furthermore, a dimming process is performed on the lighting pattern which the lighting pattern determination unit25determines so as to satisfy a required luminance value for each block. In the dimming process, the obtained lighting pattern and a lighting pattern outputted the last time are compared. If there is a light source56whose luminance suddenly changes by an amount greater than a determined value, then a correction is made to control the amount of the change. The dimming process prevents the luminance of the surface light source device50from changing suddenly.

A lighting pattern is determined through the above procedure.

FIG. 14illustrates an example of a lighting pattern in the second embodiment.

In a lighting pattern (light source lighting amount)280illustrated inFIG. 14, a lighting pattern of the sidelight light source52determined by the lighting pattern determination unit25, that is to say, a lighting amount of each of the light sources56A,56B,56C,56D,56E,56F,56G,56H,56I, and56J is set.

The lighting pattern determination unit25outputs the determined lighting pattern (light source lighting amount)280to the light source drive unit60as a light source control signal SBL. The light source drive unit60controls the drive of each light source56on the basis of the determined lighting pattern (light source lighting amount)280. The lighting pattern determination unit25also outputs the lighting pattern (light source lighting amount)280to the luminance information calculation unit26. In the above description, a tentative lighting pattern is set and a correction is made repeatedly. However, if an optimum lighting pattern is obtained by performing a calculation once, then comparison between actual luminance distribution information based on the lighting pattern and a required luminance value and correction of the lighting pattern may be omitted.

A process performed by the luminance information calculation unit26will now be described. The luminance information calculation unit26generates luminance information for each pixel on the surface light source device50on the basis of the lighting pattern (light source lighting amount)280acquired from the lighting pattern determination unit25and the light-source-specific lookup tables240stored in the light source data storage unit24. To be concrete, the luminance information calculation unit26uses the light-source-specific lookup tables240for calculating actual luminance distribution information for each light source at the time of lighting the sidelight light source52according to the determined lighting pattern280. If the obtained actual luminance distribution information for each light source is not pixel-by-pixel information, then the luminance information calculation unit26calculates a luminance value for each pixel from luminance values for representative pixels. For example, the luminance information calculation unit26uses luminance information on representative pixels included in the light-source-specific lookup tables240for performing interpolation calculation based on linear interpolation or polynomial interpolation to generate actual luminance distribution information for each light source and for each pixel. The polynomial interpolation is cubic interpolation or the like.

The actual luminance distribution information for each light source and for each pixel calculated in this way is added to obtain actual luminance distribution information on the entire surface light source device50.FIG. 15illustrates an example of luminance distribution calculated by the luminance information calculation unit in the second embodiment. Luminance distribution illustrated inFIG. 15is obtained by superimposing luminance distribution at the time of driving each light source56.

The calculated actual luminance distribution information indicates a luminance value of the surface light source device50calculated for each pixel. The image processing unit22acquires luminance information on the surface light source device.50for each pixel on the basis of the actual luminance distribution information.

A process performed by the image processing unit22will now be described. The image processing unit22calculates an output signal SRGBW for each pixel on the basis of the actual luminance distribution information calculated by the luminance information calculation unit26. To be concrete, the expansion coefficient α for an input signal SRGB to a pixel (p, q) is the reciprocal of the index 1/α for reducing corresponding luminance (p, q) of the surface light source device50. The image processing unit22finds the expansion coefficient α for the pixel (p, q) on the basis of luminance information (p, q) on the surface light source device50for the pixel (p, q) included in the actual luminance distribution information. The image processing unit22calculates the expansion coefficient α for the pixel (p, q) in this way and obtains an output signal SRGBW by performing expansion calculation by the use of α. The image processing unit22performs this expansion calculation by the use of, for example, expressions (1), (2), (3), and (4). The index 1/α is an example of the first pixel correspondence index and the expansion coefficient α is an example of the second pixel correspondence index.

As has been described, the expansion coefficient α is used for exercising division drive control of the luminance of the surface light source device50and image display control of the image display panel30. By doing so, the luminance of the surface light source device50is set to the smallest value that enables color reproduction by the display device10in the reproduction HSV color space. This reduces the power consumption of the display device10. Furthermore, by controlling image display according to the luminance for each pixel of the surface light source device50, image quality is maintained and contrast is improved.

A display control process performed by the display device10will now be described by the use ofFIGS. 16 through 20.

FIG. 16is a flow chart of a display control process performed by the display device according to the second embodiment. The display device10starts a display control process every image display frame. An input signal SRGB is inputted via the image output unit11to the signal processing unit20.

(Step S01) The signal processing unit20acquires the input signal SRGB.

(Step S02) The signal processing unit20gamma-converts the input signal SRGB to linearize it.

(Step S03) The image analysis unit23acquires the linearized input signal SRGB and performs an image analysis subprocess. In the image analysis subprocess, the image analysis unit23calculates a required luminance value of the surface light source device50on the basis of the input signal SRGB for each of the blocks obtained by dividing the display surface of the image display panel30. The details of the image analysis subprocess will be described later.

(Step S04) The lighting pattern determination unit25acquires a required luminance value for each block, refers to the light-source-specific lookup tables240stored in the light source data storage unit24, and determines a lighting pattern of the sidelight light source52which satisfies the required luminance value. In addition, the lighting pattern determination unit25outputs to the light source drive unit60a light source control signal SBL corresponding to the lighting pattern. The details of the lighting pattern determination subprocess will be described later by the use ofFIG. 18.

(Step S05) On the basis of the light-source-specific lookup tables240, the luminance information calculation unit26generates actual luminance distribution information at the time of driving the sidelight light source52according to the determined lighting pattern. The generated actual luminance distribution information includes pixel-by-pixel luminance information on the surface light source device50. The details of the luminance information calculation subprocess will be described later.

(Step S06) The image processing unit22generates from the input signal SRGB an output signal SRGBW for each pixel in which corresponding luminance information on the surface light source device50is reflected. The details of the output signal SRGBW generation subprocess will be described later.

(Step S07) The image processing unit22performs reverse gamma conversion on the output signals SRGBW and outputs them to the image display panel drive unit40.

(Step S08) Display is performed. In synchronization with a synchronization signal STM generated by the timing generation unit21, the image display panel drive unit40outputs the output signals SRGBW to the image display panel30and the light source drive unit60drives the light sources56of the surface light source device50.

By performing the above process, an image of the input signal SRGB is reproduced on the image display panel30. The luminance of the surface light source device50which lights the image display panel30is controlled for each block according to the input signal SRGB. This reduces the luminance of the surface light source device50and reduces power consumption. Furthermore, luminance information on the surface light source device50calculated for each pixel is reflected in each output signal SRGBW. This maintains image quality and improves contrast.

The image analysis subprocess will now be described by the use ofFIG. 17.FIG. 17is a flow chart of the image analysis subprocess in the second embodiment. The image analysis unit23acquires the input signal SRGB and starts the subprocess. The emission surface of the surface light source device50is divided into (I×J) blocks.

(Step S31) The image analysis unit23initializes a block number (i, j) by which a block to be processed is designated (sets a block number (i, j) to (1, 1)).

(Step S32) The image analysis unit23reads an input signal SRGB corresponding to each pixel included in a designated block (i, j).

(Step S33) The image analysis unit23detects an α value for each pixel. To be concrete, the image analysis unit23finds saturation S(p, q)and value V(S)(p, q)in the cylindrical HSV color space from an input signal SRGB corresponding to a target pixel by the use of expressions (5) and (6). The image analysis unit23finds an α value for the pixel from the saturation S(p, q)and the value V(S)(p, q)obtained in this way by the use of expression (9). The image analysis unit23repeats the same procedure to find α values for all pixels included in the block (i, j).

(Step S34) The image analysis unit23determines a required luminance value for the block (i, j) on the basis of at least one of the α values for all the pixels. For example, the image analysis unit23selects the smallest α value from among the α values for all the pixels included in the block (i, j), and considers the reciprocal 1/α of the smallest α value as a required luminance value for the block (i, j).

(Step S35) The image analysis unit23compares the block number (i, j) with the last block number (I, J) and determines whether or not the block (i, j) is the last block. If (i, j)=(I, J), then the image analysis unit23determines that the block (i, j) is the last block. In this case, the image analysis unit23has calculated required luminance values for all the blocks. Accordingly, the image analysis unit23ends the image analysis step. If the block (i, j) is not the last block, then the image analysis unit23proceeds to step S36.

(Step S36) The image analysis unit23increases the block number (i, j) by 1 and returns to step S32.

Luminance required values for the (I×J) blocks are calculated through the above procedure. By calculating a required luminance value in this way on the basis of an input signal SRGB expanded into the reproduction HSV color space, the required luminance value corresponds to an image whose luminance is increased by the fourth subpixel which displays the fourth color. Therefore, the luminance of the surface light source device50is low and power consumption is low, compared with a case where a required luminance value is simply found on the basis of an input signal SRGB. Furthermore, a required luminance value is determined for each block, so power consumption is reduced efficiently compared with a case where required luminance values are determined for the entire display surface.

The lighting pattern determination subprocess will now be described by the use ofFIG. 18.FIG. 18is a flow chart of the lighting pattern determination subprocess in the second embodiment. After a required luminance value is determined for each block, the lighting pattern determination subprocess is started.

(Step S41) The lighting pattern determination unit25sets a tentative lighting pattern which determines a lighting amount of each light source56of the sidelight light source52.

(Step S42) The lighting pattern determination unit25generates tentative luminance distribution information (luminance distribution information obtained while tentatively driving each light source56) on each light source56at the time of lighting the sidelight light source52according to the set tentative lighting pattern. The lighting pattern determination unit25calculates tentative luminance distribution information on each light source56by referring to a corresponding light-source-specific lookup table240and converting luminance information at the time of lighting each light source56at a determined lighting amount, which is set in the light-source-specific lookup table240, to luminance information at the time of lighting each light source56at a lighting amount of the tentative lighting pattern.

(Step S43) The lighting pattern determination unit25combines the tentative luminance distribution information obtained for each light source in step S42to obtain tentative luminance distribution information on the surface light source device50.

(Step S44) The lighting pattern determination unit25compares the tentative luminance distribution information on the surface light source device50for the tentative lighting pattern obtained in step S43with required luminance values. For example, the lighting pattern determination unit25compares each piece of luminance information included in the tentative luminance distribution information with a required luminance value for a corresponding block, and detects whether or not the difference between them is in a determined range.

(Step S45) If the tentative luminance distribution information on the surface light source device50for the tentative lighting pattern satisfies the required luminance values as a result of the comparison in step S44, then the lighting pattern determination unit25proceeds to step S47. If the tentative luminance distribution information on the surface light source device50for the tentative lighting pattern does not satisfy the required luminance values, then the lighting pattern determination unit25proceeds to step S46.

(Step S46) If the tentative luminance distribution information on the surface light source device50for the tentative lighting pattern does not satisfy the required luminance values, then the lighting pattern determination unit25corrects the tentative lighting pattern according to the difference between them. The lighting pattern determination unit25repeats the subprocess from step S42for a tentative lighting pattern after the correction.

(Step S47) If the tentative luminance distribution information on the surface light source device50for the tentative lighting pattern satisfies the required luminance values, then the lighting pattern determination unit25also performs dimming to determine a lighting pattern. In the dimming, the lighting pattern determination unit25refers to the luminance of each light source56in the previous image display frame and corrects a lighting amount of each light source56so that a sudden change in luminance will not take place.

As has been described, a tentative lighting pattern is set, tentative luminance distribution information is calculated for the tentative lighting pattern, the tentative luminance distribution information is compared with required luminance values, and the tentative lighting pattern is corrected. This operation is repeated. That is to say, by performing simple calculations, an optimum lighting pattern of the sidelight light source52is set. Furthermore, tentative luminance distribution information is calculated by referring to the light-source-specific lookup tables240in place of performing calculations by the use of expression (10), so the amount of calculation is reduced.

The luminance information calculation subprocess will now be described by the use ofFIG. 19.FIG. 19is a flow chart of the luminance information calculation subprocess in the second embodiment. After a lighting pattern of the sidelight light source52is determined, the luminance information calculation subprocess is started.

(Step S51) The luminance information calculation unit26generates actual luminance distribution information (luminance distribution information obtained while actually driving each light source56) for each light source at the time of driving the sidelight light source52according to the determined lighting pattern. The luminance information calculation unit26calculates actual luminance distribution information for each light source by referring to a corresponding light-source-specific lookup table240and converting luminance information set in the light-source-specific lookup table240to luminance information at the time of lighting a light source56at a lighting amount of the lighting pattern. The luminance information calculation unit26obtains in this way actual luminance distribution information for each light source at the time of driving the sidelight light source52according to the lighting pattern. The actual luminance distribution information for each light source obtained consists of luminance information on a representative pixel in each of the (m×n) areas obtained by dividing the display surface of the image display panel30.

(Step S52) The luminance information calculation unit26performs interpolation calculation by the use of luminance information on a representative pixel included in the actual luminance distribution information for each light source found in step S51to calculate actual luminance distribution information for each light source and for each pixel.

(Step S53) The luminance information calculation unit26combines the actual luminance distribution information obtained for each light source and for each pixel in step S52to find actual luminance distribution information on the entire surface light source device50.

The actual luminance distribution information including luminance information for each pixel on the surface light source device50is obtained in this way.

The output signal SRGBW generation subprocess will now be described by the use ofFIG. 20.FIG. 20is a flow chart of the output signal SRGBW generation subprocess in the second embodiment. After actual luminance distribution information including luminance information for each pixel on the surface light source device50is generated, the output signal SRGBW generation subprocess is started.

(Step S61) The image processing unit22initializes a pixel number (p, q) by which a pixel to be processed is designated (sets a pixel number (p, q) to (1, 1)).

(Step S62) The image processing unit22reads luminance information on a pixel (p, q) to be processed included in the actual luminance distribution information including the luminance information for each pixel on the surface light source device50.

(Step S63) The image processing unit22calculates from the luminance information on the pixel (p, q) to be processed the expansion coefficient α for expanding an input signal SRGB. If the luminance of light which the surface light source device50directs at the pixel (p, q) to be processed is 1/α, then the luminance of an image is increased α-fold in order to reproduce the input signal SRGB on the display surface. Accordingly, the image processing unit22calculates the reciprocal of the read luminance information on the pixel (p, q) to be processed as the expansion coefficient α.

(Step S64) The image processing unit22uses the expansion coefficient α for expanding an input signal SRGB corresponding to the pixel (p, q) to be processed and generating an output signal SRGBW. To be concrete, the image processing unit22applies expressions (1), (2), (3), and (4) to an input signal value x1(p, q)for the first subpixel, an input signal value x2(p, q)for the second subpixel, and an input signal value x3(p, q)for the third subpixel included in the input signal SRGB to calculate an output signal value X1(p,q)for the first subpixel, an output signal value X2(p,q)for the second subpixel, an output signal value X3(p,q)for the third subpixel, and an output signal value X4(p,q)for the fourth subpixel.

(Step S65) The image processing unit22compares the pixel number (p, q) with the last pixel number (P, Q) to determine whether or not the pixel (p, q) is the last pixel. If (p, q) is (P, Q), then the image processing unit22determines that the pixel (p, q) is the last pixel. In this case, output signals SRGBW for all pixels have been generated, so the image processing unit22ends the output signal SRGBW generation subprocess. If the pixel (p, q) is not the last pixel, then the image processing unit22proceeds to step S66.

(Step S66) The image processing unit22increases the pixel number (p, q) by 1 and returns to step S62.

By performing the above subprocess, a proper output signal SRGBW corresponding to the luminance of the surface light source device50which lights each pixel is calculated. As a result, proper display is performed.

The above processing functions can be realized with a computer. In that case, a program in which the contents of the functions that the display device has are described is provided. By executing this program on the computer, the above processing functions are realized on the computer. This program may be recorded on a computer readable record medium. A computer readable record medium may be a magnetic recording device, an optical disk, a magneto-optical recording medium, a semiconductor memory, or the like. A magnetic recording device may be a HDD (Hard Disk Drive), a FD (Flexible Disk), a magnetic tape, or the like. An optical disk may be a DVD (Digital Versatile Disc), a DVD-RAM (Random Access Memory), a CD-ROM (Compact Disc Read Only Memory), a CD-R (Recordable)/RW (ReWritable), or the like. A magneto-optical recording medium may be a MO (Magneto-Optical disk) or the like.

To place the program on the market, portable record media, such as DVDs or CD-ROMs, on which it is recorded are sold. Alternatively, the program is stored in advance in a storage unit of a server computer and is transferred from the server computer to another computer via a network.

When a computer executes this program, it will store the program, which is recorded on a portable record medium or which is transferred from the server computer, in, for example, its storage unit. Then the computer reads the program from its storage unit and performs processes in compliance with the program. The computer may read the program directly from a portable record medium and perform processes in compliance with the program. Furthermore, each time the program is transferred from the server computer connected via a network, the computer may perform processes in order in compliance with the program it receives.

In addition, at least a part of the above processing functions may be realized by an electronic circuit such as a DSP (Digital Signal Processor), an ASIC (Application Specific Integrated Circuit), or a PLD (Programmable Logic Device).

According to one aspect, there is provided a display device that includes: an image display panel that includes a plurality of pixels, each of which includes a first subpixel which displays a first primary color, a second subpixel which displays a second primary color, a third subpixel which displays a third primary color, and a fourth subpixel which displays a fourth color; a lighting unit which emits light to the image display panel from the rear of the image display panel; and a control unit which calculates a required luminance value for each block obtained by dividing the display surface of the image display panel on the basis of an input image signal, which determines a light source lighting amount of the lighting unit on the basis of luminance distribution information on the lighting unit stored in advance so as to satisfy the required luminance value, which generates luminance information on each pixel on the basis of the luminance distribution information and the light source lighting amount, which generates an output image signal that drives the first subpixel, the second subpixel, the third subpixel, and the fourth subpixel on the basis of the luminance information and the input image signal, which controls the lighting unit by the light source lighting amount, and which controls the image display panel by the output image signal.

In the display device, the control unit calculates a block correspondence index corresponding to each block for adjusting luminance of the lighting unit on the basis of at least one of saturation and a value of the input image signal corresponding to pixels included in each block, and calculates the required luminance value on the basis of the block correspondence index.

Further, in the display device, the control unit calculates a first pixel correspondence index corresponding to each pixel for reducing luminance of the lighting unit on the basis of the luminance information, and generates the output image signal using a second pixel correspondence index corresponding to the first pixel correspondence index for increasing luminance of each pixel.

Still further, in the display device, the lighting unit includes a plurality of light sources which can operate independently of one another, and the control unit determines lighting patterns of the plurality of light sources so as to satisfy the required luminance value.

Still further, in the display device, the control unit sets tentative lighting patterns of the plurality of light sources, generates, on the basis of the tentative lighting patterns and the luminance distribution information, tentative luminance distribution information at the time of driving the lighting unit using the tentative lighting patterns, corrects the tentative lighting patterns by comparing the tentative luminance distribution information with the required luminance value, and determines the lighting patterns.

Still further, in the display device, the luminance distribution information is stored by light source units with one light source or a combination of two or more light sources, of the plurality of light sources, as one light source unit, and the control unit generates tentative luminance distribution information for each of the light source units on the basis of the tentative lighting patterns and the luminance distribution information for each of the light source units, and combines the tentative luminance distribution information for the light source units to generate the tentative luminance distribution information on the entire lighting unit.

Still further, in the display device, the luminance distribution information includes luminance information on a representative pixel which represents pixels in a determined area of the display surface, and the control unit generates luminance information for each pixel on the lighting unit by performing interpolation calculation by the use of the luminance information on the representative pixel.

Still further, in the display device, the fourth subpixel included in each pixel displays white, and an output value is determined on the basis of at least one of a value of the first primary color, a value of the second primary color, and a value of the third primary color corresponding to the input image signal, and luminance of each pixel of the image display panel is adjusted on the basis of the output value and output values for the first subpixel, the second subpixel, and the third subpixel determined according to the output value.

In addition, according to one aspect, there is provided a display device that includes: an image display panel including a plurality of pixels, each of which includes a first subpixel which displays red, a second subpixel which displays green, a third subpixel which displays blue, and a fourth subpixel which displays white; a lighting unit which emits light to the image display panel from a rear of the image display panel; and a control unit which calculates a required luminance value for each of blocks obtained by dividing a display surface of the image display panel on the basis of an input image signal corresponding to the red, the green, and the blue, which determines a light source lighting amount of the lighting unit on the basis of luminance distribution information on the lighting unit stored in advance so as to satisfy the required luminance value, which generates luminance information on each pixel on the basis of the luminance distribution information and the light source lighting amount, which generates an output image signal corresponding to the red, the green, the blue, and the white on the basis of the luminance information and the input image signal, which controls the lighting unit by the light source lighting amount, and which controls the image display panel by the output image signal.

In addition, there is provided a method for driving a display device that includes: an image display panel including a plurality of pixels each of which includes a first subpixel which displays a first primary color, a second subpixel which displays a second primary color, a third subpixel which displays a third primary color, and a fourth subpixel which displays a fourth color; and a lighting unit which emits light to the image display panel from a rear of the image display panel. The method includes: calculating a required luminance value for each of blocks obtained by dividing a display surface of the image display panel on the basis of an input image signal; determining a light source lighting amount of the lighting unit on the basis of luminance distribution information on the lighting unit stored in advance so as to satisfy the required luminance value; generating luminance information on each pixel on the basis of the luminance distribution information and the light source lighting amount; generating an output image signal which drives the first subpixel, the second subpixel, the third subpixel, and the fourth subpixel on the basis of the luminance information and the input image signal; controlling the lighting unit by the light source lighting amount; and controlling the image display panel by the output image signal.