1. Field of the Invention
The present invention relates to a display apparatus which provides a capability of displaying a color image.
2. Description of the Related Art
As a color image display apparatus, a liquid crystal display has been proposed which includes a liquid crystal panel served to control a light transmittance of each pixel and a backlight unit combined therewith so that a color image may be displayed.
To display a color image, it is necessary to include at least three primary color components of RGB (Red, Green and Blue) in the backlight and locate sub-pixels each of which has one of at least RGB color filters as the pixels composing the liquid crystal panel. This arrangement makes it possible to control a light quantity on the overall range of a wavelength. Herein, the sub-pixel provides any one of RGB color filters and corresponds to a minimum unit of transmittance control. The pixel, termed herein, designates a combination of three sub-pixels of the RGB. Lots of pixels are ranged on the screen plane of the display apparatus. The other display apparatuses of a CRT (Cathode-ray Tube) system, a plasma system, a projector system and so forth have the same fundamental principle of display as that of the liquid crystal display apparatus, that is, those apparatuses also display a color image by properly arranging the pixels.
In the meantime, illumination brightness is considered as an environmental condition of a place where the display apparatus is located. An observer of a display screen watches an image in the synthesized light of ambient light reflected on the display screen and the light displayed by the display apparatus itself.
Letting a contrast R be a ratio of a maximum to a minimum of brightness of a display screen, the relation between the display light and the reflected light is represented as follows:R=(Maximum Display Light Quantity+Reflected Light Quantity)/(Minimum Display Light Quantity+Reflected Light Quantity)
In general, as the contrast R becomes greater, a visibility is made better. Herein, the maximum display light quantity designates a display light quantity corresponding with a maximum value of a display signal and the minimum display light quantity designates a display light quantity corresponding with a minimum value of a display signal. To improve the contrast, it is effective to make the maximum display light quantity greater or the reflected light quantity smaller.
To manage the contrast of the display screen, a method has been known in which a light sensor for sensing a brightness of ambient light is prepared so that an intensity (luminance) of the display light may be variably set according to the output of the light sensor.
The JP-A-2006-106294 discloses a technique of varying a luminous quantity of the backlight unit depending upon the ambient light, for example, in a gloomy room or a bright outdoor place. Concretely, during daylight, the technique is caused to increase the maximum display light quantity of the display screen by raising the luminous quantity of the backlight unit based on the output signal of the light sensor. This technique makes the contrast R higher and thereby the visibility better.
The US 2005/0225562 proposes a technique of adding a sub-pixel of W (White) to a combination of three RGB sub-pixels of RGB in order to improve the luminance of the display panel itself. The W sub-pixel provides no color filter, so that it has a high light transmittance and thus is effective in improving the luminance. Concretely, when the conventional pixels each composed of the RGB sub-pixels are compared with the pixels each composed of the RGBW sub-pixels on the same area, the area ratio of the sub-pixels of the former to the latter is made to be 4:3. The RGB color filters cut a wavelength distribution of a light source into one third, while the W color filter transmits a light quantity of a light source as it is. In light of this relation, the ratio of the maximum display light quantity of the RGB panel to the RGBW panel is made to be ((4+4+4)/3):((3+3;3)/3+3×1)=1:1.5
As a method of generating a RGBW signal to be used for driving the RGBW pixels, the following process has been proposed. The minimum values of the input color signals of the RGB are set to a W=MIN (R, G, B) signal or the values derived by subtracting W from the RGB color signals are newly set to a R′ G′ B′(R′=R−W, G′=G−W, B′=B−W) signal. By multiplying a proper amplification factor by the W signal, it is effective to improve the luminance.
Further, the technical background of the color reproducibility is discussed in detail in “Chromatic Science Handbook, Second edition, edited by Chromatic Academic Society of Japan, published by Tokyo University Publication, 1998”.