As for liquid crystal display devices, it is possible to realize thinness, low-power consumption, and high definition. Recently, liquid crystal display devices have been becoming larger rapidly due to progress of manufacturing technology and diffuse in the field of television in which cathode-ray tubes (CRT) are mainly used conventionally.
However, it has been pointed out that display images of liquid crystal display devices have problems in image quality such as a low contrast feeling and blurred moving images due to its display method, compared to images of CRT.
To begin with, the low contrast feeling of images of the liquid crystal display device is described, comparing with CRT with reference to FIG. 10 (a).
As for CRT, peak brightness changes dynamically according to average luminance level (APL) of image signals in one frame. For instance, contrast between bright parts and dark parts in a display screen is enhanced by displaying images of high APL (i.e., an entire display screen is bright) with low peak brightness or displaying images of low APL (i.e., an entire display screen is dark) with high peak brightness. Thus, a CRT can display sharp images giving a high contrast feeling.
In contrast, general liquid crystal display devices employs a display method in which the intensity of illumination light from a backlight is always constant and a liquid crystal panel controls the light transmittance of the illumination light. As a result, an image gives a low contrast feeling and lacks sharpness compared to an image of a CRT because a peak brightness does not change according to an average luminance level (APL) in the general liquid crystal display devices.
The causes why the performance of moving image display of the liquid crystal display devices is said to be inferior to that of CRT is described as to each of their display methods below with reference to FIG. 10 (b).
CRT adopts so-called an impulse type display method. In the impulse type display method, the image is displayed by scanning pixels with an electron beam in one frame thereby causing fluorescent material of the pixels to emit light. Thus, the pixels do not emit light except when they receive the electron beam. As a result, observer's eyeballs smoothly follow a moving object in a moving picture.
In contrast, the general liquid crystal display device adopts a hold type display method. In the hold type display method, an image is displayed by controlling transmittance of the illumination light from the light source by applying an electric field on liquid crystal in a liquid crystal panel. In each pixel, the transmittance is “held” by applying a voltage on the liquid crystal in one frame period. Thus, the moving picture, which changes time to time, is displayed as one still image in each frame period. As a result, there is a difference between the moving picture to be displayed and the still image that is actually displayed. A viewer perceives this difference as blurring.
In order to solve the problems of liquid crystal display devices above, Japanese Unexamined Patent Publication No. 40390/2002 (Tokukai 2002-40390 (published on Feb. 6, 2002)) discloses a liquid crystal display device which improves a contrast feeling by changing the ratio of an illumination period to a non-illumination period according to a display image in order to control the intensity of illumination light and has excellent performance in moving image display.
FIG. 11 is a block diagram of a liquid crystal display device disclosed in the publication above.
As illustrated in the figure, a liquid crystal display device 100 includes a liquid crystal panel 101 and a backlight 110 including a light source 111, a lightguide 112, and a liquid-crystal shutter 113.
The liquid crystal panel 101 is, what is called, an active-matrix liquid crystal panel. Pixels are arrayed in a matrix pattern corresponding to intersecting points of a plurality of scanning lines with a plurality of signal lines. Switching elements are provided corresponding to the pixels. An input image signal is supplied from a signal line to each pixel on one row selected by a scanning line. This is repeated by performing sequential scanning so as to apply the electric field to the liquid crystal of each pixel thereby controlling the light transmittance of the illumination light from the backlight. In this way, the display images are displayed.
The liquid crystal shutter 113 is divided into four parts in a stripe pattern in the direction of scanning lines of the liquid crystal panel 101 (in a horizontal direction). The electric field is applied on the liquid crystal in the liquid crystal shutter 113 in order to control transmittance and absorption of light thereof. Thus, the backlight 110 has four divided illumination areas in a stripe pattern and controls turning on/off of illumination in the divided illumination areas independently.
A peak luminance level detector circuit 121 is a circuit for measuring a peak luminance level of a display image signal. The peak luminance level detector circuit 121 measures a peak luminance level of an image displayed in a frame period in each divided area of the liquid crystal display device 100 corresponding to each of the divided illumination areas of the backlight 110.
The peak luminance level detector circuit 121 is connected to a backlight illumination control circuit 122. According to the measured peak luminance level, the backlight illumination control circuit 122 controls illumination ratio (illumination lump), that is, the intensity of illumination light in one frame period each divided illumination area of the backlight 110 independently.
A gradation converter circuit 123 converts a gradation of a display image signal on the basis of relation between the display image signal and the intensity of illumination light of each divided illumination area of the backlight 110, thereby generating an input image signal to be inputted to the liquid crystal panel 101.
FIG. 12 (a) and FIG. 12 (b) illustrate timing of when to input an input image signal to the liquid crystal panel 101 and the timing of when to turn on/off the backlight 110 in the liquid crystal display device 100. In FIG. 12 (a) and FIG. 12 (b), the horizontal axis indicates time; the vertical axis indicates vertical display positions of the liquid crystal display device.
As illustrated in FIG. 12 (a), in every one frame, the liquid crystal display device 100 controls the intensity of illumination light by changing the ratio of an illumination period to a non-illumination period of each of the divided illumination areas of the backlight 110 according to the peak luminance level of an image displayed on each of the divided areas of the liquid crystal display device 100. In other words, an illumination period of the backlight 110 is long and thereby the intensity of illumination light of the backlight is high in that divided area of the liquid crystal display device 100 whose display image is bright. On the other hand, in a divided area whose display image is dark, an illumination period of the backlight 110 is short and thereby the intensity of illumination light of the backlight 110 is low. Thus, it is possible to display sharp images giving a high contrast feeling due to enhanced contrast between the bright parts and the dark parts in a display screen.
In a case of moving image display, there are an illumination period and a non-illumination period of the backlight 110 according to the peak luminance levels in one frame period when brightness of a display image in each of the divided areas changes every frame period. With this, the liquid crystal display device of, so-called, the hold type display method can act like a liquid crystal display device of the impulse type display method, and thus attain better performance of moving image display.
However, the conventional liquid crystal display device 100 has problems described below.
As illustrated in FIG. 10 (b), with regard to the timing of when to and not to cause light emission of a specific pixel of the CRT of the impulse type display method, the specific pixel repeats illumination and non-illumination in a specific timing in one frame period. This timing does not change depending on a display image.
On the other hand, as for the conventional liquid crystal display device 100, the timing when to and not to display (i.e., the timing of when to and not to cause illumination of the backlight 110) in a specific divided area changes in one frame period according to a display image as illustrated in FIG. 12 (b). That is to say, a time center of a display image in a frame period, in other words, a time center of the intensity of illumination light fluctuates depending on a display image. This causes flickers perceivable to a viewer.