Conventionally, a liquid crystal display device (LCD) or the like is known as a flat display device having a thin thickness, a small size, and a low power consumption. Recently, display devices have been developed which use a light emitting element (electroluminescence element) in each pixel. In particular, organic light emitting display devices (hereinafter referred to as “OLED display devices”) which use an organic light emitting element (hereinafter referred to as “OLED element”) in which an organic material is used in a light emitting material or the like are being developed and researched.
The OLED element is a current-driven, self-emissive element which emits light at a luminance corresponding to a current flowing through the element. Therefore, the OLED elements have advantages that the viewing angle dependency which is observed in an LCD is low, that the visibility is high because no light source is required, that a display device with a low power consumption can be achieved with a smaller space, etc., and expectations are rising.
In the display devices which use the OLED element, a current which is approximately proportional to the average luminance of a displayed image flows in the display panel as a whole. Thus, when a darker image is to be displayed, the power consumption is very low, but the power consumption of the panel is increased as the image becomes brighter. When light emission of luminance near the maximum luminance continues in all pixels, the advantage of the low power consumption with the OLED panel is reduced.
Moreover, currently, the OLED elements are known to have problems regarding lifetime. The lifetime and the power consumption of the element depend on a product of the light emission luminance and the light emission period. In consideration of this, U.S. Pat. No. 6,806,852 (hereinafter referred to as “'852 reference”) discloses limitation of the light emission luminance in order to extend the lifetime of the element and reduce the power consumption. The '852 reference discloses that pixel data is stored in units of frames, average brightness or the like is calculated for the data, and a brightness reduction process is applied to image frame data according to the calculation result.
Japanese Patent Laid-Open Publication No. Hei 7-322179 (hereinafter referred to as “'179 reference”) discloses that pixel data is stored in units of frames, a histogram is calculated, a correction value of γ correction with respect to pixel data is adjusted based on the result of the calculation, and the brightness is adjusted for the pixel data in order to inhibit black and white saturations and improve contrast in LCDs and in PDPs.
With the brightness reduction process as described in the '852 reference, driving data can be created to not exceed a limitation current of a panel, the current flowing through each OLED element can be reduced, and the power consumption can be reduced. However, in the process of the '852 reference, a frame memory must be provided and accurate control cannot be applied unless the displayed frame and the frame used for calculation are identical. From a technical point of view, the frame memory may be omitted in order to reduce the size of the circuit or reduce the cost. However, when the frame memory is omitted, the response is delayed by one frame. In other words, rapid change in brightness cannot be handled and the current exceeds the limit current for at least one frame period. In this structure, sufficient reduction of power consumption and sufficient elongation of the lifetime of the OLED element cannot be achieved. In addition, when the current exceeds the limit current, the display luminance may rise rapidly and the raised luminance continues for approximately one frame period, and thus, a viewer may notice the high brightness, resulting in degradation of the display quality.
In a brightness adjusting method of the '179 reference also, a frame memory is required. When no frame memory is provided, accurate control cannot be applied. If a frame memory is omitted, when a brightness level of pixel data rises rapidly, such a case cannot be handled and the inhibition advantage of the power consumption is reduced. Moreover, because a correction value which is set to achieve a superior contrast near the black level until immediately before the rise of the brightness level, for example, is applied, gradation at the white level side is lost and there is a problem in that an image of white saturation tends to be displayed, resulting in a problem of degradation of display quality.
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USPTO Backgrounds