1. Field of the Invention
The present invention relates to a display apparatus and a control method of the display apparatus.
2. Description of the Related Art
A liquid crystal display apparatus is a display apparatus using the light transmissivity of a liquid crystal panel, and displays an image by allowing light, exited from a light emitting unit (backlight) that is provided on a back surface of a liquid crystal panel, to be transmitted through the liquid crystal panel or to be blocked by the liquid crystal panel.
According to a general transmissive liquid crystal panel, voltage is applied between transparent conductive films formed on two glass substrate modules, and the alignment of liquid crystal in a liquid crystal layer provided between the substrates is controlled. The transmissive liquid crystal panel functions as a liquid crystal shutter by actions of the liquid crystal layer, and a polarizing plate and a light distribution film that are provided on the front and the back of the liquid crystal layer. As the light emitted from the backlight is mostly absorbed by the light distribution film and the polarizing plate, light utilization efficiency is as low as several percent. The light utilization efficiency is the ratio of the light transmitted through the liquid crystal panel, to the light emitted from the backlight, for example.
In recent years, however, a liquid crystal panel having high light utilization efficiency has been developed, such as a liquid crystal panel of a micro electro-mechanical-systems (MEMS) shutter type, a scattering type or the like. There is a liquid crystal panel using polymer dispersed liquid crystal (PDLC) that allows small particles of nematic liquid crystal to be dispersed in polymer material, for example. In the polymer dispersed liquid crystal, the refractive index difference between the polymer material and the nematic liquid crystal is controlled by a voltage. By controlling the refractive index difference, the scattered state and the unscattered state are switched with each other. In other words, by controlling the refractive index difference, the polymer dispersed liquid crystal functions as the liquid crystal shutter. Therefore, such a liquid crystal panel does not require the polarizing plate and the light distribution film, and is able to obtain a light utilization efficiency as high as about 60 to 80 percent.
In the backlight, a plurality of light-emitting diodes (hereinafter referred to as LEDs) are used as light sources. The LEDs to be used as the light sources include, for example, a white LED as W (white), and color LEDs as R (red), G (green) and B (blue). Methods of adjusting the light emission brightness and the light emission color of each LED include a method of controlling a current value or a voltage value to be applied (PHM control), a method of controlling the application time of the current or voltage (that is, a light emission period of the LED) (PWM control), a method of performing both of the PHM control and the PWM control, and the like. By performing the PHM control and the PWM control, it is possible to obtain light having the desired light emission brightness and light having a desired color (white balance).
However, the brightness of the LEDs changes due to temperature characteristics and aging degradation. In addition, as the LEDs possess individual differences, the light emission brightness is different among the LEDs when the plurality of LEDs are driven under the same condition. In view of the above, such technology has been proposed that a brightness sensor provided inside a backlight housing is used to perform feed-back control of the light emission brightness of the backlight, in order to maintain the light emission brightness of the backlight constantly (for example, Japanese Patent Application Laid-open No. 2006-278107 and Japanese Patent Application Laid-open No. 2006-276725).
In order to perform feed-back control so that the light emission brightness of the backlight is maintained constantly, it is necessary to accurately detect the light emission brightness of the backlight (amount of light emission of the LEDs). However, the brightness sensor provided inside the backlight housing detects light containing direct light from the backlight (LEDs) and return light from the liquid crystal panel (reflected light that is reflected and returned from the liquid crystal panel, scattered light that is propagated and scattered in the liquid crystal panel and is returned back from the liquid crystal panel, and the like).
According to the general transmissive liquid crystal panel, the light emitted from the backlight is mostly absorbed by the polarizing plate and the light distribution film, and the return light from the liquid crystal panel is returned in minute amounts. Therefore, ignoring the brightness of the return light contained in the brightness detected by the brightness sensor does not present a big problem. However, according to a liquid crystal panel having the high light utilization efficiency, the ratio of the return light to the light detected by the brightness sensor increases, as the liquid crystal panel does not include the polarizing plate and the light distribution film. In addition, the ratio of the return light changes significantly, as the scattered light from the liquid crystal panel changes significantly, depending on a light distribution state of the liquid crystal corresponding to an image to be displayed (a transmission state and a blocking state, for example). In other words, a brightness value acquired in the brightness sensor changes, depending on the image to be displayed, even when the light emission brightness of the backlight does not change. For this reason, according to the liquid crystal panel having the high light utilization efficiency, it is not possible to ignore the brightness of the return light that is contained in the brightness detected by the brightness sensor.
As the above-described conventional technology does not give consideration to the return light from the liquid crystal panel, it is not possible to detect the light emission brightness of the backlight (the amount of light emission of the LEDs) with high precision. According to the liquid crystal display apparatus having the liquid crystal panel with the high light utilization efficiency, in particular, it is not possible to detect the light emission brightness of the backlight with high precision. For this reason, accurate feed-back control may not be made, and the light emission brightness of the backlight may not be maintained constantly.