1. Field of the Invention
The present invention relates to a touch panel including a photo sensor and a driving method thereof. In particular, the present invention relates to a touch panel including a plurality of pixels each of which is provided with a photo sensor and relates to a driving method thereof. Further, the invention relates to electronic devices including the touch panel.
2. Description of the Related Art
In recent years, a display device provided with a touch sensor has attracted attention. The display device provided with the touch sensor is called a touch panel a touch screen, and the like (hereinafter simply referred to as a “touch panel”). Examples of the touch sensor include a resistive touch sensor, a capacitance touch sensor, and an optical touch sensor, depending on its operation principle. In any of the sensors, when an object to be detected is in contact with a display device or in the vicinity of the display device, data can be input.
By providing of a sensor (also referred to as a “photo sensor”) that detects light as an optical touch sensor for a touch panel, a display screen serves as an input region. As an example of a device including such an optical touch sensor, a display device having an image capturing function, which is provided with a contact type area sensor that captures an image, is given (e.g., see Patent Document 1). In the case of a touch panel including an optical touch sensor, light is emitted from a touch panel. In the case where an object to be detected exists at an arbitrary position of the touch panel, light at the region where the object to be detected exists is blocked by the object to be detected, and part of light is reflected. A photo sensor (also referred to as a “photoelectric conversion element”) which can detect light is provided in a pixel of the touch panel, and the photo sensor detects the reflected light, so that the existence of the object to be detected in the region where light is detected can be recognized.
In addition, providing a personal authentication function or the like for an electronic device such as a mobile phone or a portable information terminal has been attempted (e.g., see Patent Document 2). A fingerprint, a face, a handprint, a palm print, a pattern of a hand vein, and the like are used for personal authentication. In the case where a portion different from the display portion has a personal authentication function, the number of components increases, and the weight or price of the electronic device could possibly increase.
In addition, in touch sensor systems, a technique for selecting an image processing method by which the position of a finger-tip is detected in accordance with brightness of external light has been known (e.g., see Patent Document 3).