Patent ID: 7936346

Claim:
A liquid crystal display device comprising: a liquid crystal display panel; a photodetector unit that is built into the liquid crystal display panel and has a TFT ambient light photosensor made of a thin film transistor (TFT) that has a source electrode, a drain electrode, and a gate electrode, and senses external light; an ambient light photosensor reader that reads a voltage due to photo-leakage from the TFT ambient light photosensor; and a control means that controls the photodetector unit and the ambient light photosensor reader, and controls an illuminating unit that illuminates the liquid crystal display panel according to an output from the ambient light photosensor reader; the photodetector unit having a switching element coupled between a source line coupled to the source electrode and a drain line coupled to the drain electrode of the TFT ambient light photosensor, and the control means short-circuiting the source line and the drain line of the TFT ambient light photosensor by keeping the switching element turned on when changing a gate voltage to be applied to a gate line coupled to the gate electrode of the TFT ambient light photosensor, wherein the gate voltage of the TFT ambient light photosensor in a negative voltage applied state is changed so as to be converted to a positive voltage and then converted to an off voltage after a certain period of time, and the switching element is kept turned on during a time period from just before the gate electrode is converted from the negative voltage to the positive voltage until just after the gate voltage is converted from the positive voltage to the off voltage.