Patent ID: 7940252

Claim:
An optical sensor, comprising: an active matrix area having a plurality of sensor pixel circuits arranged in matrix form; and a plurality of scan lines and readout lines arranged in the active matrix area such that the scan lines and the readout lines respectively cross each other at one of the sensor pixel circuits, wherein each of the sensor pixel circuits includes a first readout TFT for reading out voltage of a charge node, a second readout TFT for controllably resetting the charge node to a first reset voltage, a photo TFT for discharging the voltage at the charge node to a second reset voltage in absence of an object or objects, the second reset voltage being less than the first reset voltage, and a storage capacitor with a first end connected to the charge node and a second end connected to receive the second reset voltage; wherein the storage capacitor is reset by the second readout TFT such that the first end is at the first reset voltage and the second end is at the second reset voltage, thereby resulting in a voltage difference between the first reset voltage and the second reset voltage across the storage capacitor.