| abstract |
Photo sensors constituted by diodes can not be refreshed due to their structure, and leak characteristics thereof are unstable when not illuminated with light. Hence, diodes are not suitable for photo sensors. On the other hand, light quantity of photo sensors constituted by thin film transistors is very small, therefore difficult to be used for feedback. According to the present invention, a detection circuit for converting output current to voltage is appended to a photo sensor of thin film transistor. Thereby, a small current can be converted to a voltage within a desired range with which feedback operation is possible. Moreover, by changing number of connections of resistors, capacitors and photo sensors in the circuit, sensitivity of photo sensor can be modified. |