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The photo effect of gap-gate type hydrogenated amorphous silicon thin-film transistor (a-Si:H TFT) has been studied to be employed as light sensors owing to its outstanding photosensitivity. It can be operated in on region to provide a higher current level as the sensing signal. However, the gap-gate a-Si:H TFT suffers from some problems such as the photo-current degradation resulted from illuminations and the disturbance of the backlight source when it is used as the sensor in display panel. A new method is proposed to overcome the above issues, which gives a feasible way for this kind of TFT to be used in TFT LCD panel. In this paper, the operation of the sensing method is fully described.