A novel measurement method has been developed to optically measure ion density in TFT-LCD panels. We investigated the optical response of twisted-nematic (TN) cells when ions were transported by applying an alternating electric field. A characteristic flickering of transmitted light was observed when a rectangular voltage with a low frequency of 1 Hz was applied to the cells. A peak in light intensity was observed during voltage application. Its height was found to be related to ion density in a linear way. This density was measured by using the transient current technique. We conclude that ion density can be determined by measuring optical response.