1. A video signal monitoring method comprising:a detecting process in which a sensor installed in a display device detects at least of luminance or chromaticity in a predetermined region among regions of a video displayed on the display device;
a sensing result converting process of converting the at least one of detected luminance or chromaticity into video identification information for identifying the video; and
a sensing result reading process of outputting the video identification information and a clock time at which the video was displayed.