From the Wikipedia: "The Imperial Gate mosaic is located in the tympanum above that gate, which was used only by the emperors when entering the church. Based on style analysis, it has been dated to the late 9th or early 10th century. The emperor with a nimbus or halo could possibly represent emperor Leo VI the Wise or his son Constantine VII Porphyrogenitus bowing down before Christ Pantocrator, seated on a jeweled throne, giving His blessing and holding in His left hand an open book. The text on the book reads as follows: "Peace be with you. I am the light of the world". (John 20:19; 20:26; 8:12) On each side of Christ's shoulders is a circular medallion: on His left the Archangel Gabriel, holding a staff, on His right His Mother Mary."