Phos Hilaron (Koinē Greek: Φῶς Ἱλαρόν, romanized: Fόs Ilarόn) is an ancient Christian hymn originally written in Koine Greek. Often referred to in the Western Church by its Latin title Lumen Hilare, it has been translated into English as O Gladsome Light. It is the earliest known Christian hymn recorded outside of the Bible that is still in use today. The hymn is part of vespers in the Byzantine Rite and the Roman catholic church, and also included in some Anglican and Lutheran liturgies.