'''Phos Hilaron''' (Φως ΙλαρόνΦῶς ἱλαρὸν) is an ancient [[Christian]] [[hymn]] originally written in [[Koine Greek]]. The hymn is known in English as ''''Hail Gladdening Light,'''' or ''''O Gladsome Light.'''' It is the earliest known Christian hymn, recorded outside of the Bible, that is still being used today. The hymn is featured in the [[Vespers]] of the [[Orthodox Church]].