the Phos hilaron is an ancient trinitarian hymn. this hymn is used during the lighting of lamps in evening services. when that service fell out of use in the western church, so did the regular use of this hymn. it never stopped being used in the eastern church.
many modern revisions of the office in the west have reintroduced its use at the beginning of the evening office; the episcopal church appoints it for evening prayer every day.
o gracious Light,
pure brightness of the everliving Father in heaven,
o Jesus Christ, holy and blessed!
now as we come to the setting of the sun,
you are worthy at all times to be praised by happy voices,