I don’t know much about the origin of this hymn (also spelled Iucunda lux tu gloriae), except that it’s originally a metrical rendering of the Greek hymn Phō̂s hilarón (known to American Episcopalians from its use in their Evensong liturgy) into Latin.

The only real sources I was able to find about it, based on a Google search, are its use in Odd Hours by Michael Church, a Lutheran book of daily office liturgy; and a post on the Musica Sacra forum mentioning that its plainsong melody is that of Jesu dulcis memoria, which I have reproduced below.¹

1. O joyful glorious light thou art,
   O source of light come forth from light,
   O blessed Jesu, heav’nly King;
   begotten from the Father’s might.

2. The brightness of the day is gone,
   the fading light of Sun we see,
   and now the hour of dusk is come,
   with holy songs acknowledge thee.

3. We praise thee, one and only God:
   our Father from eternity;
   thy blessed Son and Holy Ghost,
   the glorious Holy Trinity.

4. May lips show forth thy rightful praise
   and age to age thy worship sing;
   O Son of God, who givest life,
   throughout the world thy praises ring. Amen.

1. Jucunda lux tu gloriae,
   fons luminis de lumine,
   beate Jesu caelitus
   a Patre sancto prodiens.

2. Fulgor diei lucidus
   solisque lumen occidit,
   et nos ad horam vesperam
   te confitemur cantico.

3. Laudamus unicum Deum,
   Patrem potentem,
   Filium cum Spiritu Paraclito
   in Trinitatis gloria.

4. O digne linguis qui piis
   lauderis omni tempore,
   Fili Dei, te saecula
   vitae datorem personent. Amen.