Luminous Armor doesn’t cause damage it applies stacks of Radiating Orb in a 10’ radius around the target not the equipped character, so it works at range. It can generate multiple overlapping AoEs with a single ability use for 10+ stacks of Radiating Orb depending on enemy spacing. It’s probably one of the strongest armors in the game. That said with the Gloves and Coruscation Ring equipped you will be limited to 9 stacks, if you unequip those be careful as the game will crash at about 40 stacks.
The Callous Glow Ring just makes it so that all of your damage sources deal radiant damage meaning more procs of Luminous Armor’s AoE debuff.