It’s often forgotten, overlooked, and unused. It might be time to revisit the Dungeon Masters guide and kick it back into your game.
Our friend Gary Gygax explains it as:
The ability to see radiation above violet in the normal visible spectrum.
It’s not of much use underground where radiation gets shielded, but an undead hunter above ground can see to 100 yards clearly and 300 dimly. This odd ability is glazed over heavily. It may be applied to a number of night hunter races or undead to plague your players.
Ultra vision is ruined by radiating magical weapons (Which is pretty common in AD&D). Of course when the weapons are out, you are ruining infravision with light. Players are also advertising to ALL that they are holding a valuable magical item in the open.
If you consider your undead or some breeds of kobolds having ultravision, they can see just fine at night, through fog, darkness, and many other situations. They may hunt the quarry of the party as if it were open daylight. They may see through things that would block infravision. The heroes at your table are likely unaware of this often forgotten AD&D ability.
Turn to page 56 of the AD&D DMG to revisit this ability and sprinkle it in when least expected for some unusual encounters!