Smithing. Although it is classified as Light Armor, the Diadem of the Savant is a circlet and cannot be improved at a workbench, regardless of any perks the Dragonborn may have.

Is Nightingale Armor light or heavy?

Nightingale Armor is a set of light levelled armour given as a reward for the quest Trinity Restored during The Thieves' Guild quest line. It consists of 4 pieces: chest armor, gloves, boots and a hood all of which can be improved with Void Salts.

Is there a light version of Daedric Armor?

There are two different versions. One just changes the current Heavy Daedric Armor to Light armor. The other one allows you to craft a separate set of Light Daedric armor at the Blacksmith.