Corpselight

A standby of hunters of the undead, this spell is most useful when tracking elusive types such as ghouls, ghosts, and vampires. 

You touch one object that is no larger than 10 feet in any dimension. Until the spell ends, the object sheds an invisible radiance in a 20-foot radius that only illuminates dead and undead flesh and bone, causing corpses and undead creatures whose challenge rating is less than your level to glow, shedding dim light in a 10-foot radius. The light can be colored as you like. Completely covering the object with something opaque blocks the effect. Undead creatures illuminated by this radiance continue to glow for 1 minute after leaving its radius.

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the radius of the illumination increases by 10 feet for each slot level above 1st.