Typically used with Psyche.
In short: dis illusions. While Clairvoyance is about seeing or feeling things on a supernatural level, Lucidity is about seeing through things which do not exist at all: illusions. Anything which affects the five senses in an artificial or magical way may require a Lucidity check to see if the character will experience things as they actually are and not as they were altered to be.
Note that a character must first be aware of the illusion in order to see beyond it. If there's an illusory dog bark that the character doesn't hear in the first place, there's no grounds for disproving it.
Lucidity checks are always in opposition to a Magic Spell or similar effect, so there are no set difficulties.
Lucidity checks are used against Magic Spells and similar effects.