Cal admits himself to a mental hospital on the episode "Pied Piper." His goal? To determine the innocence or guilt of a killer.
This is a scene from the Lie to Me episode "Pied Piper." During it, Lightman is witnessing the execution of a man he helped put away.
Cal watches an execution in this scene from Lie to Me. The man who is killed is someone he helped put away.

Lie to Me Season 2 Episode 19 Quotes

Agent Reynolds: Frankly you are beginning to sound a little insane.
Lightman: Really?
Reynolds: Really.
Lightman: I was hoping you would say that.

Eli: Does Lightman seem weird to you?
Tia: He's rude, impatient, distant, seems pretty normal to me.