Why does claudius cry out for more light when he is watching the play within the play?

That evening, in the castle hall now doubling as a theater, Hamlet anxiously lectures the players on how to act the parts he has written for them. Polonius shuffles by with Rosencrantz and Guildenstern, and Hamlet dispatches them to hurry the players in their preparations. Horatio enters, and Hamlet, pleased to see him, praises him heartily, expressing his affection for and high opinion of Horatio’s mind and manner, especially Horatio’s qualities of self-control and reserve. Having told Horatio what he learned from the ghost—that Claudius murdered his father—he now asks him to watch Claudius carefully during the play so that they might compare their impressions of his behavior afterward. Horatio agrees, saying that if Claudius shows any signs of guilt, he will detect them.