Chain Gang (1950)

Crusading newspaperman Cliff Roberts (Douglas Kennedy), masquerades as a prison guard and gets a job attached to the chain gang. Using a tiny camera in the shape of a cigarette lighter, he takes pictures of and documents the miserable conditions. The sadist guard captain, Captain Duncan (Emory Parnell), learns about Cliff's activities and has him beaten by the other guards. Roberts escapes to write his expose that brings about some prison reforms.