The Birth of the Lightsaber was a featurette in the bonus material of 2004 DVD release of the Star Wars original trilogy. It revealed how the lightsaber was made, from tape to rotoscoping. It was also included in a DVD that was packaged with select Hasbro lightsabers alongside two chapters of Star Wars: Clone Wars.
Sources
Star Wars Featurette: The Birth of the Lightsaber on the official Star Wars YouTube channel (backup link) (Posted on StarWars.com)