As stated in the answer to this question on the Sc-Fi and Fantasy SE:
In the video below, Mark Hamill and George Lucas briefly discuss the methods used to create the lightsaber effects in the Original Trilogy films.
The effect was largely achieved by coating a thin wooden tube with reflective material (made out of movie screen) and shining bright spotlights onto it in order to make it appear fluorescent. They then used film filters in post-production to make that effect seem substantially more pronounced and to add colouring.
Later films also used a technique called rotoscoping to add the light flares and colouration.
There is also this explanation, which appears to use similar terms as Hamill and Lucas in the video:
One of the special effects in Star Wars movies is the lightsaber. The lightsaber was made by covering a rotating pole with movie screen material all around it so it would reflect its light source back. If the lightsaber moved out of a certain area it went from being a glowing light to just a rotating pole. The special effects team was originally going to use reflective tape, but as soon as the actors hit each other the sabers would break, so they used the rotating pole instead.
As the Star Wars saga went on, ILM (Industrial Light & Magic) got more efficient and used just a pole, and later did effects and screen shots to make the lightsabers glow. The lightsaber had a lot to do with advances in special effects technology more than anything else in the Star Wars movies. The sound was created by the humming noise in a projector, because that’s what Ben Burtt thought a lightsaber might sound like.
This blog compiles a lot of the knowledge about lightsabers.