They used a triangular reflective rod. A motor in the hilt caused it to spin which gives the familiar flicker. It wasn't to great on film when trying to get other colors and they bent in dueling.
They actually PAINTED the sabers in each frame. They didn't have 'puters then. They couldn't tell photoshop to do it for them. That's why the things nightwing pointed out in the color thread happened. It wasn't too precise.
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