This question originally appeared on Quora. 
Answer by Ken Miyamoto, Screenwriter
There have been a few examples of this actually happening in real life, as far as copyright infringement centered around lightsabers.
Back in 2010, Lucasfilm Ltd. sent a cease-and-desist letter to Hong Kong-based Wicked Lasers, threatening legal action if it doesn't change its Pro Arctic Laser series or stop selling it altogether.
"It is apparent from the design of the Pro Arctic Laser that it was intended to resemble the hilts of our lightsaber swords, which are protected by copyright ... ," said a letter, provided to CNN by Wicked Lasers.
Steve Liu, CEO of Wicked Lasers, said that there was no intention of creating a handle for their lasers that resembles anything from Star Wars. He went on to say that the design is fairly typical for a handheld laser.
"Most people feel it's kind of ridiculous ...," he said. "We would never use any comparison like that to 'Star Wars' or a lightsaber or anything like that... Lucasfilm shouldn't be saying something like that," he said. "They're a big company that needs to protect their trademarks. Maybe they're having to look like they're protecting their trademark in case they need to [protect it again] later."
They never called it a lightsaber in the marketing campaign for the product, although some reviews did make mention of it being a real life lightsaber and such.
"These references make it clear that the public is being led to believe that the Pro Arctic Laser is an official lightsaber device and/or copied from our design," the Lucasfilm letter said.
Lucasfilm does own the Lightsaber copyright. Although, as mentioned, the company was not using the lightsaber name or likeness.
Under the 1976 Copyright Act, the owner of copyright has the exclusive right to reproduce the copyrighted work, to prepare derivative works, to distribute copies of the copyrighted work, to perform the copyrighted work publicly, or to display the copyrighted work publicly. It is under this principle that Lucasfilm would argue that Wicked Lasers has violated their copyright.
Wicked Lasers offered that there are several differences their product and a lightsaber. The Pro Arctic is "1 watt peak output power while Lightsaber is 1 billion watts peak output power," said the release. The Pro Arctic "has nonadjustable infinite length beam while Lightsaber has adjustable finite length blade."
Lastly, they cleverly stated that their product is "real while the lightsaber is imaginary."
Lucasfilm called the company's newest laser "a highly dangerous product with the potential to cause blindness, burns, and other damage to people and/or property."
In the end, the company added some additional safety features, a safety lens, and put a statement on their product and website regarding the fact that they are promoting their product as a lightsaber.
Now, that's a real life scenario for a real product. But what about another filmmaker incorporating lightsabers into their own film?
We've seen ENDLESS fan made films doing so, as well as utilizing many trademarked and copyrighted Star Wars material, images, designs, etc. George Lucas took a generous stance years ago and accepted the fact that fans are having fun with the world he created, therefore allowing such fan films to exist without any lawsuits.
However, if a film utilizes or incorporates any such things for profit (i.e. Another movie studio, production company, etc.), then there's likely to be an issue.
Overall, if a science fiction film were to use lightsabers (But called them lazor swords or something), I'm sure Lucasfilm would be on their ass right away, and rightfully so.
Spaceballs obviously utilized similar visuals, however their lightsabers had no handles ... they were formed through Schwartz rings. Regardless, Mel Brooks had sent George Lucas the script to get his blessing, and Lucas saw the humor in it and had no problems. ILM even did some of the special effects for that film as well.
What really comes into play is Fair Use, a legal exception to copyright law.
This law allows material to be used for parody or satire. Spaceballs, likely would have been able to go forward with legal right, however, as said above, Lucas gave Brooks his blessing.
Explanation of Fair Use, in the case of films, Taken from Wikipedia:
Producers or creators of parodies of a copyrighted work have been sued for infringement by the targets of their ridicule, even though such use may be protected as fair use. These fair use cases distinguish between parodies (using a work in order to poke fun at or comment on the work itself) and satires (using a work to poke fun at or comment on something else). Courts have been more willing to grant fair use protections to parodies than to satires, but the ultimate outcome in either circumstance will turn on the application of the four fair use factors.
In Campbell v. Acuff-Rose Music Inc[20] the Supreme Court recognized parody as a potential fair use, even when done for profit. Roy Orbison's publisher, Acuff-Rose Music Inc, had sued 2 Live Crew in 1989 for their use of Orbison's "Oh, Pretty Woman" in a mocking rap version with altered lyrics. The Supreme Court viewed 2 Live Crew's version as a ridiculing commentary on the earlier work, and ruled that when the parody was itself the product rather than used for mere advertising, commercial sale did not bar the defense. The Campbell court also distinguished parodies from satire, which they described as a broader social critique not intrinsically tied to ridicule of a specific work, and so not deserving of the same use exceptions as parody because the satirist's ideas are capable of expression without the use of the other particular work.
A number of appellate decisions have recognized that a parody may be a protected fair use, including both the Second (Leibovitz v. Paramount Pictures Corp.) and Ninth Circuits (Mattel v. Walking Mountain Productions). Most recently, in Suntrust v. Houghton Mifflin, a suit was brought unsuccessfully against the publication of The Wind Done Gone, which reused many of the characters and situations from Gone with the Wind, but told the events from the point of view of the slaves rather than the slaveholders. The Eleventh Circuit, applying Campbell, recognized that The Wind Done Gone was fair use, and vacated the district court's injunction against its publication.
So as you can see, there are many things to consider here.
Basically, in my opinion, if someone were to use a lightsaber in their film without the permission of Lucas, and without any Fair Use rights (i.e. It being a non-parody or non-satire science fiction film), that'd be both a poor business decision and especially a poor creative decision. Make your own concept.