A radiator style emitter refers to a series of evenly stacked and spaced rings with alternating grooves near the top of a lightsaber hilt. The Leia Organa lightsaber in Star Wars: The Rise of Skywalker features a radiator style emitter. Several Prequel Trilogy clan sabers feature a radiator style emitter as well, including: the Luminara Unduli lightsaber, Plo Koon lightsaber, Mace Windu lightsaber (Episode I), and Kit Fisto lightsaber.
The ‘radiator style emitter’ somewhat resembles a real world, old fashioned, cast iron household radiator heater.
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COVER IMAGE CREDIT Lucasfilm/Disney
Image depicts the Leia Organa lightsaber (left) and the emitter section of the Luminara Unduli lightsaber (right)