The Corps was the peacekeeping tool of the Guardians of the Universe, incredibly old beings who promoted an orderly, rational cosmos. The 3,600 members of the Corps came from almost as many worlds. They were each armed with a ring of power and given a sector of space to protect. Except for weakness against yellow and the need to be recharged every 24 hours, the rings were limited only by the bearers will. told to Kyle Rayner by Alan Scott in Green Lantern #55, September 1994.
There were some changes after Kyle got a hold of the ring, not the least of which that the rings held a charge for more than 24 hours and the yellow impurity no longer existed. This was partially explained in the mini-series Rebirth of Hal Jordan when it was revealed that Parallax was an ancient enemy that the Guardians had trapped within the central power battery on Oa. The creature was composed of yellow energy and it was the root cause of the weakness in the rings. Once it escaped into Hal Jordan, thus beginning his path to corruption, the yellow impurity no longer existed. The first evidence that was seen of this was when Kyle Rayner had the ring and faced off against the tyrant Mongul, who used yellow energy weapons and had a yellow pigmentation.
There was a time prior to this, when the Guardians were experimenting with methods of improving the rings. They briefly created a small number of them that had no yellow impurity and didn't require recharging, presumably the impurity didn't exist as they weren't connected to the central power battery.
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