What was the spell did harry potter use in prisoner of Azkaban to make light?

Harry Potter didn't use a spell to make light in "Prisoner of Azkaban." He used his wand to light his Patronus Charm, which takes the form of a silvery, stag-like creature. The Patronus is a powerful charm that can repel Dementors, the dark creatures that feed on happiness and can cause despair.

The Patronus itself is a source of light, allowing Harry to see in the dark and ward off the Dementors.