“have seen a great light…the light has shined.” This is an example of the prophetic perfect idiom, when a future event is spoken of as if it had already happened. The NLT translation catches the correct sense in modern English: “The people who walk in darkness will see a great light. For those who live in a land of deep darkness, a light will shine.”

[For more on the idiom of the prophetic perfect see commentary on Ephesians 2:6, “seated.”]