High-bay lights are used to illuminate spaces with high ceilings. That usually means ceilings ranging from 20 feet to approximately 45 feet. Low-bay lights, on the other hand, are used for ceilings 20 feet and under. Because a high-ceiling location has more space to fill, a high-bay, by definition, is a powerful light source that can brighten up a large area. The term UFO comes from the round shape of this style of high bay light.