What is BU?
BU is an abbreviation for “base up.” This refers to the bulb’s ideal—or sometimes mandatory—burn position. A bulb marked with “BU” must be burned with the base up and the lamp pointing down.
What Happens if I Position a BU Lamp Base Down?
Base position specifications are most common among MH lamps, and the consequences of disregarding base position varies. Problems may include color consistency issues, shortened lifespan (for the bulb), inferior lumen output, reduced efficiency, and—in some cases—a shattered bulb. In other words, it’s best to just follow the directions.