Burn Base Up. A designation specifying that the lamp should be operated with the bottom (base) of the lamp above the filament. Non- tungsten-halogen lamps were particularly finicky about orientation, due to how the coils of the filament were supported.
There is usually a number following, giving maximum deviation from vertical. Thus BBU45 (or BU45) means base up +/- 45 degrees. The BBU lamp was/is used in the radial ERS, and is in fact the reason for the lamp penetrating the reflector in a non-axial location. With the fixture above and pointing 45° downward, the lamp is vertical.
For the opposite, see BBD.
With today's modern lamps, it's an unnecessary specification, as almost all T/H lamps allow "burn in any position."
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