Burn Base Down. A designation specifying that the lamp should be operated with the bottom (base) of the lamp below 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 BBD30 (or BD30) means base down +/- 30 degrees. The BBD lamp was/is used in the Fresnel and beam projector.
For the opposite, see BBU.
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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