prefocus

(Literally, Pre Focus.) A type of lamp base designed so that the lamp can only be seated on one orientation. This was more important with the planar filaments of pre-TH lamps (as TH lamps tend to have symmetrical filaments), but the base type remains and is primarily used today with the BTL family of lamps in most 6" Fresnels.

A medium prefocus socket. Notice the slots for two different sized fins.

Medium Prefocus Lamp Socket

See this thread: http://www.controlbooth.com/forums/lighting/11026-altman-6-fresnel-lamp-base-problems.html?highlight=prefocus.

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