(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/...el-lamp-base-problems.html?highlight=prefocus.
A medium prefocus socket. Notice the slots for two different sized fins.
Medium Prefocus Lamp Socket
See this thread: http://www.controlbooth.com/forums/...el-lamp-base-problems.html?highlight=prefocus.