The distance from the base of the bulb to the center.
The distance between the center of the filament or arc tube in a lamp and a reference plane-usually the bottom of the lamp.
The distance between the center of the filament, or arc tube, and a reference plane - usually the bottom of the lamp base. Refer to the following chart for reference plane locations.
The distance from a specified reference point on a lamp base to its light center, typically expressed in inches.
This dimension defines the location of the filament to the lamp base. It is measured from the geometric center of the filament to a specified point of, or plane through, the base.