It is also sometimes convenient to split the conversion factor β r into a "macroscopic part" that relates to the overall geometry of the emitter and its surroundings, and a "local part" that relates to the ability of the very-local structure of the emitter surface to enhance the electric field. This is usually done by defining a "macroscopic field" F M that is the field that would be present at the emitting site in the absence of the local structure that causes enhancement. This field F M is related to the applied voltage by a "voltage-to-macroscopic-field conversion factor" β M defined by: