In the context of electron emission from surfaces, the area efficiency of emission (alpha) is defined as the ratio of the effective area of high-density emission (called the notional emission area) to the apparent area of an emitting sample as judged by its geometrical size (called the macroscopic area). For practical large-area field emitters, values of alpha are always very small in comparison with 1, perhaps often less than 10(-6). This article argues that to avoid errors of interpretation, it is best to write the equations that describe cold field electron emission from large-area emitters in forms that explicitly include alpha. Also, definitions of areas and current densities should be stated with care. It is shown that electrostatic arguments can provide a rough upper limit for alpha, above which the design of an array of identical postlike emitters becomes inefficient. This upper limit (alpha(max)) is of the order of 2/gamma(2), where gamma is the field enhancement factor for one of these emitters when standing isolated. Practical emitters will normally have alpha

Keywords:current density;electron field emission;electron sources