Ordinarily, the performance of electron lenses in electron imaging devices employing photoelectric surfaces can be evaluated only after sealed‐off vacuum tubes containing the processed photoelectric surfaces are made. After processing, such tubes cannot be opened to air for alterations without destroying the photoelectric surfaces. Three distinct types of structures for providing area sources of low‐energy electrons simulating photoelectric sources are discussed. These structures can be inserted in demountable vacuum systems and are not destroyed by subsequent exposure to air.