  Thermionic emission from monolayer graphene grown on representative
transition metals, Ir and Ru, is characterized by low-energy electron
microscopy (LEEM). Work functions were determined from the temperature
dependence of the emission current and from the electron energy spectrum of
emitted electrons. The high-temperature work function of the strongly
interacting system graphene/Ru(0001) is sufficiently low, 3.3 \pm 0.1 eV, to
have technological potential for large-area emitters that are spatially
uniform, efficient, and chemically inert. The thermionic work functions of the
less strongly interacting system graphene/Ir(111) are over 1 eV larger and vary
substantially (0.4 eV) between graphene orientations rotated by 30{\deg}.
