where ν ν F [= ν ν F (φ, F)] is the exponent correction factor for a barrier of unreduced height φ. This is the most general equation of the Fowler–Nordheim type. Other equations in the family are obtained by substituting specific expressions for the three correction factors ν ν F , P F and λ Z it contains. The so-called elementary Fowler–Nordheim-type equation, that appears in undergraduate textbook discussions of field emission, is obtained by putting λ Z →1, P F →1, ν ν F →1; this does not yield good quantitative predictions because it makes the barrier stronger than it is in physical reality. The so-called standard Fowler–Nordheim-type equation, originally developed by Murphy and Good, and much used in past literature, is obtained by putting λ Z → t F, P F →1, ν ν F → v F, where v F is v (f), where f is the value of f h obtained by putting h = φ, and t F is a related parameter (of value close to unity).