In practice, the theory of the actual Fowler–Nordheim tunneling process is much the same for all materials (though details of barrier shape may vary, and modified theory has to be developed for initial states that are localized rather than are travelling-wave-like). However, notwithstanding such differences, one expects (for thermodynamic equilibrium situations) that all CFE equations will have exponents that behave in a generally similar manner. This is why applying Fowler–Nordheim-type equations to materials outside the scope of the derivations given here often works. If interest is only in parameters (such as field enhancement factor) that relate to the slope of Fowler–Nordheim or Millikan–Lauritsen plots and to the exponent of the CFE equation, then Fowler–Nordheim-type theory will often give sensible estimates. However, attempts to derive meaningful current density values will usually or always fail.