At present, the only parameter that is considered important is the correction factor ν ν F relating to the barrier shape, and the only barrier for which there is any well-established detailed theory is the Schottky–Nordheim barrier. In this case, σ FN is given by a mathematical function called s. This function s was first tabulated correctly (as a function of the Nordheim parameter y) by Burgess, Kroemer and Houston in 1953; and a modern treatment that gives s as function of the scaled barrier field f for a Schottky–Nordheim barrier is given in. However, it has long been clear that, for practical emitter operation, the value of s lies in the range 0.9 to 1.