In equation (46), for completeness purposes, λ λ d is not approximated by unity as in (29) and (30a), although for most practical cases it is a very good approximation. Apart from this, equations (43), (44) and (46) coincide with the corresponding ones of the standard Fowler–Nordheim theory (3), (9), and (30a), in the limit R → → ∞ ∞ ; this is expected since the former equations generalise the latter.