Coupling factor as a function of the normalized radial extent of the evanescent region in Type-b evanescent regions according to Eqs. (19), (20) and (28). The cyan, blue, magenta and red colors have the same meaning as in Fig. 6. For comparison, the domain between both dashed lines represents the predictions obtained for Type-a evanescent regions in the formalism of Shibahashi (1979) such as plotted in Fig. 6.