A ray optic description of evanescent wave sensing allows a homogeneous tre
atment of the sensitivity of free space and guided wave slab sensors. It is
shown in particular that the sensitivity of evanescent wave sensors is gen
erally smaller or much smaller than that of a Free space wave traversing th
e probed medium except in thc remarkable case of large contrast slab wavegu
ides propagating a restricted number of TM modes where the sensitivity can
exceed that of a free space wave.