On the PML concept: a view from the outside

The idea that an absorbing half space can be reflectionless is most intriguing. We present an exposition of the wave theory using impedance concepts. Some limitations are also mentioned. The PML (perfectly matched layer) has received considerable attention lately. The idea is not entirely new, because the concept of "invisible scatterers" was put forth over twenty years ago. The idea then, and to some extent now, is that the absorbing region should be active in some sense. An equivalent approach is to stretch the coordinates so that Maxwell's equations are modified within the absorber. The author favours the active-medium approach, designed in such a manner that Maxwell's equations don't need to be modified. As Sacks et al. (see IEEE Transactions on Antennas and Propagation, AP-43, p.1460, 1995) showed, this is a great convenience in applying the finite-element method to open-region problems.