Kuperman and Ingenito [49] obtained the following interface condition
for an interface with RMS roughness 
:
where the elements of A are given by:
with,
and,
The subscripts 1 and 2 denote properties in the upper and lower media,
respectively. For a free surface the appropriate result is obtained by
letting 
and 
go to zero. Note that this formula also
breaks down in a catastrophic fashion if
is too large.