DDSCAT uses three angles, 
, 
, and 
,
to specify the directions
of unit vectors 
and 
in the LF
(see Fig. 5).
is the angle between and 
.
When 
, will lie in the 
plane.
When is nonzero, it will
refer to the rotation of around : e.g.,
puts in the 
plane.
When 
, will lie in the 
plane,
in such a way that when 
and , is in the
direction: e.g, 
, , has
and
.
Nonzero introduces an additional rotation of around
: e.g., , , 
has and 
.
Mathematically:
or, equivalently: