Recall that we define a ``Lab Frame'' (LF) in which the incident radiation
propagates in the +x direction.
In ddscat.par one specifies the first
polarization state 
(which obviously must lie in the y,z plane in the LF);
DDSCAT automatically constructs a second polarization state
orthogonal to (here 
is the unit vector
in the +x direction of the LF.
For purposes of discussion we will
always let unit vectors , 
,
be the three coordinate axes of the LF.
Users will often find it convenient to let polarization
vectors 
,
(although this is not mandatory -
see §20).
Figure 5: Target orientation in the Lab Frame. is
the direction of propagation of the incident radiation, and is
the direction of the ``real'' component of the
first incident polarization mode.
In this coordinate system, the orientation of target axis 
is specified by angles 
and 
.
With target axis fixed, the orientation of target axis
is then determined by angle 
specifying rotation of
the target around .
When 
, lies in the ,
plane.
Recall that definition of a target involves specifying two unit vectors,
and , which are imagined to be ``frozen'' into the target.
We require to be orthogonal to .
Therefore we may define a ``Target Frame" (TF) defined by the three unit
vectors , , and 
.
For example, when DDSCAT creates a 8
64
rectangular solid, it fixes to be
along the longest dimension of the solid, and to be
along the next-longest dimension.
Orientation of the target relative to the incident radiation can in principle be determined two ways:
and in the LF, or
(incidence direction) and
in the TF.
, , and
are specified in the file ddscat.par.
The target is oriented such that the polar angles
and specify the direction of
relative to the incident direction , where
the , plane has 
.
Once the direction of is specified, the
angle than specifies how the target is to rotated
around the axis to fully specify its orientation.
A more extended and precise explanation follows:
, , and