Crystal Axis Orientation
FRED is capable of modeling uniaxial birefringent materials, characterized by two unique refractive index values and a single crystal axis of symmetry.
The material is created using the "Sampled Birefringent and/or Optically Active Material" type where the user specifies the refractive indices for the ordinary and extraordinary axes, along with the gyrotropic coefficients. The user is also asked to specify the optical axis of the crystal.
However, since a material can be assigned to multiple objects/surfaces in the model, it is not immediately obvious in what co-ordinate system this axis is defined.
When the birefringent material is assigned by drag/drop on the object tree or through the Materials tab of the edit/view dialog box the coordinate system of the birefringent material is automatically assigned to the parent node of the individual surfaces.
In the example below, dragging and dropping the Calcite material to the "Crystal plate" object links the crystal axis to the orientation of this object. This makes the most sense and also means that if the lens is then moved and/or rotated, the crystal axis also moves with it.
In the second example below, dragging and dropping the Calcite material to an individual surface (in this case, a child surface of a Custom Element) assigns the material to this surface, but the crystal axis is aligned to the axis of the "Custom Crystal plate" object because this is the parent.
A useful interface can be found under Tools / Edit/View GRIN/Birefringent Material Position/Orientation and allows the user to view and change the various relationships between the birefringent material axis and the objects/surfaces to which they have been assigned.