Crystal Orientation as Coordinate Transformation edit page

In MTEX a crystal orientation is defined as the rotation that transforms crystal coordinates, i.e., a description of a vector or a tensor with respect to the crystal reference frame, into specimen coordinates, i.e., a description of the same object with respect to a specimen fixed reference frame.

In MTEX any orientation consists of two ingredients. A rotation

and a description of the crystal lattice, which are represented in MTEX by variables of type crystalSymmetry

Combining both ingredients allows us to define an orientation

As a consequence a variable of type orientation is at the same time of type rotation and hence allows for all operations that are available for rotations.

## Crystal coordinates to specimen coordinates

Let us consider the following crystal direction

In a grain with orientation ori this direction h has the specimen coordinates

Similarly, orientations transform tensors given with respect to the crystal reference frame, e.g., the following single crystal stiffness tensor

into a stiffness tensor with respect to the specimen reference frame

Objects that can be translated by orientations from crystal into specimen coordinates and vice versa include

## Specimen coordinates into crystal coordinates

Conversely, we can go back from specimen coordinates to crystal coordinates by multiplying with the inverse orientation

Note, that in literature orientations are often defined to transform specimen coordinates into crystal coordinates, i.e., to coincide with the inverse orientations in MTEX. The consequences of this differences are exhaustively discussed in the topic orientation conventions.

## Specimen Rotation

Rotations of the specimen ,i.e., changing the specimen reference frame, do also change the orientation. Assume the specimen is rotated about the X-axis about 60 degree. We may define this rotation by

Then an orientation ori is updated to the rotated reference frame by

It should also be noted, that orientations are sensitive with respect to the alignment of the Euclidean reference frame $$\vec X$$, $$\vec Y$$, $$\vec Z$$ with respect to the crystal axes $$\vec a$$, $$\vec b$$ and $$\vec c$$. This issue is discussed in more detail in the topic Crystal Reference Frames.