The following short section is adopted from the original theory section of  and is included here for the sake of completeness.
The orientation of the calibrator crystal is described with respect to a laboratory frame coordinate system having in the direction of the incident X-rays, pointing vertically upwards and pointing horizontally to complete a right-handed system. The reference orientation is taken to be when is along and the reciprocal lattice vector points directly back along the incident beam vector . Rotations from this reference position about and are described by the angles and .
Assuming an arbitrary crystal orientation given by and , for any reciprocal lattice vector in the laboratory frame, the angle of incidence , of the beam to the diffracting plane may be expressed as
By combining this equation and the Bragg equation, taking the Bragg angle as , the following result is obtained.
where is a constant and equal to  for in Åand in . The orientation of may now be related back to its orientation in the reference position (denoted by a prime) by
and by substituting for in Eq. we arrive at
A final correction to in the above equation yields the true energy . The correction used is that proposed by Materlik and Kostroun  and takes the form
where b is an asymmetry parameter and , being the refractive index.