The diagram below illustrates the vector relationships among three quantities that we will estimate for each Bragg reflection h by measuring the scattering amplitude Fobs() for several wavelengths _{i}.
The basic idea is that if we can locate the anomalous scattering atoms within the unit cell (those contributing to F_{A}) then we can calculate the corresponding phase angle _{A}. The MAD phasing equations of Karle [1980] (which we will get to in a minute) can then be used to generate an estimate for and F_{T}. In the simplest case (we will later show more sophisticated estimates) we can then estimate the phase of the F_{T} as + _{A}. A Fourier transform of the amplitudes F_{T} and phases ( + _{A}) should yield an electron density map corresponding to all atoms in the structure.
We may further break down the anomalous scattering component into
contributions from several types of anomalous scatterers. Each anomalous
scattering type will have its own wavelength-dependent scattering
behaviour, which we will write: