3. Theory

In this chapter a number of fundamentals are outlined. Only a brief introduction to the necessary physics (which already has a rich literature) is presented, and the emphasis is instead on code and numerical examples. These are intended both to give readers an insight into the physics, and also illustrate aspects of the Photoelectron Metrology Toolkit [5] and ePSproc codebase [33, 34, 35] that can be used for these problems. These methods will form the basis for the numerical reconstruction work presented in Part II.

Readers only interested in fitting problems from an experimental perspective may wish to skip most of this section; Sect. 3.6: and Sect. 3.7: should provide sufficient background for pure reconstruction problems.