# Dielectric function tools

The `DielectricFunction`

class
inherits from `ComplexSignal`

and can
thus access complex properties. To convert a
`ComplexSignal`

to a
`DielectricFunction`

,
make sure that the signal dimension and signal type are properly set:

>>> s.set_signal_type('DielectricFunction')

Note that `DielectricFunction`

is
complex and therefore is a subclass of
`ComplexSignal1D`

.

## Number of effective electrons

The Bethe f-sum rule gives rise to two definitions of the effective number (see [Egerton2011]):

where is the number of atoms (or molecules) per unit volume of the sample, is the vacuum permittivity, is the electron mass and is the electron charge.

The
`get_number_of_effective_electrons()`

method computes both.

## Compute the electron energy-loss signal

The
`get_electron_energy_loss_spectrum()`

“naively” computes the single-scattering electron-energy loss spectrum from the
dielectric function given the zero-loss peak (or its integral) and the sample
thickness using:

where is the zero-loss peak integral, the sample thickness, the collection semi-angle and the characteristic scattering angle.