It is a number that links the Faraday rotation with the applied magnetic field. It is not a constant but depends on both photon energy and temperature.

What kind of absorption are you thinking about? (microwaves, IR, visible, x-rays...??)

And what should the "mathematical tools" you ask for be able to do? I can't get your point.

Kai Fauth ... I am trying to measure real and imaginary part of Voigt parameter of red light of ferromagnetic material.

Gillian Gehring...... how the temperature can effect on the Voigt number? do you have any explanation for this? thank you

Gillian, I thought that was the Verdet constant. I may be wrong, of course... (I'll go and look it up)

So, Ali Al wahib, you're not looking for the Voigt function!! The Voigt function is a specific spectral line shape, resulting from a convolution of a Lorentzian and Gaussian. It is frequently used to analyze temperature (e.g. through Doppler broadening -> Gaussian) and lifetime (->Lorentzian) effects simultaneously.

See e.g. here for a definition of the Voigt parameter:

http://books.google.de/books?id=N9mvytGEBtwC&pg=PA949&lpg=PA949&dq=%22magneto+optical+voigt+parameter%22&source=bl&ots=N9stVvuRFW&sig=eQKytpBlvLQQKq0m1pj_GyQwIvg&hl=de&sa=X&ei=hIYEU-Qixr3KA9SkgpAD&ved=0CEEQ6AEwAw#v=onepage&q=%22magneto%20optical%20voigt%20parameter%22&f=false

or consider this book:

http://books.google.de/books?id=Afc_AAAAQBAJ&pg=PA226&lpg=PA226&dq=%22magneto+optical+voigt+parameter%22&source=bl&ots=7X4I8bvfoH&sig=EdOmcqqGncUSWh-iblwGiI27XHs&hl=de&sa=X&ei=hIYEU-Qixr3KA9SkgpAD&ved=0CDUQ6AEwAQ#v=onepage&q=%22magneto%20optical%20voigt%20parameter%22&f=false

The "number that links the Faraday rotation with the applied magnetic field. It is not a constant but depends on both photon energy and temperature" is a Verdet constant. The Voigt damping parameter is used in a Voigt function description (read about).

O. Miloslavskaya, there is a "magneto-optical Voigt parameter", which actually appears in the off-diagonals of the permittivity tensor (see book links above) and which has nothing to do with the Voigt parameter of the (spectroscopic) Voigt function. I have not, however, taken the time to find out the relationship between this m.-o. Voigt parameter and the Verdet constant...

Many thanks Kai Fuath for your help