**How does the software compute the absorption coefficient?**

When the software re-computes optical constants, the so-called dielectric function is computed first. This material quantity (a complex number with real and imaginary part) connects the electric field of the light wave to the polarization that is caused in the material.

In order to compute the reflection and transmission properties of interfaces between two adjacent materials, the complex square root of the dielectric function is needed. This is called the refractive index. The real part is usually denoted as n, the imaginary part as k. Whereas the real part contains information about the speed of light in the material, the imaginary part is responsible for absorption.

The absorption coefficient alpha, finally, is the exponent of the intensity decay of a light wave traveling through the material, i.e. the intensity decays proportional to exp(-alpha*x) where alpha = 4*pi*k*wavenumber. The wavenumber is the inverse of the wavelength, usually measured in cm.