  We introduce an approximate method which can be used to simulate the optical
conductivity data of correlated multiband systems for normal and
superconducting cases by taking advantage of a reverse process of a usual
optical data analysis, which has been used to extract the electron-boson
spectral density function from measured optical spectra of single-band systems,
like cuprates. We applied this method to optical conductivity data of two
multiband pnictide systems (Ba0.6K0.4Fe2As2 and LiFeAs) and obtained the
electron-boson spectral density functions. The obtained electron-boson spectral
density consists of a sharp mode and a broad background. The obtained spectral
density functions of the multiband systems show similar properties as those of
cuprates in several aspects. We expect that our method helps to reveal the
nature of strong correlations in the multiband pnictide superconductors.
